Thermophilic DNA polymerases from Thermotoga neapolitana
The present invention relates to thermostable DNA polymerases derived from the hyperthermophilic eubacteria, and Thermotoga neapolitana in particular. The present invention provides means for isolating and producing the enzymes from these thermostable DNA polymerases, which are useful in many recombinant DNA techniques, especially such techniques as thermal cycle sequencing and nucleic acid amplification.
[0001] The present invention relates to thermostable DNA polymerases derived from the hyperthermophilic eubacteria Thermotoga neapolitana and means for isolating and producing the enzymes. Thermostable DNA polymerases are useful in many recombinant DNA techniques, especially thermal cycle sequencing and nucleic acid amplification.
BACKGROUND[0002] Thermophilic bacteria are organisms which are capable of growth at elevated temperatures. Unlike the mesophiles, which grow best at temperatures in the range of 25-40° C., or psychrophiles, which grow best at temperatures in the range of 15-20° C., thermophiles grow best at temperatures greater than 50° C. Indeed, some thermophiles grow best at 65-75° C., and some of the hyperthermophiles grow at temperatures up to 130° C. (See e.g., J. G. Black, Microbiology Principles and Applications, 2d edition, Prentice Hall, New Jersey, [1993] p. 145-146).
[0003] The thermophilic bacteria encompass a wide variety of genera and species. There are thermophilic representatives included within the phototrophic bacteria (e.g., the purple bacteria, green bacteria, and cyanobacteria), eubacteria (e.g., Bacillus, Clostridium, Thiobacillus, Desulfotomaculum, Thermus, lactic acid bacteria, actinomycetes, spirochetes, and numerous other genera), and the archaebacteria (e.g., Pyrococcus, Thermococcus, Thermoplasma, Thermotoga, Sulfolobus, and the methanogens). There are aerobic, as well as anaerobic thermophilic organisms. Thus, the environments in which thermophiles may be isolated vary greatly, although all of these organisms are always isolated from areas associated with high temperatures. Natural geothermal habitats have a worldwide distribution and are primarily associated with tectonically active zones where major movements of the earth's crust occur. Thermophilic bacteria have been isolated from all of the various geothermal habitats, including boiling springs with neutral pH ranges, sulfur-rich acidic springs, and deep-sea vents. For all of these organisms, it appears that the organisms present in these geothermal habitats are optimally adapted to the temperatures at which they are living (T. D. Brock, “Introduction: An overview of the thermophiles,” in T. D. Brock (ed.), Thermophiles: General, Molecular and Applied Microbiology, John Wiley & Sons, New York [1986], pp. 1-16). Basic as well as applied research on thermophiles has provided some insight into the physiology of these organisms, as well as promise for use of these organisms in industry and biotechnology.
[0004] 1. The Genus Thermotoga
[0005] The Thermotoga is a recently described genus with three recognized species, which includes the most extremely thermophilic eubacteria known. The genus was first described in 1986, by Huber et al., (R. Huber et al. Arch. Microbiol. 144:324 [1986]; and Int. J. Syst. Bacteriol., 36:575 [1986]). At this time, there was only one species described, T. maritima. T. neapolitana was first described by Jannasch et al. in 1986 (Jannasch et al., Arch. Microbiol., 150:103-104 [1986]; and Int. J. Syst. Bacteriol., 39:93 [1989]). T. thermarum was described by Windberger et al. in 1989 (Windberger et al., Arch. Microbiol., 151:506-512; and Int. J. Syst. Bacteriol., 42:327 [1992]).
[0006] These organisms were originally isolated from geothermally heated marine sediments and hot springs. For example, T. maritima has been isolated from geothermally heated sea floors in Italy, the Azores, Indonesia, and Iceland as well as from continental, solfataric springs in Africa. T. neapolitana has been isolated from a submarine thermal vent near Naples and from continental, solfataric springs in Africa.
[0007] Members of the genus Thermotoga are considered to be hyperthermophilic, as they are capable of growth at temperatures up to 90° C., although growth will occur at temperatures between 55° C. and 90° C.; the optimum growth temperature is between 70-80° C. The Thermotoga are strictly anaerobic, non-sporing, Gram-negative rods, that ferment carbohydrates, and may be motile by polar, lateral or peritrichous flagella, although some strains are non-motile. The cells are surrounded by a sheath-like outer structure which usually balloons over the ends. In all species of Thermotoga, 1-4 cells may be enclosed within one sheath.
[0008] T. maritima, T. neapolitana, and T. thermarum are distinct species as judged by the numerous differences. For example, T. maritima possesses a single subpolar flagellum and is motile, while motile strains of T. neapolitana possess peritrichous flagella (some strains are non-motile), and T. thermarum possesses lateral flagella. In addition, T. neapolitana will grow in NaCl concentrations ranging from 0.25 to 6.0%, while T. maritima will grow in NaCl concentrations ranging from 0.25 to 3.75%, and T. thermarum will grow in NaCl concentrations ranging from 0.2-0.55% (J. G. Holt et al (eds.), Bergey's Manual® of Determinative Bacteriology 9th ed., Williams & Wilkins, Baltimore, [1994], p. 333). Also, there are differences in the susceptibility of these species to rifampicin, and differences in the inhibitory effects of hydrogen and sulfur on these species. Furthermore, the rate of growth at optimum growth temperature (80° C.) is a doubling time of about 45 min for T. neapolitana and about 75 min for T. maritima The G+C content of the DNA of T. maritima and T. neapolitana is 46% and 41%, respectively. The DNA from T. maritima and T. neapolitana shows only about 25-30% homology by DNA-DNA hybridization studies.
[0009] A few of the enzymes of the Thermotoga and other thermophilic genera have been studied in varying degrees of detail. As discussed below, the use of thermophilic enzymes in industry has been viewed as providing advantages over the use of mesophilic enzymes.
[0010] II. Uses for Thermophilic Enzymes
[0011] Advances in molecular biology and industrial processes have led to an increased interest in thermophilic organisms such as Thermotoga. Of particular interest has been the development of thermophilic enzymes for use in industrial processes such as the detergent, flavor-enhancing, and starch industries. Indeed, the cost savings associated with the longer storage stability and higher activity at higher temperatures of thermophilic enzymes, as compared to mesophilic enzymes, provide good reason to select and develop thermophilic enzymes for industrial and biotechnology applications. Thus, there has been much research conducted to characterize enzymes from thermophilic organisms. However, some thermophilic enzymes have less activity than their mesophilic counterparts under similar conditions at the elevated temperatures used in industry (typically temperatures in the range of 50-100° C.) (T. K. Ng and William R. Kenealy, “Industrial Applications of Thermostable Enzymes,” in T. D. Brock (ed.), Thermophiles: General, Molecular, and Applied Microbiology, [1986], John Wiley & Sons, New York, pp. 197-215). Thus, the choice of a thermostable enzyme over a mesophilic one may not be as beneficial as originally assumed. Nonetheless, of the $400 million worth of enzymes sold worldwide in 1984, 90% were thermostable enzymes used by the detergent and starch industries (Ng and Kenealy, at p. 206). However, much research remains to be done to characterize and compare thermophilic enzymes of importance in areas such as molecular biology (e.g., polymerases, ligases, topoisomerases, restriction endonucleases, etc.).
[0012] III. Thermophilic DNA Polymerases
[0013] Extensive research has been conducted on the isolation of DNA polymerases from mesophilic organisms such as E. coli (See e.g., Bessman et al., J. Biol. Chem. 223:171 [1957]; Buttin and Kornberg, J. Biol. Chem. 241:5419 [1966]; and Joyce and Steitz, Trends Biochem. Sci., 12:288-292 [1987]). Other mesophilic polymerases have also been studied, such as those of Bacillus licheniformis (Stenesh and McGowan, Biochim. Biophys. Acta 475:3244 [1977]; Stenesh and Roe, Biochim. Biophys. Acta 272:156-166 [1972]); Bacillus subtilis (Low et al., J. Biol. Chem., 251:1311 [1976]; and Ott et al., J. Bacteriol., 165:951 [1986]; Salmonella typhimurium (Harwood et al., J. Biol. Chem., 245:5614 [1970]; Hamilton and Grossman, Biochem., 13:1885 [1974]), Streptococcus pneumoniae (Lopez et al., J. Biol. Chem., 264:4255 [1989]), and Micrococcus luteus (Engler and Bessman, Cold Spring Harbor Symp., 43:929 [1979]), to name but a few.
[0014] Somewhat less investigation has been made on the isolation and purification of DNA polymerases from thermophilic organisms. However, native (i.e, non-recombinant) and/or recombinant thermostable DNA polymerases have been purified from various organisms, as shown in Table 1 below. 1 TABLE 1 Polymerases Isolation From Thermophilic Organisms Organism Citation Thermus aquaticus Kaledin et al., Biochem., 45: 494-501 (1980); Biokhimiya 45: 644-651 (1980). Chien et al., J. Bacteriol., 127: 1550 (1976). University of Cincinnati Master's thesis by A. Chien, “Purification and Characterization of DNA Polymerase from Thermus aquaticus,” (1976). University of Cincinnati, Master's thesis by D. B. Edgar, “DNA Polymerase From an Extreme Thermophile: Thermus aquaticus,” (1974). U.S. Pat. No. 4,889,818* U.S. Pat. No. 5,352,600* U.S. Pat. No. 5,079,352* European Patent Pub. No. 258,017* PCT Pub. No. WO 94/26766* PCT Pub. No. WO 92/06188* PCT Pub. No. WO 89/06691* Thermotoga maritima PCT Pub. No. WO 92/03556* Thermotoga strain FjSS3-B.1 Simpson et al., Biochem. Cell Biol., 68: 1292-1296 (1990). Thermosipho africanus PCT Pub. No. 92/06200* Thermus thermophilus Myers and Gelfand, Biochem., 30: 7661 (1991) PCT Pub. No. WO 91/09950* PCT Pub. No. WO 91/09944* Bechtereva et al., Nucleic Acids Res., 17: 10507 (1989). Glukhov et al., Mol. Cell. Probes 4: 435-443 (1990). Thermus thermophilus Carballeira et al., BioTech., 9: 276-281 (1990) Ruttiman et al., Eur. J. Biochem., 149: 41-46 (1985). Oshima et al., J. Biochem., 75: 179-183 (1974). Sakaguchi and Yajima, Fed. Proc., 33: 1492 (1974) (abstract). Thermus flavus Kaledin et al., Biochem., 46: 1247-1254 (1981); Biokhimiya 46: 1576-1584 (1981). PCT Pub. No. WO 94/26766* Thermus ruber Kaledin et al., Biochem., 47: 1515-1521 (1982); Biokhimiya 47: 1785-1791 (1982) Thermoplasma acidophilum Hamal et al., Eur. J. Biochem., 190: 517-521 (1990). Forterre et al., Can. J. Microbiol., 35: 228-233 (1989). Sulfolobus acidocaldarius Salhi et al., J. Mol. Biol., 209: 635-641 (1989). Salhi et al., Biochem. Biophys. Res. Comm., 167: 1341-1347 (1990). Rella et al., Ital. J. Biochem., 39: 83-99 (1990). Forterre et al., Can. J. Microbiol., 35: 228-233 (1989). Rossi et al., System. Appl. Microbiol., 7: 337-341 (1986). Klimczak et al., Nucleic Acids Res., 13: 5269-5282 (1985). Elie et al., Biochim. Biophys. Acta 951: 261-267 (1988). Bacillus caldotenax J. Biochem., 113: 401-410 (1993). Bacillus stearothermophilus Sellmann et al., J. Bacteriol., 174: 4350-4355 (1992). Stenesh and McGowan, Biochim. Biophys. Acta 475: 32-44 (1977). Stenesh and Roe, Biochim. Biophys. Acta 272: 156-166 (1972). Kaboev et al., J. Bacteriol., 145: 21-26 (1981). Methanobacterium Klimczak et al., Biochem., 25: 4850-4855 (1986). thermoautotropicum Thermococcus litoralis Kong et al., J. Biol. Chem. 268: 1965 (1993); U.S. Pat. No. 5,210,036*; U.S. Pat. No. 5,322,785* Pyrococcus furiosus Lundberg et al., Gene 108: 1 (1991) PCT Pub. WO 92/09689 *Herein incorporated by reference.
[0015] Although the organisms listed in Table 1 are considered thermophiles, many are in the archaebacteria, a group that is evolutionarily distinct from the eubacterial genus Thermotoga.
[0016] In addition to native forms, modified forms of thermostable DNA polymerases having reduced or absent 5′ to 3′ exonuclease activity have been expressed and purified from T. aquaticus, T. maritima. Thermus species sps17, Thermus species Z05, T. thermophilus and T. africanus [PCT Publication No. 92/06200].
[0017] IV. Uses for Thermophilic DNA Polymerases
[0018] One application for thermostable DNA polymerases is the polymerase chain reaction (PCR). The PCR process is described in U.S. Pat. Nos. 4,683,195 and 4,683,202, the disclosures of which are incorporated herein by reference. Primers, template, nucleoside triphosphates, the appropriate buffer and reaction conditions, and polymerase are used in the PCR process, which involves denaturation of target DNA, hybridization of primers and synthesis of complementary strands. The extension product of each primer becomes a template for the production of the desired nucleic acid sequence. If the polymerase employed in the PCR is a thermostable enzyme, then polymerase need not be added after each denaturation step because heat will not destroy the polymerase activity. Use of such enzymes as Taq DNA polymerase allows repetitive heating/cooling cycles without the requirement of fresh enzyme at each cooling step. This represents a major advantage over the use of mesophilic enzymes such as Klenow, as fresh enzyme must be added to each individual reaction tube at every cooling step. The use of Taq in PCR is disclosed in U.S. Pat. No. 4,965,188, EP Publ. No. 258,017, and PCT Publ. No. 89/06691, herein incorporated by reference.
[0019] In addition to PCR, Taq DNA polymerase is widely used in other molecular biology techniques including recombinant DNA methods. For example, various forms of Taq have been used in a combination method which utilizes PCR and reverse transcription (see e.g., U.S. Pat. No. 5,322,770, herein incorporated by reference). DNA sequencing methods have also been described which utilize Taq (see e.g., U.S. Pat. No. 5,075,216, herein incorporated by reference).
[0020] However, Taq DNA polymerase has certain characteristics which are undesirable in PCR and other applications including the presence of 5′ to 3′ exonuclease activity. When thermostable DNA polymerases which have 5′ to 3′ exonuclease activity (Taq, Tma, Tsps17, 7-Z05, Tth and Taf) are used in the PCR process and other methods, a variety of undesirable results have been observed, including a limitation of the amount of PCR product produced, an impaired ability to generate long PCR products or to amplify regions containing significant secondary structure, the production of shadow bands or the attenuation in signal strength of desired termination bands during DNA sequencing, the degradation of the 5′ end of oligonucleotide primers in the context of double-stranded primer-template complex, nick-translation synthesis during oligonucleotide-directed mutagenesis and the degradation of the RNA component of RNA:DNA hybrids. When utilized in a PCR process with double-stranded primer-template complex, the 5′ to 3′ exonuclease activity of a DNA polymerase may result in the degradation of the 5′ end of the oligonucleotide primers. This activity is not only undesirable in PCR, but also in second-strand cDNA synthesis and sequencing processes.
[0021] In the choice of enzyme for sequencing, various factors must be considered. For example, large quantities of the enzyme should be easy to prepare; the enzyme must be stable upon storage for considerable time periods; the enzyme should accept all deoxy and dideoxy nucleotides and analogues as substrates with equal affinities and high fidelity; the polymerase action should be highly processive over nucleotide extensions to 1 kb and beyond, even through regions of secondary structure within the template; the activity should remain high, even in suboptimal conditions; and it should be inexpensive (A. T. Bankier, “Dideoxy sequencing reactions using Klenow fragment DNA polymerase I,” in H. and A. Griffin (eds.), Methods in Molecular Biology: DNA Sequencing Protocols, Humana Press, Totowa, N.J., [1993], pp. 83-90). Furthermore the enzyme should be able to function at elevated temperatures (i.e, greater than about 70° C.) so that non-specific priming reactions are minimized. However, there are no commercially available enzymes which fully meet all of these criteria. Thus, mutant forms of enzymes have been produced in order to address some of these needs.
[0022] For example, mutant forms of thermostable DNA polymerases which exhibit reduced or absent 5′ to 3′ exonuclease activity have been generated. The Stoffel fragment of Taq DNA polymerase lacks 5′ to 3′ exonuclease activity due to genetic manipulations which result in the production of a truncated protein lacking the N-terminal 289 amino acids (See e.g., Lawyer et al., J. Biol. Chem., 264:6427-6437 [1989]; and Lawyer et al., PCR Meth. Appl., 2:275-287 [1993]). Analogous mutant polymerases have been generated for polymerases derived from T. maritima, Tsps17, TZ05, Tth and Taf. While the generation of thermostable polymerases lacking 5′ to 3′ exonuclease activity provides improved enzymes for certain applications, some of these mutant polymerases still have undesirable characteristics including the presence of 3′ to 5′ exonuclease activity.
[0023] The 3′ to 5′ exonuclease activity is commonly referred to as a proof-reading activity. The 3′ to 5′ exonuclease removes bases which are mismatched at the 3′ end of a primer-template duplex. While the presence of 3′ to 5′ exonuclease activity may be advantageous as it leads to an increase in the fidelity of replication of nucleic acid strands it also has other undesirable characteristics. The 3′ to 5′ exonuclease activity found in thermostable DNA polymerases such as Tma (including mutant forms of Tma that lack 5′ to 3′ exonuclease activity) also degrades single-stranded DNA such as the primers used in the PCR, single-stranded templates and single-stranded PCR products. The integrity of the 3′ end of an oligonucleotide primer used in a primer extension process (e.g., PCR, Sanger sequencing methods, etc.) is critical as it is from this terminus that extension of the nascent strand begins. Degradation of the 3′ end leads to a shortened oligonucleotide which in turn results in a loss of specificity in the priming reaction (i.e., the shorter the primer the more likely it becomes that spurious or non-specific priming will occur).
[0024] The degradation of an oligonucleotide primer by a 3′ exonuclease can be prevented by the use of modified nucleotides at the 3′ terminus. For example, the use of dideoxynucleotides or deoxynucleotides having a phosphorothiolate linkage at the 3′ terminus of an oligonucleotide would prevent degradation by 3′ exonucleases. However, the need to use modified nucleotides to prevent degradation of oligonucleotides by 3′ exonuclease increases the time and cost required to prepare oligonucleotide primers.
[0025] A few examples of a thermostable polymerase which lack both 5′ to 3′ exonuclease and 3′ to 5′ exonuclease are known. As discussed above, the Stoffel fragment of Taq DNA polymerase lacks the 5′ to 3′ exonuclease activity due to genetic manipulation and no 3′ to 5′ activity is present as Taq polymerase is naturally lacking in 3′ to 5′ exonuclease activity. Likewise the Tth polymerase naturally lacks 3′ to 5′ exonuclease activity and genetic deletion of N-terminal amino acids removes the 5′ to 3′ exonuclease activity.
[0026] Despite the development of recombinant enzymes such as Stoffel fragment, there remains a need for other thermostable polymerases having improved characteristics. For example, thermostable polymerases are used in Sanger dideoxynucleotide sequencing protocols. The most commonly used enzyme is Taq polymerase or a modified form of Taq polymerase. High concentrations of the expensive dideoxynucleotides must be used in the sequencing reaction when these enzymes are employed as they have a fairly low affinity for dideoxynucleotides. The art needs a thermostable polymerase which displays a higher affinity for dideoxynucleotides as this would result in considerable cost savings. In addition, the art needs additional thermostable polymerases having novel properties to improve the results obtained when using techniques such as DNA amplification, sequencing and nick-translation.
SUMMARY OF THE INVENTION[0027] The present invention relates to purified thermostable DNA polymerases derived from the eubacteria Thermotoga neapolitana (Tne). Nucleic acid sequences encoding the full-length Tne DNA polymerase is provided. In addition, nucleic acid sequences encoding several modified forms of the Tne DNA polymerase are provided herein. The present invention provides methods for the isolation of purified preparations of Tne DNA polymerases. The Tne DNA polymerases may be isolated from Thermotoga neapolitana cells or from host cells containing nucleic sequences encoding a Tne DNA polymerase.
[0028] In one embodiment, the present invention contemplates a purified thermostable DNA polymerase derived from the eubacterium Thermotoga neapolitana which is capable of DNA synthetic activity. In another embodiment, the purified Tne DNA polymerase has 3′ exonuclease activity. In yet another embodiment, the purified Tne DNA polymerase has 5′ exonuclease activity. In one preferred embodiment, the purified Tne DNA polymerase comprises the amino acid sequence of SEQ ID NO:2. In a particularly preferred embodiment, the specific activity of the synthetic activity of the purified Tne DNA polymerase is approximately 100,000 units/mg.
[0029] In another embodiment, the purified thermostable Tne DNA polymerase is a non-naturally occurring or recombinant DNA polymerase. The recombinant Tne DNA polymerase may further contain 3′ exonuclease activity and/or 5′ exonuclease activity.
[0030] In a preferred embodiment, the non-naturally occurring Tne DNA polymerase has reduced levels of 3′ exonuclease activity. In another embodiment, the non-naturally occurring Tne DNA polymerase lacks significant 5′ exonuclease activity. In a particularly preferred embodiment, the non-naturally occurring Tne DNA polymerase comprises an amino acid sequence selected from the group consisting of SEQ ID NOS: 8, 16, 19, 23, 26, 29, 33 and 35.
[0031] The present invention provides nucleic acid sequences encoding thermostable DNA polymerases. In a preferred embodiment, an oligonucleotide comprising the nucleic acid sequence of SEQ ID NO:1 encodes the thermostable DNA polymerase. These nucleic acid sequences encoding thermostable DNA polymerases may be modified to encode a polymerase which lacks significant 5′ exonuclease activity. In a preferred embodiment, the modified nucleic acid sequences encoding a thermostable DNA polymerase comprise the nucleotide sequence of SEQ ID NO:7. In one embodiment, the polymerase encoded by the modified nucleic acid sequences displays reduced levels of 3′ exonuclease activity. In a particularly preferred embodiment, the modified nucleic acid sequences encoding a polymerase having reduced levels of 3′ exonuclease activity are selected from the group consisting of SEQ ID NOS:7, 15, 18, 22, 25, 28, 32 and 34.
[0032] The present invention provides recombinant DNA vectors containing nucleic acid sequences which encode a thermostable DNA polymerase having DNA synthetic activity. In a preferred embodiment the polymerase-encoding nucleic acid sequences are set forth in SEQ ID NO:1. The recombinant DNA vector may contain a modified nucleic sequence encoding a thermostable DNA polymerase which lacks significant 5′ exonuclease activity. In a preferred embodiment, the recombinant DNA vector contains a modified nucleic acid sequence which comprises SEQ ID NO:7.
[0033] In a preferred embodiment, the recombinant DNA vector contains modified nucleic acid sequences encoding a thermostable DNA polymerase which exhibits reduced levels 3′ exonuclease activity. In a particularly preferred embodiment, the modified sequences encoding a thermostable DNA polymerase which exhibits reduced levels 3′ exonuclease activity are selected from the group consisting of SEQ ID NOS:7, 15, 18, 22, 25, 28, 32 and 34.
[0034] The present invention further contemplates the transformation of host cells with the recombinant DNA vectors containing nucleic acid sequences encoding Tne DNA polymerases. The invention is not limited by the choice of host cell; host cells may comprise procaryotic or eucaryotic cells. In a preferred embodiment, the host cell is an E. coli host cell.
[0035] The invention further provides methods for determining the DNA sequence of a segment or portion of a DNA molecule using the Tne DNA polymerases of the invention. Dideoxynucleotide (ddNTP) chain termination sequencing protocols are used in conjunction with the polymerases of the invention. Traditional (i.e., Sanger) as well as other methods, including but not limited to, chain termination sequencing or thermal cycle sequencing protocols benefit from the use of the Tne DNA polymerases of the invention. The claimed Tne DNA polymerases have a high affinity for dideoxynucleotides; accordingly the following ratios of dNTPs and ddNTPs are contemplated for use in either thermal cycling or Sanger chain termination protocols when Tne DNA polymerases are employed: dATP:ddATP is 1:1.67±50%; dCTP:ddCTP is 1:0.83±50%; dGTP:ddGTP is 1:0.67±50% and TTP:ddTTP is 1:2.5±50% where each dNTP is present at a final concentration of about 1 &mgr;M to 120 &mgr;M.
DESCRIPTION OF THE DRAWINGS[0036] FIG. 1 provides a schematic representation of the 5′ exonuclease, 3′ exonuclease and polymerase domains in several DNA polymerases.
[0037] FIG. 2 shows an alignment of amino acid residues from three regions within the 3′ exonuclease domain of selected DNA polymerases.
[0038] FIG. 3 shows the alignment of the amino acid residues (using the one letter code for the amino acids) from E. coli DNA polymerase I, Tma DNA polymerase and Tne DNA polymerase.
[0039] FIG. 4 provides a schematic depicting the full length and mutant Tne DNA polymerases of the present invention.
[0040] FIG. 5A shows an autoradiograph of a sequencing gel.
[0041] FIG. 5B shows an autoradiograph of a sequencing gel.
[0042] FIG. 6A shows an autoradiograph of a sequencing gel.
[0043] FIG. 6B shows an autoradiograph of a sequencing gel.
[0044] FIG. 6C shows an autoradiograph of a sequencing gel.
DEFINITIONS[0045] To facilitate understanding of the invention, a number of terms are defined below.
[0046] The term “gene” refers to a DNA sequence that comprises control and coding sequences necessary for the production of a polypeptide or precursor. The polypeptide can be encoded by a full length coding sequence or by any portion of the coding sequence so long as the desired enzymatic activity is retained.
[0047] The term “wild-type” refers to a gene or gene product which has the characteristics of that gene or gene product when isolated from a naturally occurring source. A wild-type gene is that which is most frequently observed in a population and is thus arbitrarily designated the “normal” or “wild-type” form of the gene. In contrast, the term “modified” or “mutant” refers to a gene or gene product which displays modifications in sequence and or functional properties (i.e., altered characteristics) when compared to the wild-type gene or gene product. It is noted that naturally-occurring mutants can be isolated; these are identified by the fact that they have altered characteristics when compared to the wild-type gene or gene product. The wild-type form of the coding region for the Tne DNA polymerase is listed in SEQ ID NO:1; the wild-type form of the Tne DNA polymerase protein is listed in SEQ ID NO:2. The Tne DNA polymerase proteins encoded by “modified” or “mutant” genes are referred to as non-naturally occurring Tne DNA polymerases.
[0048] The term “recombinant DNA vector” as used herein refers to DNA sequences containing a desired coding sequence and appropriate DNA sequences necessary for the expression of the operably linked-coding sequence in a particular host organism. DNA sequences necessary for expression in procaryotes include a promoter, optionally an operator sequence, a ribosome binding site and possibly other sequences. Eukaryotic cells are known to utilize promoters, polyadenlyation signals and enhancers.
[0049] As used herein, the terms “cell,” “cell line,” and cell culture” are used interchangeably and all such designations include progeny. The words “transformants” or “transformed cells” include the primary transformed cell and cultures derived from that cell without regard to the number of transfers. All progeny may not be precisely identical in DNA content, due to deliberate or inadvertent mutations. Mutant progeny that have the same functionality as screened for in the originally transformed cell are included in the definition of transformants.
[0050] As used herein, the term “vector” is used in reference to nucleic acid molecules that transfer DNA segment(s) from one cell to another. The term “vehicle” is sometimes used interchangeably with “vector.”
[0051] The term “expression vector” as used herein refers to a recombinant DNA molecule containing a desired coding sequence and appropriate nucleic acid sequences necessary for the expression of the operably linked coding sequence in a particular host organism. Nucleic acid sequences necessary for expression in procaryotes usually include a promoter, an operator (optional), and a ribosome binding site, often along with other sequences. Eucaryotic cells are known to utilize promoters, enhancers, and termination and polyadenylation signals.
[0052] The terms “in operable combination”, “in operable order” and “operably linked” as used herein refer to the linkage of nucleic acid sequences in such a manner that a nucleic acid molecule capable of directing the transcription of a given gene and/or the synthesis of a desired protein molecule is produced. The term also refers to the linkage of amino acid sequences in such a manner so that a functional protein is produced.
[0053] The term “transfection” as used herein refers to the introduction of foreign DNA into eucaryotic cells. Transfection may be accomplished by a variety of means known to the art including calcium phosphate-DNA co-precipitation, DEAE-dextran-mediated transfection, polybrene-mediated transfection, electroporation, microinjection, liposome fusion, lipofection, protoplast fusion, retroviral infection, and biolistics.
[0054] As used herein, the terms “complementary” or “complementarity” are used in reference to polynucleotides (i.e., a sequence of nucleotides) related by the base-pairing rules. For example, for the sequence “A-G-T,” is complementary to the sequence “T-C-A.” Complementary may be “partial,” in which only some of the nucleic acids' bases are matched according to the base pairing rules. Or, there may be “complete” or “total” complementary between the nucleic acids. The degree of complementary between nucleic acid strands has significant effects on the efficiency and strength of hybridization between nucleic acid strands. This is of particular importance in amplification reactions, as well as detection methods which depend upon binding between nucleic acids.
[0055] The term “homology” refers to a degree of complementary. There may be partial homology or complete homology (i.e., identity). A partially complementary sequence is one that at least partially inhibits a completely complementary sequence from hybridizing to a target nucleic acid is referred to using the functional term “substantially homologous.” The inhibition of hybridization of the completely complementary sequence to the target sequence may be examined using a hybridization assay (Southern or Northern blot, solution hybridization and the like) under conditions of low stringency. A substantially homologous sequence or probe will compete for and inhibit the binding (i.e., the hybridization) of a completely homologous to a target under conditions of low stringency. This is not to say that conditions of low stringency are such that non-specific binding is permitted; low stringency conditions require that the binding of two sequences to one another be a specific (i.e., selective) interaction. The absence of non-specific binding may be tested by the use of a second target which lacks even a partial degree of complementarity (e.g., less than about 30% identity); in the absence of non-specific binding the probe will not hybridize to the second non-complementary target.
[0056] Low stringency conditions comprise conditions equivalent to binding or hybridization at 42° C. in a solution consisting of 5×SSPE (43.8 g/l NaCl, 6.9 g/l NaH2PO4.H2O and 1.85 g/l EDTA, pH adjusted to 7.4 with NaOH), 0.1% SDS, 5× Denhardt's reagent [50× Denhardt's contains per 500 ml: 5 g Ficoll (Type 400, Pharamcia), 5 g BSA (Fraction V; Sigma)] and 100 &mgr;g/ml denatured salmon sperm DNA followed by washing in a solution comprising 5×SSPE, 0.1% SDS at 42° C. when a probe of about 500 nucleotides in length is employed.
[0057] The art knows well that numerous equivalent conditions may be employed to comprise low stringency conditions; factors such as the length and nature (DNA, RNA, base composition) of the probe and nature of the target (DNA, RNA, base composition, present in solution or immobilized, etc.) and the concentration of the salts and other components (e.g., the presence or absence of formamide, dextran sulfate, polyethylene glycol) are considered and the hybridization solution may be varied to generate conditions of low stringency hybridization different from, but equivalent to, the above listed conditions. In addition, the art knows conditions which promote hybridization under conditions of high stringency (e.g., increasing the temperature of the hybridization and/or wash steps, the use of formamide in the hybridization solution, etc.).
[0058] When used in reference to a double-stranded nucleic acid sequence such as a cDNA or genomic clone, the term “substantially homologous” refers to any probe which can hybridize to either or both strands of the double-stranded nucleic acid sequence under conditions of low stringency as described above.
[0059] When used in reference to a single-stranded nucleic acid sequence, the term “substantially homologous” refers to any probe which can hybridize (i.e., it is the complement of) the single-stranded nucleic acid sequence under conditions of low stringency as described above.
[0060] As used herein, the term “hybridization” is used in reference to the pairing of complementary nucleic acids. Hybridization and the strength of hybridization (i.e., the strength of the association between the nucleic acids) is impacted by such factors as the degree of complementary between the nucleic acids, stringency of the conditions involved, the Tm of the formed hybrid, and the G:C ratio within the nucleic acids.
[0061] As used herein, the term “Tm” is used in reference to the “melting temperature.” The melting temperature is the temperature at which a population of double-stranded nucleic acid molecules becomes half dissociated into single strands. The equation for calculating the Tm of nucleic acids is well known in the art. As indicated by standard references, a simple estimate of the Tm value may be calculated by the equation: Tm=81.5+0.41(% G+C), when a nucleic acid is in aqueous solution at 1 M NaCl (see e.g., Anderson and Young, Quantitative Filter Hybridization, in Nucleic Acid Hybridization (1985). Other references include more sophisticated computations which take structural as well as sequence characteristics into account for the calculation of Tm.
[0062] As used herein the term “stringency” is used in reference to the conditions of temperature, ionic strength, and the presence of other compounds such as organic solvents, under which nucleic acid hybridizations are conducted. With “high stringency” conditions, nucleic acid base pairing will occur only between nucleic acid fragments that have a high frequency of complementary base sequences. Thus, conditions of “weak” or “low” stringency are often required with nucleic acids that are derived from organisms that are genetically diverse, as the frequency of complementary sequences is usually less.
[0063] As used herein, the term “amplifiable nucleic acid” is used in reference to nucleic acids which may be amplified by any amplification method. It is contemplated that “amplifiable nucleic acid” will usually comprise “sample template.”
[0064] As used herein, the term “sample template” refers to nucleic acid originating from a sample which is analyzed for the presence of “target” (defined below). In contrast, “background template” is used in reference to nucleic acid other than sample template which may or may not be present in a sample. Background template is most often inadvertent. It may be the result of carryover, or it may be due to the presence of nucleic acid contaminants sought to be purified away from the sample. For example, nucleic acids from organisms other than those to be detected may be present as background in a test sample.
[0065] As used herein, the term “primer” refers to an oligonucleotide, whether occurring naturally as in a purified restriction digest or produced synthetically, which is capable of acting as a point of initiation of synthesis when placed under conditions in which synthesis of a primer extension product which is complementary to a nucleic acid strand is induced, (i.e., in the presence of nucleotides and an inducing agent such as DNA polymerase and at a suitable temperature and pH). The primer is preferably single stranded for maximum efficiency in amplification, but may alternatively be double stranded. If double stranded, the primer is first treated to separate its strands before being used to prepare extension products. Preferably, the primer is an oligodeoxyribonucleotide. The primer must be sufficiently long to prime the synthesis of extension products in the presence of the inducing agent. The exact lengths of the primers will depend on many factors, including temperature, source of primer and the use of the method.
[0066] As used herein, the term “probe” refers to an oligonucleotide (i.e., a sequence of nucleotides), whether occurring naturally as in a purified restriction digest or produced synthetically, recombinantly or by PCR amplification, which is capable of hybridizing to another oligonucleotide of interest. A probe may be single-stranded or double-stranded. Probes are useful in the detection, identification and isolation of particular gene sequences. It is contemplated that any probe used in the present invention will be labeled with any “reporter molecule,” so that is detectable in any detection system, including, but not limited to enzyme (e.g., ELISA, as well as enzyme-based histochemical assays), fluorescent, radioactive, and luminescent systems. It is not intended that the present invention be limited to any particular detection system or label.
[0067] As used herein, the term “target,” when used in reference to the polymerase chain reaction, refers to the region of nucleic acid bounded by the primers used for polymerase chain reaction. Thus, the “target” is sought to be sorted out from other nucleic acid sequences. A “segment” is defined as a region of nucleic acid within the target sequence.
[0068] As used herein, the term “polymerase chain reaction” (“PCR”) refers to the method of K. B. Mullis U.S. Pat. Nos. 4,683,195 and 4,683,202, hereby incorporated by reference, which describe a method for increasing the concentration of a segment of a target sequence in a mixture of genomic DNA without cloning or purification. This process for amplifying the target sequence consists of introducing a large excess of two oligonucleotide primers to the DNA mixture containing the desired target sequence, followed by a precise sequence of thermal cycling in the presence of a DNA polymerase. The two primers are complementary to their respective strands of the double stranded target sequence. To effect amplification, the mixture is denatured and the primers then annealed to their complementary sequences within the target molecule. Following annealing, the primers are extended with a polymerase so as to form a new pair of complementary strands. The steps of denaturation, primer annealing and polymerase extension can be repeated many times (i.e., denaturation, annealing and extension constitute one “cycle”; there can be numerous “cycles”) to obtain a high concentration of an amplified segment of the desired target sequence. The length of the amplified segment of the desired target sequence is determined by the relative positions of the primers with respect to each other, and therefore, this length is a controllable parameter. By virtue of the repeating aspect of the process, the method is referred to as the “polymerase chain reaction” (hereinafter “PCR”). Because the desired amplified segments of the target sequence become the predominant sequences (in terms of concentration) in the mixture, they are said to be “PCR amplified”.
[0069] With PCR, it is possible to amplify a single copy of a specific target sequence in genomic DNA to a level detectable by several different methodologies (e.g., hybridization with a labeled probe; incorporation of biotinylated primers followed by avidin-enzyme conjugate detection; incorporation of 32P-labeled deoxynucleotide triphosphates, such as dCTP or dATP, into the amplified segment). In addition to genomic DNA, any oligonucleotide sequence can be amplified with the appropriate set of primer molecules. In particular, the amplified segments created by the PCR process itself are, themselves, efficient templates for subsequent PCR amplifications.
[0070] “Amplification” is a special case of nucleic acid replication involving template specificity. It is to be contrasted with non-specific template replication (i.e., replication that is template-dependent but not dependent on a specific template). Template specificity is here distinguished from fidelity of replication (i.e., synthesis of the proper polynucleotide sequence) and nucleotide (ribo- or deoxyribo-) specificity. Template specificity is frequently described in terms of “target” specificity. Target sequences are “targets” in the sense that they are sought to be sorted out from other nucleic acid. Amplification techniques have been designed primarily for this sorting out.
[0071] Template specificity is achieved in most amplification techniques by the choice of enzyme. Amplification enzymes are enzymes that, under conditions they are used, will process only specific sequences of nucleic acid in a heterogeneous mixture of nucleic acid. For example, in the case of Q&bgr; replicase, MDV-1 RNA is the specific template for the replicase [D. L. Kacian et al., Proc. Natl. Acad. Sci USA 69:3038 (1972)]. Other nucleic acid will not be replicated by this amplification enzyme. Similarly, in the case of T7 RNA polymerase, this amplification enzyme has a stringent specificity for its own promoters [M. Chamberlin et al., Nature 228:227 (1970)]. In the case of T4 DNA ligase, the enzyme will not ligate the two oligonucleotides where there is a mismatch between the oligonucleotide substrate and the template at the ligation junction [D. Y. Wu and R. B. Wallace, Genomics 4:560 (1989)]. Finally, Taq and Pfu polymerases, by virtue of their ability to function at high temperature, are found to display high specificity for the sequences bounded and thus defined by the primers; the high temperature results in thermodynamic conditions that favor primer hybridization with the target sequences and not hybridization with non-target sequences [PCR Technology, H. A. Erlich (ed.) (Stockton Press 1989)].
[0072] As used herein, the terms “PCR product”, “PCR fragment” and “amplification product” refer to the resultant mixture of compounds after two or more cycles of the PCR steps of denaturation, annealing and extension are complete. These terms encompass the case where there has been amplification of one or more segments of one or more target sequences.
[0073] As used herein, the term “amplification reagents” refers to those reagents (deoxyribonucleotide triphosphates, buffer, etc.), needed for amplification except for primers, nucleic acid template and the amplification enzyme. Typically, amplification reagents along with other reaction components are placed and contained in a reaction vessel (test tube, microwell, etc.).
[0074] As used herein, the terms “restriction endonucleases” and “restriction enzymes” refer to bacterial enzymes, each of which cut double-stranded DNA at or near a specific nucleotide sequence.
[0075] As used herein, the term “recombinant DNA molecule” as used herein refers to a DNA molecule which is comprised of segments of DNA joined together by means of molecular biological techniques.
[0076] DNA molecules are said to have “5′ ends” and “3′ ends” because mononucleotides are reacted to make oligonucleotides in a manner such that the 5′ phosphate of one mononucleotide pentose ring is attached to the 3′ oxygen of its neighbor in one direction via a phosphodiester linkage. Therefore, an end of an oligonucleotides referred to as the “5′ end” if its 5′ phosphate is not linked to the 3′ oxygen of a mononucleotide pentose ring and as the “3′ end” if its 3′ oxygen is not linked to a 5′ phosphate of a subsequent mononucleotide pentose ring. As used herein, a nucleic acid sequence, even if internal to a larger oligonucleotide, also may be said to have 5′ and 3′ ends. In either a linear or circular DNA molecule, discrete elements are referred to as being “upstream” or 5′ of the “downstream” or 3′ elements. This terminology reflects the fact that transcription proceeds in a 5′ to 3′ fashion along the DNA strand. The promoter and enhancer elements which direct transcription of a linked gene are generally located 5′ or upstream of the coding region However, enhancer elements can exert their effect even when located 3′ of the promoter element and the coding region. Transcription termination and polyadenylation signals are located 3′ or downstream of the coding region.
[0077] As used herein, the term “an oligonucleotide having a nucleotide sequence encoding a gene” means a nucleic acid sequence comprising the coding region of a gene or in other words the nucleic acid sequence which encodes a gene product. The coding region may be present in either a cDNA, genomic DNA or RNA form. When present in a DNA form, the oligonucleotide may be single-stranded (i.e., the sense strand) or double-stranded. Suitable control elements such as enhancers/promoters, splice junctions, polyadenylation signals, etc. may be placed in close proximity to the coding region of the gene if needed to permit proper initiation of transcription and/or correct processing of the primary RNA transcript. Alternatively, the coding region utilized in the expression vectors of the present invention may contain endogenous enhancers/promoters, splice junctions, intervening sequences, polyadenylation signals, etc. or a combination of both endogenous and exogenous control elements.
[0078] As used herein, the term “regulatory element” refers to a genetic element which controls some aspect of the expression of nucleic acid sequences. For example, a promoter is a regulatory element which facilitates the initiation of transcription of an operably linked coding region. Other regulatory elements are splicing signals, polyadenylation signals, termination signals, etc. (defined infra).
[0079] Transcriptional control signals in eucaryotes comprise “promoter” and “enhancer” elements. Promoters and enhancers consist of short arrays of DNA sequences that interact specifically with cellular proteins involved in transcription [Maniatis, T. et al., Science 236:1237 (1987)]. Promoter and enhancer elements have been isolated from a variety of eucaryotic sources including genes in yeast, insect and mammalian cells and viruses (analogous control elements, i.e., promoters, are also found in procaryotes). The selection of a particular promoter and enhancer depends on what cell type is to be used to express the protein of interest. Some eucaryotic promoters and enhancers have a broad host range while others are functional in a limited subset of cell types [for review see Voss, S. D. et al., Trends Biochem. Sci., 11:287 (1986) and Maniatis, T. et al., supra (1987)]. For example, the SV40 early gene enhancer is very active in a wide variety of cell types from many mammalian species and has been widely used for the expression of proteins in mammalian cells [Dijkema, R. et al., EMBO J. 4:761 (1985)]. Two other examples of promoter/enhancer elements active in a broad range of mammalian cell types are those from the human elongation factor la gene [Uetsuki, T. et al., J. Biol. Chem., 264:5791 (1989), Kim, D. W. et al., Gene 91:217 (1990) and Mizushima, S. and Nagata, S., Nuc. Acids. Res., 18:5322 (1990)] and the long terminal repeats of the Rous sarcoma virus [Gorman, C. M. et al., Proc. Natl. Acad. Sci. USA 79:6777 (1982)] and the human cytomegalovirus [Boshart, M. et al., Cell 41:521 (1985)].
[0080] As used herein, the term “promoter/enhancer” denotes a segment of DNA which contains sequences capable of providing both promoter and enhancer functions (i.e., the functions provided by a promoter element and an enhancer element, see above for a discussion of these functions). For example, the long terminal repeats of retroviruses contain both promoter and enhancer functions. The enhancer/promoter may be “endogenous” or “exogenous” or “heterologous.” An “endogenous” enhancer/promoter is one which is naturally linked with a given gene in the genome. An “exogenous” or “heterologous” enhancer/promoter is one which is placed in juxtaposition to a gene by means of genetic manipulation (i.e., molecular biological techniques) such that transcription of that gene is directed by the linked enhancer/promoter.
[0081] The presence of “splicing signals” on an expression vector often results in higher levels of expression of the recombinant transcript in eucaryotic host cells. Splicing signals mediate the removal of introns from the primary RNA transcript and consist of a splice donor and acceptor site [Sambrook, J. et al., Molecular Cloning: A Laboratory Manual, 2nd ed., Cold Spring Harbor Laboratory Press, New York (1989) pp. 16.7-16.8]. A commonly used splice donor and acceptor site is the splice junction from the 16S RNA of SV40.
[0082] Efficient expression of recombinant DNA sequences in eucaryotic cells requires expression of signals directing the efficient termination and polyadenylation of the resulting transcript. Transcription termination signals are generally found downstream of the polyadenylation signal and are a few hundred nucleotides in length. The term “poly A site” or “poly A sequence” as used herein denotes a DNA sequence which directs both the termination and polyadenylation of the nascent RNA transcript. Efficient polyadenylation of the recombinant transcript is desirable as transcripts lacking a poly A tail are unstable and are rapidly degraded. The poly A signal utilized in an expression vector may be “heterologous” or “endogenous.” An endogenous poly A signal is one that is found naturally at the 3′ end of the coding region of a given gene in the genome. A heterologous poly A signal is one which is one which is isolated from one gene and placed 3′ of another gene. A commonly used heterologous poly A signal is the SV40 poly A signal. The SV40 poly A signal is contained on a 237 bp BamHI/BclI restriction fragment and directs both termination and polyadenylation [J. Sambrook, supra, at 16.6-16.7].
[0083] Eucaryotic expression vectors may also contain “viral replicons” or “viral origins of replication.” Viral replicons are viral DNA sequences which allow for the extrachromosomal replication of a vector in a host cell expressing the appropriate replication factors. Vectors which contain either the SV40 or polyoma virus origin of replication replicate to high copy number (up to 104 copies/cell) in cells that express the appropriate viral T antigen. Vectors which contain the replicons from bovine papillomavirus or Epstein-Barr virus replicate extrachromosomally at low copy number (˜100 copies/cell).
[0084] The Tne polymerases may be expressed in either procaryotic or eucaryotic host cells. Nucleic acid encoding the Tne polymerase may be introduced into bacterial host cells by a number of means including transformation of bacterial cells made competent for transformation by treatment with calcium chloride or by electroporation. If the Tne polymerases are to be expressed in eucaryotic host cells, nucleic acid encoding the Tne polymerase may be introduced into eucaryotic host cells by a number of means including calcium phosphate co-precipitation, spheroplast fusion, electroporation and the like. When the eucaryotic host cell is a yeast cell, transformation may be affected by treatment of the host cells with lithium acetate or by electroporation.
[0085] As used herein, the terms “nucleic acid molecule encoding,” “DNA sequence encoding,” and “DNA encoding” refer to the order or sequence of deoxyribonucleotides along a strand of deoxyribonucleic acid. The order of these deoxyribonucleotides determines the order of amino acids along the polypeptide (protein) chain. The DNA sequence thus codes for the amino acid sequence.
[0086] The term “isolated” when used in relation to a nucleic acid, as in “an isolated oligonucleotide” refers to a nucleic acid sequence that is identified and separated from at least one contaminant nucleic acid with which it is ordinarily associated in its natural source. Isolated nucleic acid is such present in a form or setting that is different from that in which it is found in nature. In contrast, non-isolated nucleic acids as nucleic acids such as DNA and RNA found in the state they exist in nature. For example, a given DNA sequence (e.g., a gene) is found on the host cell chromosome in proximity to neighboring genes; RNA sequences, such as a specific mRNA sequence encoding a specific protein, are found in the cell as a mixture with numerous other mRNA s which encode a multitude of proteins. However, isolated nucleic acid encoding a Tne polymerase includes, by way of example, such nucleic acid in cells ordinarily expressing a Tne polymerase where the nucleic acid is in a chromosomal location different from that of natural cells, or is otherwise flanked by a different nucleic acid sequence than that found in nature. The isolated nucleic acid or oligonucleotide may be present in single-stranded or double-stranded form. When an isolated nucleic acid or oligonucleotide is to be utilized to express a protein, the oligonucleotide will contain at a minimum the sense or coding strand (i.e., the oligonucleotide may single-stranded), but may contain both the sense and anti-sense strands (i.e., the oligonucleotide may be double-stranded).
[0087] As used herein the term “coding region” when used in reference to structural gene refers to the nucleotide sequences which encode the amino acids found in the nascent polypeptide as a result of translation of a mRNA molecule. The coding region is bounded on the 5′ side by the nucleotide triplet “ATG” which encodes the initiator methionine and on the 3′ side by one of the three triplets which specify stop codons (i.e., TAA, TAG, TGA).
[0088] As used herein, the term “purified” or “to purify” refers to the removal of contaminants from a sample. For example, recombinant Tne DNA polymerases are expressed in bacterial host cells and the polymerases are purified by the removal of host cell proteins; the percent of recombinant Tne DNA polymerase is thereby increased in the sample.
[0089] The term “recombinant DNA molecule” as used herein refers to a DNA molecule which is comprised of segments of DNA joined together by means of molecular biological techniques.
[0090] The term “recombinant protein” or “recombinant polypeptide” as used herein refers to a protein molecule which is expressed from a recombinant DNA molecule.
[0091] The term “native protein” is used herein to indicate a protein isolated from a naturally occurring (i.e., a nonrecombinant) source. Molecular biological techniques may be used to produce a recombinant form of a protein which has identical properties when compared to the native form of the protein. The term “rTne” is used to designate a recombinant form of Tne polymerase. The terms “nTne” and “nTaq” are used to designate the native forms of Tne polymerase and Taq polymerase, respectively.
[0092] As used herein the term “portion” when in reference to a protein (as in “a portion of a given protein”) refers to fragments of that protein. The fragments may range in size from four amino acid residues to the entire amino acid sequence minus one amino acid.
[0093] As used herein, the term “fusion protein” refers to a chimeric protein containing the protein of interest (i.e., Tne DNA polymerases and fragments thereof) joined to an exogenous protein fragment (the fusion partner which consists of a non-Tne polymerase protein). The fusion partner may enhance solubility of the Tne polymerase protein as expressed in a host cell, may provide an affinity tag to allow purification of the recombinant fusion protein from the host cell or culture supernatant, or both. If desired, the fusion protein may be removed from the protein of interest (i.e., Tne DNA polymerase or fragments thereof) by a variety of enzymatic or chemical means known to the art.
[0094] The term “5′ exonuclease activity” refers to the presence of an activity in a protein which is capable of removing nucleotides from the 5′ end of an oligonucleotide. 5′ exonuclease activity may be measured using any of the assays provided herein.
[0095] The term “3′ exonuclease activity” refers to the presence of an activity in a protein which is capable of removing nucleotides from the 3′ end of an oligonucleotide. 3′ exonuclease activity may be measured using any of the assays provided herein.
[0096] The terms “DNA polymerase activity,” “synthetic activity” and “polymerase activity” are used interchangably and refer to the ability of an enzyme to synthesize new DNA strands by the incorporation of deoxynucleoside triphosphates. The examples below provide assays for the measurement of DNA polymerase activity.
[0097] The term “reduced levels of 3′ exonuclease” is used in reference to the level of 3′ exonuclease activity displayed by the wild-type Tne DNA polymerase (i.e., the polymerase of SEQ ID NO:2) and indicates that the modified or “non-naturally occurring” polymerase exhibits lower levels of 3′ exonuclease than does the full-length or unmodified enzyme.
[0098] The phrase “lacks significant 5′ exonuclease activity” is used relative to the level of 5′ exonuclease activity displayed by the wild-type Tne DNA polymerase (i.e., the polymerase of SEQ ID NO:2) and indicates that the modified or “non-naturally occurring” polymerase exhibits such low levels of 5′ exonuclease that the measurement is at background levels in the assay.
DESCRIPTION OF THE INVENTION[0099] The present invention provides purified thermostable DNA polymerase I enzymes derived from Thermotoga neapolitana (Tne). These thermostable enzymes comprise the wild-type form of the enzyme as well as mutant forms which posses altered characteristics relative to the wild-type enzyme. In particular, the present invention provides deletion mutants which lack 5′ exonuclease activity. Further the present invention provides modified forms of Tne DNA polymerases which lack 5′ exonuclease activity and have reduced or absent 3′ exonuclease activity.
[0100] The present invention also relates to an improved method of determining the nucleic sequence of a DNA molecule using chain terminating dideoxynucleotides in conjunction with the modified Tne DNA polymerases. The novel properties of the polymerases of the invention provide improved enzymes for a variety of applications which utilize thermostable DNA polymerases.
[0101] The description of the invention is divided into: I. General Structural Features of Type A DNA Polymerases, II. Generation of Tne DNA Polymerases, III. Use of Tne DNA Polymerases in the PCR and IV. Use of Tne DNA Polymerases in DNA Sequencing Methods.
[0102] I. General Structural Features of DNA Polymerases
[0103] DNA polymerases (DNAPs), such as those isolated from E. coli or from thermophilic bacteria of the genera Thermus or Thermotoga, are enzymes that synthesize new DNA strands. Several of the known DNAPs contain associated nuclease activities in addition to the synthetic or polymerization activity of the enzyme.
[0104] Some DNAPs are known to remove nucleotides from the 5′ and 3′ ends of DNA chains [Kornberg, DNA Replication, W. H. Freeman and Co., San Francisco, pp. 127-139 (1980)]. These nuclease activities are usually referred to as 5′ exonuclease and 3′ exonuclease activities, respectively. For example, the 5′ exonuclease activity located in the N-terminal domain of several DNAPs participates in the removal of RNA primers during lagging stand synthesis during DNA replication and the removal of damaged nucleotides during repair. Some DNAPs, such as the E. coli DNA polymerase, also have a 3′ exonuclease activity responsible for proof-reading during DNA synthesis (Kornberg, supra).
[0105] DNAPs isolated from Thermus aquaticus (Taq), Thermus flavus (Tfl) and Thermus thermophilus (Tth) have a 5′ exonuclease activity, but lack a functional 3′ exonucleolytic domain [Tindall and Kunkell, Biochem. 27:6008 (1988)]. However, the lack of a 3′ exonuclease domain is not a general feature of DNAPs derived from thermophilic bacteria as DNA polymerases from the thermophiles Thermotoga maritima (Tma), Bacillus caldotenax, Thermococcus litoralis (Tli) and Pyrococcus furiosus (Pfu) do contain 3′ exonuclease activity.
[0106] The 5′ nuclease activity associated with a number of eubacterial Type A DNA polymerases has been found to reside in the one-third N-terminal region of the protein as an independent functional domain. In these polymerase molecules, the C-terminal two-thirds of the molecule constitute the polymerization domain which is responsible for the synthesis of DNA. Some Type A DNA polymerases also have a 3′ exonuclease activity associated with the two-third C-terminal region of the molecule. FIG. 1 provides a schematic showing the location of the 5′ exonuclease, 3′ exonuclease and polymerase domains of a number of eubacterial DNAPs. As noted above, not all DNAPs contain both 5′ and 3′ exonuclease domains.
[0107] FIG. 1 provides a schematic depicting the arrangement of the 5′ exonuclease (“5′ EXO”), 3′ exonuclease (“3′ EXO”) and polymerase (“POL”) domains in the DNA polymerases from phage T4 (“&phgr;T4”), phage T7 (“&phgr;T7”), E. coli (DNA polymerase I; “Eco Pol I”), T. aquaticus (“Taq”), T. maritima (“Tma”) and T. neapolitana (“Tne”). The absence of a 3′ exonuclease domain in Taq DNA polymerase is indicated by the use of the line between the boxed 5′ exonuclease and polymerase domains; the absence of a 5′ nuclease domain in phage T4 polymerase is indicated by the absence of the term “5′ EXO” in the first boxed region of the molecule.
[0108] The 5′ exonuclease activity and the polymerization activity of DNAPs have been separated by proteolytic cleavage or genetic manipulation of the polymerase molecule. The Klenow or large proteolytic cleavage fragment of E. coli DNA polymerase I contains the polymerase and 3′ exonuclease activity but lacks the 5′ nuclease activity [Brutlag et al., Biochem. Biophys. Res. Commun. 37:982 (1969)]. The Stoffel fragment of DNAP Taq lacks the 5′ nuclease activity due to a genetic manipulation which deleted the N-terminal 289 amino acids of the polymerase molecule [Erlich et al., Science 252:1643 (1991)].
[0109] The removal of the 5′ exonuclease domain from a DNAP may effect the activity of the remaining domains. For example, removal of the 5′ exonuclease domain from the E. coli polymerase I protein to generate the Klenow fragment affects the fidelity of the remaining large polymerase domain. The fidelity of a DNA polymerase involves several functions including the ability to discriminate against errors when nucleotides are initially inserted, discriminate against extension from misaligned or mispaired primer termini and exonucleolytic removal of errors.
[0110] In comparison to the full-length enzyme, the Klenow fragment exhibits altered base substitution error specificity and is less accurate for minus one base frameshift errors at reiterated template nucleotides [Bebenek et al., J. Biol. Chem. 265:13878 (1990)]. Thus, the removal of the 5′ exonuclease domain of E. coli DNA polymerase I adversely affects the fidelity of the remaining 3′ exonuclease and synthetic domains.
[0111] Removal of a 5′ exonuclease domain does not always adversely affect the fidelity of the resultant polymerase fragment KlenTaq, a truncated version of Taq DNA polymerase lacks the first 235 N-terminal amino acids (which includes the 5′ exonuclease domain) has been reported improved the fidelity of the polymerase two-fold [Barnes, Gene 112:29 (1992)].
[0112] Comparison of amino acid sequence in the 3′ exonuclease domain of a number DNAPs has identified three domains, termed Exo I-III, which are highly conserved between a variety of mesophilic and thermophilic organisms [Bernad et al. Cell 59:219 (1989)]. FIG. 2 provides a schematic drawing which aligns the amino acid residues from a number of DNAPs over the 3′ exonuclease domain. In FIG. 2, the one letter code is used for the amino acids; the numbers represent the amino acid residue in a given polymerase. In FIG. 2, residues which are highly conserved are indicated by the use of white letters within a black box. Portions of the 3′ exonuclease domain of following polymerases are shown: Bacillus subtillus (Bsu) polymerase III; E. coli (Eco) polymerase III&egr;; phage T4, phage T7, E. coli polymerase I, T. maritima (Tma) polymerase and T. neapolitana (Tne) polymerase. The “∇” indicates amino acid residues involved in single strand DNA binding; the “&Dgr;” indicates amino acid residues involved in metal binding and catalysis.
[0113] Site-directed mutagenesis experiments have identified a subset of these conserved residues as being critical for 3′ exonuclease activity in E. coli polymerase I. The critical residues include D355, D424, D501 which are known to bind divalent metal ions and are essential for 3′ exonuclease activity; mutation of these residues reduces 3′ exonuclease activity several thousand fold. L361, F473 and Y497 are also important for 3′ exonuclease activity and are believed to ensure correct positioning of the substrate in the active site. Mutation of L361 and Y497 reduces 3′ exonuclease activity 12.5 to 25-fold; mutation of F473 reduces 3′ exonuclease activity about 3000-fold.
[0114] PCT Publ. No. WO 92/03556 states that three characteristic domains are critical for 3′ exonuclease activity in thermostable DNA polymerases; however, no site-directed mutagenesis is shown for any of the “critical” residues and no 3′ exonuclease activity is reported for any of the mutant forms of Tma DNA polymerase (primarily deletion mutants) shown. The three domains identified in PCT Publ. No. WO 92/03556 are Domain A, which comprises D-X-E-X3-L; Domain B, which comprises N—X3-D-X3-L and Domain C, which comprises Y—X3-D where XN represents the number (N) of non-critical amino acids between the specified amino acids. As shown in FIG. 2, the location, sequence and spacing of these three domains found in polymerases derived from thermophilic organisms is consistent with the three domains identified in polymerases derived from mesophilic organisms.
[0115] While identification of residues which are highly conserved between a number of species provides a starting point for the design of site-directed mutagenesis experiments, it does not provide an absolute prediction of the effect of a given mutation in a particular protein. For example, the present invention shows that substitution of the aspartate at position 468 of the Tne DNA polymerase with a asparagine virtually eliminates the 3′ exonuclease activity [Tne M284(D468N)]. The analogous mutation in the Klenow fragment of DNA polymerase I (D501N) reduces 3′ exonuclease activity only by 2-fold [Derbyshire et al., EMBO J. 10:17 (1991)]. These results underscore the fact that much remains to be learned about structure-function relationships and that one cannot predict, with certainty, the effect of a given mutation based on analogy to other proteins.
[0116] II. Generation of Tne DNA Polymerases
[0117] The present invention provides wild-type and modified forms of Tne DNA polymerases. The modified forms lack 5′ exonuclease activity and some modified forms also display reduced or absent 3′ exonuclease activity.
[0118] By the term “reduced or absent 3′ exonuclease activity” it is meant that the modified enzyme has less than the level of 3′ exonuclease activity found in the wild-type or unmodified enzyme whose protein sequence is listed in SEQ ID NO:2. The modified Tne polymerases of the present invention are advantageous in situations where the polymerization (i.e., synthetic) activity of the enzyme is desired but the presence of 5′ exonuclease and/or 3′ exonuclease activity is not.
[0119] The present invention is not intended to be limited by the nature of the alteration (e.g., deletion, insertion, substitution) necessary to render the Tne polymerase deficient in 5′ exonuclease or 3′ exonuclease activity. The present invention contemplates a variety of methods, including but not limited to proteolysis and genetic manipulation.
[0120] 1. Reduction of Exonuclease Activity by Proteolysis
[0121] Tne DNA polymerases having a reduced level of either or both 5′ exonuclease and 3′ exonuclease activity are produced according to the present invention by physically cleaving the unmodified enzyme with proteolytic enzymes to produce fragments of the enzyme that are deficient in 5′ and/or 3′ exonuclease activity but retain synthetic activity. The proteolysis can remove the N-terminal one third of the protein (about residues 1 to 297 in SEQ ID NO:2) to remove 5′ exonuclease activity. Proteolytic cleavage which removes all or a portion of the 3′ exonuclease domain (about residues 298 to 482 in SEQ ID NO:2) will render the resulting enzyme deficient in 3′ exonuclease activity. Following proteolytic digestion, the resulting fragments are separated by standard chromatographic techniques and assayed for the ability to synthesize DNA and to act as a 5′ or 3′ exonuclease. The assays to determine synthetic activity and 5′ and 3′ exonuclease activity are described in the experimental sections below.
[0122] 2. Reduction of Exonuclease Activity by Genetic Manipulation
[0123] The examples below describe preferred methods for creating a construct (i.e., a vector) encoding a polymerase derived from Tne DNA polymerase I. The wild-type Tne polymerase is cloned by isolating genomic DNA using molecular biological methods from T. neapolitana cells. The genomic DNA is cleaved into fragments about 3 kb or larger using restriction enzymes and the fragments are inserted into a suitable cloning vector such as a plasmid or bacteriophage vector; the vectors containing fragments of T. neapolitana genomic DNA are then transformed into a suitable E. coli host. Clones containing DNA encoding the Tne polymerase may be isolated using functional assays (i.e., presence of thermostable polymerase in lysates of transformed cells) or by hybridization using a probe derived from a region of conservation among DNA polymerases derived from thermostable organisms. Alternatively, the T. neapolitana genomic DNA may be used as the target in a polymerase chain reaction (PCR) where the primers are selected from regions of high sequence conservation among the genes encoding thermostable DNA polymerases. Such a PCR may not amplify the entire coding region of the Tne polymerase I gene; in such a case, the full-length Tne gene could be isolated by using the amplified fragment as a probe to screen a genomic library containing T. neapolitana DNA.
[0124] Once the full-length Tne polymerase gene is obtained, regions encoding the 5′ exonuclease and/or 3′ exonuclease may be altered by a variety of means to reduce or eliminate these activities. Suitable deletion and site-directed mutagenesis procedures are described below in the examples.
[0125] Deletion of amino acids from the protein can be done either by deletion of the encoding genetic material, or by introduction of a translational stop codon by mutation or frame shift. In addition, proteolytic treatment of the protein molecule can be performed to remove segments of the protein.
[0126] In the examples below, specific alterations of the Tne polymerase gene were: a deletion between residues 1-849, a deletion between residues 1-945, a deletion between residues 1-966, deletion between residues 1-966, a deletion between residues 1-849 and residues 925-1272 and substitutions at residues 946, 947, 967, 968, 969, 975, 1166, 1167, 1391, 1402, 1407 and 1410. These modified sequences are described below in the examples and at SEQ ID NOS:7, 10, 15, 18, 22, 25, 28, 32, 34, 36 and 38.
[0127] Those skilled in the art know that single base changes can be innocuous in terms of enzyme structure and function. Similarly small additions and deletions can be present without substantially changing the exonuclease or polymerase function of the wild-type or modified Tne DNA polymerases. To test whether a particular change is innocuous in terms of the effect upon enzymatic activity, the polymerase encoded by a given DNA sequence is tested for the presence of synthetic activity, 5′ exonuclease activity and 3′ exonuclease activity as in the assays described in the examples below. DNA sequence which contain alterations other than those listed in SEQ ID NOS:7, 10, 15, 18, 22, 25, 28, 32, 34, 36 and 38 but which encode a polymerase molecule having the properties associated with the polymerases encoded by the above SEQ ID NOS are contained within the present invention.
[0128] Other deletions and substitutions are also suitable to create modified Tne DNA polymerases lacking 5′ and/or 3′ exonuclease activity. For example, given the degeneracy of the genetic code, several DNA sequences may be used to introduce substitutions which result in the expression of the same amino acid. It is preferable, that the alteration decrease the 5′ and/or 3′ exonuclease activity to a level which is low enough to provide an improved enzyme for a variety of applications such as PCR and chain termination sequencing (including thermal cycle sequencing) as discussed below in the examples. These modifications will preferably not reduce the synthetic activity of the modified enzyme. Modified polymerases are tested for the presence of synthetic activity and 5′ and 3′ exonuclease activity as in assays described below. Thoughtful consideration of these assays allows for the screening of candidate enzymes whose structure is heretofore as yet unknown. In other words, construct “X” can be evaluated according to the protocol described below to determine whether it is a member of the genus of modified Tne polymerases of the present invention as defined functionally, rather than structurally.
[0129] The present invention contemplates that the nucleic acid construct of the present invention be capable of expression in a suitable host. In particular it is preferable that the expression system chosen utilize a tightly controlled promoter such that expression of the Tne polymerase is prevented until expression is induced. In this manner, potential problems of toxicity of the expressed polymerases to the host cells (and particularly to bacterial host cells) is avoided. Those in the art know methods for attaching various promoters and 3′ sequences to a gene structure to achieve efficient and tightly controlled expression. The examples below disclose a number of suitable vectors and vector constructs. Of course, there are other promoter/vector combinations that would be suitable. The choice of a particular vector is also a function of the type of host cell to be employed (i.e., procaryotic or eucaryotic).
[0130] It is not necessary that a host organism be used for the expression of the nucleic acid constructs of the invention. For example, expression of the protein encoded by a nucleic acid construct may be achieved through the use of a cell-free in vitro transcription/translation system. An example of such a cell-free system is the commercially available TnT™ Coupled Reticulocyte Lysate System (Promega; this cell-free system is described in U.S. Pat. No. 5,324,637, the disclosure of which is herein incorporated by reference).
[0131] The nucleic acid construct containing DNA encoding the wild-type or a modified Tne polymerase may provide for the addition of exogenous sequences (i.e., sequences not encoded by the Tne polymerase coding region) to either the 5′ or 3′ end of the Tne polymerase coding region to allow for ease in purification of the resulting polymerase protein (the resulting protein containing such an affinity tag is termed a fusion protein). Several commercially available expression vectors are available which provide for the addition of affinity tags (an example of an exogenous sequence) to either the amino or carboxy-termini of a coding region; in general these affinity tags are short stretches of amino acids which do not alter the characteristics of the protein to be expressed (i.e., no change to enzymatic activities).
[0132] For example, the pET expression system (Novagen) utilizes a vector containing the T7 promoter which encodes the fusion protein containing a short stretch of histidine residues at either end of the protein and a host cell which can be induced to express the T7 DNA polymerase (i.e., a DE3 host strain). The production of fusion proteins containing a histidine tract is not limited to the use of a particular expression vector and host strain. Several commercially available expression vectors and host strains can be used to express protein sequences as a fusion protein containing a histidine tract (For example, the pQE series (pQE-8, 12, 16, 17, 18, 30, 31, 32, 40, 41, 42, 50, 51, 52, 60 and 70) of expression vectors (Qiagen) which are used with the host strains M15[pREP4] (Qiagen) and SG13009[pREP4] (Qiagen) can be used to express fusion proteins containing six histidine residues at the amino-terminus of the fusion protein). Additional expression systems which utilize other affinity tags are known to the art.
[0133] Once a suitable nucleic acid construct has been made, the Tne polymerase may be produced from the construct The examples below and standard molecular biological teachings enable one to manipulate the construct by different suitable methods.
[0134] Once the desired Tne polymerase has been expressed, the polymerase is tested for both synthetic and exonuclease activity as described below.
[0135] III. Use of Tne DNA Polymerases in the PCR
[0136] The wild-type and modified Tne polymerases of the present invention provide suitable and in some cases superior enzymes for use in the PCR. As shown in the examples below, the wild-type and modified forms of Tne polymerase were found to require the use of fewer units of polymerase activity to produce a given amount of product DNA in PCRs as compared to wild-type Taq DNA polymerase (i.e., nTaq) or a modified form of Tma DNA polymerase (i.e., UlTma™). In addition, modified forms of Tne polymerase were found to tolerate a broader range of dNTP concentrations and a broader range of magnesium ion concentrations in the PCR. The ability to tolerate a broad range of dNTP is important as it allows flexibility in the range of dNTPs to be used in a reaction; additionally, the ability to tolerate a wide range of dNTP concentrations demonstrates that the enzymes of the invention provide for a robust PCR (i.e., the enzyme is not sensitive to small variations in dNTP concentration). The ability to produce only specific amplification products over a wide range of magnesium ion concentration is advantageous for use in multiplexing PCR reactions.
[0137] Several of the modified Tne polymerases provide enzymes having greater resistance to thermal inactivation as compared to nTaq or UlTma™ DNA polymerases. Greater thermal stability is important for PCR applications as the greater the thermal stability of the enzyme, the fewer units of enzyme must be used in the PCR.
[0138] IV. Use of Tne DNA Polymerases in DNA Sequencing Methods
[0139] The sequence of a deoxyribonucleic acid molecule can be elucidated using chemical [Maxam and Gilbert, Proc. Natl. Acad Sci. USA 74:560 (1977)] or enzymatic [Sanger et al., Proc. Natl. Acad Sci. USA 74:5463 (1977)] methods. The enzymatic method of sequencing is based on the ability of a DNA polymerase to extend a primer, hybridized to the template that is to be sequenced, until a chain-terminating nucleotide is incorporated (referred to as chain terminating sequencing). Each sequence determination is carried out as a set of four separate reactions, each of which contains all four deoxyribonucleoside triphosphates (dNTP) supplemented with a limiting amount of a different dideoxyribonucleoside triphosphate (ddNTP). Because ddNTPs lack the 3′-OH group necessary for chain elongation, the growing oligonucleotide is terminated selectively at G, A, T, or C, depending on the respective dideoxy analog in the reaction.
[0140] The relative concentrations of each of the dNTPs and ddNTPs can be adjusted to give a nested set of terminated chains over several hundred to a few thousand bases in length. The resulting fragments, each with a common origin but ending in a different nucleotide, are separated according to size by high-resolution denaturing gel electrophoresis.
[0141] Incorporation of a radiolabel into the oligonucleotide chain permits the visualization of the sequencing products by autoradiography. The end-labeled primer protocol, a modification of that described by Heiner et al.[(1988) Applied Biosystems, Inc. DNA Sequencer Model 370 User Bulletin-Taq Polymerase: Increased Enzyme Versatility in DNA Sequencing], uses [&ggr;-32P]ATP, [&ggr;-33P]ATP or [&ggr;-3S]ATP to label the sequencing primer. The DNA template and labeled primer are repeatedly annealed and enzymatically extended/terminated in thermal cycled sequencing. The end-labeled primer protocol is the most versatile sequencing method and is useful when working with lambda DNA [Kaledin et al., Biokhimiya 45:494 (1980)], PCR templates, and any template where false priming may be a problem. This protocol generates sequence data very close to the primer and is recommended when this is needed. The reaction also contains deaza nucleotide mixes that substitute 7-deaza dGTP for dGTP. The deaza mixes resolve band compressions associated with GC-rich regions [Mizusawa et al., Nucl, Acids Res. 14:1319 (1986) and Barr et al., Biotechniques 4:428 (1986)].
[0142] Thermal cycled sequencing is an alternative method for enzymatic sequence analysis which takes advantage of the intrinsic properties of thermophilic DNA polymerases, such as the one isolated from Thermus aquaticus (Taq DNA polymerase). Because the protocol utilizes a thermocycling apparatus, several advantages are realized over conventional sequencing strategies. First, the protocol yields a linear amplification of the template DNA, reducing the amount of template required to achieve a detectable sequence ladder. Using a 32P end-labeled primer, greater than 500 bases of sequence can be obtained from as little as 4 fmol (4×10−15 moles) of template after an overnight exposure. Secondly, the high temperatures employed during each denaturation cycle eliminate the requirement for alkaline denaturation and ethanol precipitation of double-stranded DNA (dsDNA) templates. The denaturation cycles also help to circumvent the problems associated with rapid reannealing of linear dsDNA templates such as PCR reaction products. Third, high annealing temperatures increase the stringency of primer hybridization. Fourth, the high polymerization temperature decreases the secondary structure of DNA templates and thus permits polymerization through highly structured regions [Innis et al., Proc. Natl. Acad. Sci USA 85:9436 (1988)]. This system is useful for sequencing a wide variety of templates such as amplified DNA, large double-stranded DNA templates such as lambda, GC-rich templates and palindrome-rich templates.
[0143] Existing thermostable polymerases which are used in chain termination methods of sequencing (both traditional and thermal cycling protocols) require fairly high concentrations of ddNTPs as the affinity of these polymerases for ddNTPs is somewhat low. For example, when Taq DNA polymerase is employed for chain termination sequencing protocols, the optimal concentrations of ddNTPs in the ddNTP termination mixtures (3× mixtures): 180 &mgr;M ddGTP, 1 mM ddATP, 1.5 mM ddTTP and 500 &mgr;M ddCTP [as described in U.S. Pat. No. 5,075,216, the disclosure of which is herein incorporated by reference]. When the polymerase employed is a modified form of Taq DNA polymerase, sTaq (sequencing grade Taq), the optimal concentrations of ddNTPs in the ddNTP termination mixtures: 30 &mgr;M ddGTP, 350 &mgr;M ddATP, 600 &mgr;M ddTTP and 200 &mgr;M ddCTP. In contrast, a modified form of Tne DNA polymerase provided herein utilizes the following concentrations of ddNTPs in the termination mixtures (3× mixtures): 20 &mgr;M ddGTP, 50 &mgr;M ddATP, 75 &mgr;M ddTTP and 25 &mgr;M ddCTP. Because ddNTPs are expensive, the use of a thermostable polymerase having a higher affinity for ddNTPs (i.e., the modified Tne polymerase of the invention) will result in considerable cost savings in DNA sequencing applications.
[0144] Experimental
[0145] The following examples serve to illustrate certain preferred embodiments and aspects of the present invention and are not to be construed as limiting the scope thereof.
[0146] In the disclosure which follows, the following abbreviations apply: ° C. (degrees Centigrade); g (gravitational field); vol (volume); w/v (weight to volume); v/v (volume to volume); BSA (bovine serum albumin); CTAB (cetyltrimethylammonium bromide); fmol (femtomole); HPLC (high pressure liquid chromatography); DTT (dithiothreitol); DMF (N,N dimethyl formamide); DNA (deoxyribonucleic acid); i.d. (internal diameter); p (plasmid); Al (microliters); ml (milliliters); &mgr;g (micrograms); pmoles (picomoles); mg (milligrams); MOPS (3-[N-Morpholino]propanesulfonic acid); M (molar); mM (milliMolar); &mgr;M (microMolar); nm (nanometers); kdal (kilodaltons); OD (optical density); EDTA (ethylene diamine tetra-acetic acid); FITC (fluorescein isothiocyanate); SDS (sodium dodecyl sulfate); NaPO4 (sodium phosphate); Tris (tris(hydroxymethyl)-aminomethane); PMSF (phenylmethylsulfonylfluoride); TBE (Tris-Borate-EDTA, i.e., Tris buffer titrated with boric acid rather than HCl and containing EDTA); PBS (phosphate buffered saline); PPBS (phosphate buffered saline containing 1 mM PMSF); PAGE (polyacrylamide gel electrophoresis); Tween (polyoxyethylene-sorbitan); Boehringer Mannheim (Boehringer Mannheim, Indianapolis, Ind.); Epicentre (Epicentre Technologies, Madison, Wis.); New England Biolabs (New England Biolabs, Beverly, Mass.); Novagen (Novagen, Inc., Madison, Wis.); Pharmacia (Pharmacia Biotech Inc., Piscataway, N.J.); Perkin Elmer (Perkin Elmer, Norwalk, Conn.); Promega (Promega Corp., Madison, Wis.); Qiagen (Qiagen Inc., Chatsworth, Calif.); Spectra (Spectra, Houston, Tex.); Stratagene (Stratagene Cloning Systems, La Jolla, Calif.); USB (U.S. Biochemical, Cleveland, Ohio).
EXAMPLE 1 Isolation of the Tne DNA Polymerase Gene[0147] a) Growth of T. neapolitana Cells
[0148] T. neapolitana cells (obtained from V. A. Svetlichny, The Institute of Microbiology, Russian Academy of Sciences, Moscow) were grown in a medium containing (per 100 ml): 0.1 ml K-phosphate solution [300 g/l K2HPO4 and 200 g/l KH2PO4]; 1 ml of Solution 1 [27 g/l NH4CI, 27 g/l CaCl2 and 31 g/l MgCl2.6H2O]; 1 ml of a 10% solution of yeast extract (Difco); 2.5% natural sea salt; 0.1 ml of a 2% solution of resazurin; 1 ml of 5% Na2SO3; 150 mg NaHCO3 and 0.5% glucose.
[0149] b) Isolation of Genomic DNA
[0150] Large scale cultures (10 liters) of T. neapolitana cells were grown in the above medium in a 10 l fermentation vessel under nitrogen (i.e., anaerobic conditions) at 75° C. for 28 hours (early stationary phase). The cells were then collected by centrifugation at 10,000×g and the cell pellet was washed once with a solution comprising 0.9% NaCl. The washed cell pellet was frozen at −70° C. DNA was isolated from the frozen cells as follows. The frozen cells (3 g) were thawed in 30 ml of a solution containing 100 mM Tris-HCl (pH 9.0), 50 mM EDTA and 2 mg/ml lysozyme. The mixture was incubated for 30 min at 0° C. and then SDS and proteinase K was added to a final concentration of 1% and 100 &mgr;g/ml, respectively. The mixture was incubated for 1.5 hours at 45° C. with light shaking. Following the incubation, the mixture was cooled to room temperature (about 25° C.) and NaCl was added to a concentration of 0.5 M. An equal volume of phenol/chloroform was added and the mixture was extracted and the aqueous and organic phases were separated by centrifugation for 10 min at 6,000×g at room temperature. The supernatant was transferred to a fresh tube using a wide-bore pipet. A total of six phenol/chloroform extractions were performed (until the interphase disappeared). The DNA was precipitated by the addition of ethanol and gently mixing the solution. The precipitated DNA was washed with 70% ethanol. The DNA was then centrifuged for 5 min at 10,000×g and the supernatant was discarded. The pellet was resuspended in a buffer containing 10 mM Tris-HCl (pH 7.4), 0.1 mM EDTA and stored at −20° C. until used.
[0151] c) Construction of a T. neapolitana Genomic DNA Library
[0152] The T. neapolitana DNA was then digested with Sau3A under conditions which promoted the generation of fragments 3-8 kb in length. Briefly, 10 &mgr;g of genomic DNA was digested with 1.6 units of Sau3A in a volume of 15 &mgr;l for 1 hour at 37° C. The reaction was stopped by the addition of 5 &mgr;l of sample buffer [70% glycerol, 50 mM EDTA] and the digested DNA was run on a low melting temperature agarose gel (BioRad). Fragments 3-8 kb in length were isolated from the gel using standard procedures [Sambrook et al., Molecular Cloning: A Laboratory Manual, 2nd ed., Cold Spring Harbor Laboratory Press, NY (1989) pp. 6.30-6.31]. The DNA recovered from the gel was precipitated with ethanol, dried and resuspended in 100 &mgr;l of 10 mM Tris-HCl (pH 7.5), 0.1 mM EDTA. Five microliters of the DNA mixture (about 0.1 &mgr;g) was ligated to 0.25 &mgr;g of the pTZ19R vector (Pharmacia) which had been digested with BamHI and treated with bacterial alkaline phosphatase. The ligation products were used to transform competent TG1 cells [TG1 cells are an EcoK− derivative of JM101 cells; a commercially available equivalents include NM522 cells (Pharmacia) and XL1-Blue cells (Stratagene)] and the cells were plated onto MacConkey agar plates (Difco).
[0153] White colonies (i.e., those containing plasmids having an insert) were picked onto 30 master plates (96 colonies/plate; a total of about 3,000 colonies were screened). Replica plates were generated to provide cells for DNA polymerase analysis. The library was screened by functional assay; the cells from each replica plate were removed and pooled by rinsing the plate with 2 ml of 0.9% NaCl. The cells were then collected by centrifugation (12,000 rpm for 3 min) in a microcentrifuge (Eppendorf). The pellets were then washed with 1.5 ml of 0.9% NaCl. The washed cell pellets were then lysed by sonication in 0.5 ml of a solution containing 50 mM Tris-HCl (pH 7.4), 1 mM EDTA and 5 mM PMSF. The lysates were then heated to 72° C.-75° C. in a water bath for 20 min. Following the incubation, the lysates were clarified by centrifugation in a microfuge at 12,000 rpm for 10 min. The supernatant was removed to a fresh tube. DNA polymerase activity was assayed using the supernatant as follows.
[0154] Denatured bovine thymus DNA was prepared as follows. A solution comprising 6 mM bovine thymus DNA (BioLAR, Olaine, Latvia; equivalent preparations of calf thymus DNA are available from Sigma, St. Loius, Mo.) in 1 mM NaOH was incubated for 15 min at 20° C. The solution was then neutralized by the addition of HCl to a final concentration of 100 mM and Tris-HCl, pH 8.0 to a final concentration of 50 mM.
[0155] The following components were mixed: 2.5 &mgr;l 0.5 M Tris-HCl (pH 7.6), 5 &mgr;l 100 mM MgCl2, 2 &mgr;l denatured bovine thymus DNA (2 mg/ml), 0.2 &mgr;l 12.5 mM of each of the dNTPs and 1.5 &mgr;lCi of &agr;-32P-dTTP and H2O to a volume of 25 &mgr;l. Twenty-five microliters of supernatant from each of the pools of lysed cells were mixed with 25 &mgr;l of the above assay mixture in the well of a 96 well microtiter plate. The mixture was incubated for 1 hour at 75° C. The reaction was stopped by the addition of 5 &mgr;l of 200 mM EDTA (pH 8.0). Five microliters of the reaction mixture was then loaded onto a 1×1 inch square of DEAE paper (Whatman). The samples were dried at 80° C. and then washed with 0.5 M sodium phosphate (pH 7.2) (wash solution) using about 5 ml of wash solution per sample for 10 min with light shaking. Three washes were performed. The samples were then rinsed with water (10 ml/sample) for 1 min followed by an ethanol rinse. The ethanol rinsed samples were then dried at 80° C. and DEAE-absorbent radioactivity was counted using a liquid scintillation counter. The results of the initial DNA polymerase assays revealed that a single pool produced DNA polymerase activity.
[0156] To isolate clones containing Tne genomic DNA encoding the DNA polymerase activity, the colonies on the positive master plate were grown as smaller pools comprising either a single row or a single column of colonies. The smaller pools of colonies were grown, lysates were prepared and DNA polymerase activity was determined as described above. A single row and a single column contained DNA polymerase activity; the intersection of this row and column identified the single individual clone containing DNA encoding DNA polymerase activity. This single colony was grown and assayed for DNA polymerase activity to confirm the presence of thermostable DNA polymerase activity. This clone was called pTen.
[0157] DNA was prepared from the pTen clone using standard techniques of molecular biology; this clone was found to contain a insert of approximately 3.5 kb. Restriction enzyme digests were performed with a battery of enzymes to create a restriction map of the Tne genomic DNA insert. Subclones were generated from the positive clone and a series of nested deletions were generated using Exonuclease III and standard molecular biology techniques to facilitate DNA sequencing [Short Protocols in Molecular Biology, 2nd ed. (1992) Ausubel et al. Eds, John Wiley & Sons, New York, pp.7-8 to 7-16 and 7-29 to 7-37]. The DNA sequence of the insert was determined using the Sanger dideoxy sequencing method and Sequenase® (USB). The DNA sequence of the coding region for the full-length Tne DNA polymerase gene is listed in SEQ ID NO:1. The deduced amino acid sequence of the Tne DNA polymerase is listed in SEQ ID NO:2.
[0158] Analysis of the deduced amino acid sequence was performed using protein analysis software (DNAStar, Inc., Madison, Wis.). The open reading frame encodes a protein of 893 amino acids; the predicted molecular weight of the protein is 102,054 (however, as shown in Example below, the full-length protein migrates with an apparent molecular weight of 97,000 on SDS-PAGE gels). The predicted isolelectric point is 6.19 and the charge at pH 7.0 is −7.56.
[0159] The nucleotide and amino acid sequences of the Tne DNA polymerase were compared with the reported sequences for E. coli DNA polymerase I and the thermostable DNA polymerase from T. maritima. FIG. 3 provides an alignment of the amino acid residues of these three polymerases. In FIG. 3 the following abbreviations are used: Eco (E. coli DNA polymerase I); Tma (Tma DNA polymerase) and Tne (Tne DNA polymerase). Shading is used to indicate residues which differ from the amino acid sequence of Tne DNA polymerase.
[0160] In E. coli DNA polymerase I, the 5′ exonuclease domain comprises approximately residues 1-323; the 3′ exonuclease domain comprises approximately residues 324-517 and the synthetic or polymerization domain comprises approximately residues 521-928. Alignment of the amino acid sequences of E. coli DNA polymerase I with the sequence of the Tne DNA polymerase molecule of the present invention reveals that the 5′ exonuclease domain of Tne DNA polymerase comprises approximately residues 1-297; the 3′ exonuclease domain comprises approximately residues 298482 and the polymerization domain comprises approximately residues 486-893.
[0161] Alignment of amino acid residues present in E. coli DNA polymerase I and Tne DNA polymerase shows that the two enzymes are 44% identical overall and 51% identical over the polymerase domain (residues 521-928 in E. coli and residues 486-893 in Tne). The alignment was performed using the Lipman-Pearson algorithm as provided by DNASTAR, Inc. (Madison, Wis.); gaps were introduced into the two sequences to provide for maximum alignment.
[0162] Comparison of the nucleotide and amino acid sequences of the Tne and Tma polymerases revealed that these two polymerases share 78% identity at the nucleotide level and 88% identity at the amino acid level.
EXAMPLE 2 Efficient Expression of Tne DNA Polymerase in E. coli[0163] In order to express the Tne DNA polymerase in large amounts in host cells, the DNA sequences encoding the polymerase (i.e., the coding region) were removed from pTen (described in Example 1) and inserted into an expression vector.
[0164] Restriction enzyme analysis revealed that the Tne polymerase gene was present in the opposite transcriptional orientation relative to the T7 promoter present on the pTZ19R vector used to generate pTen. In order to produce Tne polymerase, sequences encoding the Tne polymerase gene were removed from pTen by digestion with SmaI and XbaI and an approximately 3.5 kb SmaI/XbaI fragment was isolated by electrophoresis of the digestion products on an agarose gel followed by excision of the desired band. DNA was recovered from the agarose block using the Wizard™ PCR Preps DNA Purification System (Promega). Briefly, 0.5 to 1.0 ml of Wizard™ PCR Preps DNA Purification Resin was added to the agarose block and the mixture was incubated at 42° C. for 5 minutes to melt the agarose. DNA was extracted using the protocol provided in the kit.
[0165] The 3.5 kb SmaI/XbaI fragment was ligated into the pGEM®-3Zf(+) vector (Promega) which had been digested with SmaI and XbaI to generate pGTne. This construction placed the 5′ end (i.e., encodes the N terminal portion) of the Tne polymerase gene downstream of the T7 promoter in the same transcriptional orientation. The ligation mixture was used to transform competent JM109(DE3) cells (Promega). Recombinant clones were isolated, confirmed by restriction digestion using standard recombinant molecular biology techniques [Sambrook et al., Molecular Cloning, A Laboratory Manual (1989)].
[0166] Expression of the Tne DNA polymerase from the pGTne construct was next examined. JM109(DE3) cells containing pGTne were grown at 37° C. and induced with 1 mM IPTG. Parallel cultures of TG1 cells containing pTen were grown and induced. After a few hours (i.e, 1-3) of growth in the presence of IPTG, the cells were collected by centrifugation and crude lysates were prepared as follows. A 1 ml aliquot of each of the cultures containing the Tne constructs was centrifuged in a microcentrifuge at 14,000×g for 3 min at room temperature to pellet the cells. The cells were then resuspended in 200 &mgr;l of a solution comprising 50 mM Tris-HCl (pH 8.0), 50 mM glucose, 1 mM EDTA and the cells were pelleted again. The cells were next resuspended in 50 &mgr;l of the previous buffer containing 4 mg/ml lysozyme and the mixture was incubated at room temperature for 10 minutes. Following the incubation, 50 82 l of a solution comprising 10 mM Tris-HCl (pH 8.0), 50 mM KCl, 1 mM EDTA, 1 mM PMSF, 0.5% Tween-20, 0.5% Nonidet P40 was added and the mixture was incubated at 75° C. for 10 minutes. The lysate was then clarified by centrifugation in a microcentrifuge at 14,000×g for 5 minutes. Eighty microliters of the supernatant was removed and stored in a separate tube at 4° C. The crude lysates were analyzed for polymerase activity at 74° C. as described in Example 5(b), below.
[0167] The following results were obtained. The pTen construct gave polymerase activity levels at or about 2 fold higher than the background level for the assay. The pGTne construct gave activity levels of about 50 times background levels. While the expression of Tne polymerase seen using pGTne was much improved relative to the level seen using pTen, this expression level was not sufficient to produce large amounts of the enzyme.
[0168] These above result suggested that the Tne polymerase promoter was non-functional in E. coli (very low level of activity present when pTen is used). Furthermore, the presence of the Tne polymerase promoter appeared to be detrimental to expression when transcription was initiated from the T7 promoter in the pGTne construct (perhaps due to transcriptional read-through interference). In order to remove the Tne polymerase gene promoter from the Tne polymerase coding region, the following experiments were conducted.
[0169] The DNA sequence of the 5′ end of the Tne polymerase gene was sequenced using the M13 forward primer in conjunction with the fmol® DNA Sequencing System (Promega); sequencing was conducted according to the manufacturer's instructions. The sequence analysis revealed that a unique BglI site was found 43 bp into the coding region (i.e., 43 bp following the A of the initiator ATG codon). To remove the coding region of the Tne polymerase gene from pGTne, pGTne was digested with BglI and XbaI and the approximately 3.0 kb BglI/XbaI fragment was isolated (as described above). The 3.0 kb BglI/XbaI fragment was ligated directly downstream of either the T7 promoter or the tac promoter present in pALTER®-Ex1 (Promega); pALTER®-Ex1 contains both the T7 and the tac promoters positioned in opposite transcriptional orientations relative to one another. These two ligations were performed as follows.
[0170] To insert the 3.0 kb BglI/XbaI fragment downstream of the T7 promoter, a 43 bp synthetic linker having a NcoI overhanging end at one end and a BglI overhanging end at the other end was ligated to the Tne polymerase coding region. This linker was formed by annealing of the following two oligonucleotides: JH64 which comprises 5′-CATGGCGAGACTATTTCTCTTTGATGGCACAGCCCTGGCCTACA-3′ (SEQ ID NO:3) and JH65 which comprises 5′-AGGCCAGGGCTGTGCCATCAAAGAGAA ATAGTCTCGC-3′ (SEQ ID NO:4). This synthetic linker regenerates the native sequence of the Tne polymerase gene located upstream of the BglI site and allows insertion of the coding region into pALTER®-Ex1. pALTER®-Ex1 was digested with NcoI and XbaI and the coding region containing the synthetic linker was ligated to the digested vector to generate pATne2.
[0171] To insert the 3.0 kb BglI/XbaI fragment downstream of the tac promoter, a 43 bp synthetic linker having a NdeI overhanging end at one end and a BglI overhanging end at the other end was ligated to the Tne polymerase coding region. This linker is formed by annealing of the following two oligonucleotides: JH62 which comprises 5-AGGCCAGGGCTGTGCCATCAAAGAGAAATAGTCTCGCCA (SEQ ID NO:5) and JH63 which comprises 5′-TATGGCGAGACTATTTCTCTTTGTGGCACAGCCCT GGCCTACA-3′ (SEQ ID NO:6). This synthetic linker regenerates the native sequence of the Tne polymerase gene located upstream of the BglI site and allows insertion of the coding region into pALTER®-Ex1. pALTER®-Ex1 was digested with NdeI and XbaI and the coding region containing the synthetic linker was ligated to the digested vector to generate pATne1.
[0172] Competent E. coli cells were transformed with the above ligation mixtures corresponding to either pATne1 (JM109 cells; Promega) and pATne2 [JM109(DE3) cells; Promega]. Recombinant clones were isolated, confirmed by restriction digestion using standard recombinant molecular biology. Cells harboring either pATne1 or pATne2 were grown and induced as described above. Crude lysates were prepared and DNA polymerase assays were performed (as described above). The results of these polymerase assays showed that both pATne1 and pATne2 gave significantly better yields of Tne polymerase than pGTne (at least 2-3 fold higher).
EXAMPLE 3 Construction of Tne Deletion Mutants Lacking 5′ to 3′ Exonuclease Activity[0173] As noted above, the presence of 5′ to 3′ exonuclease activity in a thermostable DNA polymerase is undesirable for certain applications. To construct mutant Tne polymerases lacking 5′ to 3′ exonuclease activity, two deletion mutants of the Tne polymerase gene were generated. Both mutants contain deletions which remove sequences encoding a large portion of the 5′ to 3′ exonuclease domain located at the N terminus of the Tne polymerase molecule.
[0174] a) Construction of a Vector Encoding Deletion Mutant Tne M284
[0175] The deletion mutant Tne M284 is a truncated form of the Tne polymerase which uses the naturally occurring methionine at amino acid position 284 in the full-length protein (SEQ ID NO:2) as the initiating methionine for translation initiation. FIG. 4 provides a schematic representation of several modified Tne polymerases (constructed as described in Examples 3 and 4) along the map of the full-length Tne polymerase protein. The scale represents length in increments of 100 amino acid residues. The full length Tne polymerase (SEQ ID NO:2) contains 893 amino acids. The thick open boxes represent the presence of amino acid residues; thin lines between two regions of thick boxes indicates that amino acids were deleted between the two open boxes. Circles containing a single letter indicate the location of a mutated amino acid residue (the single letter code is used for the amino acid residues indicated).
[0176] To generate a construct containing the Tne M284 mutant, pGTne was digested with BspHI (generates ends compatible with NcoI ends) and KpnI and a 1.05 kb BspHI/KpnI fragment (containing the 5′ portion of the coding region) was isolated as described in Example 2. A second aliquot of pGTne was digested with KpnI and XbaI and a 1.3 kb fragment containing the 3′ portion of the Tne polymerase coding region was isolated. pALTER-Ex1 was digested with NcoI-and XbaI. A three-way ligation was performed using the digested pALTEREx1 vector, 1.05 kb BspHI/KpnI fragment and the 1.3 kb KpnI/XbaI fragment. Competent JM109(DE3) cells were transformed with the ligation mixture and recombinant clones were isolated, confirmed by restriction digestion using standard recombinant molecular biology. The resulting plasmid was called pM284. The nucleotide sequence of the region encoding the Tne M284 gene is listed in SEQ ID NO:7. The amino acid sequence of Tne M284 is listed in SEQ ID NO:8.
[0177] b) Construction of a Vector Encoding Deletion Mutant Tne M316
[0178] The deletion mutant Tne M316 is a truncated form of the Tne polymerase protein which uses an artificially created methionine at amino acid position 316 as the initiator methionine. Tne M316 was created by introducing a methionine residue at position 316 (and a corresponding NcoI site) via site-directed mutagenesis using the Altered Sites® II in vitro Mutagenesis System (Promega) in conjunction with mutagenesis oligonucleotide JH68 [5′-ATCGAAAAGCTGACCATGGTTCCATCTT TTG-3′ (SEQ ID NO:9)] and pATne2. The manufacturer's protocol was followed exactly. Briefly, pATne2 was denatured using alkali and the JH68 mutagenic oligonucleotide was annealed to the denatured plasmid along with the ampicillin repair oligonucleotide (provided in the kit). The mutant strand was synthesized using T4 DNA polymerase and T4 DNA ligase. ES1301mutS cells (provided in the kit) were then cotransformed with the mutagenized pATne2 and R408 DNA (provided in the kit). Small scale DNA preparations were prepared from the transformed ES1301mutS cells and the DNA was used to transform JM109 cells. Mutants were selected by growth on ampicillin plates and the desired recombinant were confirmed by restriction enzyme analysis (i.e., presence of an additional NcoI site). The resulting plasmid containing the engineered NcoI site was then digested with NcoI which deleted the 5′ to 3′ exonuclease domain as a 948 bp fragment and the large fragment was isolated (as described above) and religated to itself to create the pM316 construct. The nucleotide sequence of the region encoding the Tne M316 gene is listed in SEQ ID NO:10. The amino acid sequence of Tne M316 is listed in SEQ ID NO:11.
[0179] c) Expression of Tne M284 and Tne M316 in E. coli
[0180] The pM284 and pM316 constructs (in JM109 cells) were grown, induced and crude lysates were prepared as described in Example 2. DNA polymerase activity was measured in crude lysates as described in Example 2.
[0181] The results of the polymerase assays showed that the Tne M284 mutant (pM284) contained 17 units per &mgr;l of polymerase activity from the crude lysate and the Tne M316 mutant (pM316) produced no detectable polymerase activity. No detectable polymerase activity was found when the Tne M316 polymerase was expressed from the trc promoter either (to express the Tne M316 coding region from the trc promoter, a 2.5 kb NcoI/PstI fragment was isolated from pM316 and ligated to pTrc 99 A (Pharmacia) digested with NcoI and PstI).
[0182] Aliquots (5 &mgr;l) of each crude lysate were electrophoresed on a pre-cast 4-20% denaturing gradient polyacrylamide gel (Novex, San Diego, Calif.); following electrophoresis, the gel was stained with Coomassie blue to visualize the separated proteins. A single, sharp protein band corresponding to the expected size was visible in lysates produced from cells containing the pATne1 (full-length Tne polymerase) and pM284 (Tne M284 deletion mutant) constructs. No protein band was observed for deletion mutant Tne M316 when expressed from either the T7 or trc promoters.
[0183] In order to increase the level of expression of Tne M284 protein in E. coli, the Tne M284 coding region was placed downstream of the strong tac promoter present in the JHEX3 vector to create pJM284. JHEX3 was created as follows. pALTER-1 (Promega) was digested with ClaI and StyI and the ends were made blunt by incubation with the Klenow fragment. The 1.345 kb ClaI/StyI fragment was isolated and ligated into pTrc 99 A (Pharmacia) which had been digested with BsaAI. This ligation inserted the tetracycline-resistance gene into the pTrc 99 A vector; the resulting vector was called JHEXa. The ampicillin-resistance gene was then removed from the JHEXa by digestion with SspI, DraI and PvuI; this digestion cut the ampicillin gene into four small fragments (483 bp, 227 bp, 209 bp and 19 bp). The large fragments (3.93 kp and 652 bp) were isolated and ligated together to create JHEXb. The Trc promoter was removed from JHEXb as an 89 bp SspI/NcoI fragment and replaced with the tac promoter. The tac promoter was inserted into the SspI/NcoI-digested JHEXb vector as a 141 bp BsrBI fragment from pALTER-Ex1 (Promega) together with a 30 bp linker formed by the oligonucleotide pair listed in SEQ ID NOS:45 and 46.
[0184] To generate pJM284 construct was made as follows. pGTne was digested with BspHI (generates ends compatible with NcoI ends) and KpnI and a 1.05 kb BspHI/KpnI fragment (containing the 5′ portion of the′ coding region) was isolated as described in Example 2. A second aliquot of pGTne was digested with KpnI and XbaI and a 1.3 kb fragment containing the 3′ portion of the Tne polymerase coding region was isolated. JHEX3 was digested with NcoI and XbaI. A three-way ligation was performed using the digested JHEX3 vector, 1.05 kb BspHI/KpnI fragment and the 1.3 kb KpnI/XbaI fragment. Competent JM109 cells were transformed with the ligation mixture and recombinant clones were isolated, confirmed by restriction digestion using standard recombinant molecular biology. The resulting plasmid was called pJM284.
[0185] Crude lysates were prepared from a small scale culture of JM109 cells containing the pJM284 construct or the pM284 construct. DNA polymerase assays were performed as described in Example 2. The level of Tne M284 polymerase produced by pJM284 was found to be about 50% greater than the level produced by expression from the pM284 construct.
EXAMPLE 4 Construction of Tne Polymerase Mutants Having Altered 3′ to 5′ Exonuclease Activity[0186] In order to produce modified forms of Tne polymerase which possess varying amounts of 3′ to 5′ exonuclease activity, seven different point mutants and two deletion mutants were created using the pM284 construct as the starting material. FIG. 4 provides a schematic drawing of these mutant Tne polymerases.
[0187] All nine mutagenic changes also involved a change in the restriction digest pattern of the starting pM284 plasmid to allow for easy selection of the mutants. In all cases, a small portion of the mutagenized region was exchanged into an Tne M284 gene that did not undergo mutagenesis and the exchanged region was sequenced not only to confirm the mutation, but also to show that there were no second site mutations. DNA sequencing was performed using the fmol® DNA Sequencing System (Promega) in conjunction with using primers JH61. [5′-TGCCGTACACCTCC GAGAGC-3′ (SEQ ID NO:12)] or JH66 [5′-CTCGTTTGGCTCCAGCAAATATGC-3′ (SEQ ID NO:13)]. The mutants were constructed as follows.
[0188] a) Construction of pD323E
[0189] pD323E produces a modified form of the Tne polymerase which lacks the first 283 amino acids from the N-terminus of the full-length protein and contains an amino acid substitution at residue 323 (number indicates position of the residue in the full length protein). At amino acid residue 323 the wild-type aspartic acid is replaced with glutamic acid. pM284 was used in conjunction with the mutagenic oligonucleotide JH74 [5′-TTTGCCCTGGAaCTTGAAACG-3′ (SEQ ID NO:14)]; the mutagenic residues are indicated by the lower case letter] and the Altered Sites® II in vitro Mutagenesis System (Promega) to generate pD323E as described in Example 3. The desired mutants were confirmed by restriction analysis (absence of one of the SinI restriction sites present in pM284. The DNA sequence of pD323E was obtained as described above using the JH66 (SEQ ID NO: 13) primer. The DNA sequence of the polymerase coding region present in pD323E is listed in SEQ ID NO: 15. The corresponding amino acid sequence of the Tne M284(D323E) protein is listed in SEQ ID NO:16.
[0190] b) Construction of pE325D
[0191] pE325D produces a modified form of the Tne polymerase which lacks the first 283 amino acids from the N-terminus of the full-length protein and contains an amino acid substitution at residue 325. At amino acid residue 325, the wild-type glutamic acid residue is replaced with aspartic acid. pM284 was used in conjunction with the mutagenesis oligonucleotide JH75 [5′-GACCTTGAcACGTCCTC (SEQ ID NO:17);the mutagenic residue is indicated by the lower-case letter] and the Altered Sites® II in vitro Mutagenesis System (Promega) to generate pE325D as described in Example 3. The desired mutant was confirmed by restriction analysis (the presence of additional AflIII restriction site). The DNA sequence of pD323 was obtained as described above using the JH66 (SEQ ID NO:13) primer. The DNA sequence of the polymerase coding region present in pE325D is listed in SEQ ID NO: 18. The corresponding amino acid sequence of the Tne M284(E325D) protein is listed in SEQ ID NO:19.
[0192] c) Construction of pY464F
[0193] pY464F produces a modified form of the Tne polymerase which lacks the first 283 amino acids from the N-terminus of the full-length protein and contains an amino acid substitution at residue 464. At amino acid residue 464, the wild-type tyrosine residue is replaced with phenylalanine. pY464F was constructed by replacing a 39 bp FokI fragment present in pM284 with a 39 bp synthetic region. The 39 bp synthetic region was formed by the following two oligonucleotides: JH81 [5′-TAAGTGATATC TGCATCCTCGCAGGAGAAGTTCGCAGCC-3′ (SEQ ID NO:20) and JH82 [5′-ACAAGGCTGCGAACTTCTCCTGCGAGGATGCAGATATCA-3′ (SEQ ID NO:21)]. This synthetic 39 bp oligonucleotide contains the mutation. The desired mutant was confirmed by restriction analysis (the presence of additional EcoRV restriction site). The DNA sequence of pY464F was obtained as described above using the JH61 (SEQ ID NO:12) primer. The DNA sequence of the polymerase coding region present in pY464F is listed in SEQ ID NO:22. The corresponding amino acid sequence of the Tne M284(Y464F) protein is listed in SEQ ID NO:23.
[0194] d) Construction of pD468N
[0195] pD468N produces a modified form of the Tne polymerase which lacks the first 283 amino acids from the N-terminus of the full-length protein and contains an amino acid substitution at residue 468. At amino acid residue 468, the wild-type aspartic acid residue is replaced with asparagine. pM284 was used in conjunction with the mutagenesis oligonucleotide JH79 [5′-ACTCCTGCGAGaATGCtGACATCACTTAT AGG-3′ (SEQ ID NO:24); the mutagenic residues are indicated by the use of lower case letters] and the Altered Sites® II in vitro Mutagenesis System (Promega) to generate pD468N as described in Example 3. The desired mutant was confirmed by restriction analysis (the presence of an additional BsmI restriction site). The DNA sequence of pD468N was obtained as described above using the JH61 (SEQ ID NO:12) primer. The DNA sequence of the polymerase coding region present in pD468N is listed in SEQ ID NO:25. The corresponding amino acid sequence of the Tne M284(D468N) protein is listed in SEQ ID NO:26.
[0196] e) Construction of pD323A
[0197] pD323A produces a modified form of the Tne polymerase which lacks the first 283 amino acids from the N-terminus of the full-length protein and contains an amino acid substitution at residue 323. At amino acid residue 323, the wild-type aspartic acid residue is replaced with alanine. pM284 was used in conjunction with the mutagenesis oligonucleotide JH70 [5′-TTTGCCCTGGcCCTTGAAACG-3′ (SEQ ID NO:27); the mutagenic residue is indicated by the use of the lower case letter] and the Altered Sites® II in vitro Mutagenesis System (Promega) to generate pD323A as described in Example 3. The desired mutant was confirmed by restriction analysis (the absence of a SinI restriction site). The DNA sequence of pD323A was obtained as described above using the JH66 (SEQ ID NO:13) primer. The DNA sequence of the polymerase coding region present in pD323A is listed in SEQ ID NO:28. The corresponding amino acid sequence of the Tne M284(D323A) protein is listed in SEQ ID NO:29.
[0198] f) Construction of pD389A
[0199] pD389A produces a modified form of the Tne polymerase which lacks the first 283 amino acids from the N-terminus of the full-length protein and contains an amino acid substitution at residue 389. At amino acid residue 389, the wild-type aspartic acid residue is replaced with alanine.
[0200] To construct pD389A, the PCR was used to amplify two overlapping fragments independently; the PCR products were combined and the resulting large fragment was reamplified. Mutagenesis oligonucleotide JH80 [5′-CCTGAAGTACGcgTACAAGGT TCTTATGG-3′ (SEQ ID NO:30); the mutagenic residues are indicated by the use of lower case letters] and sequencing primer JH61(SEQ ID NO: 12) were used to prime a first PCR to create a 425 bp fragment which incorporates the desired mutation. The sequencing primers JH66 (SEQ ID NO:13) and M13 reverse (Promega Q5401; SEQ ID NO:31) were used to amplify a 564 bp fragment using pM284 as the template in a second PCR When these two PCR products were combined using the M13 reverse and JH61 primers, a 889 bp fragment was made. A 348 bp BglII fragment was then removed from the 889 bp product and was exchanged with the analogous, but, non-mutagenic BglII fragment of pM284. The desired mutants was confirmed by the presence of an extra MluI restriction site. All PCR reactions were performed using a Perkin-Elmer 480 thermal cycler.
[0201] For generation of the 564 bp product, the PCR was conducted by performing 15 cycles comprising a denaturation step (95° C. for 15 sec) and an annealing/extension step (70° C. for 1 min). JM284 was used as the template in a reaction containing 1 &mgr;M of each of the primers (JH66 and M13 reverse), 1.5 MM MgCl2 and 3 units Tli DNA polymerase (Promega).
[0202] For generation of the 425 bp product, the PCR was conducted by performing 20 cycles comprising a denaturation step (95° C. for 15 sec), an annealing step (55° C. for 30 sec; cycles 1-5) or an annealing step (70° C. for 15 sec; cycles 6-20) and an extension step (70° C. for 45 sec). JM284 was used as the template in a reaction containing 1 &mgr;M of each of the primers (JH80 and JH61), 1.5 mM MgCl2 and 3. units Tli DNA polymerase (Promega).
[0203] For the generation of the 889 bp product, the 564 bp product and the 425 bp product were used as the template in a reaction containing 1.5 mM MgCl2 and 3 units Tli DNA polymerase (Promega). The cycling conditions were: denaturation (95° C. for 15 sec) and annealing/extension at 70° C. for 1 min; no primers were present for cycles 1-5. One &mgr;M of the M13 reverse primer was present for cycles 5-10 and I &mgr;M of the M13 reverse and JH61 primers were present in cycles 11-25.
[0204] The DNA sequence of pD389A was obtained as described above using the JH61 and JH66 (SEQ ID NOS:12 and 13) primer. The DNA sequence of the polymerase coding region present in pD389A is listed in SEQ ID NO:32. The corresponding amino acid sequence of the Tne M284(D389A) protein is listed in SEQ ID NO:33.
[0205] g) Construction of pD323,389A
[0206] pD323,389A produces a modified form of the Tne polymerase which lacks the first 283 amino acids from the N-terminus of the full-length protein and contains two amino acid substitutions at residues 323 and 389. At amino acid residue 323, the wild-type aspartic acid residue is replaced with alanine and at amino acid residue 389, the wild-type aspartic acid residue is replaced with alanine.
[0207] To create pD323,389A, the 215 bp Csp45I fragment of pD323A was exchanged with the analogous fragment in pD389A bringing the two single mutations into the same construct. The desired mutants were selected as having the two restriction site changes of the individual mutants (described above). The DNA sequence of pD323,389A was obtained as described above using the JH66 (SEQ ID NO:13) primer. The DNA sequence of the polymerase coding region present in pD323,389A is listed in SEQ ID NO:34. The corresponding amino acid sequence of the Tne M284(D323A,D389A) protein is listed in SEQ ID NO:35; this enzyme is referred to as the triple mutant Tne polymerase.
[0208] When all of the above point mutant constructs (sections a-g) were induced to express the modified Tne polymerases in an E. coli host, the crude extracts showed polymerase activity comparable with the activity observed using the original pM284 construct (cultures were grown, induced, lysates prepared and assayed for DNA polymerase activity as described in Example 2).
[0209] h) Construction of Deletion Mutants pM323 and pJM284&Dgr;B
[0210] Two mutants were created which contained deletions into the putative 3′ to 5′ exonuclease domain of the Tne polymerase gene. It was predicted that these two deletion mutations would remove all 3′ to 5′ exonuclease activity. Surprisingly, when these two mutants were expressed in E. coli no polymerase activity was detected.
[0211] i) Construction of pM323
[0212] pM323 was constructed by digestion of pJM284 with SinI and EcoRV followed by removal of the 3′ overhanging (i.e., sticky) ends with Mung Bean nuclease. The 1.9 kp SinI/EcoRV(polished ends) fragment was isolated as described in Example 2. JHEX3 (Example 3) was digested with NcoI and SmaI and the NcoI overhanging ends were made blunt by incubation with the Klenow fragment. The 1.9 kp SinI/EcoRV(polished ends) fragment was then blunt end ligated into the prepared JHEX3 vector. When the blunted SinI end ligates to the blunted NcoI end 39 amino acids are removed from the N terminus of the protein encoded by the pM284 construct; this junction also creates an initiating:methionine at amino acid position 323 that is in-frame with the rest of the coding region. The polymerase coding region present in the pM323 construct was sequenced to confirm that no undesirable mutations were introduced (i.e., insertion of stop codons, frame-shift mutations). The DNA sequence of the polymerase coding region present in pM323 is listed in SEQ ID NO:36. The corresponding amino acid sequence of the Tne M323 protein is listed in SEQ ID NO:37.
[0213] ii) Construction of pJM284&Dgr;B
[0214] pJM284&Dgr;B was constructed by digestion of the JM284 construct with BglII followed by religation of the large fragment (6.6 kb) back on itself. Digestion of JM284 with BglII created two fragments, the smaller being 348 bp which contains DNA sequences which encode a portion of the 3′ to 5′ exonuclease domain. The pJM284&Dgr;B construct removed 116 amino acids of the 3′ to 5′ exonuclease domain which correspond to residues 309 through 424 in SEQ ID NO:2. pJM284&Dgr;B contains the same 25 amino acids which encode the N terminus of the protein encoded by pM284. This deletion does not change the reading frame for the polymerase domain. Restriction digest analysis was conducted on the pJM284&Dgr;B construct to confirm the proper construction was made. The DNA sequence of the polymerase coding region present in pJM284&Dgr;B is listed in SEQ ID NO:38. The corresponding amino acid sequence of the Tne M284&Dgr;B protein is listed in SEQ ID NO:39.
[0215] iii) Expression of pM323 and pJM284&Dgr;B in E. coli
[0216] When both of these deletion mutants constructs were grown and induced to express the Tne polymerase in an E. coli host (JM109), no detectable polymerase activity was detected. As these two constructs were analyzed by restriction digestion or DNA sequencing to insure that no undesirable mutations were introduced, it appears, surprisingly, that deletion into the putative 3′ exonuclease domain (approximately aa residues 291-484) is deleterious either for polymerase activity or alternatively for protein stability. These results, in conjunction with those obtained using the pM316 construct, show that deletions beyond about position 849 of SEQ ID NO:1 produce proteins which either are unstable (perhaps due to improper folding) or lack polymerase activity.
EXAMPLE 5 Purification of Tne DNA Polymerases[0217] In order to produce purified preparations of the wild-type and modified Tne polymerases, cells harboring the Tne expression vectors described above were grown, induced and the Tne polymerases were isolated.
[0218] a) Growth of E. coli Cells Harboring Recombinant Tne (rTne) Constructs
[0219] E. coli strains containing a desired construct were streaked onto LB plates [10 g/l tryptone, 5 g/l yeast extract, 5 g/l NaCl, 1 ml/l 1N NaOH and 15 g/l agar] containing 10 &mgr;g/ml tetracycline to isolate single colonies and the plates were grown overnight at 37° C. A single colony was inoculated into 150 ml LB broth [10 g/l tryptone, 5 g/l yeast extract, 5 g/l NaCl and 1 ml/l 1N NaOH] containing 10 &mgr;g/ml tetracycline (divided into three flasks of 50 ml each); the three flasks were grown with shaking overnight at 37° C. The next day, 120 ml of the overnight culture was used to seed the fermentation of 6 liters of LB containing 10 &mgr;g/ml tetracycline prewarmed to 37° C. (divided into six flasks of 1 liter each). The large scale culture was grown for 5 hours at 37° C. and then IPTG was added to a final concentration of 1 mM and growth was continued for an additional 2 hours at 37° C. The induced cells were harvested by centrifugation at 9,000 rpm for 5 minutes in a Beckmann JA10 rotor. Yields were typically 2 g cell paste per liter of fermented culture.
[0220] b) Purification of rTne DNA Polymerases
[0221] Ten grams of cell paste (prepared as described above) were resuspended in 100 ml of an ice-cold solution containing 0.25 M NaCl in TEDGT buffer [50 mM Tris-HCl (pH 7.3), 1 mM EDTA, 1 mM DTT, 10% glycerol and 0.1% Tween 20] containing 2.5 mM PMSF. The resuspended cells were lysed by sonication using a Vibracell sonicator (Model VCX600; Sonics and Materials, Inc., Danbury, Conn.). The solution was kept ice-cold during sonication by placement of the beaker containing the cell suspension in a salted ice bath. Sonication was repeated ten times at 40% output for 1 minute with a 2 minute rest between the 1 minute sonication bursts. The cell lysate was heat treated to denature the bulk of E. coli proteins by incubation of the lysate at 68° C. to 70° C. for 5 to 10 minutes; following heat treatment the lysate was placed on ice.
[0222] The following purification steps were performed at 4° C. The chilled lysate was centrifuged at 15,000 rpm for 15 minutes in a Beckman JA18 rotor to remove the heat-denatured proteins. The cleared lysate supernatant was removed and 4 ml of 5% polyethylenimine (PEI) was added to the supernatant to precipitate any DNA present in the lysate. The lysate was centrifuged at 15,000 rpm for 15 minutes in a Beckman JA18 rotor to remove the precipitated DNA. The supernatant was retrieved and solid ammonium sulfate was added to 60% saturation to precipitate the DNA polymerase. After dissolution of the ammonium sulfate, the sample was centrifuged at 15,000 rpm for 1 hour in a Beckman JA18 rotor. The supernatant was discarded and the precipitated proteins were gathered and dissolved in 10 ml TEDGT buffer. The resolubilized protein was then placed in a dialysis membrane tubing having a 12,000 to 14,000 mw cutoff (Spectra, Houston, Tex.) and then dialyzed against TEDGT buffer to remove the ammonium sulfate.
[0223] The dialyzed protein solution was then loaded onto a 15 ml Heparin Sepharose (Scientific Protein Laboratory, Waunakee, Wis.) column (1.7 cm i.d.×6.5 cm height). The column was washed with 150 ml 0.05 M NaCl in TEDGT buffer. A 100 ml salt (NaCl) gradient was run over the column to elute the DNA polymerase; the gradient started at 0.05 M NaCl and ended at 1 M NaCl (all in TEDGT buffer). Fractions (1.5 ml) were collected and assayed for DNA polymerase activity at 74° C. as described below.
[0224] Thermostable DNA polymerase activity was assayed by incorporation of radiolabeled dTTP into nicked and gapped (i.e., activated) calf thymus DNA (prepared as described below). One unit of thermostable DNA polymerase is defined as the amount of enzyme required to catalyze the incorporation of 10 nmol of dNTP into an acid-insoluble form in 30 minutes at 74° C. The reaction conditions comprised: 50 mM Tris-HCl (pH 9.0 at 25° C.), 50 mM NaCl, 10 mM MgCl2, 200 &mgr;M dATP, 200 &mgr;M dCTP, 200 &mgr;M dGTP, 200 &mgr;M dTTP and 5 &mgr;Ci 3H-DTTP (Amersham) and 60 &mgr;g activated calf thymus DNA in a 250 &mgr;l final volume.
[0225] The reaction components were assembled at room temperature. Samples suspected of containing polymerase activity were added (5 &mgr;l containing 0.05 to 0.5 units) and the tube was incubated at 74° C.; aliquots (50 ∞l) were withdrawn at 6, 9, 12 and 15 minutes and placed immediately into 1.5 ml microcentrifuge tubes containing 0.5 ml of ice-cold 10% TCA on ice. After 10-30 minutes on ice, the entire TCA precipitation was filtered through a GF/A filter (Whatmann). The reaction tubes were rinsed with 3 volumes of cold 5% TCA and the filters were washed twice with 10 ml of ice-cold 5% TCA followed by a rinse with 1 ml of acetone. The filters were dried and the radioactivity bound to filters was counted in a scintillation counter.
[0226] Total and background counts were determined as follows. For total cpm, 10 &mgr;l of the reaction mix (without any polymerase added) was spotted onto duplicate GF/A filters and counted. For background counts, 50 &mgr;l of the reaction mix (without any polymerase added) was added to duplicate tubes containing 0.5 ml cold 10% TCA and the samples were filtered through GF/A filters and washed as described above.
[0227] Activated calf thymus DNA was prepared by dissolving 1 g calf thymus DNA (#D-151, Sigma, St. Louis, Mo.) in 400 ml TM buffer [10 mM Tris-HCl (pH 7.3), 5 mM MgCl2]. Four hundred microliters of a solution containing 40 units of RQ1-DNAse (Promega) in TM buffer was added to the DNA solution and incubated at 37° C. for 10 minutes. The DNAse digestion was stopped by heating the DNA solution at 68° C. for 30 minutes. The activated calf thymus DNA was stored at −20° C. until used. The activated calf thymus DNA was heated to 74° C. for 10 minutes and then cooled to room temperature before use.
[0228] As shown in Table 2 below, the Tne DNA polymerases generally eluted from the Heparin Sepharose column at a salt concentration of between 0.14 M and 0.29 M NaCl. Fractions containing the polymerase activity were pooled, placed into dialysis membrane tubing (as described above) and dialyzed against TEDGT buffer until the salt concentration was less than or equal to 0.05 M NaCl as measured by conductivity using a conductance meter (Yellow Spring Instrument Co., Yellow Springs, Ohio).
[0229] The dialyzed polymerase fraction was then loaded onto a 7 ml Cibracron Blue 3GA (Sigma, St. Louis, Mo.) column (1.25 cm i.d.×6 cm height). The Cibracron Blue 3GA column was washed with 70 ml of 0.05 M NaCl in TEDGT buffer. A 100 ml salt gradient was run over the column to elute the DNA polymerase; the gradient started at 0.05 M NaCl and ended at 1 M NaCl (all in TEDGT). Fractions (1.5 ml) were collected and assayed for DNA polymerase activity at 74° C. (as described above).
[0230] As shown in Table 2, the Tne DNA polymerases generally eluted from the Cibracron Blue 3GA column at a salt concentration of between 0.20 M and 0.46 M NaCl. Fractions containing the polymerase activity were pooled, placed in dialysis membrane (as described above) and dialyzed against storage buffer [20 mM Tris-HCl (pH 8.0), 100 mM KCl, 0.1 mM EDTA, 1 mM DTT, 50% glycerol and 0.5% Tween 20]. Aliquots of preparations of purified Tne polymerases were electrophoresed on SDS-PAGE gels and stained with Coomaisse blue. The purified DNA polymerases produced by all of the recombinant Tne polymerase constructs were judged to be at least 95% pure based on visual inspection of the Coomaisse-stained SDS PAGE gels. 2 TABLE 2 Salt Elution Characteristics For rTne DNA Polymerases SEQ ID NO Heparin Sepharose Cibracron Blue rTne DNA 2 0.26 to 0.37 M NaCl 0.5 to 0.8 M NaCl Polymerase full-length Tne M284 8 0.13 to 0.3 M NaCl 0.05 to 0.3 M NaCl Tne M284 19 0.15 to 0.28 M NaCl 0.3 to 0.48 M NaCl (E325D) Tne M284 26 0.11 to 0.23 M NaCl 0.16 to 0.35 M NaCl (D468N) Tne M284 16 0.05 to 0.32 M NaCl 0.11 to 0.4 M NaCl (D323E) Tne M284 23 0.11 to 0.28 M NaCl 0.24 to 0.4 M NaCl (Y464F) Tne M284 35 0.14 to 0.27 M NaCl 0.06 to 0.46 M NaCl (D323A, D389A)
[0231] The above results provide methods for the isolation of the full-length and modified forms of Tne DNA polymerase in a highly pure form at high yields.
EXAMPLE 6 Purification of Tne DNA Polymerase from T. neapolitana Cells[0232] The preceding example described the isolation of recombinant Tne DNA polymerases from E. coli harboring plasmids which overexpress these enzymes. The full-length Tne DNA polymerase may also be isolated from T. neapolitana cells
[0233] T. neapolitana cells are obtained from the ATCC (ATCC 49049) and grown in anaerobic culture in MMS medium containing (per liter): 6.93 g NaCl; 1.75 g MgSO4.7H2O; 1.38 g MgCl2.6H2O; 0.16 g KCl; 25 mg NaBr; 7.5 mg H3BO3; 3.8 mg SrCl2.6H2O; 0.025 mg KI; 0.38 g CaCl2; 0.5 g KH2PO4; 0.5 g Na2S.9H2O; 2 mg (NH4)2Ni(SO4)2; 15 ml trace minerals [per liter: 3.0 g MgSO4.7H2O; 1.5 g nitriloacetic acid, 1.0 g NaCl; 0.5 g MnSO4.H2O; 0.1 g FeSO4.7H2O; 0.1 g CoCl2.6H2O; 0.1 g CaCl2; 0.1 g ZnSO4.7H2O; 0.01 g CuSO4.5H2O; 0.01 g AlK(SO4)2.12H2O; 0.01 g H3BO3 and 0.01 g Na2MoO4.2H2O]; 1 mg resazurin and 5 g starch at a pH of 6.5 (adjusted with H2SO4). For growth on solid medium, 0.8% agar (Difco) was added to the above medium.
[0234] The cells are grown in a fermentation vessel maintained at 75-80° C. under anaerobic conditions (i.e., under nitrogen). The cells are grown for approximately 28 hours (early stationary phase). The cells are collected by centrifugation at 10,000×g. The cell pellet may be frozen at −70° C. until used. All of the subsequent operations should be carried out at 0 to 4° C. unless otherwise stated.
[0235] Resuspend about 50 g of frozen Thermotoga neapolitana cells in 100 ml of TEDGT [50 mM Tris-HCl (pH 7.3 at 25° C.), 1 mM EDTA, 1 mM DTT, 10% glycerol, 0.1% Tween 201 containing 2.5 mM PMSF (from 144 mM stock in DMF). The thawed and resuspended cells can be lysed in a Aminco French Pressure Cell (American Instrument Co., Silver Spring, Md., cat. no. FA-073) at 16,000 to 24,000 psi. This operation should be done twice to ensure adequate lysis. The lysate should be diluted by adding another 100 ml TEDGT containing 2.5 mM PMSF and stirring gently.
[0236] PEI (polyethyleneimine) is added to the lysate to precipitate the DNA. The exact amount of PEI is determined empirically, but in general 0.2% PEI should be adequate to precipitate most of the DNA (greater than 90%). Approximately 10 ml of 5% PEI is added to precipitate most of the DNA in the lysate. The lysate is centrifuged at 15,000 rpm for 15 minutes in a Beckman JA18 rotor to remove the precipitated DNA. The supernatant is retrieved and solid ammonium sulfate is added to 60% saturation to precipitate the DNA polymerase and other proteins. After the salt is dissolved, the sample is centrifuged at 15,000 rpm for 1 hour in a Beckmann JA18 rotor. The supernatant is discarded and the precipitated protein is gathered and dissolved in TEDGT buffer. The resolubilized protein is then placed in a dialysis membrane having a 12,000 to 14,000 mw cutoff (Spectra) and then dialyzed against TEDGT buffer to remove the ammonium sulfate.
[0237] The dialyzed protein solution is then loaded onto a 60 ml DEAE Sepharose (Sigma, St. Louis Mo.) column (2.5 cm i.d.×13 cm height). The column is washed with 300 ml 0 M NaCl in TEDGT buffer. A 300 ml salt gradient is run over the column to elute the DNA polymerase starting at 0 M NaCl and ending at 0.5 M NaCl (all in TEDGT buffer). Fractions (5.0 ml) are collected and assayed for DNA polymerase activity at 74° C. using the protocol described in Example 5. Fractions containing the polymerase activity are pooled, placed in dialysis membrane (12,000 to 14,000 mw cutoff; Spectra) and dialyzed against TEDGT buffer until the salt concentration is less than or equal to 0.05 M NaCl as measured by conductivity.
[0238] The pooled polymerase fraction is then loaded onto a 15 ml Heparin Sepharose (Scientific Protein Laboratory, Waunakee, Wis.) column (1.7 cm i.d.×6.5 cm height). The column is washed with 150 ml 0.05 M NaCl in TEDGT buffer. A 100 ml salt gradient is run over the column to elute the DNA polymerase starting at 0.05 M NaCl and ending at 1 M NaCl (all in TEDGT buffer). Fractions (1.5 ml) are collected and assayed for DNA polymerase activity at 74° C. using the assay described in Example 5. The Tne DNA polymerase should elute between 0.14 M and 0.29 M NaCl (in TEDGT buffer). Fractions containing the polymerase activity are pooled, placed in dialysis membrane (12,000 to 14,000 mw cutoff; Spectra) and dialyzed against TEDGT buffer until the salt concentration is less than or equal to 0.05 M NaCl as measured by conductivity.
[0239] The pooled polymerase fraction is then loaded onto a 15 ml DNA Agarose (Pharmacia) column (1.7 cm i.d.×6.5 cm height). The column is washed with 75 ml 0 M NaCl in TEDGT buffer. A 100 ml salt gradient is run over the column to elute the DNA polymerase starting at 0 M NaCl and ending at 0.5 M NaCl (all in TEDGT buffer). Fractions (1.5 ml) are collected and assayed for DNA polymerase activity at 74° C. using the assay described in Example 5. Fractions containing the polymerase activity are pooled, placed in dialysis membrane (12,000 to 14,000 mw cutoff; Spectra) and dialyzed against TEDGT buffer until the salt concentration is less than or equal to 0.05 M NaCl as measured by conductivity.
[0240] The pooled polymerase fraction is then loaded onto a 7 ml Cibracron Blue 3GA (Sigma, St. Louis, Mo.) column (1.25 cm i.d.×6 cm height). The column is washed with 70 ml of 0.05 M NaCl in TEDGT buffer. A 100 ml salt gradient is run over the column to elute the DNA polymerase starting at 0.05 M NaCl and ending at 1 M NaCl (all in TEDGT buffer). Fractions (1.5 ml) are collected and assayed for DNA polymerase activity at 74° C. using the assay described in Example 5. The Tne DNA polymerase should elute between 0.20 M and 0.46 M NaCl (in TEDGT buffer). Fractions containing the polymerase activity are pooled, placed in dialysis membrane (12,000 to 14,000 mw cutoff; Spectra) and dialyzed against storage buffer [20 mM Tris-HCl (pH 8.0), 100 mM KCl, 0.1 mM EDTA, 1 mM DTT, 50% glycerol, 0.5% Tween 20].
[0241] The purity and approximate molecular weight of the DNA polymerase can be assessed by SDS-PAGE gel electrophoresis using a 4-20% gradient Tris-Glycine SDS gel (Novex, San Diego, Calif.). An aliquot of the purified material is mixed with sample buffer [63 mM Tris-HCl pH 6.8, 10% glycerol, 2% SDS, 0.0025% Bromphenol Blue] and the gel is run using the following running buffer [25 mM Tris-Base, 192 mM glycine, 0.1% SDS, pH 8.3]. The gel can be run for 90 minutes at 125 V D.C. until the bromphenol blue band is just to the bottom of the gel. The apparent molecular weight for Tne DNA polymerase should be about 97,000. Using the activity assay described in Example 5, the number of units of DNA polymerase per microliter can be established. By visually assessing the quantity of DNA polymerase on the Coomassie stained SDS-PAGE gel compared to the protein standards run in the molecular weight marker lane, the specific activity of the DNA polymerase preparation may be estimated. The specific activity of the purified Tne DNA polymerase should be approximately 100,000 units/mg.
EXAMPLE 7 Characterization of the Full-Length and Modified Tne Polymerases[0242] The full-length and modified Tne polymerases were assayed for 5′ to 3′ exonuclease activity and 3′ to 5′ exonuclease activity.
[0243] a) 5′ to 3′ Exonuclease Assay
[0244] A 5′ to 3′ exonuclease assay was performed on the Tne M284 DNA polymerase (SEQ ID NO:8) present in crude lysates of cells containing the pJM284 construct (prepared as described in Example 3c) to determine whether any residual 5′ to 3′ exonuclease activity remained in this molecule. A comparison was made of the amount of 5′ to 3′ exonuclease activity present in the following thermostable DNA polymerases: Tne M284 (SEQ ID NO:8), the full-length rTne polymerase (SEQ ID NO:2), nTaq-(Promega) and UlTma (Perkin Elmer). The 5′ to 3′ exonuclease assay was performed as follows.
[0245] End-labeled substrate DNAs were prepared by digestion of pBR322 DNA with either EcoRI or EcoRV followed by incubation with &ggr;-32P-ATP and T4 polynucleotide kinase. The 5′ to 3′ exonuclease assay was performed in a final reaction volume of 25 &mgr;l and contained 1× Taq buffer (10 mM Tris-HCl (pH 9.0 at 25° C.), 50 mM KCl, 0.1% Triton X-100), 1.5 mM MgCl2, 200 &mgr;M of each of the four dNTPs, 50 ng of labeled substrate DNA and 5 units of the DNA polymerase to be tested (a no enzyme control was also conducted; water was used in place of the enzyme). The reaction was incubated for 1 hour at 74° C. The reaction was terminated by the addition of 5 &mgr;l of 0.5 M EDTA. Ten microliters of this mixture were spotted onto 2.3 cm circular DE81 filters (Whatman). The filters were dried briefly under a heat lamp. The filters were washed in 50 ml of 0.5 M sodium phosphate (pH 6.8) twice for 5 minutes/wash to remove unincorporated counts. The no enzyme control was used to permit determination of the total cpm in the sample. The washed filters were dried under a heat lamp and then the incorporated cpm and total cpm (no enzyme control filter) were determined by liquid scintillation counting.
[0246] The results of this assay showed that nTaq and the full-length Tne (SEQ ID NO:2) polymerases contained considerable 5′ to 3′ exonuclease activity, while the UlTma and Tne M284 (SEQ ID NO:8) polymerases did not contain detectable levels of 5′ to 3′ exonuclease activity.
[0247] b) 3′ to 5′ Exonuclease Assay
[0248] 3′ to 5′ exonuclease assays were performed in order to establish how mutations in the putative exonuclease domain of the Tne polymerase gene effected this activity. The assay was initially performed on crude lysates from E. coli cells containing a plasmid which produces the Tne M284 polymerase (SEQ ID NO:8). The assays were subsequently performed on purified preparations of Tne M284 polymerase (SEQ ID NO:8).
[0249] The assay was performed using either a single stranded or double stranded DNA substrate which contained a 3′ end label. The substrate DNA were prepared as follows.
[0250] To create the double stranded substrate, Lambda DNA/EcoRI Markers (Promega G1721) were 3′ end labeled with [&agr;-32P]dATP in a final reaction volume of 50 &mgr;l containing 10 &mgr;g of lambda DNA, 1× Buffer A [6 mM Tris-HCl (pH 7.5 at 37° C.), 6 mM MgCl2, 6 mM NaCl and 1 mM DTT], 5 &mgr;l [C-32P]dATP (3000 Ci/mmol; Amersham), 5 units Klenow fragment (Promega) and 10 mM of each of the four dNTPs. The reaction was incubated for 20 minutes at 37° C. The Klenow enzyme was inactivated by heating the mixture at 65° C. for 15 min. Unincorporated counts were removed by chromatography of the reaction mixture on a Nick™ Column (Pharmacia) according to the manufacturer's instructions. The labeled DNA was eluted in a volume of 400 &mgr;l. A 10 &mgr;l aliquot of the eluted DNA was counted by liquid scintillation counting and the aliquot contained approximately 2×105 cpm.
[0251] To create the single stranded substrate, a synthetic 74 nucleotide oligonucleotide, PM3074 (SEQ ID NO:41) is 3′ end labeled with [&agr;-32P]dATP in a final reaction volume of 10 &mgr;l containing 10 pmoles of the PM3074 oligonucleotide, 1× TdT Buffer [50 mM Tris-HCl (pH 7.5), 10 mM MgCl2, 5 mM DTT and 0.1 mM spermidine], 3 &mgr;l [&agr;-32P]dATP (3000 Ci/mmol; Amersham) and 15 units terminal deoxynucleotidyl transferase (TdT) (Promega). The reaction was incubated for 60 minutes at 37° C. The TdT enzyme was inactivated by heating the mixture at 65° C. for 15 min. Unincorporated counts were removed by chromatography of the reaction mixture on a Nick™ Column (Pharmacia) according to the manufacturer's instructions. The labeled DNA was eluted in a volume of 400 &mgr;l. A 10 &mgr;l aliquot of the eluted DNA was counted by liquid scintillation counting and the aliquot contained approximately 1×105 cpm.
[0252] The 3′ to 5′ exonuclease assay was performed as follows. In a final volume of 50 &mgr;l, the following components were assembled, 1 or 2 units of the DNA polymerase to be assayed, 5 &mgr;l of 10× Toga buffer [10 mM Tris-HCl (pH 8.8 at 25° C.), 10 mM KCl, 0.002% Tween 20] and nuclease-free water (Promega). One tube was also set up which did not contain any polymerase (Le., a no enzyme control). The reaction mixtures were prewarmed to the reaction temperature (reactions were performed at either 25° C. or 74° C.) and 10 &mgr;l of either the labeled single stranded or double stranded substrates were added to start the assay. Ten microliter fractions were removed at the following time points: 2, 4 or 6 minutes. The aliquots were spotted onto 2.3 cm circular DE81 filters and processed as described in section a) above.
[0253] The definition of one unit of 3′ to 5′ exonuclease activity is defined as the amount of enzyme required to remove 1 pmol of labeled 3′ end from the substrate in 30 minutes.
[0254] In addition to assaying the polymerases for 3′ exonuclease activity, all samples were also analyzed for DNA polymerase activity using the assay described in Example 5(b). In this manner, the ratio of 3′ exonuclease activity to polymerase activity could be determined for each polymerase.
[0255] When the 3′ to 5′ exonuclease assay was performed using on crude lysates containing the Tne M284 enzyme, the assay was run using only the single stranded substrate at 25° C. Under these conditions, a comparison was made between purified UlTma (a modified form of Tma polymerase which lacks 5′ to 3′ exonuclease activity) and UlTma spiked into a lysate derived from E. coli cells shown to lack 3′ to 5′ exonuclease activity; the spiked sample showed a 22% lower 3′ exonuclease: polymerase activity ratio (polymerase activity was measured using the assay described in Example ). As E. coli lysate alone shows no 3′ exonuclease activity, this result indicated that results obtained with crude lysates containing the Tne M284 protein (SEQ ID NO:8) should underestimate the 3′ to 5′ exonuclease levels that would be seen using purified Tne polymerase preparations.
[0256] When purified UlTma™ was compared with crude lysates containing the full-length (SEQ ID NO:2) or Tne M284 (SEQ ID NO:8) polymerases, the highest exonuclease:polymerase activity ratio was seen for the full-length Tne enzyme (1.37), then M284 Tne (0.83), then UlTma™ (0.45). These results were unexpected as it was not predictable that a deletion in the 5′ to 3′ exonuclease domain (Tne M284 polymerase) would result in a 39% decrease in 3′ to 5′ exonuclease activity.
[0257] Purified Tne M284 polymerase was used in 3′ exonuclease assays in comparison to purified UlTma™ using both single stranded and double stranded substrates at 25° C. and 74° C. Surprisingly, both enzymes displayed the same level of 3′ exonuclease activity on single stranded substrate at 25° C. Results obtained using crude lysates containing Tne M284 polymerase suggested that Tne M284 would have a higher activity. Both enzymes (purified Tne M284 and UlTma™) gave nearly identical results when the 3′ exonuclease assay was performed using the double stranded substrate at 25° C. (and both showed very low activity under-these conditions).
[0258] When the 3′ exonuclease assay was performed using a single stranded substrate at 74° C., UlTma™ had no activity, whereas purified Tne M284 showed an exonuclease: polymerase ratio of 0.32.
[0259] These results demonstrate that purified Tne M284 polymerase and UlTma™ have significantly different 3′ to 5′ exonuclease activities.
EXAMPLE 8 Characterization of the Purified Tne Polymerases[0260] In order to ascertain the characteristics of the full length and modified forms of the rTne polymerases, a number of determinations were made as described below.
[0261] a) Molecular Weight on SDS-PAGE Gels
[0262] The apparent molecular weight of the full length and modified rTne polymerases were determined by SDS-PAGE. An aliquot (10 units) of each purified polymerase was mixed with sample buffer [63 mM Tris-HCl pH 6.8, 10% glycerol, 2% SDS, 0.0025% bromophenol blue] and applied to a precast 4-20% gradient Tris-Glycine SDS gel (Novex, San Diego, Calif.). Molecular weight markers (Promega) were run on the same gel to allow calculation of the molecular weight of the Tne polymerases. The gel was run using the following running buffer [25 mM Tris-Base, 192 mM glycine, 0.1% SDS, pH 8.3] for 90 minutes at 125 V D.C. until the bromphenol blue band was just to the bottom of the gel.
[0263] Following electrophoresis, the gel was stained with Coomaisse blue to visualize the proteins. The full-length rTne polymerase migrated with an apparent molecular weight of 97,000 daltons. Tne M284, Tne M284(D323E), Tne M284(E325D), Tne M284(Y464F), Tne M284(D468N), and Tne M284(D323A, D389A) all migrated with an apparent molecular weight of 70,000 daltons.
[0264] b) Specific Activity of Purified Tne Polymerases
[0265] Preparations of purified full-length rTne and Tne M284, Tne M284(D323E), Tne M284(E325D), Tne M284(Y464F), Tne M284(D468N), and Tne M284(D323A, D389A) were all found to have a specific activity of 100,000 units/mg when the enzymes were purified and DNA polymerase assays were conducted as described in Example 5.
[0266] c) 3′ to 5′ Exonuclease Activity
[0267] Purified preparations of full-length rTne and Tne M284, Tne M284(D323E), Tne M284(E325D), Tne M284(Y464F), Tne M284(D468N) and Tne M284(D323A, D389A) were assayed for 3′ to 5′ exonuclease activity. In the same experiment, the 3′ to 5′ exonuclease activity of the UlTma™ (Perkin Elmer) and nTaq (Promega) polymerases were also measured for comparison to the Tne polymerases.
[0268] The 3′ exonuclease assay involved the incubation of the above enzymes with a 3′ end-labeled DNA substrate; the loss of radioactivity from the labeled substrate was measured after a 10 minute incubation period.
[0269] i) Preparation of the 3′ End-Labeled Substrate
[0270] Ten micrograms of lambda phage DNA was digested with 50 u of MluI (Promega) in a 50 &mgr;l reaction volume in 1× Buffer D [6 mM Tris-HCL (pH 7.9 at 37° C.), 6 mM MgCl2, 150 mM NaCl and 1 mM DTT]. The reaction was incubated for 3 hours at 37° C. The 3′ ends were then filled in using [&agr;-32P]-dCTP and unlabelled dGTP in a reaction containing 5 u of Klenow Exo-(USB) and 1× Buffer D in a volume of 100 &mgr;l; the reaction mixture was incubated for 20 min at 25° C. The reaction was terminated by heating the sample to 74 C for 15 min. The bulk of the unincorporated [&agr;-32P]-dCTP was removed by passage of the sample over a Nick™ column (Pharmacia) and the labeled DNA was eluted in 400 &mgr;l TE [10 mM Tris-HCl (pH 7.4), 1 mM EDTA]. An aliquot (10 &mgr;l) of the eluted DNA was counted in a scintillation counter and the aliquot contained approximately 2×105 cpm.
[0271] ii) 3′ to 5′ Exonuclease Assay
[0272] The 3′ to 5′ exonuclease assay was performed as follows. In a final volume of 50 &mgr;l, the following components were assembled, 1 or 2 units of the DNA polymerase to be assayed, 5 &mgr;l of 10× Toga buffer [10 mM Tris-HCl (pH 8.8 at 25° C.), 10 mM KCl, 0.002% Tween 20] and nuclease-free water (Promega). One tube was also set up which did not contain any polymerase (i.e., a no enzyme control). The reaction mixtures were prewarmed to the reaction temperature (reactions were performed at either 25° C. or 74° C.) and 10 &mgr;l of the labeled double stranded substrates were added to start the assay. Ten microliter fractions were removed at the following time points: 2, 4, 6 and 10 minutes (In a subsequent experiment, aliquots were withdrawn at 30 and 60 minutes to permit the detection of activity for enzymes showing very low levels of activity). The aliquots were spotted onto 2.3 cm circular DE81 filters and processed as described in Example 7, section a) above.
[0273] The definition of one unit of 3′ to 5′ exonuclease activity is defined as the amount of enzyme required to remove 1 pmol of labeled 3′ end from the substrate in 30 minutes.
[0274] In addition to assaying the polymerases for 3′ exonuclease activity, all samples were also analyzed for DNA polymerase activity using the assay described in Example 5(b). In this manner, the ratio of 3′ exonuclease activity to polymerase activity could be determined for each polymerase. The results are reported as the ratio of 3′ exonuclease activity to polymerase activity. The value obtained for the full-length Tne polymerase was assigned a value of 100% and all other values are expressed relative to this value. 3 TABLE 3 3′ Exonuclease Activity DNA Polymerase Exo:Pol Ratio rTne 100 ± 9.0 UlTma ™ 23.3 ± 0.8 Tne M284 28.0 ± 0.08 Tne M284(Y464F) 1.9 ± 0.2 Tne M284(D468N) 0.47 ± 0.02 Tne M284(D323E) 0.0 Tne M284(E325D) 0.0 Tne M284(D323A, D389A) 0.0 nTaq 0.0
[0275] The results shown in Table 3 demonstrate that deletions which remove 5′ exonuclease activity (e.g., Tne M284) also affect 3′ exonuclease activity in the Tne polymerases (compare rune with Tne M284; activity of Tne M284 is roughly one third that seen in rTne). Mutation of amino acid residues suspected of being critical for 3′ exonuclease activity (by analogy to similar residues in E. coli polymerase I) further reduces or eliminates 3′ exonuclease. The virtual elimination of 3′ exonuclease activity seen with the Tne M284(D468N) polymerase was unexpected. The analogous mutation in the Klenow fragment of DNA polymerase I (D501N) reduces 3′ exonuclease activity only by 2-fold [Derbyshire et al., EMBO J. 10:17 (1991)]. These results underscore the fact that much remains to be learned about structure-function relationships and that one cannot predict, with certainty, the effect of a given mutation based on analogy to other proteins.
[0276] d) Thermostability of the Purified Tne Polymerases at 97.5° C.
[0277] Thermostability was measured by incubating a DNA polymerase at 97.5° C. for various amounts of time and measuring the remaining DNA polymerase activity at 74° C. Samples were withdrawn after 0, 5, 10, 30, 60, 90, and 120 minutes after exposure to 97.5° C. and DNA polymerase assays were conducted as described in Example 5b. The time necessary to reduce the activity by one-half the initial value was determined by plotting the remaining activity versus the time of incubation at 97.5° C.
[0278] The incubation buffer contained 10 mM Tris-HCl (pH 8.8 at 25° C.), 10 mM KCl, 0.002% Tween 20 and 1.5 mM MgCl2 and 2-3 units of the enzyme to be tested. All enzymes, including commercially available enzymes (e.g., nTaq, UlTma™), were assayed using the DNA polymerase assay described in Example 5. The results of the thermostability assays are summarized in Table 4 below. 4 TABLE 4 Thermostability of Tne Polymerases Half-life at DNA polymerase SEQ ID NO: 97.5° C. (min) full-length rTne 2 5 Tne M284 (E325D) 19 5 native Taq 8 UlTma ™ 12 Tne M284(D323E) 16 12.5 Tne M284(Y464F) 23 16 Tne M284 8 18 Tne M284(D323A, 35 22 D389A) Tne M284 (D468N) 26 66
[0279] The results shown in Table 4 demonstrate that deletion of the amino terminal 5′ to 3′ exonuclease domain of Tne polymerase increases resistance to thermal degradation (2.5 to 13.2 fold). While amino-terminal deletions have been shown to increase thermostability (i.e., thermal tolerance) of certain modified forms of Taq DNA polymerase (e.g., the Stoffel fragment) and Tma DNA polymerase, the increase seen is generally about 2-3 fold. The increase in thermostability seen by the introduction of single or double point mutations into the Tne M284 deletion mutant was unexpected, especially the dramatic increase in thermostability caused by the single point mutation present in Tne M284(D468N) (SEQ ID NO:26).
[0280] The Tne M284(D468N) protein is identical to the Tne M284 protein with the exception that an asparagine is substituted for an aspartate at position 468; the carbon backbones of these two molecules should be identical, yet there is a greater than 5-fold resistance to thermal degradation seen when asparagine is present at position 468. The only difference between these two molecules should be a negatively charged carbonyl group (aspartate) and a neutral amide group (asparagine); both of these groups can participate in hydrogen-bonding. Substitutions of solvent-exposed amino acids (e.g., aspartate or asparagine) have been shown to have little effect on protein stability or structure [Matthews, Ann. Rev. Biochem. 62:139 (1993)], leading to the view that the rigid parts of proteins are critical for folding and stability. Based on analogy to the E. coli DNA polymerase I molecule at position 501, it is expected that position 468 in the Tne DNA polymerase is accessible to solvent Furthermore, since Tne M284(D468N) lacks the carbonyl group to coordinate a metal ion, it was predicted that a slight destabilization (in response to heat) would be seen rather than a dramatic stabilization. This data show that a subtle change in the 3′ exonuclease active site can dramatically alter the thermostability (i.e., thermotolerance) of the Tne DNA polymerase.
[0281] The above results demonstrate that several of the modified Tne polymerases have superior thermostability; accordingly, when these enzymes are used in PCR and other reactions run at elevated temperature, less polymerase activity needs to be used as less enzyme is inactivated by exposure to elevated temperature.
[0282] g) Optimal Temperature for DNA Polymerase Activity
[0283] The DNA polymerase activity of several Tne polymerases and nTaq DNA polymerase was measured at various temperatures using the assay described in Example 5b (with the exception that the temperature of incubation was varied). The results are summarized in Table 5. The temperature which gave the highest activity for a given enzyme was assigned a value of 100% and all other values given are expressed relative to the 100% value.
[0284] The results shown in Table 5 demonstrate that for nTaq DNA polymerase, the maximal DNA polymerase activity was present when the reaction was run at 78° C. Optimal temperature for DNA polymerase activity for the full-length rTne DNA polymerase was 74° C. Optimal temperature for DNA polymerase activity for the Tne M284 and Tne M284(D323A, D389A) DNA polymerases was 75° C. 5 TABLE 5 Optimal Temperature For Tne Polymerase Activity Tne M284(D323A, Temp. (° C.) Tne M284 D389A) nTaq rTne 40 12 15 n.d. n.d. 45 15 17 n.d. n.d. 50 18 20 n.d. n.d. 55 24 26 n.d. n.d. 60 33 36 n.d. n.d. 65 53 52 n.d. n.d. 70 71 80 n.d. 77 72 n.d. n.d. n.d. 86 74 n.d. n.d. n.d. 100 75 100 100 n.d. n.d.
[0285] 6 TABLE 5 Optimal Temperature For Tne Polymerase Activity Tne M284(D323A, Temp. (° C.) Tne M284 D389A) nTaq rTne 76 n.d. n.d. 93 94 77 n.d. n.d. 97 n.d. 78 n.d. n.d. 100 96 79 n.d. n.d. 97 n.d. 80 53 56 85 85 81 n.d. n.d. 82 n.d. 82 n.d. n.d. 68 68 85 16 16 n.d. n.d. 90 8 8 n.d. n.d.
EXAMPLE 9 The Tne DNA Polymerases Provide Improved Enzymes for use in the Polymerase Chain Reaction[0286] The use of the full-length and modified forms of Tne polymerase in the PCR was examined. The results of the following experiments show that the Tne polymerases provide improved enzymes for a variety of PCR applications.
[0287] a) The Modified Tne Polymerases Utilize a Broader Range of Optimal dNTP Concentrations in the PCR
[0288] PCR reactions were conducted using a range of dNTP concentrations to determine the optimal range of dNTP concentrations utilized by either the UlTma™ DNA polymerase (Perkin Elmer) or the Tne M284 and Tne M284(D323A, D389A) polymerases. The modified Tne polymerases were found to produce high yields of amplified product over a much broader range of nucleotide concentrations. The PCRs were conducted as follows.
[0289] The three enzyme preparations were assayed for DNA polymerase activity using the assay in Example 5b so that the same amount of enzyme was used in the PCRs. PCRs were performed in a buffer optimized for each type of enzyme; the UlTma™ and Tne enzymes were both found to give optimal results in the following buffer [10 mM Tris-HCl (pH 8.8 at 25° C.), 10 mM KCl, 0.002% Tween 20]. All reactions contained 1 ng of pGEM-luc (Promega) as the template, 20 pmol of each primer [the primers used were LME41(SEQ ID NO:42) and LME43(SEQ ID NO:43)], 1.5 mM MgCl2 (this concentration was chosen as it was optimal for both the UlTma™ and the modified Tne enzymes as shown below in section b) and 2.5 units of each enzyme. The final reaction volume was 50 &mgr;l.
[0290] A dilution series was created for the mixture of all four dNTPs ranging from 20 to 200 &mgr;M (20, 40, 60, 80, 100, 120, 140, 160 and 200 &mgr;M). The PCRs were thermal cycled using the following conditions, an initial denaturation at 96° C. for 2 min, followed by 30. cycles comprising denaturation at 94° C. for 30 sec, annealing/extension at 65° C. for 2 min; following the last cycle the tubes were incubated at 65° C. for 10 min and then the tubes were incubated at 4° C. Thermal cycling was conducted on a Perkin Elmer Thermocycler Model 480. The PCR products were resolved on 1% agarose gels, stained with EtBr and quantitated by visual inspection of the stained gels. The experiment was repeated a second time and similar results were obtained.
[0291] The optimal dNTP concentration for UlTma™ was found to be 40 &mgr;M (the same value as reported to be optimal by Perkin Elmer in the UlTma™ product insert); slight levels of product could be detected using the UlTma™ enzyme at a concentration of 20 &mgr;M dNTPs but not at all at other concentrations tested. The optimal range of concentrations of dNTPs for Tne M284(D323A, D389A) was found to be from 60 &mgr;M to 200 &mgr;M with slight amounts of product being detected at both 20 and 40 &mgr;M dNTP. The optimal range of concentrations of dNTPS for Tne M284 was from 40 &mgr;M to 200 &mgr;M with slight amounts of product being detected at 20 &mgr;M dNTP.
[0292] The ability to use a wide range of dNTP concentrations is advantageous. The modified Tne polymerases allow PCR users wide flexibility in the design of the reaction conditions. Because the modified Tne polymerases are not sensitive to small variations in dNTP concentration, PCRs using these enzymes are more robust. Additionally the ability to use a higher concentration of dNTPs may also allow a higher yield of product to be generated in a PCR using the modified Tne polymerases as compared to the use of the UlTma™ polymerase.
[0293] b) The Tne Polymerases Tolerate a Broader Range of Mg++ Concentrations in PCR
[0294] In side-by-side tests with UlTma™ DNA polymerase and Tne M284 or Tne M284(D323A, D389A), the Tne variants were found to produce high yields of amplified product over a broader range of magnesium ion concentrations. A dilution series was created for magnesium (MgCl2) ranging from 0.5 to 5 mM (0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, and 5.0 mM). The other components of the PCR were as described above in section a) with the exception that a single, dNTP concentration was used (40 &mgr;M dNTP was used for the UlTma™ enzyme and the modified Tne enzymes as this value was within the operable range for all three enzymes; it is noted that 40 &mgr;M dNTP is not optimal for the Tne M284(D323A, D389A enzyme and that even higher product yields would be expected if 60 &mgr;M dNTP were used for this enzyme) and the concentration of MgCl2 was varied. Thermal cycling conditions were as described above in section a). The reaction products were treated as described above in section a).
[0295] The results of these assays showed that the UlTma™ enzyme (Perkin Elmer) had an optimal concentration of 1.5 MM MgCl2 and a great deal of non-specific background products were seen on the gels (as a smear). The optimal concentration for Tne M284 enzyme was found to be from 1 to 3 mM MgCl2 and some nonspecific background products were observed. The optimal concentration for M284(D323A, D389A) was from 1 to 3 mM MgCl2 and very little or no detectable non-specific background products were observed.
[0296] The ability of a thermostable enzyme to produce only specific PCR products over a wide range of Mg++ concentrations is important for certain PCR applications. For example, multiplexing PCRs utilize several pairs of primers to amplify several different targets in the sample. As each primer pair and target combination will have a optimum Mg++ concentration and this value may vary considerably from primer set to primer set, the availability of thermostable polymerases having a wide optimum for Mg++ is advantageous. Therefore, the Tne M284 and Tne M284(D323A, D389A) polymerases provide improved thermostable DNA polymerases.
[0297] c) Modified Tne Polymerases Produce High PCR Product Yields
[0298] A comparison was made between the yield of PCR product obtained using a consistent amount of polymerase activity when the following enzymes were used in the PCR: Tne M284, Tne M284(D323E), Tne M284(E325D), Tne M284(Y464F), Tne M284(D468N), Tne M284(D323A, D389A), AmpliTaq (Perkin Elmer), nTaq (Promega) and UlTma™ (Perkin Elmer). All enzymes were assayed using the DNA polymerase assay described in Example 5b so that the same number of units of enzyme were added to the PCRs.
[0299] PCRs which used UlTma™ polymerase contained 10 mM Tris-HCl (pH 8.8 at 25° C.), 10 mM KCl, 0.002% Tween 20 and 1.5 mM MgCl2. This buffer gives optimal PCR results for the UlTma™ enzyme (as reported by Perkin Elmer). For the results shown in Table 6, PCRs which used the Tne DNA polymerases contained 10 mM Tris-HCl (pH 8.8 at 25° C.), 10 mM KCl, 0.002% Tween 20 and 1.5 mM MgCl2. This buffer gives acceptable results for the Tne polymerases.
[0300] For the results shown in Table 7, PCRs which used the Tne DNA polymerases contained 10 mM Tris-HCl (pH 9.0 at 25° C.), 10 mM KCl, 0.01% Tween 20 and 1.5 mM MgCl2 as it was found that the Tne DNA polymerases perform better when the pH of the buffer is raised to 9.0 and the amount of detergent (i.e., Tween 20) is increased to 0.01%.
[0301] PCRs which used the UlTma™ enzyme contained 40 &mgr;M dNTPs as this value was determined to be optimal for this enzyme (see section a, above). PCRs which used the modified Tne enzymes contained 140 &mgr;M dNTPs as this value was determined to be optimal for these enzymes (see section a, above).
[0302] PCRs which used nTaq or AmpiTaq polymerase contained 10 mM Tris-HCl (pH 9.0 at 25° C.), 50 mM KCl, 1.5 mM MgCl2, 0.1% Triton X-100; this buffer gives optimal PCR results for the nTaq and AmpliTaq polymerases. PCRs which used the nTaq and AmpliTaq enzymes contained 200 &mgr;M dNTPs as this value is reported to be optimal for these enzymes.
[0303] A single template, pGEM-luc was tested in conjunction with two different primer pairs. The primer pair comprising LME41 (SEQ ID NO:42) and LME45 (SEQ ID NO:44) amplifies a 500 bp target on the template. The primer pair comprising LME41 (SEQ ID NO:42) and LME43 (SEQ ID NO:43) amplifies a 1.5 kb target on the template.
[0304] All PCR reactions contained 3.0 units of the enzyme to be tested, 2 ng of pGEM-luc (Promega) as the template, 50 pmol of each primer pair, 1.5 mM MgCl2 (this concentration was chosen as it was determined to be optimal for all three types of enzyme tested using this template and these primer pairs). The final reaction volume was 100 &mgr;l.
[0305] The PCRs which amplified the 500 bp target were thermal cycled using the following conditions, an initial denaturation at 96° C. for 2 min, followed by 25 cycles comprising denaturation at 95° C. for 30 sec, annealing at 65° C. for 30 sec and extension at 72° C. for 1 min; following the last cycle the tubes were incubated at 4° C.
[0306] The PCRs which amplified the 1500 bp target were thermal cycled using the following conditions, an initial denaturation at 94° C. for 2 min, followed by 30 cycles comprising denaturation at 94° C. for 1 min and annealing/extension at 65° C. for 2 min; following the last cycle, the tubes were incubated at 68° C. for 10 min and then the tubes were brought to 4° C.
[0307] Thermal cycling was conducted on a Perkin Elmer Thermocycler Model 480. Each reaction was run in duplicate. The PCR products were resolved by electrophoresis through 1% agarose gels, followed by staining with EtBr and quantitated by scanning the gels with a fluoroimager (Molecular Dynamics); product yields were quantitated using ImageQuant software (Molecular Dynamics). The results (average of the duplicates) are shown below in Tables 6 and 7. The enzyme which gave the highest yield was assigned a value of 100% and all other values are expressed as a percentage of the maximal yield.
[0308] The results shown above in Tables 6 and 7 demonstrate the following. Regardless of the target size, the modified Tne enzymes produced higher yields as compared to the UlTma™ enzyme (2.5 to 6.7 fold more product). The modified Tne enzymes also performed better than nTaq when the 500 bp target was used (about 2 fold more product) and better than the recombinant Taq polymerase (AmpliTaq) when the 1.5 kb target was used (about 30% more product).
[0309] Thus, to produce the same amount of PCR product, fewer units of modified Tne polymerases are required compared to the use of the UlTma™, nTaq and AmpliTaq enzymes. These modified Tne polymerases allow the user to use smaller amounts of enzyme in the PCR and therefore will provide significant cost savings in PCR applications. 7 TABLE 6 Product Yields For Amplification Of A 500 bp Target Enzyme % Yield nTaq 59 UlTma ™ 16 Tne M284 100 Tne M284(D323E) 89 Tne M284(E325D) 87 Tne M284(Y464F) 91 Tne M284(D468N) 89 Tne M284(D323A, D389A) 91
[0310] 8 TABLE 7 Product Yields For Amplification Of A 1.5 kb Target Enzyme % Yield AmpliTaq 67 UlTma ™ 40 Tne M284 81 Tne M284 (D323E) 70 Tne M284 (E325D) 86 Tne M284 (Y464F) 93 Tne M284 (D468N) 100 Tne M284 (D323A, D389A) 87
EXAMPLE 10 Thermal Cycle Sequencing using Modified Tne Polymerases[0311] The modified Tne polymerases were further characterized by examining their ability to be used in thermal cycle sequencing reactions. Initial sequencing reactions were performed using with the Tne M284(D323E) and Tne M284(E325D) mutants (SEQ ID NOS:16 and 19, respectively) using the buffer and dideoxy mixes from the fmol® DNA Sequencing System kit (Promega) according to the manufacturer's instructions. The buffer and concentrations of dideoxy and deoxyribonucleotides in this kit were optimized for use with sTaq DNA polymerase (Promega; sTaq is a modified form of Taq DNA polymerase which has very low 5′ to 3′ exonuclease activity).
[0312] When the sequencing reactions were performed using purified preparations of either Tne M284(D323E) (SEQ ID NO:16) or Tne M284(E325D) (SEQ ID NO:19), the resulting reaction products were very short and uneven and created light bands on the sequencing gel in every position and in every lane. The light bands in every position were consistent with the presence of residual 3′ to 5′ exonuclease activity in the modified Tne polymerases.
[0313] Thermal cycle sequencing reactions were then performed using the triple mutant Tne M284(D323A, D389A) (SEQ ID NO:35). All thermal cycle sequencing was carried out using a Perkin-Elmer 9600 thermal cycler in conjunction with the fmol® DNA Sequencing System reagents (Promega). The template used in the sequencing reactions was the Tne clone pE325D (Example 4b) and the 5′ labeled primer was the JH66 primer (SEQ ID NO:12). The reactions were run at 95° C. for 15 seconds and 70° C. for 60 seconds for 30 cycles.
[0314] The results of this experiment showed that the use of the triple mutant enzyme (SEQ ID NO:35) eliminated the light bands in every position which were seen with Tne M284(D323E) and Tne M284(E325D); however the sequence ladders were still shifted toward short extension products. These short intense reads were indicative of a DNA polymerase having a higher affinity for the dideoxynucleotides than Taq DNA polymerase.
[0315] To examine if the triple mutant Tne enzyme (SEQ ID NO:35) had a higher affinity for dideoxynucleotides, the sequencing reactions were repeated using a lower ratio of dNTPs to ddNTPs in the sequencing reaction mixtures (all other conditions remained the same). Table 8 lists the 3× mixes used for sTaq and Tne M284(D323A, D389A) polymerases in this experiment. The use of lower concentration of dideoxynucleotides in the sequencing reaction run using the triple mutant Tne enzyme (SEQ ID NO:35) produced reads as long as those obtained using sTaq. 9 TABLE 8 Tne M284, D323A, sTaq D389A dNTPs ddNTPs dNTPs ddNTPs G mix 20 &mgr;M 30 &mgr;M 20 &mgr;M 30 &mgr;M A mix 20 &mgr;M 350 &mgr;M 20 &mgr;M 75 &mgr;M T mix 20 &mgr;M 600 &mgr;M 40 &mgr;M 40 &mgr;M C mix 20 &mgr;M 200 &mgr;M 40 &mgr;M 20 &mgr;M
EXAMPLE 11 Optimization of Nucleotide Mixtures for Thermal Cycle Sequencing using Tne M284(D323A, D389A) Polymerase[0316] The results discussed in Example 10 demonstrated that the Tne M284(D323A, D389A) polymerase (SEQ ID NO:35) has a higher affinity for dideoxynucleotides than does sTaq polymerase. Further testing with various concentrations of dideoxy- and deoxynucleotides, including modified dNTPs, in sequencing reactions was performed to ascertain the optimal concentration for these reagents. The optimized Tne dNTP/ddNTP mixes are detailed in Table 9 below. The values reported in Table 9 represent 3× mixtures; these mixtures are diluted 3-fold in the final reaction mixture as described below. To obtain the final concentration of dNTPs and ddNTPs in the reactions, the values in Table 9 are divided by 3. 10 TABLE 9 Optimized Nucleotide Mix Formulation for Tne Polymerase G Nucleotide A Nucleotide T Nucleotide C Nucleotide Component Mix Mix Mix Mix ddGTP 20 &mgr;M — — — ddATP — 50 &mgr;M — — ddTTP — — 75 &mgr;M — ddCTP — — — 25 &mgr;M 7-deaza dGTP 30 &mgr;M 30 &mgr;M 30 &mgr;M 30 &mgr;M dATP 30 &mgr;M 30 &mgr;M 30 &mgr;M 30 &mgr;M dTTP 30 &mgr;M 30 &mgr;M 30 &mgr;M 30 &mgr;M dCTP 30 &mgr;M 30 &mgr;M 30 &mgr;M 30 &mgr;M
[0317] To illustrate the fact that the Tne M284(D323A,D389A) enzyme has a higher affinity for ddNTPs as compared to sTaq, the following thermal cycle sequencing reactions were performed. The template used was pGEM-3Zf(+) (Promega) and the primer was the &ggr;-32P end labeled pUC/M13 Forward Primer (Promega; SEQ ID NO:40). Reactions using sTaq polymerase or the Tne M284(D323A, D389A) polymerase were performed using dNTP/ddNTP mixes optimized for sTaq as provided in the fmol(D DNA Sequencing System.
[0318] The extension products of thermal cycle sequencing reactions performed using either sTaq or the Tne M284(D323A, D389A) polymerase were resolved by electrophoresis on a 6% denaturing polyacrylamide gel; following electrophoresis, the gel was exposed to X-ray film. The resulting autoradiograph is shown in FIG. 5, Panel A.
[0319] In FIG. 5, Panel A, lanes 1-4 contain reaction products generated using sTaq and lanes 5-8 contain reaction products generated using the Tne M284(D323A, D389A) polymerase. In each set of four lanes, reactions run in the presence of ddGTP, ddATP, ddTTP and ddCTP were loaded left to right.
[0320] The results shown in FIG. 5, Panel A show that the sequence ladders generated using the modified Tne polymerase were shifted toward short extension products. These short intense reads were indicative of a DNA polymerase having a higher affinity for the dideoxynucleotides than Taq DNA polymerase.
[0321] Sequencing reactions were then performed using sTaq in conjunction with dNTP/ddNTP mixes optimized for sTaq as provided in the fmol® DNA Sequencing System or the Tne M284(D323A, D389A) polymerase and the dNTP/ddNTP mixes shown in Table 9. The template used was pGEM-3Zf(+) (Promega) and the primer was the &ggr;-32P end labeled pUC/M13 Forward Primer (Promega; SEQ ID NO:40). An autoradiograph of reaction products is shown in FIG. 5, Panel B.
[0322] In FIG. 5, Panel B, lanes 1-4 contain reaction products generated using sTaq and sequencing mixes optimized for sTaq and lanes 5-8 contain reaction products generated using the Tne M284(D323A, D389A) polymerase and sequencing mixes shown in Table 9 for the Tne M284(D323A, D389A) polymerase. In each set of four lanes, reactions run in the presence of ddGTP, ddATP, ddTTP and ddCTP were loaded left to right.
[0323] The results shown in FIG. 5, Panel B demonstrate that the Tne M284(D323A, D389A) polymerase has a higher affinity for ddNTPs than does sTaq polymerase and therefore lower concentrations of ddNTPs must be used in the sequencing reactions.
[0324] Using the optimized mixes shown in Table 9, the Tne M284(D323A,D389A) enzyme was compared with Sequencing Grade Taq (sTaq; Promega) for its ability to sequence three different templates. sTaq (Promega) was used in conjunction with the fmol DNA Sequencing System (Promega) and all protocols were followed as per the instructions. The Tne M284(D323A,D389A) polymerase was used in conjunction with the fmol® DNA Sequencing System kit (Promega), except that the dNTP's/ddNTP's mixes used were the Tne optimized mixes shown in Table 9.
[0325] The following three DNA templates used in the indicated amounts in the thermal cycle sequencing reactions. Forty femtomoles of pGEM-3Zf(+) (Promega) was sequenced using the &ggr;-32P end labeled pUC/M13 Forward Primer (Promega; SEQ ID NO:40). Four femtomoles of a 500 bp PCR fragment was sequenced using the gamma 32P end labeled LME-28 primer (5′-TAATACGACTCACTATAGGGC GAAT-3′ (SEQ ID NO:47). Four femtomoles of &lgr;gt11 phage DNA (Promega) was sequenced using a &ggr;-32P end labeled &lgr;gt11 Forward Primer (Promega).
[0326] The 500 bp PCR product used as template was generated by amplification of the template pGEM-luc with primers LME41 (SEQ ID NO:42) and LME45 (SEQ ID NO:44) as described in Example 9.
[0327] The thermal profile of all sequencing reactions was 95° C. for 2 min, then 30 cycles of 95° C. for 30 sec, 70° C. for 60 sec; following the last cycle, the reactions were brought to 4° C. The thermal cycling was performed using a Perkin-Elmer 480 Thermal Cycler. The extension products were separated on a 6% denaturing polyacrylamide gel; following electrophoresis, the gel was exposed to X-ray film. The resulting autoradiographs are shown in FIG. 6.
[0328] In FIG. 6, three panels are shown. Panel A contains the extension products generated using pQEM-3Zf(+) as the template; Panel B contains the extension products generated using the 500 bp PCR product as the template; Panel C contains the extension products generated using &lgr;gt11 phage DNA as the template. In all three panels, eight lanes are shown. The first four lanes correspond to extension products generated using sTaq polymerase; the last four lanes correspond to extension products generated using the Tne M284(D323A, D389A) polymerase. In each set of four lanes, the termination mixes were used in the following order (left to right): G, A, T and C.
[0329] The results shown in FIG. 6 demonstrated that the Tne M284(D323A, D389A) polymerase produced sequencing ladders which were virtually identical in terms of quality and quantity to those produced using sTaq (Promega); both enzymes provided the correct DNA sequence for each template. While both enzymes produced similar results in thermal cycle sequencing reactions, the Tne M284(D323A, D389A) polymerase required lower concentrations of dideoxynucleotides. Thus, the use of the use of the Tne M284(D323A, D389A) polymerase will result in considerable cost savings for thermal cycle sequencing applications.
[0330] The optimal ddNTP concentrations shown in Table 9 for the Tne M284(D323A, D389A) polymerase (SEQ ID NO:35) as compared to the optimal concentrations for sTaq (Table 8) demonstrate that the modified Tne polymerase has a greater affinity four all four ddNTPs. In particular this modified Tne polymerase requires 8-fold less ddTTP or ddCTP, 7-fold less ddATP and 1.5-fold less ddGTP than does sTaq in thermal cycle sequencing reactions. As dideoxynucleotides are an expensive component of the sequencing reaction mixtures, the use of the Tne M284(D323A, D389A) polymerase (SEQ ID NO:35) in place of enzymes such as Taq polymerase which have lower affinities for ddNTPS will result in considerable cost savings. These results also demonstrate that the Tne M284(D323A, D389A) enzyme can utilize the nucleotide analog 7-deaza dGTP which is used to resolve band compression artifacts generated when sequencing G+C-rich regions of DNA.
EXAMPLE 12 Preferred Sequencing Protocol using Tne M284(D323A, D389A) Polymerase[0331] The preferred sequencing protocol uses a thermal cycling format. A detectable signal may be generated using either an end radiolabeled primer or a radiolabeled dNTP that is incorporated into the extension products.
[0332] a) Sequencing Protocol using an End-Labeled Primer
[0333] i) Primer Radiolabeling Reaction
[0334] To generate a radiolabeled primer for use in the sequencing reaction, the following reaction components are assembled in a 0.5 ml microcentrifuge tube: 10 pmol of the desired sequencing primer; 10 pmol of &ggr;-labeled ATP (see Table 10 for amount); 1 &mgr;l of 10× T4 polynucleotide kinase Buffer [500 mM Tris-HCl (pH 7.5); 100 mM MgCl2; 50 mM DTT; 1.0 mM spermidine]; 5 units T4 polynucleotide kinase and sterile H2O to a final volume of 10 &mgr;l. The reaction mixture is incubated at 37° C. for 10-30 min (if end-labeling is to be performed using [&ggr;-35S]ATP, 20 units of polynucleotide kinase are used and the reaction is incubated for 4 hours at 37° C.). The reaction is then terminated by inactivation of the kinase by incubation at 90° C. for 2 minutes. The tube is then briefly spun in a microcentrifuge to collect any condensation. The labeled primer may be used directly in the sequencing reaction without further purification. 11 TABLE 10 Amount of Radiolabeled Nucleotide Needed To Equal 10 pmol [&ggr;-32P]ATP: 3.0 &mgr;l of 3,000 Ci/mmol, 10 &mgr;Ci/&mgr;l 5.0 &mgr;l of 5,000 Ci/mmol, 10 &mgr;Ci/&mgr;l 0.5 &mgr;l of 6,000 Ci/mmol, 135 &mgr;Ci/&mgr;l [&ggr;-35S]ATP: 1.4 &mgr;l of 1,326 Ci/mmol, 10 &mgr;Ci/&mgr;l
[0335] ii) Extension/Termination Reactions
[0336] For each set of sequencing reactions, label four 0.5 ml microcentrifuge tubes (G, A, T, C). Add 2 &mgr;l of the appropriate 3× dNTP/ddNTP Mix to each tube (see Table 9 for components of the 3× mixes). Cap the tubes and store on ice or at 4° C. until needed. For each set of four sequencing reactions, mix the following reagents in a microcentrifuge tube: 440 fmol of template DNA (see Table 11 below for recommended amounts); 5 &mgr;l fmol® Sequencing 5× Buffer [250 &mgr;M Tris-HCl (pH 9.0 at 25° C.), 10 mM-MgCl2]; 1.5 &mgr;l labeled primer (1.5 pmol); sterile H2O to a final volume of 16 &mgr;l (this comprises the primer/template mix).
[0337] Add 1.0 &mgr;l of Tne M284 (D323A,D389A) DNA Polymerase (5 u/&mgr;l) to the primer/template mix. Mix briefly by pipetting up and down (this comprises the enzyme/primer/template mix). Add 4 &mgr;l of the enzyme/primer/template mix to the inside wall of each tube containing d/ddNTP mix. Add one drop (approximately 20 &mgr;l) of mineral oil to each tube and briefly spin in a microcentrifuge. Place the reaction tubes in a thermal cycler that has been preheated to 95° C. and start the cycling program. 12 TABLE 11 Recommended Amounts Of Template DNA (ng) for End-Labeled Primer Protocol Template Length ng of Template 200 bp (PCR product) 0.5 ng (4 fmol) 3,000-5,000 bp (supercoiled plasmid DNA) 100 ng (40 fmol) 48,000 bp (lambda DNA) 130 ng (4 fmol)
[0338] When the primer used is less than 24 nucleotides in length or has a G+C-content less than 50%, the following cycling profile is used: 95° C. for 2 minutes followed by 30 cycles of 95° C. for 30 seconds (denaturation); 42° C. for 30 seconds (annealing); 70° C. for 1 minute (extension); the tubes are then brought to 4° C.
[0339] When the primer used is greater than or equal to 24 nucleotides in length or when shorter primers having a G+C-content greater than or equal to 50%, the following cycling profile is used: 95° C. for 2 minutes followed by 30 cycles of 95° C. for 30 seconds (denaturation); 70° C. for 30 seconds(annealing/extension); the tubes are then brought to 4° C.
[0340] After the thermocycling program has been completed, add 3 &mgr;l of fmol′ Sequencing Stop Solution [10 mM NaOH; 95% formamide; 0.05% bromophenol blue; 0.05% xylene cyanol] to the inside wall of each tube. Briefly spin the tubes in a microcentrifuge to terminate the reactions. Heat the reactions at 70° C. for 2 minutes immediately before loading onto a sequencing gel. Load 2.5-3.0 &mgr;l of each reaction on the gel.
[0341] b) Sequencing Protocol using Direct Incorporation
[0342] i) Extension/Termination Reactions
[0343] The following procedure requires about 500 fmol of plasmid templates and about 40 fmol of PCR product. The end-labeled primer protocol (section a) is recommended for PCR templates. This procedure is not recommended for the sequencing of lambda templates.
[0344] For each set of sequencing reactions, label four 0.5 ml microcentrifuge tubes (G, A, T, C). Add 2 &mgr;l of the appropriate d/ddNTP Mix to each tube. Cap the tubes and store on ice or at 4° C. until needed. For each set of four sequencing reactions, mix the following reagents in a microcentrifuge tube: 500 fmol template DNA (approx. 1 &mgr;g of a 3-5 kb template); 3.0 pmol primer (approx. 25 ng of a 24 mer); 0.5 l [&agr;-35S]dATP (>1,000 Ci/mmol, 10 &mgr;Ci/&mgr;l) or [&agr;-32P]dATP (800 Ci/mmol, 10 &mgr;Ci/&mgr;l); 5 &mgr;fmol® Sequencing 5× Buffer and sterile H2O to final volume of 16 &mgr;l.
[0345] Add 1.0 &mgr;l of Tne M284 (D323A,D389A) DNA Polymerase (5 u/&mgr;l) to the primer/template mix. Mix briefly by pipetting up and down. Add 4 &mgr;l of the enzyme/primer/template mix to the inside wall of each tube containing d/ddNTP Mix. Add one drop (approximately 20 &mgr;l) of mineral oil to each tube and briefly spin in a microcentrifuge. Place the reaction tubes in a thermal cycler that has been preheated to 95° C. and start the cycling program. The cycling profile chosen depends upon the characteristics of the primer used; see section a(i) above.
[0346] After the thermocycling program has been completed, add 3 &mgr;l of fmol® Sequencing Stop Solution to the inside wall of each tube. Briefly spin in a microcentrifuge to terminate the reactions. Heat the reactions at 70° C. for 2 minutes immediately before loading on a sequencing gel. Load 2.5-3.0 &mgr;l of each reaction on the gel.
EXAMPLE 12 Use of Tne M284(D323A, D389A) Polymerase in Sanger Sequencing Protocols[0347] The above examples described the use of the Tne M284(D323A, D389A) polymerase in thermal cycle sequencing protocols. The Tne M284(D323A, D389A) Polymerase may also be used in traditional Sanger sequencing protocols.
[0348] If a double stranded DNA template is used, the template is first denatured using alkali as follows. Four micrograms (approximately 2 pmol) of supercoiled plasmid DNA is added to a microcentrifuge tube and deionized H2O is added to a final volume of 18 &mgr;l. Two microliters of 2 M NaOH, 2 mM EDTA is added and the mixture is incubated for 5 minutes at room temperature. To neutralize the reaction, add 8 &mgr;l of 5M ammonium acetate, pH 7.5, and vortex. Add 112 &mgr;l of 100% ethanol and vortex. Centrifuge the tube for 10 minutes at top speed in a microcentrifuge. Decant the supernatant. Wash the pellet with 1 ml of 70% ethanol and centrifuge for 1 minute. Remove the supernatant and dry the pellet. Resuspend the dried pellet in 18 &mgr;l of distilled water for sequencing. Proceed to either section i) or ii) depending on whether an end-labeled primer is employed or whether radiolabeled nucleotides are employed in the sequencing reaction.
[0349] a) Sequencing Protocol using an End-Labeled Primer
[0350] The primer is end labeled using the protocol described in Example 11 (a)(i). The template and primer are annealed as follows. When a single-stranded DNA template is employed the following reaction is used. For each set of four sequencing reactions, mix the following reagents in a microcentrifuge tube: 0.8 pmol ssDNA (approx. 2 &mgr;g of an M13 template); 5.0 &mgr;l of 5× Taq DNA Polymerase [250 mM Tris-HCL (pH 9.0 at 25° C.]; 50 mM MgCl2; 1.0 &mgr;l labeled primer (1 pmol); sterile dH2O to a final volume of 25 &mgr;l. Incubate at 37° C. for 10 minutes. During the incubation, prepare the extension/termination reaction tubes as described in section c) below.
[0351] When a double-stranded plasmid is-used as the template, the following reaction is used. For each set of four sequencing reactions, mix the following reagents in a microcentrifuge tube: 1.6 pmol denatured plasmid dsDNA (approx. 4 &mgr;g of a 3-5 kb template); 5.0 &mgr;l Taq DNA Polymerase 5× Buffer; 2.0 &mgr;l labeled primer (2 pmol) and sterile dH2O to a final volume of 25 &mgr;l. Incubate at 37° C. for 10 minutes. During the incubation, prepare the extension/termination reaction tubes as described in section c) below.
[0352] b) Extension/Termination Reactions
[0353] For each set of sequencing reactions, label four microcentrifuge tubes (G,A,T and C) and add 1 ∥l of the 8× Tne optimized d/ddNTP Mix (see Table 12 for components of the 8× mixes) to each tube. Cap the tubes and store on ice or at 4° C. until needed. Add 1 &mgr;l of Tne M284 (D323A,D389A)DNA Polymerase (5 u/&mgr;l) to the annealed primer/template mix (prepared as described above) and mix briefly by pipetting up and down.
[0354] Add 6 &mgr;l of the enzyme/primer/template mix to each of the four tubes containing the d/ddNTP Mixes. Mix briefly by pipetting up and down. A brief spin may be needed to ensure that no liquid is left on the tube walls. Incubate at 70° C. for 15 minutes. Add 4 &mgr;l of Stop Solution to each tube and set at room temperature. Heat the reactions to ≧70° C. for 2-5 minutes before loading the sequencing gel. Load 2.5-3.0 &mgr;l of each reaction on the gel (6% denaturing polyacrylamide). Following electrophoresis of the sequencing gel, the gel is exposed to X-ray film to generate an autoradiograph.
[0355] If the extension products seen on the autoradiograph are too short, the ddNTP concentrations should be lowered and conversely if the extension products are all skewed to high molecular weight products, the ddNTP concentrations should be raised. 13 TABLE 12 8X Nucleotide Mix Formulation for Sanger Sequencing Using Tne M284(D323A, D389A) Polymerase G A T C Nucleotide Nucleotide Nucleotide Nucleotide Nucleotide Component Mix Mix Mix Mix ddGTP 20 &mgr;M — — — ddATP — 50 &mgr;M — — ddTTP — — 75 &mgr;M — ddCTP — — — 25 &mgr;M 7-deaza dGTP 30 &mgr;M 250 &mgr;M 250 &mgr;M 250 &mgr;M dATP 250 &mgr;M 30 &mgr;M 250 &mgr;M 250 &mgr;M dTTP 250 &mgr;M 250 &mgr;M 30 &mgr;M 250 &mgr;M dCTP 250 &mgr;M 250 &mgr;M 250 &mgr;M 30 &mgr;M
[0356] c) Sequencing Protocol using Direct Incorporation
[0357] i) Annealing the Template and Primer
[0358] When a single-stranded DNA template is employed the following reaction is used to anneal the template and primer. For each set of four sequencing reactions, mix the following reagents in a microcentrifuge tube: 0.8 pmol ssDNA (approx. 2 &mgr;g of an M13 template); 1.0 pmol primer (approx. 8 ng of a 24 mer); 5.0 &mgr;l Taq DNA Polymerase 5× Buffer; 2.0 &mgr;l Extension/Labeling Mix [7.5 &mgr;M each of dGTP, dTTP and dCTP];sterile dH2O to a final volume of 25 &mgr;l. Incubate at 37° C. for 10 minutes. During the incubation, prepare the nucleotide tubes for the termination reaction as described in section iii, below.
[0359] When a double-stranded DNA template is employed the following reaction is used to anneal the template and primer. For each set of four sequencing reactions, mix the following reagents in a microcentrifuge tube: 1.6 pmol denatured plasmid dsDNA (approx. 4 &mgr;g of a 3-5 kb template); 2 pmol primer (approx. 16 ng of a 24 mer); 5.0 &mgr;l Taq DNA Polymerase 5× Buffer; 2.0 &mgr;l Extension/Labeling Mix; sterile dH2O to a final volume of 25 &mgr;l. Incubate at 37° C. for 10 minutes. During the incubation, prepare the nucleotide tubes for the termination reaction as described in section iii, below.
[0360] ii) Extension/Labeling Reaction
[0361] Add 0.5 &mgr;l of [&agr;-35S]dATP (1,000 Ci/mmol, approximately 10 &mgr;Ci/&mgr;l) or 0.5 &mgr;l of [&agr;-32P]dATP (800 Ci/mmol, approximately 10 &mgr;Ci/&mgr;l) to the annealed primer/template mixture. Add 1 l of Tne M284 (D323A,D389A) DNA Polymerase (5 u/&mgr;l) and mix briefly by pipetting up and down. Incubate at 37° C. for 5 minutes.
[0362] iii) Termination Reaction
[0363] For each set of sequencing reactions, label four microcentrifuge tubes (G,A,T,C) and add &mgr;l of the 8× Tne optimized d/ddNTP Mix to each tube (see Table 12 above). Store on ice or at 4° C. until just before completion of the extension/labeling reaction. When the extension/labeling reaction is complete, aliquot 6 &mgr;l to each tube (G,A,T,C) containing d/ddNTP Mix. Mix briefly by pipetting up and down. A brief spin may be needed to ensure that no liquid is left on the tube walls. Incubate at 70° C. for 15 minutes. Add 4 &mgr;l of Stop Solution to each tube and store at −20° C. Heat the reactions to >70° C. for 2-5 minutes immediately before loading on a sequencing gel. Load 2.5-3.0 &mgr;l of each reaction on the gel. Following electrophoresis of the sequencing gel, the gel is exposed to X-ray film to generate an autoradiograph.
[0364] If the extension products seen on the autoradiograph are too short, the ddNTP concentrations should be lowered and conversely if the extension products are all skewed to high molecular weight products, the ddNTP concentrations should be raised.
EXAMPLE 12[0365] Fidelity of Tne DNA Polymerases
[0366] The fidelity of the Tne polymerases (full-length and modified forms) is measured using a PCR fidelity assay. This assay is based on the amplification, circularization, and transformation of the pUC19 derivative pLACIQ, which contains a functional lacIq allele [Frey and Suppmann, Biochemica 2:8 (1995)]. PCR-derived mutations in lacI result in a de-repression of the expression of lacZ&agr; and subsequent formation of a functional &bgr;-galactosidase enzyme, which can be easily detected on X-Gal indicator plates.
[0367] a) Construction of pLACIQ
[0368] The truncated lacI gene present in pUC19 is replaced by a functional copy of lacIq. pUC19 (GibcoBRL) is digested with PvuII and AflIII and the 180 bp PvuII-AflIII fragment of pUC19 is replaced by a 1189 bp DNA fragment encoding lacIq. The 1189 bp lacIq fragment is created by PCR amplification of residues 2972-4142 of pTrc 99 A (Pharmacia). The following primer pair is used in the PCR: 5′-CATGCCATGCATGCATTTACGTTGACACCA-3′ (SEQ ID NO:48) and 5′-TCCCCCGGGTTGCGCTCACTGCCCGCTTTCCAGT-3′ (SEQ ID NO:49). The oligonucleotide of SEQ ID NO:48 contains a NcoI recognition site and the oligonucleotide of SEQ ID NO:49 contains a SmaI recognition site. The PCR is performed using 18 cycles of denaturation at 94° C. for 30 sec; annealing at 57° C. for 30 sec and extension at 72° C. for 4 min. The PCR is performed using Pfu DNA polymerase (Stratagene) in the buffer recommended by the manufacturer. The PCR products are digested with NcoI and SmaI to generate a 1189 bp fragment having a blunt end (compatible with PvuII ends) and a 5′ overhanging end compatible with the AflIII digested end of pUC19. The 1189 bp fragment is ligated into the digested pUC19 using standard techniques.
[0369] The ligation products are used to transform the &agr;-complementing E. coli strain DH5&agr; (GibcoBRL) and the desired plasmid, termed pLACIQ (3695 bp), is isolated using standard techniques. DH5′ cells containing pLACIQ produce white (LAC1+) colonies on LB plates containing ampicillin (100 &mgr;g/ml) and X-Gal (0.004% w/v).
[0370] b) The PCR Fidelity Assay
[0371] The template used in the PCR fidelity assay is prepared as follows. pLACIQ is linearized by digestion with Dra II. A typical PCR reaction contains 5 or 10 ng of linearized, gel-purified plasmid DNA.
[0372] The following primers are used in the PCR fidelity assay to amplify the lacIq sequences of pLACIQ; both PCR primers used have Cla I cleavage sites at their 5′ ends: 5′-AGCTTATCGATGGCACTTTTCGGGGAAATGTGCG-3′ (SEQ ID NO:50) and 5′-AGCTTATCGATAAGCGATGCCGGGAGCAGACAAGC-3′ (SEQ ID NO:51). The length of the resulting PCR product is 3,561 bp.
[0373] The PCRs which employ the Tne DNA polymerases or UlTma DNA polymerase are performed using the following buffer [10 mM Tris-HCl (pH 8.8 at 25° C.), 10 mM KCl, 0.002% Tween 20, 1.5 mM MgCl2 and 40 &mgr;M all four dNTPs]. PCRs which employ nTaq DNA polymerase are performed using the following buffer [10 mM Tris-HCl (pH 9.0 at 25° C.), 50 mM KCl, 1.5 mM MgCl2, 0.1% Triton X-100 and 140 &mgr;M all four dNTPs]. All reactions contain 5-10 ng of template, 20 pmol of each primer and 2.5 units of polymerase (all polymerases are assayed using the assay described in Example 5 to ensure the same amount of activity is used for each enzyme tested). The following cycling conditions are used: denaturation for 10 sec at 94° C.; annealing for 30 sec at 57° C. and extension for 4 min at 72° C. for 18 cycles.
[0374] Following the PCR, the amplification products are PEG-precipitated as follows. The PCR amplification mixtures are frozen at −20° C. and the mineral oil is removed by rinsing twice with cold chloroform. The mixtures are then thawed and 10 &mgr;g of glycogen and ½ volume of 30% PEG 3350/1.5 M NaCl is added. The mixture is allowed to stand for a minimum of 1 hr at 0-4° C. The mixture is then centrifuged in a microcentrifuge for 8 min and the supernatant is removed. The pellet is then rinsed with 75% ethanol and dried. The DNA is then digested with ClaI and the digested DNA is purified by gel electrophoresis. The purified DNA is then ligated to recircularize the plasmid in a reaction containing less than or equal to 30 ng DNA.
[0375] The resulting PCR-derived plasmids are transformed into competent E. coli DH5&agr; and plated on LB Amp100 X-Gal plates [LB plates containing 100 &mgr;g/ml ampicillin and 0.004% X-Gal (w/v)]. After incubation overnight at 37° C., blue and white colonies are counted. The error rate (f) per bp is calculated using the rearranged equation published by Keohavong and Thilly [Proc. Natl. Acad. Sci. USA 86:9253 (1989): f=InF/d×b bp; where F is the fraction of white colonies: F=white (LACI+)/total colony number; 2d is the number of DNA duplications: 2d=output DNA/input DNA; and b is the effective target size of the (1080 bp) lacI gene, which is 349 bp according to Provost et al. [Mut. Research 288:133 (1993)]; there are 349 phenotypically identified (by color screening) single-based substitutions (nonsense and mis-sense) at 179 codons (approximately 50% of the coding region) within the lacI gene [Provost et al., supra]. Frameshift errors, which may occur at every position in the 1080 bp open reading frame of lac, are not taken into account.
[0376] A religation control is prepared as follows. Fifty nanograms of Dra II-linearized, gel-purified pLACIQ DNA is religated, and an aliquot of the ligation reaction is transformed into DH5&agr;. After incubation overnight, the number of growing colonies (0.027%) showing a blue (LACI−) phenotype on LB Amp X-Gal plates is measured to assess the rate of the formation of concatameric ligation products (with subsequent intramolecular recombination in E. coli that eliminates an additional origin of replication), which seems to be a very rare event. Restriction analysis of PCR-derived plasmids isolated from blue colonies is also performed to confirm that the LACI− phenotype originates in PCR-derived mutations of lac, but not in deleterious recombination events after transformation of the ligated DNA in DH5&agr;.
[0377] From the above, it is clear that the enzymes of the present invention provide thermostable DNA polymerase having novel features. In particular, these enzymes provide superior polymerases for use in PCR applications. In addition, these polymerases have higher affinities for ddNTPs which results in significant cost savings for users of chain termination sequencing protocols.
[0378] Other modifications of the embodiments of the invention described above that are obvious to those of ordinary skill in the areas of molecular biology, biochemistry and related disciplines are intended to be within the scope of the accompanying claims.
Claims
1. A purified thermostable DNA polymerase capable of DNA synthetic activity, said enzyme derived from the eubacterium Thermotoga neapolitana.
2. The polymerase of claim 1, wherein said polymerase is further capable of 3′ exonuclease activity.
3. The polymerase of claim 1, wherein said polymerase is further capable of 5′ exonuclease activity.
4. The polymerase of claim 1, wherein said polymerase comprises the amino acid sequence of SEQ ID NO:2.
5. The polymerase of claim 1, wherein the specific activity of said synthetic activity is approximately 100,000 units/mg.
6. A purified thermostable non-naturally occurring DNA polymerase derived from the eubacterium Thermotoga neapolitana.
7. The polymerase of claim 6, wherein said polymerase is capable of 3′ exonuclease activity.
8. The polymerase of claim 6, wherein said polymerase is capable of 5′ exonuclease activity.
9. The polymerase of claim 6, wherein said polymerase has reduced levels of 3′ exonuclease activity.
10. The polymerase of claim 6, wherein said polymerase lacks significant 5′ exonuclease activity.
11. The polymerase of claim 6, wherein said polymerase comprises an amino acid sequence selected from the group consisting of SEQ ID NOS:8, 16, 19, 23, 26, 29, 33 and 35.
12. An oligonucleotide comprising the nucleic acid sequence of SEQ ID NO: 1, said nucleotide sequence encoding a thermostable DNA polymerase.
13. The oligonucleotide of claim 12, wherein said sequence is modified such that the encoded polymerase lacks significant 5′ exonuclease activity.
14. The oligonucleotide of claim 13, wherein said modified sequence comprises the nucleotide sequence of SEQ ID NO:7.
15. The oligonucleotide of claim 12, wherein said sequence is modified such that the encoded polymerase displays reduced levels of 3′ exonuclease activity.
16. The oligonucleotide of claim 15, wherein said modified sequence is selected from the group consisting of SEQ ID NOS:7, 15, 18, 22, 25, 28, 32 and 34.
17. A recombinant DNA vector comprising an oligonucleotide having the nucleic acid sequence set forth in SEQ ID NO:1, said nucleotide sequence encoding a thermostable DNA polymerase having DNA synthetic activity.
18. The recombinant DNA vector of claim 17, wherein said nucleic acid sequence is modified such that the encoded thermostable DNA polymerase lacks significant 5′ exonuclease activity.
19. The recombinant vector of claim 18, wherein said modified sequence comprises SEQ ID NO:7.
20. The recombinant DNA vector of claim 17, wherein said nucleic acid sequence is modified such that the encoded thermostable DNA polymerase exhibits reduced levels 3′ exonuclease activity.
21. The recombinant vector of claim 20, wherein said modified sequence is selected from the group consisting of SEQ ID NOS:7, 15, 18, 22, 25, 28, 32 and 34.
Type: Application
Filed: Nov 12, 2003
Publication Date: Jul 8, 2004
Inventors: Elena Bolchakova (Foster City, CA), Alexander Novikov (Foster City, CA), Michael R. Slater (Madison, WI), Fen Huang (Madison, WI), James R. Hartnett (Fitchburg, WI), Galina A. Velikodvorskaya (Moscow)
Application Number: 10706229
International Classification: C12Q001/68; C07H021/04; C12N009/22; C12N001/21; C12N015/74;
