POLYMERASE
The present invention relates to an engineered polymerase with an expanded substrate range characterised in that the polymerase is capable of incorporating an enhanced occurrence of detection agent-labelled nucleotide analogue into nucleic acid synthesised by that engineered polymerase as compared with the wild type polymerase from which it is derived.
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The present invention relates to DNA polymerases. In particular the invention relates to a method for the generation of DNA polymerases with an enhanced ability to incorporate nucleotide analogues bearing detection agent labelled substituents. Uses of engineered polymerases produced using the methods of the invention are also described.
BACKGROUNDEfficient and precise replication of DNA is pivotal to the maintenance, transmission and expression of genetic information. High-fidelity DNA polymerases are the key enzymes responsible for maintaining genome integrity. To avoid the negative consequences of mutations (hereditary and sporadic diseases) high-fidelity DNA polymerases perform an astonishing feat of molecular recognition, selecting the correct nucleotide triphosphate (dNTP) molecule from a pool of very similar substrates and catalysing its incorporation as specified by the template base. DNA synthesis by exonucleolytic proof-reading deficient DNA polymerases occurs with error rates ranging from 10−3 to >10−6 per base-pair (Kunkel and Bebenek, 2000; Tippen et al., 2004). Although in nature high polymerase fidelity is vital for accurate DNA replication it has serious drawbacks for many biotechnological applications. Specifically, it restricts the use of unnatural or modified nucleotide bases and the applications they enable.
Fluorescence-based technologies have superseded radioisotopic detection as the preferred choice for labelling and detecting biomolecules. The incorporation of fluorescent labelled nucleotides into nucleic acids is central to such techniques as DNA sequencing, gene expression microarray analyses (Schena et al., 1995) tissue microarrays (TMA; Kononen et al., 1998), comparative genome hybridisations (CGH; Kallioniemi et al., 1992) and Fluorescent In-Situ Hybridisation (FISH; McNeil et al., 1991). Nucleic acid fluorescent labelling methodologies have been developed based on either enzymatic (direct incorporation of the fluorescent dye) or chemical modification (indirect incorporation of the fluorescent dye).
Direct incorporation of dye labelled nucleotides employs polymerase enzymes and is limited by the fact that polymerase enzymes have evolved to preserve a high selectivity for their correct nucleotide substrate. As such, most naturally occurring or commercially available polymerase enzymes discriminate against nucleotides bearing bulky side groups such as fluorescent moieties or incorporate them to a low level exhibiting significant sequence bias (Zhu and Waggoner, 1997; Zhu et al., 1994). Most enzymatic fluorescent labelling protocols, therefore, use the dye-labelled nucleotide, spiked at low percentage into a standard reaction mix (Reid et al., 1992). Even under these conditions, however, the polymerase enzyme still favours the natural nucleotide over the modified nucleotide (Zhu et al., 1994).
To overcome the low fluorophore densities achieved with direct labelling methods indirect labelling technologies have been developed whereby a less bulky amine-modified nucleotide is directly incorporated into the nucleic acid and the fluorescent label is chemically coupled to the nucleotide via the reactive amine group post nucleic acid synthesis (Cox and Singer, 2004). Although higher fluorescent nucleic acid labelling densities can be attained by indirect labelling methods complete substitution of every reactive nucleotide has not been achieved.
Nucleic acid probes with a higher density of labels (up to 100% substitution) are desirable as they would be expected to increase detection sensitivity. Furthermore, 100% substitution of every base with its fluorescently modified counterpart is a prerequisite of many single molecule sequencing techniques (Shendure et al., 2004). With current indirect DNA labelling methods unable to label 100% of the available positions research efforts have focussed on identifying naturally occurring or mutant DNA polymerase enzymes that are less stringent with regard to their substrate specificity.
Such efforts have met with modest success. Specifically, several members of the A (PolI-like; Brakmann and Nieckchen, 2001; Anderson et al., 2005; Yu et al., 1994; Tasara et al., 2003; Augustin et al., 2001; Ghadessy et al., 2004; Ramanathan et al., 2005) or B (PolII-like; Anderson et al., 2005; Augustin et al., 2001; Tasara et al., 2003; Földes-Papp et al., 2001; Glick et al., 2002; Jäger and Famulok, 2004; Obayashi et al., 2002; Ono et al., 1997) evolutionary families of DNA polymerase enzymes (Zhu and Ito, 1994) have been identified as able to incorporate fluorescent labelled nucleotides. In the case of enzymes harbouring polymerase as well as proofreading activity the yield of dye-labelled DNA was improved by using their exonuclease deficient mutants.
The ability of the majority of the polymerase enzymes to incorporate fluorescently labelled nucleotides was only investigated by primer extension analyses generating single stranded fluorescently labelled DNA. PCR incorporation of fluorescently labelled nucleotides allows simultaneous labelling and amplification of DNA. However, in contrast to primer extension reactions, following a few cycles of PCR amplification the fluorescent nucleotide is also present in the template strand. This has consequences for PCR amplification as the polymerase frequently pauses or aborts copying and the yield of labelled DNA decreases as the fluorescent nucleotide incorporation increases presumably due to steric crowding effects (Zhu and Waggoner, 1994). Consequently there is a need for polymerase enzymes able to efficiently incorporate fluorescent nucleotide analogues to high density by PCR and there remains a need in the art for polymerases, in particular DNA polymerases which are capable of incorporating a high density of detection label and/or capable of incorporating the detection label into large double stranded DNA fragments.
SUMMARY OF THE INVENTIONThe present invention modified the principles of compartmentalised self replication (CSR; Ghadessy et al., 2001) to reduce discrimination against dye labelled nucleotide analogues (detection-agent labels), in particular Cy5-dCTP and Cy3-dCTP (the dye labelled nucleotides most commonly used for labelling of microarray probes). In this way the inventors were able to identify a number of polymerases which allowed the synthesis of nucleic acid probes with high detection agent labels, in particular fluorophore detection agents.
Thus, in the first aspect the present invention provides an engineered (eg. a mutated) polymerase with an expanded substrate range characterised in that the polymerase is capable of incorporating an enhanced density of detection agent-labelled nucleotide analogue into nucleic acid synthesised by that engineered polymerase, as compared with the wild type polymerase from which it is derived.
The term “mutant” is used herein to mean a polypeptide or nucleotide sequence having a sequence which differs from the wild type sequence by one or more additions, substitutions or deletions.
In one embodiment, the engineered polymerase is derived from a Pol B family polymerase. More advantageously, it is a Pfu mutant polymerase. In another embodiment, the engineered polymerase is a Pfu mutant polymerase expressed by the clones selected from the group consisting of the following: 23, AH12, 55, 15, 33, 34, 35 and E10.
In another embodiment of the invention the Pfu mutant is E10, as described herein selected from repertoires of Pfu genes mutated in the A- or A- and B- or A- and C-motifs by a variant of CSR, short-patch CSR (spCSR), for an enhanced ability to incorporate Cy5-dCTP. For example, E10 incorporates Cy5-dCTP when substituted to 100% for dCTP in ELISA extension reactions and PCR (see examples). This is significant since during PCR amplification both the template and product strand become decorated with bulky dye molecules. Advantageously, E10 also incorporates Cy3-dCTP when substituted to 100% for dCTP in ELISA extension reactions and PCR (see examples). DNA labelled by E10 in PCR reactions where 100% of the dCTP had been replaced by Cy3-dCTP resulted in a 7-fold higher fluorescent signal when used in microarray hybridisation experiments (see examples). Surprisingly DNA labelled by E10 in PCR reactions where 10% of the dCTP had been replaced by Cy3-dCTP results in a 4 fold higher fluorescent signal in microarray experiments suggesting that E10 has an enhanced affinity for Cy3 modified dCTP (see examples).
According to the invention described herein, the presence of a detection agent-labelled nucleotide at a given position in the newly synthesised nucleic acid effectively labels that nucleotide position. Such labelling then facilitates the use of the synthesised nucleic acid in any of the following functions: polymerase chain reaction (PCR); single strand DNA sequencing and microarray detection. Those skilled in the art will be aware of other suitable uses for detection-labelled nucleic acid synthesised according to the invention.
Thus in a further aspect the present invention provides a method for enhancing the sensitivity of any one or more of the following: PCR, ELISA, FISH, fibre FISH and microarrays which method comprises the step of utilising a nucleic acid probe which has been Cy5-CTP or Cy3-CTP labelled using a nucleic acid polymerase, suitably, an E10 polymerase according to the invention in that method.
In another embodiment the detection agent label is a dye label. More suitably, it is a fluorescent dye label. Suitable dye labels will be familiar to those skilled in the art and are described in more detail in the detailed description of the invention. In a preferred embodiment of the methods of the invention, the fluorescent label is Cy3 and/or Cy5 labelled dCTP.
According to the present invention, the term ‘engineered (eg. mutated) DNA polymerase’ refers to a DNA polymerase which has a nucleic acid sequence which is not 100% identical at the nucleic acid level to the one or more DNA polymerase/s or fragments thereof, from which it is derived, and which has been generated using one or more synthetic methods. Advantageously, an engineered DNA polymerase according to the invention is a pol-B family DNA polymerase. Most suitably the engineered polymerase is a Pfu enzyme mutant. As referred to above the term ‘engineered DNA polymerase’ also includes within its scope fragments, derivatives and homologues of an ‘engineered DNA polymerase’ as herein defined so long as it exhibits the requisite property of possessing an expanded substrate range as defined herein. In addition, it is an essential feature of the present invention that an engineered DNA polymerase according to the invention includes no more than 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1% or substantially 0% (compared to wild type polymerase) of a polymerase without a 3-5′ exonuclease activity under the conditions used for the polymerisation reaction. Such a proofreading activity would remove any 3′ mismatches incorporated according to the method of the invention, and thus would prevent a polymerase according to the invention possessing an expanded substrate range as defined herein.
As defined herein the term ‘expanded substrate range’ (of an engineered DNA polymerase) refers to the ability of an engineered polymerase according to the present invention to incorporate an enhanced density of detection agent-labelled nucleotide analogue into nucleic acid synthesised by that engineered polymerase as compared with the wild type polymerase from which it is derived.
In one embodiment, an engineered polymerase according to the invention can incorporate 10% detection-agent labelled nucleotides expressed as a percentage of total nucleotides in the newly synthesised nucleic acid—such as DNA. In a preferred embodiment still, an engineered polymerase according to the invention can incorporate 20, 30, 40, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95% detection labelled nucleotides expressed as a percentage of total nucleotides in the newly synthesised nucleic acid-such as DNA. In one embodiment, the engineered polymerase incorporates 100% detection labelled nucleotides expressed as a percentage of total nucleotides in the newly synthesised nucleic acid—such as DNA.
In a further aspect, there is provided a method for the generation of an engineered polymerase with an expanded substrate range characterised in that the polymerase is capable of incorporating an enhanced occurrence of detection agent-labelled nucleotide analogue into nucleic acid synthesised by that engineered polymerase as compared with the wild type polymerase from which it is derived, which method comprises the steps of:
- (a) generating a polymerase repertoire;
- (b) performing compartmentalised self replication (CSR) using one of the repertoires of step (a), wherein the CSR is performed in emulsion, utilising primers which anneal 3′ and 5′ of the region diversified in the library according to step (a) and wherein the emulsion comprises detection agent-labelled dCTP in place of dNTP;
- (c) expressing those engineered polymerase repertoire members selected according to step (b) to obtain the protein product (mutant enzyme eg. mutant Pfu enzyme),
- (d) selecting those expressed mutant enzymes which are capable of incorporating one or more detection agent-labelled nucleotide analogue/s; and
- (e) optionally isolating and/or purifying the selected mutant polymerase.
In one embodiment, the polymerase repertoire according to step (a) is generated from any of the following: a pol A polymerase; a pol B polymerase and a Pfu polymerase or a combination thereof.
In one embodiment, the compartmentalised self replication used in step (b) in the method above is short patch self replication as described herein.
According to the method described above, in one embodiment the detection agent label is a dye label. In another embodiment it is a fluorescent dye label. Suitable dye labels will be familiar to those skilled in the art. In a preferred embodiment of the above aspect of the invention the fluorescent label is either Cy3 or Cy5 labelled dCTP.
In another embodiment an engineered polymerase selected according to the method of the invention can incorporate 10% detection-agent labelled nucleotides expressed as a percentage of total nucleotides in the newly synthesised nucleic acid—such as DNA. In a preferred embodiment still, an engineered polymerase according to the invention can incorporate 20, 30, 40, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95% detection labelled nucleotides expressed as a percentage of total nucleotides in the newly synthesised nucleic acid—such as DNA. In another embodiment, an engineered polymerase incorporates 100% detection labelled nucleotides expressed as a percentage of total nucleotides in the newly synthesised nucleic acid—such as DNA.
In a further aspect still the present invention provides a method for the incorporation of detection agent-labelled nucleotide analogues into newly synthesised nucleic acid which method comprises the use of an engineered polymerase according to the invention.
In yet a further aspect the present invention provides the use of an engineered polymerase according to the invention in the synthesis of nucleic acid which comprises fluorescently labelled nucleotide analogues.
In a further aspect the present invention provides a Pfu mutant library selected from the following: Pfu A motif library; Pfu A+B motif library and Pfu A+C motif library.
In a further aspect the present invention provides the use of an engineered polymerase with an expanded substrate range according to the invention in one or more techniques in the group consisting of the following: polymerase chain reaction (PCR); microarray analysis (such as gene expression microarray analyses, tissue microarrays, array Comparative Genome Hybridisations); fluorescent in-situ hybridisation (FISH); fibre FISH; comparative genome hybridisations; DNA sequencing, nucleic acid sequencing, (eg. single-stranded nucleic acid sequencing) and single molecule detection.
In another embodiment the engineered polymerase is a Pfu mutant polymerase expressed by the clones selected from the group consisting of the following: 23, AH12, 55, 15, 33, 34, 35 and E10.
In further aspect, there is provided a nucleotide sequence wherein at least 90%, suitably 100% of the Cytidine residues are labelled with Cy5 and/or Cy3. In another embodiment, the nucleotide sequence does not comprise a linker between the C and Cy residues.
The nucleotide sequence may be of synthetic or recombinant origin. The nucleotide sequence may be double-stranded or single-stranded whether representing the sense or antisense strand or combinations thereof. The nucleotide sequence may be DNA. The nucleotide sequence may be prepared by use of recombinant DNA techniques (eg. recombinant DNA).
The nucleotide sequences may include within them synthetic or modified nucleotides. A number of different types of modification to oligonucleotides are known in the art. These include methylphosphonate and phosphorothioate backbones and/or the addition of acridine or polylysine chains at the 3′ and/or 5′ ends of the molecule.
In a further aspect, there is provided a method for analysing the nucleotide sequence, comprising the use of Atomic Force Microscopy, wherein said method comprises the step of depositing the nucleotide sequence onto poly-L-lysine coated mica.
There is also provided the nucleotide sequence bound to poly-L-lysine coated mica.
The use of poly-L-lysine coated mica for analysing the nucleotide sequence is also described.
DESCRIPTION OF THE FIGURESPCR was carried out in solution or emulsion (CSR) under modified conditions with cells expressing an active Pfu variant (encoded by pASKpfuexo-2) and an inactive Pfu variant (encoded by pASKpfuexo-7) present at a ratio of 1:100 and primers 40 and 41.
The inactive Pfu variant contains a unique XhoI restriction enzyme site at 1218 bp. Restriction digestion with XhoI allows PCR products derived from the inactive and active polymerase genes to be distinguished. In solution there is no compartmentalisation of active and inactive clones and the active Pfu polymerase enzyme amplifies both its own encoding gene and the gene encoding the inactive Pfu variant. After XhoI digestion of reamplified PCR products fragments resistant to digestion (2.4 kb) derived from the active polymerase gene and digested fragments (1.238 kb and 1.166 kb) are observed following agarose gel electrophoresis. By contrast in emulsion individual bacteria expressing either active or inactive or inactive Pfu polymerases are isolated in separate aqueous compartments ensuring that the active polymerase enzyme only amplifies its own encoding gene. PCR products derived from reactions performed in emulsions are resistant to XhoI digestion indicating that the fragment originates from the active polymerase gene and demonstrates a clear enrichment of the active clone over the inactive clone.
Activity normalised lysates from Pfu and Pfu variants 15, 23, 55 and AH12 selected with Cy5-dCTP from the Pfu A motif repertoire library were assessed for their ability to incorporate (A) 4; primer 33, (B) 6; primer 34 or (C) 8; primer 35 consecutive Cy3- or Cy5-dCTPs. All selected clones are significantly better than Pfu at incorporating Cy5-dCTP, the activity for which they were selected. Additionally, all selected clones are also better able to incorporate Cy3-dCTP than Pfu.
ELISAs performed with primers 33, 34 and 35 identified Pfu variants 15, 23, 55 and AH12 as having a significantly enhanced ability to incorporate Cy5-dCTP and Cy3-dCTP (see
Crude lysates from Pfu variants selected with Cy5-dCTP from the Pfu A-B motif repertoire library, Pfu variants 15, 23 and 55 (selected with Cy5-dCTP from the Pfu A motif repertoire library) and Pfu were assessed for their ability to incorporate 8 (primer 35) consecutive Cy3- or Cy5-dCTPs. Clones isolated from the Pfu A-B motif repertoire library are significantly better than Pfu and clones selected from the first round of Cy5-dCTP selection (Pfu A motif repertoire: clones 15, 23 and 55) at incorporating Cy5-dCTP, the activity for which they were selected. Additionally, all selected clones are also better able to incorporate Cy3-dCTP than Pfu and Cy5-dCTP selected A motif library variants (15, 23, and 55).
ELISAs performed with primer 35 identified Pfu variants 2-13 as having a significantly enhanced ability to incorporate Cy5-dCTP or Cy3-dCTP (see
Crude lysates from Pfu variants selected with Cy5-dCTP from the Pfu A-C motif repertoire library, Pfu variants 15, 23, 55 and AH12 (selected with Cy5dCTP from the Pfu A motif repertoire library) were assessed for their ability to incorporate 8 (primer 35) consecutive Cy3- or Cy5-dCTPs. Clones isolated from the Pfu A-C motif repertoire library are significantly better than clones selected from the first round of Cy5-dCTP selection (Pfu A motif repertoire: clones 15, 23, 55 and AH12) at incorporating Cy5-dCTP, the activity for which they were selected. Additionally, all selected clones are also better able to incorporate Cy3-dCTP than Cy5-dCTP selected A motif library variants (15, 23, 55 and AH12).
ELISAs performed with primer 35 identified Pfu variants E10, 5, 9, 10, 12, 13, 15, 22, 23, 25, 27 and 28 as having a significantly enhanced ability to incorporate Cy5-dCTP or Cy3-dCTP (see
Clone E10 selected with Cy5-dCTP from the Pfu A-C motif repertoire library was identified as having a significantly enhanced ability to incorporate both Cy3- and Cy5-dCTP. Sequencing of this clone with primers 36, 37, 39, 44, 45 and 46 identified 14 point mutations of which 9 are silent. The remaining 5 introduce the following mutations: V337I, E399D, N400D, R407I, and Y546H.
Crude lysates from Pfu variants selected with Cy3-dCTP from the Pfu A-C motif repertoire library, Pfu and Pfu variants 15, 23, 55 and AH12 (selected with Cy5dCTP from the Pfu A motif repertoire library) were assessed for their ability to incorporate 8 (primer 35) consecutive Cy3- or Cy5-dCTPs. All clones isolated from the Pfu A-C motif repertoire library are significantly better than Pfu at incorporating Cy3-dCTP, the activity for which they were selected. Additionally, all selected clones are also better able to incorporate Cy5-dCTP than Pfu.
ELISAs performed with primer 35 identified Pfu variants 1-11 as having a significantly enhanced ability to incorporate Cy5-dCTP or Cy3-dCTP (see
Crude lysates from Pfu variants selected with Cy5-dCTP and Biotin-16-dUTP from the Pfu A-B motif repertoire library, Pfu variants 15, 23, 55 and AH12 (selected with Cy5-dCTP from the Pfu A motif repertoire library) and Pfu were assessed for their ability to incorporate 8 (primer 35) consecutive Cy3- or Cy5-dCTPs. Clones isolated from the Pfu A-B motif repertoire library are significantly better than Pfu at incorporating Cy5-dCTP, the activity for which they were selected. Additionally, all selected clones are also better able to incorporate Cy3-dCTP than Pfu.
ELISAs performed with primer 35 identified Pfu variants D1, E1, F1, A2, C2, D2. E2, F2, G2 and A3 as having an enhanced ability to incorporate Cy5-dCTP or Cy3-dCTP (see
PCRs were performed with heparin purified activity normalised Pfu V93Q and Pfu variants selected with Cy5-dCTP from the A motif Pfu repertoire library (variant 15), Pfu variants selected with Cy5-dCTP and Biotin-16-dUTP from the A-B Pfu repertoire library (variants A3 and D2) or Pfu variants selected with Cy5-dCTP from the A-C motif Pfu repertoire library (variants 1, 9, 3, 10, 4 and E10). 100% of the dCTP in the PCR reaction was replaced by Cy3-dCTP (3) or Cy5-dCTP (5). A fragment of size 410 bp is amplified with primers 38 and 39 requiring the incorporation of 287 Cy-dye labelled dCTPs.
Fluorescent output measured using ArrayWoRx E scanner (Applied Precision instruments) from experiments where equimolar quantities of PfuCy510 DNA were hybridised with either PfuCy310 DNA (A), E10Cy310 DNA (B) or E10Cy3100 DNA (C) to microarray slides on which had been printed a 2-fold dilution series of Pfu polymerase DNA (50-6.25 ng/μL). A 4-fold increase in fluorescent signal was detected from E10Cy310 DNA and a 7-fold increase in Cy3 signal was detected from E10Cy3100 DNA (D).
Cy5-(
Boxplots of fluorescent signal obtained from co-hybridisation experiments of 270 bp and 1.3 kb fragments directly labelled by E10 (50% replacement of dCTP) with indirectly Klenow-labelled samples scanned using ArrayWoRx E scanner. E=E10; K=Klenow; 3=Cy3-dCTP; 5=Cy5-dCTP.
Scatterplots of fluorescent signal obtained from co-hybridisation experiments of 270 bp fragments directly labelled by E10 in PCRs where 10% (E1010), 50% (E1050) or 100% (E10100), of the dCTP has been replaced by Cy3- or Cy5-dCTP labelled dCTP with 270 bp fragments directly labelled by Pfuexo- in PCRs where 10% (Pfu10) of the dCTP has been replaced by Cy-dye labelled dCTP. Each point represents an individual array feature. Red points=Cy3 and Cy5 signal from 10% Pfuexo- labelled DNA; Green points=Cy5 signal from E10-labelled DNA; Blue points=Cy3 signal from E10-labelled DNA.
Lanes 1 & 7 Marker ØX174 HaeIII, Lanes 2 & 5 DdeI; Lanes 3 & 6 MseI; Lanes 4 & 7 uncut.
A) ELISAs performed with activity normalised crude lysates of E10 (E10 B and F; E399D, N400G, R407I, V337I, Y546H), clone 15 (15 G and C; E399D, N400G, R407I and V337I), pASKpfuexo−6Y546H (Y546H D and H) or Pfuexo− (A and E; Stratagene) and primer 35 and Cy3 or Cy5-dCTP.
B) Agarose gel (1.2% w/v) electrophoresis of the products of PCRs performed with activity normalised Pfuexo− (1 and 5; Stratagene), E10 (2 and 6), clone 15 (3 and 7) and Y546H (4 and 8) and extension times of either 30 seconds or 10 seconds. With a 10 second extension time only E10 (6) is able to amplify the 270 bp product. Marker=ØX174 HaeIII (HT Biotechnology Ltd).
A) Sequence alignment of Tgo V93Q exo- (Tgo), Tgo V93Q exo− into which the E10 A motif mutations have been introduced (Tgo E10AMo; V93Q, Exo−, E398D, N399D, R406I), Tgo V93Q exo− Therminator into which the E10 A motif mutations have been introduced (TgoT E10AMo; V93Q, Exo-, A485L, E398D, N399D, R406I).
B) Agarose gel (1.2% w/v) electrophoresis of PCRs performed with crude lysates of Tgo E10AMo (Lane 1; V93Q, exo−, E398D, N399D, R406I), Tgo (Lane 2; V93Q, exo−), TgoT E10AMo (Lane 3; V93Q, Exo−, A485L, E398D, N399D, R406I) TgoT (Lane 4: V93Q, Exo−, A485L). While both Tgo and Tgo E10AMo are able to amplify the 410 bp fragment TgoT is only able to do so in the presence of the E10 A motif mutations.
Agarose gel (1.2% w/v) electrophoresis of PCRs performed E10 or Pfuexo- in the presence of 100% FITC-12-dATP or Biotin-16-dUTP. Marker=ØX174 HaeIII (HT Biotechnology Ltd).
Crude lysates from Pfu variants selected with Biotin-16-dUTP from the Pfu A motif repertoire library and Pfu were assessed for their ability to incorporate 12 (primer 60) consecutive Biotin-16-dUTPs. Clones isolated from the Pfu A motif repertoire library are significantly better than Pfu at incorporating Biotin-16-dUTP, the activity for which they were selected.
ELISAs performed with primer 60 identified Pfu variants Bio187, Bio120, Bio94, Bio80, Bio56, Bio33 and Bio32 as having a significantly enhanced ability to incorporate Biotin-16-dUTP (see
Fluorescent in situ hybridisation of probes to NRG1 (A), WRN (B) or DCTN6 (C) to metaphase spreads of the human pancreatic cancer cell line Suit-2 detects co-localisation of FITC, Cy3 and Cy5 signal to a single chromosome (D; merged image) without the need for secondary signal amplification.
Atomic force microscopy (AFM) imaging of DNA and CyDNA. (A) Mica-adsorbed DNA and CyDNA (DNA, Cy3-DNA, Cy5-DNA from left to right) fragments of identical length and sequence were imaged using AFM in tapping mode. (B) AFM was used to determine average contour length, stiffness (persistence length) and height. While Cy-DNA displayed were similar values to standard DNA for contour length and stiffness, Cy3- and in particular Cy5-DNA displayed substantially increased average contour height, presumably due to the dense array of cyanine dyes surrounding the DNA helix.
A Direct detection of FITC or Cy5 signal on a DAPI stained DNA fibre.
B Organisation of DNA probes along the Neuregulin1 gene The distance of probes 1, 4 and 6 from probes 2, 3, 5 and 7 is indicated. Probes 2, 3, 5 and 7 are spaced 10 Kb apart.
C Probe information.
A solution of 5 pg/ul each of 50% Cy3-dCTP-labelled and of 100% Cy5-dCTP-labelled 270mer PCR products was pumped at 30 μl/hr in a fused silica capillary past a sensitive fluorescence detection system (see text). The representative output shows fluorescence (counts per second—vertical axis) at 670 nm (Cy5 maximum—blue trace) and 570 nm (Cy3 maximum—red trace) recorded in consecutive 20 μusec periods (numbered along horizontal axis) and averaged over a sliding window of 40 such periods. Note that Cy3 fluorescence gives a smaller “bleed-through” signal in the Cy5 channel.
DETAILED DESCRIPTION OF THE INVENTION (A) Principles Underlying Compartmentalised Self-Replication TechnologyThe techniques of directed evolution and compartmentalised self replication are detailed in GB 97143002 and GB 98063936 and GB 01275643, in the name of the present inventors. These documents are herein incorporated by reference.
The inventors modified the methods of compartmentalised self replication and surprisingly generated DNA polymerases which exhibited an expanded substrate range as herein defined.
In particular, the inventors realised that for self-replication of Pfu polymerase, compartments must remain stable at the high temperatures of PCR thermocycling. Encapsulation of PCRs has been described previously for lipid vesicles (Oberholzer, T., Albrizio, M. & Luisi, P. L. (1995) Chem. Biol. 2, 677-82 and fixed cells and tissues (Haase, A. T., Retzel, E. F. & Staskus, K. A. (1990) Proc. Natl. Acad. Sci. USA 87, 4971-5; Embleton, M. J., Gorochov, G., Jones, P. T. & Winter, G. (1992) Nucleic Acids) but with low efficiencies.
Further details of the method of compartmentalised self replication in general are given below. Of particular importance in the selection of polymerases which exhibit an enhanced ability to incorporate dye-labelled nucleotide analogues as compared with the polymerase from which they are derived is that the compartmentalised self replication method was modified. These modifications are detailed in section (B) below and also in Example 3 herein.
(i) MicrocapsulesThe microcapsules used according to the method of the invention require appropriate physical properties to allow the working of the invention.
First, to ensure that the nucleic acids and gene products may not diffuse between microcapsules, the contents of each microcapsule must be isolated from the contents of the surrounding microcapsules, so that there is no or little exchange of the nucleic acids and gene products between the microcapsules over the timescale of the experiment.
Second, the method of the present invention requires that there are only a limited number of nucleic acids per microcapsule. This ensures that the gene product of an individual nucleic acid will be isolated from other nucleic acids. Thus, coupling between nucleic acid and gene product will be highly specific. The enrichment factor is greatest with on average one or fewer nucleic acids per microcapsule, the linkage between nucleic acid and the activity of the encoded gene product being as tight as is possible, since the gene product of an individual nucleic acid will be isolated from the products of all other nucleic acids. However, even if the theoretically optimal situation of, on average, a single nucleic acid or less per microcapsule is not used, a ratio of 5, 10, 50, 100 or 1000 or more nucleic acids per microcapsule may prove beneficial in sorting a large library. Subsequent rounds of sorting, including renewed encapsulation with differing nucleic acid distribution, will permit more stringent sorting of the nucleic acids. In one embodiment, there is a single nucleic acid, or fewer, per microcapsule.
Third, the formation and the composition of the microcapsules must not abolish the function of the machinery the expression of the nucleic acids and the activity of the gene products.
Consequently, any microencapsulation system used must fulfil these three requirements. The appropriate system(s) may vary depending on the precise nature of the requirements in each application of the invention, as will be apparent to the skilled person.
A wide variety of microencapsulation procedures are available (see Benita, 1996) and may be used to create the microcapsules used in accordance with the present invention. Indeed, more than 200 microencapsulation methods have been identified in the literature (Finch, 1993).
These include membrane enveloped aqueous vesicles such as lipid vesicles (liposomes) (New, 1990) and non-ionic surfactant vesicles (van Hal et al., 1996). These are closed-membranous capsules of single or multiple bilayers of non-covalently assembled molecules, with each bilayer separated from its neighbour by an aqueous compartment.
In the case of liposomes the membrane is composed of lipid molecules; these are usually phospholipids but sterols such as cholesterol may also be incorporated into the membranes (New, 1990). A variety of enzyme-catalysed biochemical reactions, including RNA and DNA polymerisation, can be performed within liposomes (Chakrabarti et al., 1994; Oberholzer et al., 1995a; Oberholzer et al., 1995b; Walde et al., 1994; Wick & Luisi, 1996).
With a membrane-enveloped vesicle system much of the aqueous phase is outside the vesicles and is therefore non-compartmentalised. This continuous, aqueous phase should be removed or the biological systems in it inhibited or destroyed (for example, by digestion of nucleic acids with DNase or RNase) in order that the reactions are limited to the microcapsules (Luisi et al., 1987).
Enzyme-catalysed biochemical reactions have also been demonstrated in microcapsules generated by a variety of other methods. Many enzymes are active in reverse micellar solutions (Bru & Walde, 1991; Bru & Walde, 1993; Creagh et al., 1993; Haber et al., 1993; Kumar et al., 1989; Luisi & B., 1987; Mao & Walde, 1991; Mao et al., 1992; Perez et al., 1992; Walde et al., 1994; Walde et al., 1993; Walde et al., 1988) such as the AOT-isooctane-water system (Menger & Yamada, 1979).
Microcapsules can also be generated by interfacial polymerisation and interfacial complexation (Whateley, 1996). Microcapsules of this sort can have rigid, nonpermeable membranes, or semipermeable membranes. Semipermeable microcapsules bordered by cellulose nitrate membranes, polyamide membranes and lipid-polyamide membranes can all support biochemical reactions, including multienzyme systems (Chang, 1987; Chang, 1992; Lim, 1984). Alginate/polylysine microcapsules (Lim & Sun, 1980), which can be formed under very mild conditions, have also proven to be very biocompatible, providing, for example, an effective method of encapsulating living cells and tissues (Chang, 1992; Sun et al., 1992).
Non-membranous microencapsulation systems based on phase partitioning of an aqueous environment in a colloidal system, such as an emulsion, may also be used.
In one embodiment, the microcapsules are formed from emulsions; heterogeneous systems of two immiscible liquid phases with one of the phases dispersed in the other as droplets of microscopic or colloidal size (Becher, 1957; Sherman, 1968; Lissant, 1974; Lissant, 1984).
(ii) EmulsionsEmulsions may be produced from any suitable combination of immiscible liquids. In one embodiment the emulsion has water (containing the biochemical components) as the phase present in the form of finely divided droplets (the disperse, internal or discontinuous phase) and a hydrophobic, immiscible liquid (an ‘oil’) as the matrix in which these droplets are suspended (the nondisperse, continuous or external phase). Such emulsions are termed ‘water-in-oil’ (W/O). This has the advantage that the entire aqueous phase containing the biochemical components is compartmentalised in discreet droplets (the internal phase). The external phase, being a hydrophobic oil, generally contains none of the biochemical components and hence is inert.
The emulsion may be stabilised by addition of one or more surface-active agents (surfactants). These surfactants are termed emulsifying agents and act at the water/oil interface to prevent (or at least delay) separation of the phases. Many oils and many emulsifiers can be used for the generation of water-in-oil emulsions; a recent compilation listed over 16,000 surfactants, many of which are used as emulsifying agents (Ash and Ash, 1993). Suitable oils include light white mineral oil and non-ionic surfactants (Schick, 1966) such as sorbitan monooleate (Span™80; ICI) and polyoxyethylenesorbitan monooleate (Tween™ 80; ICI) and Triton-X-100.
The use of anionic surfactants may also be beneficial. Suitable surfactants include sodium cholate and sodium taurocholate. Particularly preferred is sodium deoxycholate, suitably at a concentration of 0.5% w/v, or below. Inclusion of such surfactants can in some cases increase the expression of the nucleic acids and/or the activity of the gene products. Addition of some anionic surfactants to a non-emulsified reaction mixture completely abolishes translation. During emulsification, however, the surfactant is transferred from the aqueous phase into the interface and activity is restored. Addition of an anionic surfactant to the mixtures to be emulsified ensures that reactions proceed only after compartmentalisation.
Creation of an emulsion generally requires the application of mechanical energy to force the phases together. There are a variety of ways of doing this which utilise a variety of mechanical devices, including stirrers (such as magnetic stir-bars, propeller and turbine stirrers, paddle devices and whisks), homogenisers (including rotor-stator homogenisers, high-pressure valve homogenisers and jet homogenisers), colloid mills, ultrasound and ‘membrane emulsification’ devices (Becher, 1957; Dickinson, 1994).
Aqueous microcapsules formed in water-in-oil emulsions are generally stable with little, if any exchange of nucleic acids or gene products between microcapsules. Additionally, we have demonstrated that several biochemical reactions proceed in emulsion microcapsules. Moreover, complicated biochemical processes, notably gene transcription and translation are also active in emulsion microcapsules. The technology exists to create emulsions with volumes all the way up to industrial scales of thousands of litres (Becher, 1957; Sherman, 1968; Lissant, 1974; Lissant, 1984).
The preferred microcapsule size will vary depending upon the precise requirements of any individual selection process that is to be performed according to the present invention. In all cases, there will be an optimal balance between gene library size, the required enrichment and the required concentration of components in the individual microcapsules to achieve efficient expression and reactivity of the gene products.
Details of emulsion/s used when performing the method of the present invention are provided in the Examples.
(iii) Expression within Microcapsules
The processes of expression usually occur within each individual microcapsule provided by the present invention. Both in vitro transcription and coupled transcription-translation become less efficient at sub-nanomolar DNA concentrations. Because of the requirement for only a limited number of DNA molecules to be present in each microcapsule, this therefore sets a practical upper limit on the possible microcapsule size. In one embodiment, the mean volume of the microcapsules is less that 5.2×10−16 m3, (corresponding to a spherical microcapsule of diameter less than 10 μm, more suitably less than 6.5×10−17 m3 (5 μm), more suitably about 4.2×10−18 m3 (2 μm) and ideally about 9×10−18 m3 (2.6 μm).
The effective DNA or RNA concentration in the microcapsules may be artificially increased by various methods that will be well-known to those versed in the art. These include, for example, the addition of volume excluding chemicals such as polyethylene glycols (PEG) and a variety of gene amplification techniques, including transcription using RNA polymerases including those from bacteria such as E. coli (Roberts, 1969; Blattner and Dahlberg, 1972; Roberts et al., 1975; Rosenberg et al., 1975), eukaryotes e.g. (Weil et al., 1979; Manley et al., 1983) and bacteriophage such as T7, T3 and SP6 (Melton et al., 1984); the polymerase chain reaction (PCR) (Saiki et al., 1988); Qβ replicase amplification (Miele et al., 1983; Cahill et al., 1991; Chetverin and Spirin, 1995; Katanaev et al., 1995); the ligase chain reaction (LCR) (Landegren et al., 1988; Barany, 1991); and self-sustained sequence replication system (Fahy et al., 1991) and strand displacement amplification (Walker et al., 1992). Even gene amplification techniques requiring thermal cycling such as PCR and LCR could be used if the emulsions and the in vitro transcription or coupled transcription-translation systems are thermostable (for example, the coupled transcription-translation systems could be made from a thermostable organism such as Thermus aquaticus).
Increasing the effective local nucleic acid concentration enables larger microcapsules to be used effectively. This allows a preferred practical upper limit to the microcapsule volume of about 5.2×10−16 m3 (corresponding to a sphere of diameter 10 um).
The microcapsule size must be sufficiently large to accommodate all of the required components of the biochemical reactions that are needed to occur within the microcapsule. For example, in vitro, both transcription reactions and coupled transcription-translation reactions require a total nucleoside triphosphate concentration of about 2 mM.
For example, in order to transcribe a gene to a single short RNA molecule of 500 bases in length, this would require a minimum of 500 molecules of nucleoside triphosphate per microcapsule (8.33×10−22 moles). In order to constitute a 2 mM solution, this number of molecules must be contained within a microcapsule of volume 4.17×10−19 litres (4.17×10−22 m3 which if spherical would have a diameter of 93 nm.
Furthermore, particularly in the case of reactions involving translation, it is to be noted that the ribosomes necessary for the translation to occur are themselves approximately 20 nm in diameter. Hence, the preferred lower limit for microcapsules is a diameter of approximately 100 nm.
Therefore, the microcapsule volume is suitably of the order of between 5.2×10−22 m3 and 5.2×10−16 m3 corresponding to a sphere of diameter between 0.1 um and 10 um, more suitably of between about 5.2×10−19 m3 and 6.5×10−17 m3 (1 um and 5 um). Sphere diameters of about 2.6 um are most advantageous.
It is no coincidence that the preferred dimensions of the compartments (droplets of 2.6 um mean diameter) closely resemble those of bacteria, for example, Escherichia are 1.1-1.5×2.0-6.0 um rods and Azotobacter are 1.5-2.0 um diameter ovoid cells. In its simplest form, Darwinian evolution is based on a ‘one genotype one phenotype’ mechanism. The concentration of a single compartmentalised gene, or genome, drops from 0.4 nM in a compartment of 2 um diameter, to 25 pM in a compartment of 5 um diameter. The prokaryotic transcription/translation machinery has evolved to operate in compartments of ˜1-2 um diameter, where single genes are at approximately nanomolar concentrations. A single gene, in a compartment of 2.6 um diameter is at a concentration of 0.2 nM. This gene concentration is high enough for efficient translation. Compartmentalisation in such a volume also ensures that even if only a single molecule of the gene product is formed it is present at about 0.2 nM, which is important if the gene product is to have a modifying activity of the nucleic acid itself. The volume of the microcapsule should thus be selected bearing in mind not only the requirements for transcription and translation of the nucleic acid/nucleic acid, but also the modifying activity required of the gene product in the method of the invention.
The size of emulsion microcapsules may be varied simply by tailoring the emulsion conditions used to form the emulsion according to requirements of the selection system. The larger the microcapsule size, the larger is the volume that will be required to encapsulate a given nucleic acid/nucleic acid library, since the ultimately limiting factor will be the size of the microcapsule and thus the number of microcapsules possible per unit volume.
The size of the microcapsules is selected not only having regard to the requirements of the transcription/translation system, but also those of the selection system employed for the nucleic acid/nucleic acid construct. Thus, the components of the selection system, such as a chemical modification system, may require reaction volumes and/or reagent concentrations which are not optimal for transcription/translation. As set forth herein, such requirements may be accommodated by a secondary re-encapsulation step; moreover, they may be accommodated by selecting the microcapsule size in order to maximise transcription/translation and selection as a whole. Empirical determination of optimal microcapsule volume and reagent concentration, for example as set forth herein, is preferred.
A “nucleic acid” in accordance with the present invention is, typically, a molecule or construct selected from the group consisting of a DNA molecule, an RNA molecule, a partially or wholly artificial nucleic acid molecule consisting of exclusively synthetic or a mixture of naturally-occurring and synthetic bases, any one of the foregoing linked to a polypeptide.
The nucleic acid portion of the nucleic acid may comprise suitable regulatory sequences, such as those required for efficient expression of the gene product, for example promoters, enhancers, translational initiation sequences, polyadenylation sequences, splice sites and the like.
(iv) Product Selection.Details of a preferred method of performing the method of the invention are provided in the Examples. However, those skilled in the art will appreciate that the examples given are non-limiting and methods for product selection are discussed in more general terms below.
A ligand or substrate can be connected to the nucleic acid by a variety of means that will be apparent to those skilled in the art (see, for example, Hermanson, 1996). According to the method of the present invention, the ligand or substrate is a ‘detection agent label’—such as a dye-labelled nucleotide analogue, in particular Cy3CTP and/or Cy5CTP.
Sorting can be by any method which allows the preferential separation, amplification or survival of the detection agent labelled nucleic acid. Examples include selection by binding (including techniques based on magnetic separation, for example using Dynabeads™), and by resistance to degradation (for example by nucleases, including restriction endonucleases).
When all reactions are stopped and the microcapsules are combined, the nucleic acids encoding the active engineered polymerases selected can be enriched using an antibody or other molecule which binds, or reacts specifically with the “detection agent label”. Although both substrates and product have the detection agent label, only the nucleic acids encoding active gene product will co-purify.
The terms “isolating”, “sorting” and “selecting”, as well as variations thereof, are used herein. Isolation, according to the present invention, refers to the process of separating an entity from a heterogeneous population, for example a mixture, such that it is substantially, suitably totally, free of at least one substance with which it was associated before the isolation process. In one embodiment, isolation refers to purification of an entity essentially to homogeneity. Sorting of an entity refers to the process of preferentially isolating desired entities over undesired entities. In as far as this relates to isolation of the desired entities, the terms “isolating” and “sorting” are equivalent. The method of the present invention permits the sorting of desired nucleic acids from pools (libraries or repertoires) of nucleic acids which contain the desired nucleic acid. Selecting is used to refer to the process (including the sorting process) of isolating an entity according to a particular property thereof.
Initial selection of a nucleic acid from a nucleic acid library (for example a mutant Pfu library) using the present invention will in most cases require the screening of a large number of variant nucleic acids. Libraries of nucleic acids can be created in a variety of different ways, including the following.
Pools of naturally occurring nucleic acids can be cloned from genomic DNA or cDNA (Sambrook et al., 1989); for example, mutant Pfu libraries or other DNA polymerase libraries, made by PCR amplification repertoires of Pfu or other DNA polymerase genes have proved very effective sources of DNA polymerase fragments. Further details are given in the examples.
Libraries of genes can also be made by encoding all (see for example Smith, 1985; Parmley and Smith, 1988) or part of genes (see for example Lowman et al., 1991) or pools of genes (see for example Nissim et al., 1994) by a randomised or doped synthetic oligonucleotide. Libraries can also be made by introducing mutations into a nucleic acid or pool of nucleic acids ‘randomly’ by a variety of techniques in vivo, including; using ‘mutator strains’, of bacteria such as E. coli mutD5 (Liao et al., 1986; Yamagishi et al., 1990; Low et al., 1996). Random mutations can also be introduced both in vivo and in vitro by chemical mutagens, and ionising or LTV irradiation (see Friedberg et al., 1995), or incorporation of mutagenic base analogues (Freese, 1959; Zaccolo et al., 1996). ‘Random’ mutations can also be introduced into genes in vitro during polymerisation for example by using error-prone polymerases (Leung et al., 1989). In a preferred embodiment of the method of the invention, the repertoire of nucleic fragments used is a mutant Taq repertoire which has been mutated using error prone PCR. Details are given in Examples 1. According to the method of the invention, the term ‘random’ may be in terms of random positions with random repertoire of amino acids at those positions or it may be selected (predetermined) positions with random repertoire of amino acids at those selected positions.
Further diversification can be introduced by using homologous recombination either in vivo (see Kowalczykowski et al., 1994 or in vitro (Stemmer, 1994a; Stemmer, 1994b)).
(V) Microcapsules/SortingIn addition to the nucleic acids described above, the microcapsules according to the invention will comprise further components required for the sorting process to take place. Other components of the system will for example comprise those necessary for transcription and/or translation of the nucleic acid. These are selected for the requirements of a specific system from the following; a suitable buffer, an in vitro transcription/replication system and/or an in vitro translation system containing all the necessary ingredients, enzymes and cofactors, RNA polymerase, nucleotides, nucleic acids (natural or synthetic), transfer RNAs, ribosomes and amino acids, and the substrates of the reaction of interest in order to allow selection of the modified gene product.
A suitable buffer will be one in which all of the desired components of the biological system are active and will therefore depend upon the requirements of each specific reaction system. Buffers suitable for biological and/or chemical reactions are known in the art and recipes provided in various laboratory texts, such as Sambrook et al., 1989.
The in vitro translation system will usually comprise a cell extract, typically from bacteria (Zubay, 1973; Zubay, 1980; Lesley et al., 1991; Lesley, 1995), rabbit reticulocytes (Pelham and Jackson, 1976), or wheat germ (Anderson et al., 1983). Many suitable systems are commercially available (for example from Promega) including some which will allow coupled transcription/translation (all the bacterial systems and the reticulocyte and wheat germ TNT™ extract systems from Promega). The mixture of amino acids used may include synthetic amino acids if desired, to increase the possible number or variety of proteins produced in the library. This can be accomplished by charging tRNAs with artificial amino acids and using these tRNAs for the in vitro translation of the proteins to be selected (Ellman et al., 1991; Benner, 1994; Mendel et al., 1995).
After each round of selection the enrichment of the pool of nucleic acids for those encoding the molecules of interest can be assayed by non-compartmentalised in vitro transcription/replication or coupled transcription-translation reactions. The selected pool is cloned into a suitable plasmid vector and RNA or recombinant protein is produced from the individual clones for further purification and assay.
(Vi) Microcapsule IdentificationMicrocapsules may be identified by virtue of a change induced by the desired gene product which either occurs or manifests itself at the surface of the microcapsule or is detectable from the outside as described in section iii (Microcapsule Sorting). This change, when identified, is used to trigger the modification of the gene within the compartment. In a preferred aspect of the invention, microcapsule identification relies on a change in the optical properties of the microcapsule resulting from a reaction leading to luminescence, phosphorescence or fluorescence within the microcapsule. Modification of the gene within the microcapsules would be triggered by identification of luminescence, phosphorescence or fluorescence. For example, identification of luminescence, phosphorescence or fluorescence can trigger bombardment of the compartment with photons (or other particles or waves) which leads to modification of the nucleic acid. A similar procedure has been described previously for the rapid sorting of cells (Keij et al., 1994). Modification of the nucleic acid may result, for example, from coupling a molecular “fluorescent detection agent label”, caged by a photolabile protecting group to the nucleic acids: bombardment with photons of an appropriate wavelength leads to the removal of the cage. Afterwards, all microcapsules are combined and the nucleic acids pooled together in one environment. Nucleic acids encoding gene products exhibiting the desired activity can be selected by affinity purification using a molecule that specifically binds to, or reacts specifically with, the “fluorescent label”.
(Vii) Multi Step ProcedureIt will also be appreciated that according to the present invention, it is not necessary for all the processes of transcription/replication and/or translation, and selection to proceed in one single step, with all reactions taking place in one microcapsule. The selection procedure may comprise two or more steps. First, transcription/replication and/or translation of each nucleic acid of a nucleic acid library may take place in a first microcapsule. Each gene product is then linked to the nucleic acid which encoded it (which resides in the same microcapsule). The microcapsules are then broken, and the nucleic acids attached to their respective gene products optionally purified. Alternatively, nucleic acids can be attached to their respective gene products using methods which do not rely on encapsulation. For example, phage display (Smith, G. P., 1985), polysome display (Mattheakkis et al., 1994), RNA-peptide fusion (Roberts and Szostak, 1997) or lac repressor peptide fusion (Cull, et al., 1992).
In the second step of the procedure, each purified nucleic acid attached to its gene product is put into a second microcapsule containing components of the reaction to be selected. This reaction is then initiated. After completion of the reactions, the microcapsules are again broken and the modified nucleic acids are selected. In the case of complicated multistep reactions in which many individual components and reaction steps are involved, one or more intervening steps may be performed between the initial step of creation and linking of gene product to nucleic acid, and the final step of generating the selectable change in the nucleic acid.
(Viii) AmplificationIn all the above configurations, genetic material comprised in the nucleic acids may be amplified and the process repeated in iterative steps. Amplification may be by the polymerase chain reaction (Saiki et al., 1988) or by using one of a variety of other gene amplification techniques including; Qβ replicase amplification (Cahill, Foster and Mahan, 1991; Chetverin and Spirin, 1995; Katanaev, Kurnasov and Spirin, 1995); the ligase chain reaction (LCR) (Landegren et al., 1988; Barmy, 1991); the self-sustained sequence replication system (Fahy, Kwoh and Gingeras, 1991) and strand displacement amplification (Walker et al., 1992).
(B) Modifications to the Standard CSR ProcedureModifications to the previously described CSR protocol are required to enable selection for Pfu variants and especially variants of Pfu able to incorporate dye labelled nucleotide analogues. CSR conditions described previously (Ghadessy et al., 2001), when performed in 1×Pfu buffer (10 mM KCl, 10 mM (NH4)2SO4, 20 mM Tris-Cl pH 8.75, 2 mM MgSO4, 0.1% Triton X-100, 100 μg/mL BSA; Stratagene Ltd), did not enrich for an active Pfu variant (pASKpfuexo−2; see example 1) over an inactive variant (pASKpfuexo−7; see example 1) when present at a 1:100 ratio. The aqueous phase of the emulsion had to be modified to include primers (1 μM), dNTPs (0.1 mM each), RNase (10 μg/mL), glycerol (10% v/v), formamide (1% v/v), DTT (1 mM) in 1×Pfu buffer.
Cells expressing the active Pfu variant (pASKpfuexo−2) were mixed at a ratio of 1:100 with the inactive variant pASKpfuexo−7 and subjected to CSR either in solution or in emulsion under the modified conditions described. In solution no significant enrichment of the active variant was observed whereas in emulsion the active variant was enriched for (
In a first aspect the present invention provides an engineered polymerase with an expanded substrate range characterised in that the polymerase is capable of incorporating an enhanced occurrence of detection agent-labelled nucleotide analogue/s into nucleic acid synthesised by that engineered polymerase as compared with the wild type polymerase from which it is derived.
As defined herein the term ‘expanded substrate range’ (of an engineered DNA polymerase) refers to the ability of an engineered polymerase according to the present invention to incorporate an enhanced occurrence of detection agent-labelled nucleotide analogue into nucleic acid synthesised by that engineered polymerase as compared with the wild type polymerase from which it is derived.
In a further embodiment an engineered polymerase according to the invention can incorporate 10% detection-agent labelled nucleotides expressed as a percentage of total nucleotides in the newly synthesised nucleic acid—such as DNA. In a preferred embodiment still, an engineered polymerase according to the invention can incorporate 20, 30, 40, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95% detection labelled nucleotides expressed as a percentage of total nucleotides in the newly synthesised nucleic acid—such as DNA. In another embodiment, the engineered polymerase incorporates 100% detection labelled nucleotides expressed as a percentage of total nucleotides in the newly synthesised nucleic acid—such as DNA.
In another embodiment the detection agent labelled nucleotide analogues are dye labelled analogues. Dye labels may be attached to any of the following nucleotides dATP, dCTP, dGTP and dTTP. Advantageously, the dye label is a fluorescent label.
More advantageously the fluorescent label is Carbocyanaine (Cy). Details of this fluorescently labelled nucleotide are provided in U.S. Pat. No. 6,974,873 which is herein incorporated by reference. Most advantageously, the fluorescent label is Cy1 (Cy3) or Cy5 (Cy5) and the nucleotide is dCTP. Other suitable fluorescently labelled nucleotides will be familiar to those skilled in the art and include those labelled with Alexa Fluor™ dyes (Invitrogen, UK), diethylaminocoumarin, tetramethylrhodamine, N-methylanthraniloyl (MANT), trinitrophenyl, etheno derivatives, biotin or fluorescein.
In one aspect, there is provided a nucleotide sequence wherein at least 50%, 60%, 70%, 80%, 90%, 95%, 99% or 100% of the Cytidine residues are labelled with Cy5 and/or Cy3.
Suitably, the nucleotide sequence is at least about 1 kb in length, at least about 2 kb in length or at least about 3 kb in length—such as from 1 kb to 3 kb in length.
Accordingly, there is also described a nucleotide sequence wherein at least 50% of the Cytidine residues are labelled with Cy5 and/or Cy3 and wherein the nucleotide sequence is at least about 1 kb in length, at least about 2 kb in length or at least about 3 kb in length—such as from 1 kb to 3 kb in length.
There is also described a nucleotide sequence wherein at least 60% of the Cytidine residues are labelled with Cy5 and/or Cy3 and wherein the nucleotide sequence is at least about 1 kb in length, at least about 2 kb in length or at least about 3 kb in length—such as from 1 kb to 3 kb in length.
There is also described a nucleotide sequence wherein at least 70% of the Cytidine residues are labelled with Cy5 and/or Cy3 and wherein the nucleotide sequence is at least about 1 kb in length, at least about 2 kb in length or at least about 3 kb in length—such as from 1 kb to 3 kb in length.
There is also described a nucleotide sequence wherein at least 80% of the Cytidine residues are labelled with Cy5 and/or Cy3 and wherein the nucleotide sequence is at least about 1 kb in length, at least about 2 kb in length or at least about 3 kb in length—such as from 1 kb to 3 kb in length.
There is also described a nucleotide sequence wherein at least 90% of the Cytidine residues are labelled with Cy5 and/or Cy3 and wherein the nucleotide sequence is at least about 1 kb in length, at least about 2 kb in length or at least about 3 kb in length—such as from 1 kb to 3 kb in length.
There is also described a nucleotide sequence wherein at least 95% of the Cytidine residues are labelled with Cy5 and/or Cy3 and wherein the nucleotide sequence is at least about 1 kb in length, at least about 2 kb in length or at least about 3 kb in length—such as from 1 kb to 3 kb in length.
There is also described a nucleotide sequence wherein at least 99% of the Cytidine residues are labelled with Cy5 and/or Cy3 and wherein the nucleotide sequence is at least about 1 kb in length, at least about 2 kb in length or at least about 3 kb in length—such as from 1 kb to 3 kb in length.
There is also described a nucleotide sequence wherein 100% of the Cytidine residues are labelled with Cy5 and/or Cy3 and wherein the nucleotide sequence is at least about 1 kb in length, at least about 2 kb in length or at least about 3 kb in length—such as from 1 kb to 3 kb in length.
(Ci) Pfu Mutants Selected from the A Motif Library with Cy5-dCTP.
Cy5-dCTP selections (see example 5) from the Pfu repertoire library diversified at the A-motif (see example 4) results in the selection of 4 Pfu variants 23, AH12, 55 and 15 which exhibit a significantly enhanced ability to incorporate Cy5 labelled dCTP over the wild-type enzyme as determined in an ELISA extension assay (see example 7) with primer 33, 34 and 35 (
(Cii) Pfu Variants Selected from the A and B Motif Library with Cy5-dCTP
Pfu variants able to significantly incorporate Cy5-dCTP compared to the ability of the wild-type enzyme are selected from the A-B motif Pfu repertoire library (see example 4) with primers that anneal 5′ to the A-motif and 3′ of the B-motif (see example 5). Clones exhibiting a significantly enhanced ability to incorporate Cy5-dCTP are ranked by ELISA extension assay with primer 35 (see example 7;
(Ciii) Pfu Variants Selected from the A and C Motif Library with Cy5-dCTP
Pfu variants able to significantly incorporate Cy5-dCTP compared to the ability of the wild-type enzyme are selected from the A-C motif Pfu repertoire library (see example 4) with primers that anneal 5′ to the A-motif and 3′ of the C-motif (see example 5). Clones exhibiting a significantly enhanced ability to incorporate Cy5-dCTP are ranked by ELISA extension assay with primer 35 (see example 7;
(Civ) Pfu Variant Able to Incorporate Cy5dCTP or Cy3dCTP.
One clone, E10, exhibited a significantly enhanced ability to incorporate either Cy3-dCTP or Cy5-dCTP (
The present inventors have shown that the use of short-patch compartmentalised self-replication can be used to select for a number of Pfu variant polymerases (engineered DNA polymerases) with a relaxed substrate specificity with respect to the incorporation of dye labelled nucleotide analogue/s.
The unusual properties of the DNA polymerases according to the present invention, in particular E10 as described herein may have immediate uses for example for the improved incorporation of dye-modified nucleotides in sequencing and array labelling and/or the amplification of ultra-long DNA targets. They may prove useful in the amplification of damaged DNA templates in forensics or paelobiology, may permit an expansion of the chemical repertoire of aptamers or deoxi-ribozymes (Benner, Barbas, ribozyme review).
In addition, DNA polymerases according to the invention, in particular E10 polymerase as herein described may serve as a useful framework for mutagenesis and evolution towards polymerases capable of utilizing an ever wider array of modified nucleotide substrates.
(E) Atomic Force MicroscopyAdvantageously, the structure of cyDNA can be determined using Atomic Force Microscopy (AFM). However, it has proved difficult to prepare CyDNA samples for AFM analysis. It was, however, possible to analyse samples on mica stubs that had been functionalised with Poly-L-Lysine. Accordingly, AFM can be used to analyse (eg. to determine the structural characteristics of) cyDNA described herein.
Using AFM it was confirmed that CyDNA exists as double stranded B-form DNA.
The lengths and heights of 40 unmodified DNA molecules, 40 Cy3-dCTP modified molecules (where 100% of the dCTP was replaced by Cy3) and 71 Cy5-dCTP molecules (where 100% of the dCTP was replaced by Cy5) were determined. There is a small difference in molecule length between the unmodified and Cy3-dCTP modified DNA molecules (104.0 nm±0.3 vs 97.5 nm±0.25). The height (width) of the Cy3-dCTP and Cy5-dCTP labelled DNA molecules is substantially greater than that of the unmodified DNA molecules (see
Both the Cy5-dCTP modified and unmodified DNA fragments have similar persistence length.
(F) FISH & Fibre FISHHigh label density will not necessarily result in an increase in fluorescence due to re-absorption and re-emission effects and quenching. Model microarray experiments identified that PCR fragments labelled in PCRs where of the 50% of the dCTP was replaced by either Cy3- or Cy5-dCTP resulted in higher fluorescent signal than fragments labelled with 100% replacement. Consequently probes used in FISH and Fibre FISH were generated in PCRs where 50% of the unlabelled nucleotide was replaced by its fluorescent counterpart. Pfu exo- is able to incorporate FITC-12-dATP when present at 100% in PCRs. E10 retains this ability. 3 colour FISH with probes labelled with Cy3, Cy5 or FITC was performed on metaphase spreads. 2 colour Fibre FISH was performed with probes labelled with Cy5 or FITC.
(G) Sequence Identity or Sequence HomologyThe present invention encompasses the use of sequences having a degree of sequence identity or sequence homology with amino acid sequence(s) of a polypeptide having the specific properties defined herein or of any nucleotide sequence encoding such a polypeptide (hereinafter referred to as a “homologous sequence(s)”). Here, the term “homologue” means an entity having a certain homology with the subject amino acid sequences and the subject nucleotide sequences. Here, the term “homology” can be equated with “identity”.
The homologous amino acid sequence and/or nucleotide sequence should provide and/or encode a polypeptide which retains the functional activity and/or enhances the activity of the polymerase.
In the present context, a homologous sequence is taken to include an amino acid sequence which may be at least 70, 75, 80, 85, 90, 95 or 99% identical, preferably at least 95 or 98% identical to the subject (eg. wild type) sequence. Typically, the homologues will comprise the same active sites etc. as the subject amino acid sequence. Although homology can also be considered in terms of similarity (i.e. amino acid residues having similar chemical properties/functions), in the context of the present invention it is preferred to express homology in terms of sequence identity.
In the present context, a homologous sequence is taken to include a nucleotide sequence which may be at least 70, 75, 80, 85, 90% identical, preferably at least 95 or 99% identical to a nucleotide sequence encoding a polypeptide of the present invention (the subject sequence). Typically, the homologues will comprise the same sequences that code for the active sites etc. as the subject sequence. Although homology can also be considered in terms of similarity (i.e. amino acid residues having similar chemical properties/functions), in the context of the present invention it is preferred to express homology in terms of sequence identity.
Homology comparisons can be conducted by eye, or more usually, with the aid of readily available sequence comparison programs. These commercially available computer programs can calculate % homology between two or more sequences.
% homology may be calculated over contiguous sequences, i.e. one sequence is aligned with the other sequence and each amino acid in one sequence is directly compared with the corresponding amino acid in the other sequence, one residue at a time. This is called an “ungapped” alignment. Typically, such ungapped alignments are performed only over a relatively short number of residues.
Although this is a very simple and consistent method, it fails to take into consideration that, for example, in an otherwise identical pair of sequences, one insertion or deletion will cause the following amino acid residues to be put out of alignment, thus potentially resulting in a large reduction in % homology when a global alignment is performed. Consequently, most sequence comparison methods are designed to produce optimal alignments that take into consideration possible insertions and deletions without penalising unduly the overall homology score. This is achieved by inserting “gaps” in the sequence alignment to try to maximise local homology.
However, these more complex methods assign “gap penalties” to each gap that occurs in the alignment so that, for the same number of identical amino acids, a sequence alignment with as few gaps as possible—reflecting higher relatedness between the two compared sequences—will achieve a higher score than one with many gaps. “Affine gap costs” are typically used that charge a relatively high cost for the existence of a gap and a smaller penalty for each subsequent residue in the gap. This is the most commonly used gap scoring system. High gap penalties will of course produce optimised alignments with fewer gaps. Most alignment programs allow the gap penalties to be modified. However, it is preferred to use the default values when using such software for sequence comparisons.
Calculation of maximum % homology therefore firstly requires the production of an optimal alignment, taking into consideration gap penalties. A suitable computer program for carrying out such an alignment is the Vector NTI (Invitrogen Corp.). Examples of software that can perform sequence comparisons include, but are not limited to, the BLAST package (see Ausubel et al 1999 Short Protocols in Molecular Biology, 4th Ed—Chapter 18), BLAST 2 (see FEMS Microbiol Lett 1999 174(2): 247-50; FEMS Microbiol Lett 1999 177(1): 187-8 and tatiana@ncbi.nlm.nih.gov), FASTA (Altschul et al 1990 J. Mol. Biol. 403-410) and AlignX for example. At least BLAST, BLAST 2 and FASTA are available for offline and online searching (see Ausubel et al 1999, pages 7-58 to 7-60).
Although the final % homology can be measured in terms of identity, the alignment process itself is typically not based on an all-or-nothing pair comparison. Instead, a scaled similarity score matrix is generally used that assigns scores to each pairwise comparison based on chemical similarity or evolutionary distance. An example of such a matrix commonly used is the BLOSUM62 matrix—the default matrix for the BLAST suite of programs. Vector NTI programs generally use either the public default values or a custom symbol comparison table if supplied (see user manual for further details). For some applications, it is preferred to use the default values for the Vector NTI package.
Alternatively, percentage homologies may be calculated using the multiple alignment feature in Vector NTI (Invitrogen Corp.), based on an algorithm, analogous to CLUSTAL (Higgins D G & Sharp P M (1988), Gene 73(1), 237-244).
Once the software has produced an optimal alignment, it is possible to calculate % homology, preferably % sequence identity. The software typically does this as part of the sequence comparison and generates a numerical result.
Should Gap Penalties be used when determining sequence identity, then preferably the following parameters are used for pairwise alignment:
In one embodiment, CLUSTAL may be used with the gap penalty and gap extension set as defined above.
Suitably, the degree of identity with regard to a nucleotide sequence is determined over at least 20 contiguous nucleotides, preferably over at least 30 contiguous nucleotides, preferably over at least 40 contiguous nucleotides, preferably over at least 50 contiguous nucleotides, preferably over at least 60 contiguous nucleotides, preferably over at least 100 contiguous nucleotides.
Suitably, the degree of identity with regard to a nucleotide sequence may be determined over the whole sequence.
Suitably, the amino acid sequence that is at least 70% identical to the engineered polymerases described herein (eg. that designated as E10) retains one or more of the mutations that confer the expanded substrate range described herein (eg. one or more of the following mutations: V337I, E399D, N400D, R407I and/or Y546H).
(H) KitsThe materials for use in the methods of the present invention are ideally suited for preparation of kits.
Such a kit may comprise containers, each with one or more of the various reagents (typically in concentrated form) utilised in the methods—such as for buffers, the appropriate nucleotide triphosphates (e.g., dATP, dCTP, dGTP and dTTP; or rATP, rCTP, rGTP and UTP), DNA polymerase, and one or more oligonucleotides.
A set of instructions will also typically be included.
(I) General Recombinant DNA TechniquesThe present invention employs, unless otherwise indicated, conventional techniques of chemistry, molecular biology, microbiology, recombinant DNA and immunology, which are within the capabilities of a person of ordinary skill in the art. Such techniques are explained in the literature. See, for example, J. Sambrook, E. F. Fritsch, and T. Maniatis, 1989, Molecular Cloning: A Laboratory Manual, Second Edition, Books 1-3, Cold Spring Harbor Laboratory Press; Ausubel, F. M. et al. (1995 and periodic supplements; Current Protocols in Molecular Biology, ch. 9, 13, and 16, John Wiley & Sons, New York, N.Y.); B. Roe, J. Crabtree, and A. Kahn, 1996, DNA Isolation and Sequencing: Essential Techniques, John Wiley & Sons; M. J. Gait (Editor), 1984, Oligonucleotide Synthesis: A Practical Approach, Irl Press; and, D. M. J. Lilley and J. E. Dahlberg, 1992, Methods of Enzymology: DNA Structure Part A: Synthesis and Physical Analysis of DNA Methods in Enzymology, Academic Press. Each of these general texts is herein incorporated by reference.
EXAMPLESThe invention will now be described by the following examples which are in no way limiting of the invention claimed herein.
Example 1 Cloning of Pfu Polymerase Open Reading Frame into pASKThe Pfu polymerase open reading frame was amplified by PCR from pETpfu (Lu and Erickson, 1997) using primers 1 and 2. The amplified fragment was restriction digested with NdeI and SalI and ligated into pASKDpo4 (Skerra, 1994) from which the NdeI site at 2421 bp had been eliminated from the pASK vector backbone by site-directed-mutagenesis using primers 3 and 4 thereby creating the expression vector pASKpfu.
The 3″-5″ exonuclease activity of the Pfu enzyme was disabled by site-directed-mutagenesis of exonuclease domain I (Derbyshire et al., 1995) using primers 5 and 6 and pASKpfu as a template thereby generating pASKpfuexo−.
The XbaI site present in the Pfu sequence at 1683 bp was removed by site-directed-mutagenesis using primers 7 and 8 and pASKpfuexo− as a template thereby generating pASKpfuexo−1.
To facilitate library construction by iPCR the BsaI site present in the Pfu sequence at 1636 bp was eliminated by site-directed-mutagenesis using primers 9 and 10 and pASKpfuexo−1 as a template thereby generating pASKpfuexo−2.
In order to subclone the sequences selected by CSR the BamHI site present in the Pfu sequence at 606 bp was removed by site-directed-mutagenesis using primers 11 and 12 and pASKpfuexo−2 as a template thereby generating pASKpfuexo−3. A silent mutation was introduced by site-directed-mutagenesis into the Pfu gene to include a unique BamHI (1416 bp) restriction enzyme site using primers 13 and 14 and pASKpfuexo−3 as a template thereby generating pASKpfuexo−4.
The uracil stalling function (Fogg et al., 2002) of the Pfu enzyme was removed by site-directed-mutagenesis using primers 15 and 16 and pASKpfuexo−4 as a template thereby generating pASKpfuexo−5.
To facilitate cloning of A-C motif library selections a silent mutation was introduced by site-directed-mutagenesis into the Pfu gene to include a unique XhoI (1726 bp) restriction enzyme site using primers 17 and 18 respectively and pASKpfuexo−5 as a template thereby generating pASKpfuexo−6.
An inactive Pfu variant was constructed to allow CSR conditions to be evaluated. A XhoI site which introduces a +1 frameshift in the A motif at 1218 bp was generated by site-directed-mutagenesis using primers 19 and 20 and pASKpfuexo−4 as a template thereby generating pASKpfuexo−7.
Example 2 Pfu Protein ExpressionPlasmid constructs or libraries were transformed into E. coli Ace6 or TG1TR and expressed as described (Skerra, 1994). Briefly, transformed Ace6 cells are grown overnight at 37° C. in 2×TY, 0.1 mg/mL ampicillin. For expression overnight cultures were diluted 1:50 in 2×TY 0.1 mg/mL ampicillin, grown to an OD595 of 0.6 at 37° C. and induced for protein expression by the addition of anhydrotetracycline. Protein expression was induced for 6 hours at 37° C.
Cells were harvested by centrifugation, resuspended in 20 ml (per litre of culture) of buffer A (50 mM Tris pH8.0, 1% glucose, 1 mM EDTA), Buffer B (10 mM Tris pH8.0, 50 mM KCl, 1 mM EDTA, 0.5% NP40) was added to a final volume of 50 ml and cells were lysed for 30 min at 75° C. Debris was pelleted by centrifugation and the NaCl was added to the supernatant to 0.25M final concentration. Then neutralized polyethyleneamine (PEI) was added to a final concentration of 0.1% v/v and precipitate pelleted by centrifugation. The cleared supernatant was diluted 5× with 20 mM Tris pH7.5 and loaded onto a 6/10 Hi-Prep Heparin FF Column (Pharmacia) equilibrated with Column running buffer (CRB) (20 mM Tris pH7.5, 50 mM NaCl, 10% glycerol). The column was washed with 150 ml of CRB and bound polymerase eluted with a NaCl gradient from 0 to 1M. Pfu eluted between 0.2M-0.3M NaCl, 10% glycerol. Eluted polymerase samples were concentrated using Ultra-15 centrifugal filter devices (Amicon), filter dialyzed into 2×Pfu storage buffer (100 mM Tris pH8.0, 2 mM DTT, 0.1% CHAPS) and glycerol was added to 50% final v/v. Samples were stored at −20° C.
Example 3 Pfu Requires Modified CSR ConditionsModifications to the previously described CSR protocol were required to enable selection for Pfu variants and especially variants of Pfu able to incorporate labelled nucleotide analogues. CSR conditions described previously (Ghadessy et al., 2001), when performed in 1×Pfu buffer (10 mM KCl, 10 mM (NH4)2SO4, 20 mM Tris-Cl pH 8.75, 2 mM MgSO4, 0.1% Triton X-100, 100 μg/mL BSA; Stratagene Ltd), did not enrich for an active Pfu variant (pASKpfuexo-2; see example 1) over an inactive variant (pASKpfuexo-7; see example 1) when present at a 1:100 ratio. The aqueous phase of the emulsion had to be modified to include primers (1 μM), dNTPs (0.1 mM each), RNase (10 μg/mL), glycerol (10% v/v), formamide (1% v/v), DTT (1 mM) in 1×Pfu buffer. Cells expressing the active Pfu variant were mixed at a ratio of 1:100 with cells expressing the inactive Pfu variant and subjected to PCR with primers 40 and 41 either in solution or in emulsion (CSR) under the modified conditions described. PCR was performed in emulsion (CSR) or solution with the following cycling parameters: 94° C. 5 min 25 times (94° C. 30 sec, 50° C. 1 min, 72° C. 15 min). Successful amplification results in a product of size 2.5 kb. Following PCR amplification in emulsion the emulsions were extracted with 2 volumes of diethylether. PCR amplifications performed in emulsion and solution were purified using a Qiagen PCR purification kit including an additional wash step with 750 μL 35% guanidium hydrochloride. Purified products were eluted in 50 μL elution buffer. To remove parental plasmid DNA and primers 7 μl of column eluate was digested with DpnI and ExoSAP-IT. 2 μL of DpnI and ExoSAP-IT treated sample was reamplified with a SuperTaq (HT Biotechnology)/PfuTurbo (Stratagene) blend (68 parts Taq: 8 parts PfuTurbo; 1 μL per reaction) in 1×Taq buffer (10 mM Tris-HCl pH 9.0, 1.5 mM MgCl2, 50 mM KCl, 0.1% Triton X-100, 0.01% stabiliser) with primers 42 and 43 (1 μM each), 1% (v/v) formamide and dNTPs (0.5 mM each). Cycling parameters were as follows: 94° C. 4 min 18 times (94° C. 30 sec, 50° C. 30 sec, 72° C. 3 min) 65° C. 10 min. The inactive Pfu variant encoded by pASKpfuexo−7 contains an A>C mutation at 1220 bp and a +1 frameshift at 1218 bp resulting from the insertion of a C thereby creating a unique XhoI restriction site at 1218 bp. Restriction digestion with XhoI allows PCR products derived from the inactive and active polymerase genes to be distinguished. In solution no significant enrichment of the active variant was observed whereas in emulsion the active variant was enriched for (
Library diversity was targeted to the Pfu polymerase active site which is comprised of 3 motifs termed the A, B and C motifs these clasp the incoming nucleotide and are involved in primer template binding. Libraries were constructed by iPCR. Briefly iPCR reactions were hot-started by the addition of 3.5 U of Expand High Fidelity polymerase (Roche) to a PCR mix [10 ng of plasmid template, primers (0.4 μM), dNTPs (0.2 mM), in 1× High Fidelity Expand buffer with MgCl2 (Roche)]. Primers including diversity were PAGE purified. Reactions were thermocycled [94° C. for 4 min; 19 times (94° C. for 20 sec, 65° C. for 20 sec—1° C./cycle, and 68° C. for 10 min); 15 times (94° C. for 20 sec, 50° C. for 20 see, 68° C. for 10 min); 15 times (94° C. for 20 sec, 50° C. for 20 sec, 68° C. for 10 min+15 sec/cycle)]. iPCR reactions were purified with a QIAquick PCR purification kit (Qiagen) and eluted in 50 μL of H2O. Purified DNA was restriction digested overnight with DpnI (New England Biolabs) to remove the input plasmid template. Restricted DNA samples were purified with a QTAquick PCR purification kit (Qiagen) and eluted in 50 μL of H2O. Purified DNA was restriction digested overnight with BsaI (New England Biolabs) after which the reactions were purified with a QIAquick PCR purification kit (Qiagen) and eluted in 50 μL of H2O. 150 μL of the restriction digested DNA was ligated overnight at 16° C. with 1200 U T4 DNA ligase (New England Biolabs) in 1×T4 DNA ligase buffer (New England Biolabs) in a final volume of 200 μL. Ligation reactions were electroporated into E. coli Ace6 or TG1TR cells which were plated onto 2×TY/Amp 0.1 mg/mL plates.
The Pfu A motif library was generated by iPCR using pASKpfuexo−2 as template and PCR primer 21 and degenerate primer 22 which introduces random mutations at a rate of 10% in amino acid residues 404 and 406-412 (L and FRALYPS) avoiding the conserved catalytic aspartate (D405). Residues 399-403 and residue 415 located adjacent to the A motif (ENIVY & I) were mutated to include the sequence diversity naturally present in B-family polymerase enzymes. The library was transformed into E. coli Ace6.
The Pfu A-B motif library was generated by iPCR using the four best clones selected with Cy5-dCTP from the Pfu A motif library (15, 23, 55 and AH12; see example 6) as template and degenerate PCR primer 23 which introduces random mutations in amino acid residues 488-500 (KLLANSFYGYYGY) at a rate of 10% and primer 24. Residue K501 located adjacent to the B motif was mutated to include the sequence diversity naturally present in B-family polymerase enzymes. The library was transformed into E. coli Ace6. Plasmid DNA was isolated from the Pfu A-B repertoire library in E. coli Ace6 and transformed into E. coli TG1TR. The library in E. coli TG1TR was used in selections.
The Pfu A-C motif library was generated by iPCR using the four best clones selected with Cy5-dCTP from the Pfu A motif library (15, 23, 55 and AH12; see example 6) as template and degenerate PCR primer 25 which introduces random mutations in amino acid residues 537-540, 542 and 544 (VLYI, T and G) at a rate of 10% avoiding the catalytic aspartate residues (D541 and D543) and PCR primer 26. Residues L545 and Y546 were mutated to include the sequence diversity naturally present in B-family polymerase enzymes. The library was transformed into E. coli Ace6. Plasmid DNA was isolated from the Pfu A-C repertoire library in E. coli Ace6 and transformed into E. coli TG1TR. The library in TG1TR was used in selections.
Example 5 Selection of Pfu Variants Able to Incorporate Cy5-dCTP from Pfu RepertoiresPfu variants were selected from the repertoire libraries (see example 4) for the ability to incorporate dye-labelled nucleotides, specifically Cy5-dCTP. spCSR selections were set up as described in example 3 however, the dCTP was completely replaced by 100 μM of Cy5-dCTP. Primers annealing 5′ and 3′ of the region diversified were employed in spCSR selections. spCSR was performed due to the difficulty in incorporating dye-labelled nucleotide analogues that initially precludes the amplification of the full length Pfu sequence.
spCSR Cy5-dCTP selections from the A motif library (94° C. for 5 min; 20 times at 94° C. for 30 sec, 50° C. for 1 min, and 72° C. for 18 min followed by a final elongation step at 65° C. for 10 min) were carried out using primers 27 and 28. The aqueous phase was extracted as described in example 3 and purified selection products were reamplified by PCR (94° C. 2 min; 31 times at 94° C. for 30 sec, 54° C. for 1 min, and 72° C. for 3 min followed by a final elongation step at 65° C. for 10 min) using primers 29 and 28 as described in example 3. Reamplified selection products were digested with EcoRI and BamHI and subcloned into pASKpfuexo−4 (see Example 1) before transformation into E. coli Ace6.
spCSR Cy5-dCTP selections from the A-B motif library (94° C. for 5 min; 20 times at 94° C. for 30 sec, 50° C. for 1 min, and 72° C. for 10 min) were carried out using primers 27 and 30 as described in example 3. The aqueous phase was extracted as described in Example 3 and purified selection products were reamplified by PCR (94° C. 2 min; 29 times at 94° C. for 30 sec, 54° C. for 1 min, and 72° C. for 1 min 30 sec) using primers 29 and 31. Reamplified selection products were digested with EcoRI and SacI and subcloned into pASKpfuexo−6 before transformation into E. coli Ace6.
spCSR Cy5-dCTP selections from the A-C motif library (94° C. for 5 min; 20 times at 94° C. for 30 sec, 50° C. for 1 min, and 72° C. for 10 min) were carried out using primers 27 and 32. The aqueous phase was extracted as described (Ghadessy et al., 2001) and purified selection products were reamplified by PCR (94° C. 2 min; 29 times at 94° C. for 30 sec, 54° C. for 1 min, and 72° C. for 1 min 30 sec) using primers 29 and 32. Reamplified selection products were digested with EcoRI and XhoI and subcloned into pASKpfuexo−6.
Example 6 Selection of Pfu Variants Able to Incorporate Cy5-dCTP and Biotin-16-dUTPPfu variants were selected from the Pfu A-B repertoire libraries (see example 4) for the ability to incorporate Cy5-dCTP and Biotin-16-dUTP. spCSR selections were set up as described in Example 3 however, the dCTP was completely replaced by 100 μM of Cy5-dCTP and the dTTP was completely replaced by 100 μM Biotin-16-dUTP. Primers annealing 5′ and 3′ of the region diversified were employed in spCSR selections. spCSR was performed due to the difficulty in incorporating modified nucleotide analogues that initially precludes the amplification of the full length Pfu sequence.
spCSR Cy5-dCTP and Biotin-16-dUTP selections from the A-B motif library (94° C. for 5 min; 20 times at 94° C. for 30 sec, 50° C. for 1 min, and 72° C. for 10 min) were carried out using primers 27 and 30 as described in example 3. The aqueous phase was extracted as described in example 3. Purified selection products were treated with Dpn1 and ExoSAP (as described in example 3) and reamplified using a 2 step pull through procedure. The initial PCR was hot-started by the addition of 2.5 U of Pfuexo− (Stratagene) to a PCR mix [1 μL purified Dpn1/ExoSAP treated purified selection, primers 31 and 29 (1 μM each), 1×Pfu buffer (10 mM KCl, 10 mM (NH4)2SO4, 20 mM Tris-Cl pH 8.75, 2 mM MgSO4, 0.1% Triton X-100, 100 μg/mL BSA; Stratagene Ltd), 2% (v/v) formamide and dNTPs (0.5 mM each)]. Reactions were thermocycled (94° C. for 2 min; 20 times at 94° C. for 30 sec, 50° C. for 30 sec, and 72° C. for 2 min 30 sec). 1 μL of the Pfuexo− amplified PCR was used as template in a second PCR with primers 31 and 47 and a SuperTaq (HT Biotechnology)/PfuTurbo (Stratagene) blend (as described in example 3; 94° C. for 2 min; 20 times at 94° C. for 30 sec, 50° C. for 30 sec, and 72° C. for 1 min 30 sec). Reamplified selection products were digested with EcoRI and SacI and subcloned into pASKpfuexo−6 before transformation into E. coli Ace6.
Example 7 Colony Screening by ELISA of Selected Pfu VariantsIn order to rank the Cy5-dCTP selected Pfu variants their ability to incorporate Cy-dye labelled dCTP was assessed using an ELISA extension assay. Briefly cells expressing Pfu or selected Pfu variants were washed twice in 1×Pfu buffer (10 mM KCl, 10 mM (NH4)2SO4, 20 mM Tris-Cl pH 8.75, 2 mM MgSO4, 0.1% Triton X-100, 100 μg/mL BSA; Stratagene Ltd) and concentrated 10 fold in 1×Pfu buffer. Cells were lysed by incubation at 85° C. for 10 min and cell debris pelleted by centrifugation at 2,000×g for 10 min.
Cleared lysates can be activity normalised in an ELISA extension reaction with primer 48 [1×Pfu buffer (10 mM KCl, 10 mM (NH4)2SO4, 20 mM Tris-Cl pH 8.75, 2 mM MgSO4, 0.1% Triton X-100, 100 μg/mL BSA; Stratagene Ltd), dATP (10 μM), dGTP (10 μM), dTTP (9 μM), digoxigenin-11-dUTP (1 μM; Perkin Elmer), dCTP (10 μM), primer (1 μM), 9 μL, H2O, 2 μL cell lysate], Primer 48 contains an internal biotin and requires that 20 consecutive nucleotides be incorporated prior to the insertion of digoxigenin-11-dUTP that is used for colorimetric detection.
Crude cleared lysates or activity normalised lysates were screened for the ability to incorporate either Cy5-dCTP or Cy3-dCTP in an ELISA extension reaction [1×Pfu buffer (10 mM KCl, 10 mM (NH4)2SO4, 20 mM Tris-Cl pH 8.75, 2 mM MgSO4, 0.1% Triton X-100, 100 μg/mL BSA; Stratagene Ltd), dATP (10 μM), dGTP (10 μM), dTTP (9 μM), digoxigenin-11-dUTP (1 μM; Perkin Elmer), Cy3-dCTP or Cy5-dCTP (10 μM), primer (1 μM), 9 μL H2O, 2 μL cell lysate]. The primers used in the extension reaction contain an internal biotin and require that 4 (Primer 33), 6 (Primer 34) or 8 (Primer 35) consecutive Cy3 or Cy5 labelled dCTP be incorporated prior to the insertion of digoxigenin-11-dUTP that is used for colorimetric detection.
The extension reaction was allowed to proceed at 94° C. for 5 min, 50° C. for 5 min, 72° C. for 5 min. Thereafter 5 μL of the extension reaction was bound for 30 min at room temperature via the biotin, internal to the primer, to a StreptaWell (high capacity) streptavidin coated 96-well microtiter plate (Roche) in 200 μL of 1×PBS-Tween-20 (0.2%). The plate was washed 5× in 1×PBS-Tween-20 (0.2%) and a 1:2,000 dilution (in 200 μL 1×PBS-Tween-20) of an anti-digoxigenin Fab fragment-peroxidase conjugate (Roche) was added to each well and bound for 30 min at room temperature. The plate was then washed 5 times with 1×PBS-Tween-20 and 100 μL of 1-Step Ultra TMB-ELISA substrate was added (Pierce). Colour development was stopped by the addition of 100 μL 1M H2SO4 and quantified by measuring absorbance at 450 nm-650 nm.
Example 8 Pfu Variants Selected from the A-Motif Library with Cy5-dCTPCy5-dCTP selections (see example 5) from the Pfu repertoire library diversified at the A-motif (see example 4) resulted in the selection of 4 Pfu variants 23, AH12, 55 and 15 which exhibited a significantly enhanced ability to incorporate Cy5-dCTP over the wild-type enzyme as determined in an ELISA extension assay with activity normalised lysates (see example 7) performed with primers 33, 34 and 35 (
Pfu variants able to significantly incorporate Cy5-dCTP compared to the ability of the wild-type enzyme were selected from the A-B motif Pfu repertoire library (see example 4) with primers that anneal 5′ to the A-motif and 3′ of the B-motif (see example 5). Clones exhibiting a significantly enhanced ability to incorporate Cy5-dCTP were ranked by ELISA extension assay with primer 35 (see example 7;
Pfu variants able to significantly incorporate Cy5-dCTP compared to the ability of the wild-type enzyme were selected from the A-C motif Pfu repertoire library (see example 4) with primers that anneal 5′ to the A-motif and 3′ of the B-motif (see example 5). Clones exhibiting a significantly enhanced ability to incorporate Cy5-dCTP were ranked by ELISA extension assay with primer 35 (see example 7;
Pfu variants able to significantly incorporate Cy3-dCTP compared to the ability of the wild-type enzyme were selected from the A-C motif Pfu repertoire library (see example 4) with primers that anneal 5′ to the A-motif and 3′ of the C-motif (see example 5). Clones exhibiting a significantly enhanced ability to incorporate Cy3-dCTP were ranked by ELISA extension assay with primer 35 (see example 7;
Pfu variants able to significantly incorporate Cy5-dCTP compared to the ability of the wild-type enzyme were selected with Cy5-dCTP and Biotin-16-dUTP from the A-B motif Pfu repertoire library (see example 4) with primers that anneal 5′ to the A-motif and 3′ of the B-motif (see example 5). Clones exhibiting a significantly enhanced ability to incorporate Cy5-dCTP were ranked by ELISA extension assay with primer 35 (see example 7;
Surprisingly some clones originally selected for an enhanced ability to incorporate Cy5-dCTP or Cy5-dCTP and Biotin-16-dUTP are also able to incorporate Cy3-dCTP as assessed with an ELISA extension assay with primers 33, 34 and 35 (see example 7;
Certain Pfu variants identified as being able to incorporate both Cy5-dCTP and Cy3-dCTP in ELISA extension assays (see examples 8, 10, 12 and 13) and wild-type enzyme were expressed and purified (see example 2). Enzymes were normalised for activity in PCR with primers 36 and 37 and 10 ng of pASKpfu as template. Activity normalised enzymes were added to PCR reactions containing: 10 ng of plasmid template, 1×Pfu buffer (10 mM KCl, 10 mM (NH4)2SO4, 20 mM Tris-Cl pH 8.75, 2 mM MgSO4, 0.1% Triton X-100, 100 μg/mL BSA; Stratagene Ltd), 50 μM dGTP, 50 μM dATP, 50 μM dTTP, 50 μM Cy3-dCTP or 50 μM Cy5-dCTP, 1 μM primers, 2% formamide, H2O to 50 μL. Cycling conditions were as follows: 94° C. 2 min, 50 times at 94° C. for 10 sec, 50° C. for 30 sec, and 72° C. for 20 min. Amplified fragments were analysed on 6% polyacrylamide gels.
The ability of the selected Pfu variants to amplify a 410 bp fragment of the Taq polymerase gene (70% GC) was assessed with primers 38 and 39. This requires that the polymerase enzyme inserts 287 Cy-dye labelled dCTP (
To assess whether the highly Cy-dye labelled DNA was able to hybridise and result in significant increases in fluorescent signal model microarray experiments were performed. Pfu polymerase DNA was PCR labelled with E10 using primers 36 and 37 in reactions where 10% or 100% of the dCTP in the reaction had been replaced by Cy3-dCTP (E10Cy310 and E10Cy3100 respectively) or with wild-type Pfuexo− in reactions where 10% of the dCTP had been replaced by Cy5-dCTP (PfuCy510). DNA was purified using the “freeze squeeze” method. Briefly Cy-dye labelled DNA was electrophoresed on a 0.8% 1×TBE agarose gel. The labelled fragment was excised from the gel. The excised gel slice was placed into a 0.6 mL Eppendorf tube that had been pierced with a 25 g needle close to the base of the tube. The cap of the tube was closed and the tube dropped into liquid nitrogen. The slice was incubated for 15 min after which the tube was placed into a 1.5 mL Eppendorf tube and immediately centrifuged at 12,000 rpm for 10-20 min at 4° C. Cy-dye labelled DNA was also purified by ethanol precipitation (2.5× sample volume ethanol 1/10th sample volume sodium acetate pH 5.2) followed by purification on Autoseq G50 columns (Amersham Biosciences). The extracted labelled DNA is collected in the base of the 1.5 mL Eppendorf tube. Hybridisation of equimolar quantities of purified PfuCy510 DNA with either E10Cy310 or E10Cy3100 DNA to microarray slides on which had been printed a dilution series of Pfu polymerase DNA resulted in either 4 or 7-fold increase in Cy3 signal respectively (
Dilutions series' (100, 50, 25, 12.5, and 6.25 ng/μl) of probe molecules (Pfu and Taq sequences, and sheared salmon testis genomic DNA) were prepared in 150 mM NaPO4 pH8.5/0.01% SDS and spotted in pentuplicate onto GAPSII aminosilane-coated glass slides (Corning) using a BioRobotics MicroGrid (Genomic Solutions Ltd). Printed slides were baked for 2 hr at 80° C., incubated with agitation for 30 minutes at 42° C. in 5×SSC/1% BSA (Sigma-Aldrich)/0.1% SDS, boiled for 2 min in ultrapure water, washed in 3 changes of ultrapure water at room temperature, rinsed in propan-2-ol and dried by centrifugation.
Array HybridisationLabelled-target nomenclature is as follows: Pfu 10% Cy3 or Pfu 10% Cy5 is target DNA that has been labelled using the Pfu enzyme where 10% of the dCTP present in the labelling reaction is Cy3-dCTP or Cy5-dCTP. Likewise, E10 100% Cy5 or E10 10% Cy5 or E10 100% Cy3 or E10 10% Cy3 is target DNA that has been labelled using the E10 enzyme where 100% or 10% of the dCTP present in the labelling reaction is Cy5-dCTP or Cy3-dCTP.
The following competitive hybridisations were performed in duplicate: Pfu 10% Cy3 vs Pfu 10% Cy5 was performed as a control hybridisation; Pfu 10% Cy3 vs E10 10% Cy5 and E10 10% Cy3 vs Pfu 10% Cy5 were performed to measure the improvement in labelling by E11) in the presence of 10% Cy-labelled dCTP; Pfu 10% Cy3 vs E10 100% Cy5 and E10 100% Cy3 vs Pfu 10% Cy5 were performed to measure the improvement in hybridisation signal following labelling by E10 in the exclusive presence of Cy-labelled dCTP.
5 ng of Cy3- and Cy5-labelled products were prepared in 20 μl of hybridisation buffer (1 mM Tris-HCl pH 7.4, 50 mM tetrasodium pyrophosphate, 1×Denhardts solution, 40% deionised formamide, 1% NP-40, 10 mM DTT, 100 μg/ml sheared salmon sperm DNA). Each sample was heated to 95° C. for 5 minutes, centrifuged for 2 min, applied to the surface of an array and covered with a 22×22 mm LifterSlip (Erie Scientific). Hybridisations were performed at 37° C. for 16 hr in a hybridisation chamber (Telechem). Arrays were washed once with 2×SSC/0.1% SDS at room temperature for 5 minutes, and twice with 1×SSC at room temperature for 5 min. Slides were dried by centrifugation and scanned with an ArrayWoRx ‘e’ (Applied Precision Instruments). To ensure comparability between slides, identical scanning parameters were used for all arrays. Images were analysed using GenePix Pro 4.1 (Axon Instruments) and array features affected by local artefacts were manually excluded from further analysis.
These hybridisation and wash conditions were optimised to provide the largest fold-improvements in signal from highly-labelled DNA without compromising the specificity of hybridisation. Smaller fold-improvements were observed in more stringent conditions, namely elevated hybridisation temperature (42° C.) and increased wash stringency (0.1×SSC at room temperature).
Calculation of Fold-Improvements in SignalLog2 (mean Cy3/mean Cy5) for each array feature was calculated. For each probe type (Pfu, Taq or salmon testis DNA) in each experiment (duplicate arrays of a competitive hybridisation) the mean log2 ratio for all included array features (max. 50 features i.e. 10 replicate features each of 5 probe dilutions) was calculated.
To normalise the data to the control experiment (Pfu 10% Cy5 vs Pfu 10% Cy3), where equivalent signal is expected in both channels, the Pfu mean log2 ratio from the control experiment was subtracted from that of every other comparison. If the resulting log2 ratios are positive or negative, they represent signal improvements in Cy3 and Cy5, respectively, compared to labelling with Pfu at 10% Cy-dye concentration. Fold-improvements for Cy3 and Cy5 are therefore, respectively, the inverse of the resulting mean log 2 ratio and the reciprocal of the inverse of the resulting mean log 2 ratio.
Example 16 Highly Cy-Dye Labelled DNA Partitions to the Organic Phase in the Presence of Salt on Phenol ExtractionPCR primers 49 and 50 were used to amplify a 100 bp fragment of the Taq DNA polymerase gene (2152-2251 bp). PCR reactions containing: 10 ng of plasmid template (pASK75 Taq), 1×Pfu buffer (10 mM KCl, 10 mM (NH4)2SO4, 20 mM Tris-Cl pH 8.75, 2 mM MgSO4, 0.1% Triton X-100, 100 μg/mL BSA; Stratagene Ltd), 50 μM dGTP, 50 μM dATP, 50 μM dTTP, 50 μM Cy3-dCTP or 50 μM Cy5-dCTP, 0.4 μM each primer, 1% (v/v) formamide, H2O to 25 μL. Reactions were hot-started with E10 polymerase. Cycling conditions were as follows: 94° C. 2 min, 50 times at 94° C. for 10 sec, and 72° C. for 20 min. Amplified fragments were purified by ethanol precipitation (2.5× sample volume ethanol 1/10th sample volume sodium acetate pH 5.2) followed by purification on Autoseq G50 columns (Amersham Biosciences). Following purification NaCl was added to equivalent volumes of Cy-dye labelled DNA to give a final concentration of 0 mM NaCl, 100 mM NaCl or 150 mM NaCl or 200 mM NaCl. Each sample was vortexed with 20 μL of Tris-HCl (pH 7.4) equilibrated phenol. Samples were subsequently centrifuged for 10 min at 13,000 g. Unlike native DNA highly labelled DNA partitions to the organic phenol phase in the presence of salt concentrations commonly found in some restriction enzyme buffers (eg. NEB3; New England Biolabs) following phenol extraction (see
The influence of fragment length on fluorescent signal obtained from highly Cy-dye labelled DNA was investigated using a model array on which had been printed a dilution series (200, 100, 50, 25, and 12.5 ng/ul) of the full-length Pfu polymerase gene 2300 bp), a dilution series of salmon testis DNA (200, 100, 50, 25, and 12.5 ng/μl) and spotting buffer (the latter 2 acting as negative controls). The increase in fluorescent signal from highly Cy-dye E10 labelled DNA of defined length (270 bp or 1.3 kb) was measured relative to Klenow labelled DNA samples where the unlabelled 270 bp or 1.3 kb DNA fragments were used as templates. Klenow labelling results in an average labelled DNA length of 50-100 bp irrespective of the length of the DNA template.
Klenow DNA Labelling:Fragments of the Pfu polymerase gene of lengths 1300 bp (corresponding to the polymerase domain) and 270 bp (a portion of the polymerase domain from 1047-1312 bp) were amplified with primers 51 and 52 and 36 and 37 respectively. DNA fragments were amplified with 5 U of SuperTaq (HT Biotechnology Ltd) in reactions containing 1×PCR buffer (10 mM TrisHCl pH 9.0, 1.5 mM MgCl2, 50 mM KCl, 0.1% Triton X-100, 0.01% stabiliser; HT Biotechnology Ltd) 1 μM primer 1 and 1 μM primer 2, 10 ng pASKpfuexo−5, 250 μM dNTPs and H20 to 50 μL. Reactions hotstarted and thermocycled as follows: 94° C. 2 min, 30 times at 94° C. for 30 sec, 55° C. for 30 sec and 72° C. for 3 min. The DNA products of the PCR amplification were purified using a Qiagen PCR purification kit. 250 ng of purified PCR product of lengths 270 bp or 1.3 kb were used as templates in Klenow labelling reactions (Bioprime DNA labelling System; Invitrogen). Klenow labelling reactions containing 250 ng of PCR product 1× random primer solution (50 mM Tris-HCl pH 6.8, 12.5 mM MgCl2, 10 mM 2-mercaptoethanol, 300 μg/mL random octamer oligodeoxyribonucleotide primers; Bioprime DNA labelling System Invitrogen) in a final reaction volume of 424, were mixed by vortexing and heated to 95° C. for 5 min after which they were immediately placed on ice. To the reaction mix was added 200 μM (final concentration) of each of dATP, dTTP and dGTP, 100 μM (final concentration) dCTP, 100 μM (final concentration) Cy3- or Cy5-dCTP and 0.8 U of Klenow Large Fragment of DNA polymerase 1 (Bioprime DNA labelling System Invitrogen). Reactions were incubated for 2 h at 37° C. In these standard reactions 20% of the dCTP is replaced by Cy-dye labelled dCTP. The size distribution of the Klenow labelled DNA populations was analysed using an Agilent 2100 Bioanalyser and shown to peak between 50-100 bp irrespective of template length.
E10 DNA LabellingDNA fragments of lengths 1.3 kb or 270 bp were labelled with Cy3- or Cy5-dCTP by E10 in PCRs with primers 51 and 52 and 36 and 37 respectively. PCRs were performed in 1×Pfu buffer (10 mM KCl, 10 mM (NH4)2SO4, 20 mM Tris-Cl pH 8.75, 2 mM MgSO4, 0.1% Triton X-100, 100 μg/mL BSA; Stratagene Ltd), 1% (v/v) formamide, 1 μM each primer, 50 μM dATP, 50 μM dGTP, 50 μM dTTP, 25 μM dCTP, 25 μM Cy3- or Cy5-dCTP and 10 ng pASKpfuexo−5 in a reaction volume of 100 μL H2O. Reactions were hot started with E10 and thermocycled under the following conditions: 94° C. for 2 min 50 times at 94° C. for 10 sec, 50° C. for 10 sec and 68° C. for 20 min. Cy-dye labelled DNA products were purified by ethanol precipitation (2.5× sample volume ethanol 1/10th sample volume sodium acetate pH 5.2) followed by purification on Autoseq G50 columns (Amersham Biosciences). In these reactions 50% of the dCTP is replaced by Cy3- or Cy5-dCTP.
Model Array HybridisationsLabelled-target nomenclature is as follows:
E10270 Cy3 or E10270 Cy5 refers to a 270 bp target DNA that has been labelled using the E10 enzyme where 50% of the dCTP present in the labelling reaction is Cy3-dCTP or Cy5-dCTP. E101300 Cy3 or E101300 Cy5 is a 1300 bp target DNA that has been labelled using the E10 enzyme where 50% of the dCTP present in the labelling reaction is Cy3-dCTP or Cy5-dCTP.
Klenow270 Cy3 or Klenow270 Cy5 refers to a 270 bp target DNA that has been labelled using Klenow enzyme where 20% of the dCTP present in the reaction is Cy3-dCTP or Cy5-dCTP. Klenow1300 Cy3 or Klenow1300 Cy5 refers to a 1300 bp target DNA that has been labelled using Klenow enzyme where 20% of the dCTP present in the reaction is Cy3-dCTP or Cy5-dCTP.
The following competitive hybridisations were performed:
-
- 1) Klenow270 Cy3 vs Klenow270 Cy5
- 2) Klenow1300 Cy3 vs Klenow1300 Cy5
- 3) E10270 Cy3 vs Klenow270 Cy5
- 4) E10270 Cy5 vs Klenow270 Cy3
- 5) E101300 Cy3 vs Klenow1300 Cy5
- 6) E101300 Cy5 vs Klenow1300 Cy3
Competitive hybridisations 1 and 2 were performed as control hybridisations and competitive hybridisations 3, 4, 5 and 6 were performed to measure the influence of fragment length on the level of fluorescent signal obtained following hybridisation.
10 ng of Cy3- and Cy5-labelled products were prepared in 20 μl of hybridisation buffer (1 mM Tris-HCl pH 7.4, 50 mM tetrasodium pyrophosphate, 1×Denhardts solution, 40% deionised formamide, 1% NP-40, 10 mM DTT, 100 μg/ml sheared salmon sperm DNA). Each sample was heated to 95° C. for 5 minutes, centrifuged for 2 min, applied to the surface of an array and covered with a 22×22 mm LifterSlip (Erie Scientific). Hybridisations were performed at 37° C. for 16 h in a hybridisation chamber (Telechem). Arrays were washed once with 2×SSC/0.1% SDS at room temperature for 5 minutes, and twice with 1×SSC at room temperature for 5 min. Slides were dried by centrifugation and scanned with an ArrayWoRx ‘e’ (Applied Precision Instruments). To ensure comparability between slides, identical scanning parameters were used for all arrays. Images were analysed using GenePix Pro 4.1 (Axon Instruments) and array features affected by local artefacts were manually excluded from further analysis.
Calculation of Fold-Improvements in SignalLog2 (mean Cy3/mean Cy5) for each array feature was calculated. For each probe type (Pfu, or salmon testis DNA) in each experiment the mean log2 ratio for all included array features (max. 100 Pfu features i.e. 20 replicate features each of 5 Pfu probe dilutions; max. 100 salmon sperm DNA features i.e. 20 replicate features for each of 5 salmon sperm DNA probe dilution; max. 20 spotting buffer features; max. 20 unspotted areas) was calculated. To normalise the data to the control experiment (Klenow270 Cy3 vs Klenow270 Cy5 or Klenow1300 Cy3 vs Klenow1300 Cy5), where equivalent signal is expected in both channels, the Klenow mean log2 ratio from the control experiments was subtracted from that of every other comparison. If the resulting log2 ratios are positive or negative, they represent signal improvements in Cy3 and Cy5, respectively, compared to labelling with Klenow at 20% Cy-dye concentration. Fold-improvements for Cy3 and Cy5 are therefore, respectively, the inverse of the resulting mean log 2 ratio and the reciprocal of the inverse of the resulting mean log 2 ratio.
Hybridisation of E10270 Cy3 DNA fragment resulted in a 34-fold higher signal whereas hybridisation of the longer E101300 Cy3 DNA fragment resulted in only a 3.6-fold higher signal (see
The influence of label density on fluorescent signal obtained from highly Cy-dye labelled DNA was investigated using a model array on which had been printed a dilution series (200, 100, 50, 25, and 12.5 ng/μl) of the full-length Pfu polymerase gene 2300 bp), a dilution series of salmon testis DNA (200, 100, 50, 25, and 12.5 ng/ul) and spotting buffer (the latter 2 acting as negative controls). The increase in fluorescent signal from a 270 bp Cy-dye E10 labelled DNA fragment labelled in PCRs where 10%, 50% or 100% of the dCTP is replaced by Cy3- or Cy5-dCTP was measured relative to the equivalent 270 bp Cy3- or Cy5-dCTP Pfu labelled fragment amplified in PCRs where 10% of the dCTP is replaced by Cy-dye labelled dCTP.
DNA LabellingDNA fragments of length 270 bp were labelled with Cy3- or Cy5-dCTP by E10 or Pfu exo− (Stratagene) in PCRs with primers 36 and 37 respectively. PCRs were performed in 1×Pfu buffer (10 mM KCl, 10 mM (NH4)2SO4, 20 mM Tris-Cl pH 8.75, 2 mM MgSO4, 0.1% Triton X-100, 100 μg/mL BSA; Stratagene Ltd), 10 ng pASKpfuexo−5, 1% (v/v) formamide, 1 μM each primer, 50 μM dATP, 50 mM dGTP, 50 μM dTTP, 50 μM Cy3- or Cy5-dCTP (100% labelling) or 25 μM dCTP and 25 μM Cy3- or Cy5-dCTP (50% labelling) or 5 μM Cy3- or Cy5-dCTP and 45 μM dCTP (10% labelling) in a reaction volume of 50 μL H2O. Reactions were hot started with E10 (100%, 50% and 10% dCTP replacement) or Pfu exo− (10% dCTP replacement only) and thermocycled under the following conditions: 94° C. for 2 min 50 times at 94° C. for 10 sec, 50° C. for 10 sec and 68° C. for 20 min. Cy-dye labelled DNA products were purified by ethanol precipitation (2.5× sample volume ethanol 1/10th sample volume sodium acetate pH 5.2) followed by purification on Autoseq G50 columns (Amersham Biosciences).
Model Array HybridisationsLabelled-target nomenclature is as follows:
E10100 Cy3 or E10100 Cy5 refers to a 270 bp target DNA that has been labelled using the E10 enzyme where 100% of the dCTP present in the labelling reaction is Cy3-dCTP or Cy5-dCTP. E1050 Cy3 or E1050 Cy5 refers to a 270 bp target DNA that has been labelled using the E10 enzyme where 50% of the dCTP present in the labelling reaction is Cy3-dCTP or Cy5-dCTP. E1010 Cy3 or E1010 Cy5 refers to a 270 bp target DNA that has been labelled using the E10 enzyme where 10% of the dCTP present in the labelling reaction is Cy3-dCTP or Cy5-dCTP.
Pfu10 Cy3 or Pfu10 Cy5 refers to a 270 bp target DNA that has been labelled using Pfu exo− enzyme where 10% of the dCTP present in the reaction is Cy3-dCTP or Cy5-dCTP.
The following competitive hybridisations were performed:
-
- 1) Pfu10 Cy3 vs Pfu10 Cy5
- 2) E10100 Cy3 vs E10100 Cy5
- 3) E1050 Cy3 vs E1050 Cy5
- 4) E1010 Cy3 vs E1010 Cy5
- 5) E1010 Cy3 vs Pfu10 Cy5
- 6) E1010 Cy5 vs Pfu10 Cy3
- 7) E1050 Cy3 vs Pfu10 Cy5
- 8) E1050 Cy5 vs Pfu10 Cy3
- 9) E10100 Cy3 vs Pfu10 Cy5
- 10) E10100 Cy5 vs Pfu10 Cy3
Competitive hybridisations 1-4 were performed as control hybridisations and competitive hybridisations 5-10 were performed to measure the influence of labelling density on the level of fluorescent signal obtained following hybridisation.
10 ng of Cy3- and Cy5-labelled products were prepared in 20 μl of hybridisation buffer (1 mM Tris-HCl pH 7.4, 50 mM tetrasodium pyrophosphate, 1×Denhardts solution, 40% deionised formamide, 1% NP-40, 10 mM DTT, 100 μg/ml sheared salmon sperm DNA). Each sample was heated to 95° C. for 5 minutes, centrifuged for 2 min, applied to the surface of an array and covered with a 22×22 mm LifterSlip (Erie Scientific). Hybridisations were performed at 37° C. for 16 hr in a hybridisation chamber (Telechem). Arrays were washed once with 2×SSC/0.1% SDS at room temperature for 5 minutes, and twice with 1×SSC at room temperature for 5 min. Slides were dried by centrifugation and scanned with an ArrayWoRx ‘e’ (Applied Precision Instruments) and a GenePix 4100A (Axon). To ensure comparability between slides, identical scanning parameters were used for all arrays. Images were analysed using GenePix Pro 4.1 (Axon Instruments) and array features affected by local artefacts were manually excluded from further analysis.
Calculation of Fold-Improvements in SignalLog2 (mean Cy3/mean Cy5) for each array feature was calculated. For each probe type (Pfu, or salmon testis DNA) in each experiment the mean log2 ratio for all included array features (max. 100 Pfu features i.e. 20 replicate features each of 5 Pfu probe dilutions; max. 100 salmon sperm DNA features i.e. 20 replicate features for each of 5 salmon sperm DNA probe dilution; max. 20 spotting buffer features; max. 20 unspotted areas) was calculated. To normalise the data to the control experiment (Pfu10 Cy3 vs Pfu10 Cy5), where equivalent signal is expected in both channels, the Pfu mean log2 ratio from the control experiments was subtracted from that of every other comparison. If the resulting log2 ratios are positive or negative, they represent signal improvements in Cy3 and Cy5, respectively, compared to labelling with Pfu at 10% Cy-dye concentration. Fold-improvements for Cy3 and Cy5 are therefore, respectively, the inverse of the resulting mean log 2 ratio and the reciprocal of the inverse of the resulting mean log 2 ratio.
Scanning of slides with the GenePix 4100A (Axon) and subsequent analysis of fluorescent signal obtained from experiments where E1010 Cy3, E1050 Cy3 or E10100 Cy3 DNA fragments had been hybridised resulted in 1.7-, 2.4 or 1.6-fold higher fluorescent signal respectively. Scanning of the identical slides with an ArrayWoRx ‘e’ (Applied Precision Instruments) and subsequent analysis of fluorescent signal resulted in 1.85-fold for E1010 Cy3, 3.8-fold for E1050 Cy3 and 4.5-fold for E10100 increases in fluorescent signal. See
Scanning of slides with the GenePix 4100A (Axon) and subsequent analysis of fluorescent signal obtained from experiments where E1010 Cy5 or E1050 Cy5 DNA fragments had been hybridised all resulted in 2.3-fold or 1.65-fold higher fluorescent signal respectively. Scanning of slides with the GenePix 4100A (Axon) and subsequent analysis of fluorescent signal obtained from experiments where E10100 Cy5 DNA fragments had been hybridised all resulted in a 2.2-fold decrease in fluorescent signal. Scanning of the identical slides with an ArrayWoRx ‘e’ (Applied Precision Instruments) and subsequent analysis of fluorescent signal resulted in 2.5-fold for E1010 Cy5, 2.3-fold for E1050 Cy5 and O-fold for E10100 increases in fluorescent signal. See
These experiments identify that the increase in fluorescent signal is dependent on the type of scanner used to obtain data suggesting that fluorescent spectra are altered at higher labelling densities or that quenching becomes an issue at higher labelling densities. Additionally labelling density is important to obtain maximal signal from directly E10-labelled samples. No increase in background fluorescence is seen when hybridising highly labelled DNA samples and the fold increase in fluorescence is linear across the dilution series printed on the slide.
Example 19 Highly Cy-Dye E10 Labelled DNA can be Digested by Restriction EnzymesDNA fragments of length 270 bp were labelled with Cy3- or Cy5-dCTP by E10 in PCRs with primers 36 and 37 respectively. PCRs were performed in 1×Pfu buffer (10 mM KCl, 10 mM (NH4)2SO4, 20 mM Tris-Cl pH 8.75, 2 mM MgSO4, 0.1% Triton X-100, 100 μg/mL BSA; Stratagene Ltd), 10 ng pASKpfuexo−5, 1% (v/v) formamide, 1 μM each primer, 50 μM dATP, 50 μM dGTP, 50 μM dTTP, 50 μM Cy3- or Cy5-dCTP in a reaction volume of 50 μL H2O. Reactions were hot started with E10 and thermocycled under the following conditions: 94° C. for 2 min 50 times at 94° C. for 10 sec, 50° C. for 10 sec and 68° C. for 20 min. Cy-dye labelled DNA products were purified by ethanol precipitation (2.5× sample volume ethanol 1/10th sample volume sodium acetate pH 5.2) followed by purification on AutoSeq G50 columns (Amersham Biosciences). Labelled DNA products were digested with the enzymes MseI or DdeI (New England Biolabs). The recognition sequence for MseI is TTAA and would be expected to restrict the DNA if it is double stranded. This was found to be the case (see
To determine the contribution of the mutations selected with Cy5-dCTP from the Pfu A motif repertoire (E399D, N400G, R407I, and V337I) and the additional mutation selected from the Pfu A and C motif repertoire (Y546H) to the activity of E10 the Y546H mutation was introduced into Pfu in the absence of A motif mutations. The E10 mutation Y546H was introduced into pASKpfuexo−6 by subcloning a BamHI/XhoI fragment from E10 into pASKpfuexo−6 thereby generating pASKpfuexo−6Y546H.
Crude lysates of E10 (E399D, N400G, R407I, V337I, Y546H), clone 15 (E399D, N400G, R407I, and V337I) and pASKpfuexo−6Y546H (Y546H) were made by growing overnight cultures in 2×TY (100 μg/mL ampicillin) at 30° C. Protein expression was induced by the addition of anhydrous tetracycline (0.4 μg/mL) to the overnight culture and the cultures allowed to grow for 5 h at 37° C. Induced cells were pelleted by centrifugation (13,000 g, 5 min) and 10× lysates were made by resuspension in 1/10th of the culture volume of 1×Pfu buffer (10 mM KCl, 10 mM (NH4)2SO4, 20 mM Tris-Cl pH 8.75, 2 mM MgSO4, 0.1% Triton X-100, 100 μg/mL BSA; Stratagene Ltd). The cells were lysed by incubation at 85° C. for 10 min and cell debris was removed by centrifugation at 13,000 g for 10 min.
Crude lysates as well as Pfuexo− (5 U; Stratagene) were activity normalised with primer 48 as described in example 7. ELISA experiments designed to determine the contribution of the mutations selected from the Pfu A motif repertoire (E399D, N400G, R407I, and V337I) and Pfu A and C motif repertoire (Y546H) to the activity of E10 were performed activity with normalised crude lysates with primer 35 (requires the incorporation 8 consecutive C's prior to the incorporation of digoxigenin-11-dUTP) and Cy3- or Cy5-dCTP as described in example 7. These experiments show that the contribution of the A motif (and V337I) and C motif mutations are synergistic (
Crude lysates and Pfuexo− (5 U; Stratagene) were activity normalised in PCRs with primers 36 and 37 and 10 ng of pASKpfuexo−5 as template. Activity normalisation of the crude lysates in PCR was performed as follows: 1 μM each primer, 1% (v/v) formamide, 100 μM dNTPs, 1×Pfu buffer (10 mM KCl, 10 mM (NH4)2SO4, 20 mM Tris-Cl pH 8.75, 2 mM MgSO4, 0.1% Triton X-100, 100 μg/mL BSA; Stratagene Ltd). Reactions were thermocycled as follows: 94° C. for 2 min 25 times at 94° C. for 30 sec, 55° C. for 30 sec and 72° C. for 2 min. PCRs were set up with activity normalised crude lysates and Pfuexo− (5 U; Stratagene) as described above. Reactions were thermocycled with decreasing extension times (30 sec or 10 sec at 72° C.; see
The A motif mutations selected with Cy5-dCTP from the Pfu A motif repertoire were introduced by iPCR into the B family DNA polymerase gene from Thermococcus gorgonarius (Tgo) in which the uracil stalling function (V93Q; Fogg et al., 2002) had been removed as well as the 3′-5′ exonuclease function (D141A and E143A; Derbyshire et al., 1995) with and without the Therminator mutation (A485L; Gardener and Jack, 1999) with primers 53 and 54 as described in example 4 (see
Tgo (V93Q, Exo−), Tgo E10 A motif (V93Q, Exo−, E398D, N399D, R406I), Tgo Therminator (V93Q, Exo−, A485L) and Tgo Therminator E10 A motif (V93Q, Exo−, A485L, E398D, N399D, R406I, A485L) clones were grown overnight at 30° C. in 2×TY (Ampicillin 100 μg/mL). Protein expression was induced the next day by the addition of anhydrous tetracycline (0.4 μg/mL) to the overnight culture and the cultures allowed to grow for 4 h at 37° C. Induced cells were pelleted by centrifugation (13,000 g, 5 min) and 10× lysates were made by resuspension in 1/10th of the culture volume of 1×Pfu buffer (10 mM KCl, 10 mM (NH4)2SO4, 20 mM Tris-Cl pH 8.75, 2 mM MgSO4, 0.1% Triton X-100, 100 μg/mL BSA; Stratagene Ltd). The cells were lysed by incubation at 85° C. for 10 min and cell debris was removed by centrifugation at 13,000 g for 10 min.
PCRs were hot started by the addition of 2 μL of crude lysate to a reaction containing: 10 ng of pASKTaq, 1 μM primer 56, 1 μM primer 39, 1×Pfu buffer (10 mM KCl, 10 mM (NH4)2SO4, 20 mM Tris-Cl pH 8.75, 2 mM MgSO4, 0.1% Triton X-100, 100 μg/mL BSA; Stratagene Ltd), 1% (v/v) formamide and 100 μM dNTPs in a reaction volume of 204. Thermocycling conditions were as follows: 94° C. for 2 min 25 times at 94° C. for 30 sec, 50° C. for 30 sec and 72° C. for 2 min. PCR products were analysed by agarose gel electrophoresis on a 1.2% (w/v) gel (see
Pfuexo− is able to perform PCR with 100% substitution of dATP by FITC-12-dATP or Biotin-16-dUTP. To assess whether E10 retains these properties PCRs were hot started by the addition of 1 μL of E10 or 2.5 U Pfuexo− (Stratagene) to a PCR reaction. For PCRs with 100% replacement of dATP by FITC-12-dATP reactions contained: 10 ng of pASKTaq, 1 μM primer 56, 1 μM primer 39, 1×Pfu buffer (10 mM KCl, 10 mM (NH4)2SO4, 20 mM Tris-Cl pH 8.75, 2 mM MgSO4, 0.1% Triton X-100, 100 μg/mL BSA; Stratagene Ltd), 1% (v/v) formamide and 50 μM dNTPs (-dATP) 50 μM FITC-12-dATP in a reaction volume of 12.5 μL. Reactions with 100% replacement of dTTP with Biotin-16-dUTP were performed as described above but with dNTPs lacking dTTP and 50 μM Biotin-16-dUTP. Thermocycling conditions were as follows: 94° C. for 2 min 50 times at 94° C. for 10 sec, 60° C. for 30 sec and 68° C. for 20 min. PCR products were analysed by agarose gel electrophoresis on a 1.2% (w/v) gel (see
Pfu variants were selected from the Pfu A motif repertoire libraries (see example 4) for the ability to incorporate Biotin-16-dUTP. spCSR selections were set up as described in example 3 however, the dTTP was completely replaced by 100 μM of Biotin-16-dUTP. Primers annealing 5′ and 3′ of the region diversified were employed in spCSR selections.
spCSR Biotin-16-dUTP selections from the Pfu A motif library (94° C. for 5 min; 20 times at 94° C. for 30 sec, 50° C. for 1 min, and 72° C. for 10 min; 1 times at 65° C. for 10 min) were carried out using primers 57 and 37 as described in example 3. The aqueous phase was extracted as described as described in example 3 and purified selection products were reamplified by PCR (94° C. 5 min; 50° C. 5 min; 72° C. 5 min; 26 times at 94° C. for 30 sec, 50° C. for 1 min, and 72° C. for 3 min; 1 times 65° C. 10 min) using primers 58 and 37 as described in example 3. Reamplified selection products were gel purified (QIAquick gel extraction kit; Qiagen) and used as a primer in an iPCR reaction with an equimolar quantity of primer 59 and 10 ng of pASKpfuexo−7 as template. iPCR reactions were performed as follows: Briefly iPCR reactions were hot-started by the addition of 3.5 U of Expand High Fidelity polymerase (Roche) to a PCR mix [10 ng of plasmid template, primers (equimolar quantity dependent on yield of selection product), dNTPs (0.2 mM), in 1× High Fidelity Expand buffer with MgCl2 (Roche)]. Reactions were thermocycled [94° C. for 4 min; 19 times (94° C. for 20 sec, 65° C. for 20 sec—1° C./cycle, and 68° C. for 10 min)] at this point 40 μM each of primers 58 and 59 and a further 3.5 U of Expand High Fidelity polymerase (Roche) were added to the iPCRs which were subjected to further thermocycling [15 times (94° C. for 20 sec, 50° C. for 20 sec, 68° C. for 10 min); 15 times (94° C. for 20 sec, 50° C. for 20 sec, 68° C. for 10 min+15 sec/cycle)]. iPCR reactions were purified with a QIAquick PCR purification kit (Qiagen) and eluted in 50 μL of H2O. Purified DNA was restriction digested overnight with DpnI (New England Biolabs) to remove the input plasmid template. Restricted DNA samples were purified with a QIAquick PCR purification kit (Qiagen) and eluted in 50 μL of H2O. Purified DNA was restriction digested overnight with BsaI (New England Biolabs) after which the reactions were purified with a QIAquick PCR purification kit (Qiagen) and eluted in 50 μL of H2O. 50 μL of the restriction digested DNA was ligated overnight at 16° C. with 1600 U T4 DNA ligase (New England Biolabs) in 1×T4 DNA ligase buffer (New England Biolabs) in a final volume of 60 μL. Ligation reactions were extracted with an equal volume of phenol/chloroform (4:1 v/v), ethanol precipitated and then electroporated into E. coli Ace6 or TG1TR cells which were plated onto 2×TY/Amp 0.1 mg/mL plates.
Example 24 Pfu Variants Selected from the A-Motif Library with Biotin-16-dUTPPfu variants able to significantly incorporate Biotin-16-dUTP compared to the ability of the wild-type enzyme were selected from the A motif Pfu repertoire library (see example 23) with primers that anneal 5′ and 3′ of the A-motif (see example 5). Clones exhibiting a significantly enhanced ability to incorporate Biotin-16-dUTP were ranked by ELISA extension assay with primer 60 (see example 7;
Metaphase spreads from the human pancreatic cancer cell line Suit-2 were prepared on Super Frost glass slides (BDH), and denatured by incubations in 50% formamide/1×SSC for 1 min 20 sec at 70° C. and in ice-cold ethanol for 3 min. The chromosomes were dehydrated by 3-minute incubations each in 70%, 90% and 100% ethanol and air-dried at 37° C. until hybridisation.
DNA probes were generated to WRN (chr8:31,150,398-31,150,474; primers 61 & 62, a 766 bp fragment labelled with Cy5-dCTP 32% GC), DCTN6 (chr8:30,133,979-30,134,732; primers 63 & 64, a 754 bp fragment labelled with FITC-12-dATP 59% AT) or NRG1 (chr8:32,718,891-32-32,719,828; primers 65 & 66, a 938 bp fragment labelled with Cy3-dCTP 42% GC). Plasmids containing the fragments to be labelled were used as template in PCRs where 50% of the unlabelled dNTP was replaced by its labelled counterpart. The PCRs were hot-started by the addition of 1 μL of purified E10 to a PCR mix [10 ng plasmid DNA template, primers (0.4 μM each), 1×Pfu buffer (10 mM KCl, 10 mM (NH4)2SO4, 20 mM Tris-Cl pH 8.75, 2 mM MgSO4, 0.1% Triton X-100, 100 μg/mL BSA; Stratagene Ltd), formamide and dNTPs (0.05 mM each, 0.025 mM unlabelled counterpart), 0.025 mM Cy3- or Cy5-dCTP or FITC-dATP]. PCRs to generate labelled DNA probes to NRG1 or WRN were performed with a final formamide concentration of 2% (v/v) and PCRs to generate labelled DNA probes to DCTN6 were performed with a final formamide concentration of 1% (v/v). Reactions were thermocycled (94° C. for 2 min; 50 times at 94° C. for 10 sec, annealing for 1 min, and 72° C. for 20 min). The annealing temperatures were 60° C., 55° C. and 58° C. for the NRG1, WRN and DCT respectively. Labelled probes were purified by ethanol precipitation with 2.5×vol of ethanol 1/10 volume NaAc (pH 5.2) and resuspended in 20 μL of H2O followed by G50 column purification.
Probes labelled with Cy-dyes or FITC (100-150 ng) were mixed with 3 μL human Cot-1 DNA (100 ng/μL; Roche) in 14 μL hybridisation buffer (50% (v/v) formamide, 2×SSC 10 mM Tris-HCl pH 7.5, 0.1% (w/v) Tween 20, 10% dextran sulphate) and incubated at 37° C. for 30 min. The probe mixture was denatured at 95° C. for 10 min, cooled on ice for 2 min and incubated at 37° C. for 1 hr. The probes were applied to denatured metaphase spreads and covered with a glass cover slip. Following hybridisation at 37° C. overnight, the slides were soaked in 1×SSC to remove the cover slips, washed 2 times 5 min in 50% formamide/2×SSC at 42° C. and 2 times 5 min in 1×SSC. The sides were then incubated in 4×SSC, 0.5% BSA (Sigma), 0.05% Tween for 5 min, mounted with Vectashield Mounting Medium with DAPI (Vectashield Laboratories), and viewed with Nikon Eclipse E800 Fluorescence microscope. Cy3, Cy5 and FITC signals from these exceptionally short probes co-localised to a single chromosome in 5 out of 5 cells analysed and could be detected directly without the need for subsequent signal amplification (
The physico-chemical properties of CyDNA are significantly altered as compared to native DNA.
Analysis of the melting temperature of a 100 bp fragment (70% GC) labelled with E10 in PCRs where 100% of the dCTP was replaced by either Cy3- or Cy5-dCTP revealed that the CyDNA melts at lower temperature than native DNA. DNA fragments (100 bp, 70% GC) for melting temperature analysis were amplified with primers 67 and 68. Reactions were performed in 1×Pfu buffer (Stratagene) containing 10 ng template (pASKwtTaq; Ghadessy et al 2001), 1% formamide (v/v), primers (0.4 μM each) dNTPs (50 μM each). Cy-dye labelled DNA fragments were amplified with E10 and 100% of the dCTP was replaced by Cy3- or Cy5-dCTP. Reactions were hot-started with E10 and thermocycled as follows: 94° C. 2 min; 50 times (94° C. 10 sec, 50° C. 10 sec, 68° C. 20 min). Cy-dye labelled DNA fragments were concentrated and purified by ethanol precipitation followed by additional purification with illustra Microspin G-50 columns (GE Healthcare). Reactions to generate unlabelled control DNA were hot-started with Pfu Turbo (2.5 U Stratagene) and thermocycled as follows: 94° C. 2 min; 30 times (94° C. 30 sec, 50° C. 1 min, 72° C. 1 min). Unlabelled PCR products were purified with QIAquick PCR purification columns (QIAGEN Chatsworth Calif.) followed by gel extraction with QIAquick gel extraction kit (QIAGEN Chatsworth Calif.). Melting temperatures of the labelled and control DNA fragments (1 μM) were determined with a Perkin Elmer Lambda 40 over a temperature range of 60° C.-100° C. The unlabelled DNA melts at 90° C. whereas the Cy5-dCTP labelled DNA melts at 87° C. and the Cy3-dCTP labelled melts at 84° C. This correlates with what others have observed and what we have observed in model microarray experiments where hybridisation temperatures and wash stringencies require reduction. Additionally CyDNA melting is less cooperative than native DNA probably reflecting the presence of additional groups on the duplex.
It is not possible to efficiently purify CyDNA with silica resins in the presence of chaotropic salts or by phenol extraction in the presence of NaCl concentrations commonly found in some restriction enzyme buffers (e.g. NEB3; New England Biolabs). Unlike native DNA 100% Cy3 labelled DNA partitions entirely to the organic phase in the presence of 200 mM NaCl on phenol extraction whereas 100% Cy5 DNA partitions entirely to the organic phase in the presence of 100 mM NaCl. CyDNA can be recovered from the phenol phase into the aqueous phase by either diluting the salt containing phase with H20 or removing the aqueous salt containing phase and replacing this with an equivalent volume of H20. CyDNA cannot be cloned reflecting an inability of the native E. coli polymerase enzymes to replicate this unnatural DNA. However, it is possible to ethanol precipitate CyDNA.
CyDNA exhibits decreased electrophoretic mobility compared to a native DNA fragment amplified with the same primers. Additionally, Cy-dye modified DNA does not fluoresce with ethidium bromide. Presumably the presence of the modifications at the 5′ position of the pyrimidine sterically hinders intercalation. However, despite the absence of ethidium bromide intercalation CyDNA is, at least in part, double stranded and B-form as indicated by its digestion with restriction enzymes, such as MseI, whose recognition sequence is devoid of modified C's. The presence of a Cydye modified C in the restriction enzyme recognition site prevents restriction digestion
Further evidence that CyDNA exists as double stranded B-form DNA was obtained by Atomic Force Microscopy (AFM). The results of AFM are shown in
The lengths and heights (width) of individual CyDNA molecules amplified with E10 in PCRs where 100% of the dCTP was replaced by either Cy3- or Cy5-dCTP were measured. For AFM imaging the modified and control DNA fragments were diluted to a concentration of 1 nM in buffer X (10 mM Tris-HCl pH 7.4, 10 mM MgCl2, 10 mM NaCl). It proved difficult to prepare the CyDNA samples for analysis on mica stubs presumably due to the hydrophobic nature of the samples. It was, however, possible to analyse samples on mica stubs that had been functionalised with Poly-L-Lysine. Poly-L-lysine coated mica was formed by incubating 50 μL of 0.001% poly-L-lysine (Sigma, Poole, UK) on freshly cleaved mica (Goodfellows, Huntingdon, UK) for 10 min, followed by rinsing with 10 mL MilliQ water (Millipore, Billerica, Mass.), and blown dry with a stream of nitrogen. The DNA fragments were deposited on the poly-L-lysine coated mica by incubating a 10 μL drop of DNA containing solution on the surface for 4 min, the sample was then rinsed with 10 mL MilliQ water and blown dry with a stream of nitrogen. AFM imaging was performed using a Veeco Multimode AFM with a Nanoscope IIIa controller (Veeco, Santa Barbara, Calif.) operated in Tapping mode, using Olympus AC160TS silicon nitride cantilevers with a resonant frequency of approximately 350 kHz. The images were acquired at 512*512 pixel resolution with a scan size of 3 μm, scan rates were 1.97 Hz. Images were flattened post capture to remove Z offsets and sample tilt. The DNA contour length of individual molecules was measured by approximating the DNA backbone as a series of straight lines using the Nanoscope software (Veeco). The DNA end-end distance was measured by tracing the straight line distance between the two DNA ends.
The lengths and heights of 40 unmodified DNA molecules, 40 Cy3-dCTP modified molecules and 71 Cy5-dCTP molecules were measured. The distribution in length of the Cy5-dCTP modified DNA molecules is not Gaussian this could be due to the presence of smaller termination products present in the PCR amplification making data interpretation difficult. It is possible, however, to conclude that there is a significant difference in molecule length between the unmodified and Cy3-dCTP modified DNA molecules (104.0 nm±0.3 vs 97.5 nm±0.25). The height (width) of the Cy3-dCTP and Cy5-dCTP labelled DNA molecules is significantly greater than that of the unmodified DNA molecules (see
Cy3-dCTP labelled DNA molecules (0.620 nm±0.008) consistent with Cy5 being a larger fluorophore than Cy3. Additionally, the persistence length (stiffness) of the unmodified and Cy5-dCTP modified DNA molecules was modelled using data from the mean end-to-end distance derived in each case from 61 DNA molecules (i.e. a straight line from one end to the other rather than following the DNA backbone)—the larger the average end-to-end distance the stiffer the molecule. It can be concluded that both the Cy5-dCTP modified and unmodified DNA fragments can be modelled as having the same persistence length. This assumes that both fragments bind to the surface in the same manner, since weaker binding is likely to allow the molecule to equilibrate on the surface better resulting in a larger <R>2 Measured. It was demonstrated previously that the modified DNA binds more weakly to a nonfunctionalised mica surface therefore the persistence length data should be interpreted with caution.
Example 27 FISH & Fibre FISHHigh label density will not necessarily result in an increase in fluorescence due to re-absorption and re-emission effects and quenching. Model microarray experiments identified that PCR fragments labelled in PCRs where of the 50% of the dCTP was replaced by either Cy3- or Cy5-dCTP resulted in higher fluorescent signal than fragments labelled with 100% replacement (see Example 18). Consequently probes used in FISH and Fibre FISH were generated in PCRs where 50% of the unlabelled nucleotide was replaced by its fluorescent counterpart. Pfu exo- is able to incorporate FITC-12-dATP when present at 100% in PCRs. E10 retains this ability (see Example 22)
FISHFISH was performed as described in Example 25.
Fibre FISHFibre-FISH was based on the method described by Mann et al. (1997). Briefly, 10 μL of fixed metaphase preparation from the human skin cell line DRM was smeared across the width of a polylysine-coated slide (Polyprep, Sigma, Dorset, UK) approximately 1 cm from the top of the usable slide surface and allowed to dry for a minute before being submerged vertically in lysis buffer (0.5% (w/v) SDS, 50 mM EDTA, 0.2M Tris-HCl pH7.4) in a coplin jar and left to stand with the cell preparation just below the buffer surface. After 5 min 94% ethanol was slowly run on to the buffer surface to form an upper layer until the entire slide was covered. After incubating for 10 min, the slide was pulled out of the solutions slowly and steadily at a 30° angle, submerged in 70% ethanol, incubated for 30 min, and dehydrated in an ethanol series. Slides were hybridized and analysed as for metaphase spreads.
DNA probes of approximately 900 bp in length were generated to the NRG1 gene: Probe 1 (primers 71 and 72; chr8: 31626471+31627403), probe 2 (primers 73 and 74; chr8:31628404+31629322), probe 3 (primers 75 and 76; chr8:31639299+31640223) probe 4 (primers 77 and 78; chr8:31645286+31646222), probe 5 (primers 79 and 80; chr8:31650233+31651127), probe 6 (primers 81 and 82; chr8:31653626+31654580), probe 7 (primers 83 and 84; chr8:31661123+31662023). Probes 1, 4 and 6 were labelled with FITC-dATP (Perkin Elmer) and probes 2, 3, 5 and 7 were labelled with Cy5-dCTP (GE Healthcare). Plasmids containing the fragments to be labelled were used as template in PCRs where 50% of the unlabelled dNTP was replaced by its labelled couterpart. The PCRs were hot-started by the addition of 1 μL of purified E10 to a PCR mix [long plasmid DNA template, primers (1 μM each), 1×Pfu buffer (Stratagene Ltd), 2% formamide (v/v) and dNTPs (0.05 mM each, 0.025 mM unlabelled counterpart), 0.025 mM Cy5-dCTP or FITC-dATP]. Reactions were thermocycled (94° C. for 2 min; 50 times at 94° C. for 10 sec, annealing for 1 min, and 72° C. for 20 min). The annealing temperatures were 55° C. for probes 1, 2, 6 and 7, 60° C. for probe 3, 52° C. for probe 4 and 58° C. for probe 5. Labelled probes were purified by ethanol precipitation with 2.5× volume of ethanol 1/10 volume NaAc (pH 5.2) and resuspended in 20 μL of H2O followed by S-300 column purification (GE Healthcare). Slides were hybridized and analysed as for metaphase spreads. Fluoresence could be detected from all of these exceptionally short probes, including those separated by just 1 kb (probes 1 and 2), without the need for signal amplification (see
DNA fragments of length 270 bp were labelled with Cy3- or Cy5-dCTP by E10 in PCRs with primers 36 and 37 respectively. PCRs were performed in 1×Pfu buffer (10 mM KCl, 10 mM (NH4)2SO4, 20 mM Tris-Cl pH 8.75, 2 mM MgSO4, 0.1% Triton X-100, 100 μg/mL BSA; Stratagene Ltd), 10 ng pASKpfuexo−5, 1% (v/v) formamide, 1 μM each primer, 50 μM dATP, 50 μM dGTP, 50 μM dTTP, 50 μM Cy3- or Cy5-dCTP in a reaction volume of 50 μL H2O. Reactions were hot started with E10 and thermocycled under the following conditions: 94° C. for 2 min 50 times at 94° C. for 10 sec, 50° C. for 10 sec and 68° C. for 20 min. Cy-dye labelled DNA products were purified by ethanol precipitation (2.5× sample volume ethanol 1/10th sample volume sodium acetate pH 5.2) followed by purification on AutoSeq G50 columns (Amersham Biosciences).
For detection at single- or near-single-molecule levels, a solution was prepared containing 5 pg/μl each of 100% Cy5-dCTP labelled and 50% Cy3-dCTP labelled 270 bp PCR products in water. Based on the sequence of the PCR product, each double-stranded molecule should carry on average 102 Cy3 or 102 Cy5 labels.
This solution was driven through a fused silica capillary (internal diameter ˜40 μm) at 30 μl/hr using a syringe pump. Fluorescence was detected at a fixed point inside the lumen of the capillary, using a custom optical unit which consisted of 532/635 nm laser light fed in by a single mode (3/125 μM) silica optical fiber. On emerging from the fiber the light was collected and focused onto the sample using a combination of 0.3 NA and 0.65 NA aspheric lenses, the latter being both the focusing and collecting objective for fluorescence. The working distance of the sample objective was approximately 1.5 mm, and the intensity field in the focal volume had a Guassian profile with a 1/e diameter of approximately 2.5 μM (effective focal volume approximately 8 μm3, or 8 fl). Collected light was filtered of reflected laser energy using a cascaded series of reflective/transmissive filters and then focussed into a receiving fiber which conveyed the light to a Perkin Elmer SPCM module for the final quantitation of intensity. Counts for both Cy3 fluorescence (570 nm; bandwidth ˜25 nm) and Cy5 fluorescence (670 nm; bandwidth ˜25 nm) were collected in consecutive 20 μsec periods and averaged over a sliding window (centre-weighted) of 40 periods.
A representative section of the output, spanning a total of 0.2 s, is shown in
The frequency of detection events is approximately that expected if single labeled molecules (rather than aggregates of multiple molecules) were being detected.
All publications mentioned in the above specification are herein incorporated by reference. Various modifications and variations of the described methods and system of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention which are obvious to those skilled in molecular biology or related fields are intended to be within the scope of the claims.
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Claims
1. An engineered polymerase with an expanded substrate range characterised in that the polymerase is capable of incorporating an enhanced occurrence of detection agent-labelled nucleotide analogue into nucleic acid synthesised by that engineered polymerase as compared with the wild type polymerase from which it is derived.
2. The engineered polymerase according to claim 1 wherein the agent-labelled nucleotide analogue is a dye labelled nucleotide analogue.
3. The engineered polymerase according to claim 2 wherein the agent-labelled nucleotide analogue is a fluorescent labelled nucleotide analogue.
4. The engineered polymerase according to claim 3 wherein the fluorescent label is selected from the group consisting of Alexa Fluor™ dye, diethylaminocoumarin, tetramethylrhodamine, N-methylanthraniloyl, trinitrophenyl, etheno derivatives, biotin, fluorescein, or carbocyanine.
5. The engineered polymerase according to claim 4 wherein the agent-labelled nucleotide analogue is a carbocyanine labelled nucleotide analogue.
6. The engineered polymerase according to claim 1 which is a DNA polymerase.
7. The engineered polymerase according to claim 2, wherein the dye labelled nucleotide analogue is selected from the group consisting of Cy5-dNTP and Cy3-dNTP.
8. (canceled)
9. The engineered polymerase according to claim 6 wherein the polymerase has one or more of the following mutations: V337I, E399D, N400D, R407I and/or Y546H.
10. The engineered polymerase according to claim 1 comprising the amino acid sequence designated SEQ ID NO: 1.
11. The engineered polymerase according to claim 1 comprising an amino acid sequence selected from the group consisting of: SEQ ID NOs: 4, 6, 8, 10, 12 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38 and 40.
12. The isolated nucleic acid molecule according to claim 17 comprising a nucleotide sequence encoding an engineered polymerase and selected from the group consisting of SEQ ID NOS: 2, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37 and 39.
13. The engineered DNA polymerase according to claim 6 wherein the engineered DNA polymerase is derived from wild type DNA polymerase by substitution, deletion or insertion of one or more amino acids.
14. The engineered DNA polymerase according to claim 6, wherein the engineered DNA polymerase is derived from a DNA polymerase by substitution, deletion or insertion of one or more amino acids.
15. The engineered polymerase according to claim 14, wherein the DNA polymerase is selected from the group consisting of polA polymerases, polB polymerases and pfu polymerases.
16. The engineered polymerase according to claim 1, wherein the amino acid sequence of the polymerase is at least 70% identical to that designated herein as E1O and set forth in SEQ ID NO: 1.
17. An isolated nucleic acid molecule which encodes an engineered polymerase polypeptide wherein the nucleotide, sequence is at least 80% identical to any of the nucleotide sequences selected from the group consisting of SEQ ID NOS: 2, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37 and 39, and wherein said polypeptide is capable of incorporating an enhanced occurrence of detection agent-labelled nucleotide analogue into nucleic acid synthesised by that engineered polymerase.
18.-21. (canceled)
22. A method for the incorporation of detection agent-labelled nucleotide analogues into newly synthesised nucleic acid which method comprises the use of an engineered polymerase according to claim 1.
23. (canceled)
24. The method according to claim 22 wherein the detection agent-labelled nucleotide is a dye-labelled nucleotide analogue.
25. The method according to claim 24 wherein the dye-labelled nucleotide analogue is a fluorescent dye-labelled nucleotide analogue.
26. The method according to claim 25 wherein the fluorescent dye-labelled nucleotide analogue is selected from the group consisting of the following: Cy3-CTP, Cy5-CTP.
27.-38. (canceled)
Type: Application
Filed: Apr 9, 2013
Publication Date: Nov 7, 2013
Applicant: Medical Research Council (Swindon)
Inventors: Philipp Holliger (Cambridge), Nicola Ramsay (Cambridge), Ann-Sofie Jemth (Uppsala)
Application Number: 13/859,521
International Classification: C12N 9/12 (20060101); C12Q 1/68 (20060101);
