CELL-FREE PRODUCTION OF RIBONUCLEIC ACID
Provided herein, in some aspects, are methods and compositions for cell-free production of ribonucleic acid.
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This application claims the benefit under 35 U.S.C. § 119(e) of U.S. provisional application No. 62/319,220 filed Apr. 6, 2016 and U.S. provisional application No. 62/452,550 filed Jan. 31, 2017, each of which is incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTIONRibonucleic acid (RNA) is ubiquitous to life. RNA acts as the key messenger of information in cells, carrying the instructions from DNA for the regulation and synthesis of proteins. RNA is of interest in biotechnology as synthetically modulating mRNA levels in cells (positively through the introduction of mRNA or negatively through the introduction of siRNA or dsRNA) has applications in fields such as agricultural crop protection, anti-cancer therapeutics, and vaccines. RNA interference (RNAi), for example, refers to a cellular mechanism that uses the DNA sequence of a gene to turn the gene “off”—a process referred to as “silencing.” In a wide variety of organisms, including animals, plants, and fungi, RNAi is triggered by double-stranded RNA (dsRNA). Functional single-stranded (e.g. mRNA) and double-stranded RNA molecules have been produced in living cells and in vitro using purified, recombinant enzymes and purified nucleotide triphosphates (see, e.g., European Patent No. 1631675 and U.S. Patent Application Publication No. 2014/0271559 A1, each of which is incorporated herein by reference). Nonetheless, the production of RNA at scales enabling widespread commercial application is currently cost-prohibitive.
SUMMARY OF THE INVENTIONProvided herein are methods, compositions, cells, constructs, and systems for the production (biosynthesis) of RNA. Generally, polymeric RNA from a biomass material is enzymatically depolymerized into its constituent monomers, these monomers are then phosphorylated to their cognate triphosphorylated variants (via a series of kinases) which are subsequently polymerized into a polymeric RNA using a corresponding nuclei acid (e.g., DNA) template.
In some embodiments, the methods, compositions, cells, constructs, and systems of the present disclosure are used for the production of RNA under cell-free conditions, for example, using at least one cell lysate, a combination of purified proteins, or a combination of cell lysate(s) and purified protein(s). The present disclosure is based, in some embodiments, on the conversion of RNA from biomass (e.g., endogenous cellular RNA) to desired synthetic RNA (e.g., synthetic single-stranded or double-stranded RNA) using a cell lysate. First, RNA from biomass (e.g., endogenous RNA), such as messenger RNA (mRNA), transfer RNA (tRNA), and/or ribosomal RNA (rRNA) (e.g., present in a cell lysate) is depolymerized into its monomeric form, 5′-nucleoside monophosphates (NMPs) by one or more nucleases (
Thus, some aspects of the present disclosure provide cell-free methods of producing (biosynthesizing) ribonucleic acid (RNA), the methods comprising: (a) incubating at least one cell lysate mixture that comprises (i) RNA and (ii) at least one enzymatic activity selected from the group consisting of enzymatic activities that depolymerize RNA, thermostable kinase activities, and thermostable RNA polymerase activities, under conditions that result in depolymerization of RNA to produce a cell lysate mixture that comprises nucleoside monophosphates; (b) heating the cell lysate mixture produced in step (a) to a temperature (e.g., 50-80° C.) that inactivates or partially inactivates endogenous nucleases and phosphatases without completely inactivating the thermostable kinase activities and thermostable RNA polymerase activities, to produce a cell lysate mixture that comprises heat-inactivated nucleases and phosphatases; and (c) incubating the cell lysate mixture produced in step (b) in the presence of an energy source (e.g., an ATP regeneration system) and a deoxyribonucleic acid (DNA) template (e.g., containing a promoter operably linked to a nucleotide sequence encoding a RNA of interest), under conditions that result in production of nucleoside triphosphates and polymerization of the nucleoside triphosphates to produce a cell lysate mixture that comprises the RNA of interest.
The cell lysate mixture may comprise a single cell lysate obtained from cells that comprise RNA and express at least one enzyme (including at least one fusion enzyme) that acts as a ribonuclease, acts as a kinase, and/or acts as a RNA polymerase. Alternatively, the cell lysate mixture may comprise at least two (e.g., at least 3, 4, 5, or 6) cell lysates, wherein at least one cell lysate is obtained from cells that comprise RNA, and at least one cell lysate (e.g., at least 2, 3, 4, or 5) is obtained from cells that express at least one enzyme that acts as a nuclease, acts as a kinase, and/or acts as a RNA polymerase.
An enzyme or fusion enzyme is considered to “act as a nuclease” if the enzyme of fusion enzyme exhibits nuclease activity (cleaves or depolymerizes a nucleic acid; e.g., RNase R). An enzyme or fusion enzyme is considered to “act as a kinase” if the enzyme of fusion enzyme exhibits kinase activity (catalyzes the transfer of a phosphate group from one molecule to another molecule; e.g. polyphosphate kinase). An enzyme or fusion enzyme is considered to “act as a polymerase” if the enzyme of fusion enzyme exhibits polymerase activity (assembles nucleotides to produce nucleic acids; e.g., RNA polymerase).
In some embodiments, the RNA of step (a) is messenger RNA (mRNA), transfer RNA (tRNA), or ribosomal RNA (rRNA).
In some embodiments, the cell lysate mixture comprises at least one ribonuclease, at least one thermostable kinase, and/or at least one RNA polymerase (e.g., a thermostable RNA polymerase). The use of fusion enzymes is also encompassed by the present disclosure. For example the cell lysate mixture may comprise a fusion of a ribonuclease and a kinase, or a fusion of multiple kinases. Other fusion enzymes are encompassed by the present disclosure.
Other aspects of the present disclosure provide engineered cells, cell lysates, and cell lysate mixtures comprising at least one nucleoside monophosphate kinase (e.g., thermostable nucleoside monophosphate kinase), at least one nucleoside diphosphate kinase (e.g., thermostable nucleoside diphosphate kinase), and at least one polyphosphate kinase (e.g., thermostable polyphosphate kinase). The cells, in some embodiments, may also comprise at least one ribonuclease and/or at least one RNA polymerase (e.g., thermostable RNA polymerase).
In some embodiments, methods of producing (biosynthesizing) RNA comprise (a) lysing cultured cells (e.g., engineered cells) that comprise RNA (e.g., mRNA, tRNA, and/or rRNA), RNase R, thermostable kinases (e.g., PfPyrH, TthAdk, TthCmk, PfGmk, AaNdk, TePpk, and/or PPK2 (e.g., see Table 6), and a thermostable T7 RNA polymerase, thereby producing a cell lysate, (b) incubating the cell lysate produced in step (a) under conditions that result in depolymerization of RNA to 5′-NMPs, thereby producing a cell lysate that comprises 5′-NMPs, (c) heating the cell lysate produced in step (b) to 60-80° C. to inactivate endogenous nucleases and phosphatases without completely inactivating the thermostable kinases and thermostable RNA polymerase, thereby producing a cell lysate that comprises heat-inactivated nucleases and phosphatases, and (d) incubating the cell lysate produced in step (c) in the presence of an energy source (e.g., an ATP regeneration system comprising polyphosphate) and an engineered nucleic acid (e.g., DNA) template (e.g., containing a promoter operably linked to a nucleotide sequence encoding a RNA of interest), under conditions that result in production of nucleoside triphosphates and polymerization of the nucleoside triphosphates to produce the RNA of interest.
In some embodiments, the RNA, RNase R, thermostable kinases, and thermostable T7 RNA polymerase are contained in a single strain of cultured cells (e.g., engineered cells). In other embodiments, cultured cells (e.g., engineered cells) containing a subset of the above activities/components are lysed, and the lysates combined to generate a cell lysate mixture that comprises all the enzymatic activities described in step (a) above. In some embodiments, enzymatic activities, in the form of purified enzymes, are added to the lysates described in step (a) above. In some embodiments, lysates and/or purified proteins are combined before the heat-inactivation step described in step (c) above. In other embodiments, lysates and/or purified proteins are combined after the heat inactivation step described in step (c) above.
The RNA of interest may be any form of RNA, including single-stranded RNA and double-stranded RNA. For example, the RNA of interest may be messenger RNA (mRNA), antisense RNA, micro RNA, small interfering RNA (siRNA), or a short hairpin RNA (shRNA). Other RNA interference (RNAi) molecules are encompassed herein.
The details of several embodiments of the invention are set forth in the accompanying Figures and the Detailed Description. Other features, objects, and advantages of the invention will be apparent from the description and from the claims.
Provided herein, in some aspects, are methods, compositions, cells, constructs, and systems for the cell-free production (biosynthesis) of nucleic acid (e.g., RNA or DNA). In some embodiments, a single type of organism (e.g., a population of bacterial cells) is engineered to express at least one nuclease, at least one thermostable kinases and at least one thermostable polymerase (e.g., RNA or DNA polymerase). The engineered cells are grown (cultured) under conditions that result in enzyme expression. In some embodiments, the engineered cells may be grown to a desired cell density, and then expression of certain enzymes may be induced (activated). Thus, transcription of certain enzymes may be under the control of an inducible promoter. The cells (e.g., engineered and/or non-engineered cells) are then lysed (e.g., mechanically, chemically, or enzymatically disrupted) to produce a cell lysate that comprises the enzymatic activities required for cell-free production of RNA (e.g., ssRNA or dsRNA). In some embodiments, cells containing polymeric RNA (e.g., mRNA, tRNA, and/or rRNA) are mixed with the engineered cells containing pathway enzymes prior to the cell lysis step. In other embodiments, cell lysate(s) obtained from cells containing polymeric RNA is combined (mixed) with cell lysate(s) obtained from engineered cells containing pathway enzymes. In yet other embodiments, one or more purified pathway enzymes are combined (mixed) with cell lysate(s) obtained from engineered cells. “Pathway enzymes” are enzymes required to biosynthesize the RNA of interest (e.g., starting from polymeric RNA).
To synthesize RNA, the cell lysate (or cell lysate mixture) is incubated under conditions that result in nuclease-mediated (e.g., RNase-mediated) depolymerization of the host-derived (endogenous) RNA to a desired yield of 5′-nucleoside monophoshates (NMPs, or nucleoside monophosphates). The cell lysate (or cell lysate mixture) is then heated, in some embodiments, to inactivate the majority of host-derived enzymes, including phosphatases and nucleases (e.g., RNases), as well as any exogenous nuclease(s) previously added to the cell lysate to facilitate depolymerization of the host-derived RNA. Following the heat inactivation step, the cell lysate is incubated under conditions that result in phosphorylation of the NMPs to NTPs (nucleoside triphosphates) by thermostable kinases (e.g., thermostable nucleoside monophosphate kinases and nucleoside diphosphate kinases) using, for example, thermostable polyphosphate kinase and the addition of polyphosphate as the energy source. The resulting NTPs are subsequently polymerized to RNA by a RNA polymerase (e.g., thermostable RNA polymerase) using an engineered template (e.g., DNA template) present in the lysates (e.g., either expressed by the engineered cells and included as a cellular component of the cell lysate, or later added to the cell lysate).
Cell-Free Production“Cell-free production” is the use of biological processes for the synthesis of a biomolecule or chemical compound without using living cells. The cells are lysed and unpurified (crude) portions or partially-purified portions, both containing enzymes, are used for the production of a desired product. Purified enzymes may be added to cell lysates, in some embodiments. As an example, cells are cultured, harvested, and lysed by high-pressure homogenization or other cell lysis method (e.g., chemical cell lysis). The cell-free reaction may be conducted in a batch or fed-batch mode. In some instances, the enzymatic pathways fill the working volume of the reactor and may be more dilute than the intracellular environment. Yet substantially all of the cellular catalysts are provided, including catalysts that are membrane associated. The inner membrane is fragmented during cell lysis, and the fragments of these membranes may form membrane vesicles. See, e.g., Swartz, AIChE Journal, 2012, 58(1), 5-13, incorporated herein by reference.
Cell-free methods, compositions, and systems of the present disclosure utilize cell lysates (e.g., crude or partially purified cell lysates), discussed in greater detail herein. Cell lysates prepared, for example, by mechanical means (e.g., shearing or crushing), are distinct from chemically-permeabilized cells. As discussed above, in some embodiments, during cell lysis (e.g., mechanical cell lysis), the inner cell membrane is fragmented such that inverted membrane vesicles are formed in the cell lysates. Such inverted membrane vesicles are not produced through chemical cell permeabilization methods. Cells that are lysed (e.g., at least 75%, 80%, 85%, 90%, or 95%) are no longer intact. Thus, permeabilized cells, which are intact cells containing perforations (small holes) are not considered lysed cells.
While the methods provided herein are generally cell-free and use cell lysates, in some embodiments, it may be advantageous, at least for some steps of the methods, to use permeabilized cells. Thus, the present disclosure does not exclude the use of permeabilized cells in at least one step of the RNA production methods.
It should be understood that while many of the embodiments described herein refer to “lysing cultured cells” that comprise particular enzymes, the phrase is intended to encompass lysing a clonal population of cells obtained from a single culture (e.g., containing all the enzymes needed to synthesize RNA) as well as lysing more than one clonal population of cells, each obtained from different cell cultures (e.g., each containing one or more enzymes needed to synthesize RNA and/or the polymeric RNA substrate). For example, in some embodiments, a population of cells (e.g., engineered cells) expressing one thermostable kinase may be cultured together and used to produce one cell lysate, and another population of cells (e.g., engineered cells) expressing a different thermostable kinase may be cultured together and used to produce another cell lysate. These two cell lysates, each comprising a different thermostable kinase, may then be combined for use in a RNA biosynthesis method of the present disclosure.
Depolymerization of Ribonucleic Acid to Nucleoside MonophosphatesThe present disclosure is based on the conversion of RNA from biomass (e.g., endogenous cellular RNA) to desired synthetic RNA using a cell lysate through a cell-free process involving a series of enzymatic reactions. First, RNA (e.g., endogenous RNA) present in a cell lysate, derived from host cells, is converted to its constituent monomers by nucleases. RNA from biomass (e.g., endogenous RNA) typically includes ribosomal RNA (rRNA), messenger RNA (mRNA), transfer RNA (tRNA), other RNAs, or a combination thereof. Depolymerization or degradation of RNA results in a pool of 5′-nucleoside monophosphates (5′-NMPs), also referred to simply as “monomers.” These monomers, which are converted to nucleoside diphosphates, which are converted to nucleoside triphosphates, are used as starting material for the downstream polymerization/synthesis of a RNA of interest. In some embodiments, the RNA of interest is ssRNA (e.g., mRNA). In some embodiments, the RNA of interest is dsRNA.
The amount of RNA (e.g., endogenous RNA) required to synthesize a RNA of interest may vary, depending on, for example, the desired length and yield of the RNA of interest as well as the nucleotide composition of the RNA relative to the nucleotide composition of the RNA (e.g., endogenous RNA) of the cell (e.g., E. coli cell). Typically, for a bacterial cell, for example, RNA (e.g., endogenous RNA) content ranges from 5-50% of the total cell mass. The mass of the starting material can be calculated, for example, using the following equation: (kilogram (kg) of RNA/kilogram of dry cell weight)×100%.
Endogenous RNA may be depolymerized or degraded into its constituent monomers by chemical or enzymatic means. Chemical hydrolysis of RNA, however, typically produces 2′- and 3′-NMPs, which cannot be polymerized into RNA. Thus, the methods, compositions, and systems as provided herein primarily use enzymes for the depolymerization of endogenous RNA. An “enzyme that depolymerizes RNA” catalyzes the hydrolysis of the phosphodiester bonds between two nucleotides in a RNA. Thus, “an enzyme that depolymerizes RNA” converts RNA (polymeric RNA) into its monomeric form—nucleoside monophosphates (NMPs). Depending on the enzyme, enzymatic depolymerization of RNA may yield 3′-NMPs, 5′-NMPs or a combination of 3′-NMPs and 5′-NMPs. Because it is not possible to polymerize 3′-NTPs (converted from 3′-NDPs, which are converted from 3′-NMPs), enzymes (e.g., RNase R) that yield 5′-NMPs (which are then converted to 5′-NDPs, and then 5′-NTPs) are preferred. In some embodiments, enzymes that yield 3′-NMPs are removed from the genomic DNA of the engineered cell to increase efficiency of RNA production. In some embodiments, the enzyme used for RNA depolymerization is RNase R. In some embodiments, the concentration of RNase R used is 0.1-1.0 mg/mL (e.g., 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9. or 1.0 mg/mL). In some embodiments, the concentration of RNase R used is 0.4-0.6 mg/mL. In some embodiments, the concentration of RNase R used is 0.5 mg/mL. In some embodiments, the concentration of RNase R used is greater than 1.0 mg/mL.
Examples of enzymes that depolymerize RNA include, without limitation, nucleases, including ribonucleases (RNases, e.g., RNase R) and phosphodiesterases. Nucleases catalyze the degradation of nucleic acid into smaller components (e.g., monomers, also referred to as nucleoside monophosphates, or oligonucleotides). Phosphodiesterases catalyze degradation of phosphodiester bonds. These enzymes that depolymerize RNA may be encoded by full length genes or by gene fusions (e.g., DNA that includes at least two different genes (or fragments of genes) encoding at two different enzymatic activities).
RNase functions in cells to regulate RNA maturation and turn over. Each RNase has a specific substrate preferences—dsRNA or ssRNA. Thus, in some embodiments, a combination of different RNases, or a combination of different nucleases, generally, may be used to depolymerize biomass-derived polymeric RNA (e.g., endogenous RNA). For example, 1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9, or 1-10 different nucleases may be used in combination to depolymerize RNA. In some embodiments, at least 1, at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9, or at least 10 different nucleases may be used in combination to depolymerize RNA. Non-limiting examples of nucleases for use as provided herein are included in Table 1. In some embodiments, the nuclease used is RNase R.
Enzymes that depolymerize RNA (e.g., RNases) may be endogenous to a host cell (host-derived), or they may be encoded by engineered nucleic acids exogenously introduced into a host cell (e.g., on an episomal vector or integrated into the genome of the host cell).
In some embodiments, engineered nucleic acids encoding enzymes that depolymerize RNA are operably linked to an inducible promoter. Thus, in some embodiments, expression of an engineered nucleic acid encoding an enzyme that depolymerizes RNA is temporally or spatially regulated. For example, nucleic acids may be engineered to encode enzymes (e.g., RNases) that are relocated to or are sequestered in the periplasm of a host cell so that activity of the enzyme does not interfere with cell growth or other metabolic processes. Upon cell lysis, the relocated enzyme is released from the periplasm, brought into contact with the endogenous RNA, and depolymerizes the RNA into monomeric form. See, e.g., International Publication No. WO 2011/140516, published Nov. 10, 2011, incorporated herein by reference.
“Conditions that result in depolymerization of RNA” are known in the art or may be determined by one of ordinary skill in the art, taking into consideration, for example, optimal conditions for nuclease (e.g., RNase) activity, including pH, temperature, length of time, and salt concentration of the cell lysate as well as any exogenous cofactors. Examples including those described previously (see, e.g., Wong, C. H. et al. J. Am. Chem. Soc., 105: 115-117, 1983), EP1587947B1, Cheng Z F, Deutscher M P. J. Biol Chem. 277:21624-21629, 2002).
In some embodiments, metal ions (e.g., Mg2+) are depleted from the depolymerization reaction. In some embodiments, the concentration of metal ion (e.g., Mg2+) is 8 mM or less (e.g., less than 8 mM, less than 7 mM, less than 6 mM, less than 5 mM, less than 4 mM, less than 3 mM, less than 2 mM, less than 1 mM, less than 0.5 mM). In some embodiments, the concentration of metal ion (e.g., Mg2+) is 0.1 mM-8 mM, 0.1 mM-7 mM, or 0.1 mM-5 mM.
The pH of a cell lysate during a RNA depolymerization reaction may have a value of 3.0 to 8.0. In some embodiments, the pH value of a cell lysate is 3.0-8.0, 4.0-8.0, 5.0-8.0, 6.0-8.0, 7.0-8.0, 3.0-7.0, 4.0-7.0, 5.0-7.0, 6.0-7.0, 3.0-6.0, 4.0-6.0, 5.0-6.0, 3.0-5.0, 3.0-4.0, or 4.0-5.0. In some embodiments, the pH value of a cell lysate is 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, or 8.0. In some embodiments, the pH value of a cell lysate is 7.0, 7.1, 7.2, 7.3, 7.4, or 7.5. The pH of a cell lysate may be adjusted, as needed.
The temperature of a cell lysate during a RNA depolymerization reaction may be 15° C. to 70° C. In some embodiments, the temperature of a cell lysate during a RNA depolymerization reaction is 15-60° C., 15-50° C., 15-40° C., 15-30° C., 25-70° C., 25-60° C., 25-50° C., 25-40° C., 30-70° C., 30-60° C., or 30-50° C. In some embodiments, the temperature of a cell lysate during a RNA depolymerization reaction is 37° C. In some embodiments, the temperature of a cell lysate during a RNA depolymerization reaction is 15° C., 25° C., 32° C., 37° C., 40° C., 42° C., 45° C., 50° C., 55° C., 56° C., 57° C., 58° C., 59° C., 60° C., 61° C., 62° C., 63° C., 64° C., 65° C., 66° C., 67° C., 68° C., 69° C., or 70° C.
A cell lysate during a RNA depolymerization reaction may be incubated for 5 minutes (min) to 72 hours (hrs). In some embodiments, a cell lysate during a RNA depolymerization reaction is incubated for 5-10 min, 5-15 min, 5-20 min, 5-30 min, or 5 min-48 hrs. For example, a cell lysate during a RNA depolymerization reaction may be incubated for 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 45 min, 1 hr, 2 hrs, 3 hrs, 4 hrs, 5 hrs, 6 hrs, 7 hrs, 8 hrs, 9 hrs, 10 hrs, 11 hrs, 12 hrs, 18 hrs, 24 hrs, 30 hrs, 36 hrs, 42 hours, or 48 hours. In some embodiments, a cell lysate during a RNA depolymerization reaction is incubated for 24 hours at a temperature of 37° C. In some embodiments, a cell lysate during a RNA depolymerization reaction is incubated for 5-10 min at a temperature of 37° C. In some embodiments, a cell lysate during a RNA depolymerization reaction has a pH of 7.0 and is incubated for 15 minutes at a temperature of 37° C. In some embodiments, a cell lysate during a RNA depolymerization reaction may be incubated under conditions that result in greater than 65% conversion of RNA to 5′-NMPs. In some embodiments, RNA is converted to 5′-NMPs at a rate of (or at least) 50 mM/hr, 100 mM/hr or 200 mM/hr.
In some embodiments, salt is added to a cell lysate, for example, to prevent enzyme aggregation. For example, sodium chloride, potassium chloride, sodium acetate, potassium acetate, or a combination thereof, may be added to a cell lysate. The concentration of salt in a cell lysate during a RNA depolymerization reaction may be 5 mM to 1 M. In some embodiments, the concentration of salt in a cell lysate during a RNA depolymerization reaction 5 mM, 10 mM, 15 mM, 20 mM, 25 mM, 50 mM, 100 mM, 150 mM, 200 mM, 250 mM, 500 mM, 750 mM, or 1 M. In some embodiments, the cell lysate comprises a mixture that includes 40-60 mM potassium phosphate, 1-5 mM MnCl2, and/or 10-50 mM MgCl2 (e.g., 20 mM MgCl2).
In some embodiments, buffer is added to a cell lysate, for example, to achieve a particular pH value and/or salt concentration. Examples of buffers include, without limitation, phosphate buffer, Tris buffer, MOPS buffer, HEPES buffer, citrate buffer, acetate buffer, malate buffer, MES buffer, histidine buffer, PIPES buffer, bis-tris buffer, and ethanolamine buffer.
Depolymerization of RNA results in the production of 5′-NMP, including 5′-AMP, 5′-UMP, 5′-CMP, and 5′-GMP. While the NMP may be present in the cell lysate at relatively equimolar amounts, depolymerization of RNA does not result in any predetermined ratio of NMPs.
In some embodiments, 50-98% of the endogenous RNA in a cell upon lysis is converted to (depolymerized to) 5′-NMPs. For example, 50-95%, 50-90%, 50-85%, 50-80%, 75-98%, 75-95%, 75-90%, 75-85% or 75-80% RNA is converted to (depolymerized to) 5′-NMPs. In some embodiments, 65-70% of the endogenous RNA in a cell upon lysis is converted to (depolymerized to) 5′-NMPs. Lower yields are also acceptable.
Elimination of Futile CyclesFollowing conversion of RNA from biomass (e.g., endogenous RNA) to its monomeric constituents by endogenous and/or exogenous nucleases, there typically remains in the cell lysate several enzymes, including nucleases and phosphatases, which may have deleterious effects on RNA biosynthesis. For example, Escherichia coli has numerous phosphatases, many of which dephosphorylate NTPs, NDPs, and NMPs. Dephosphorylation of NMPs following RNA depolymerization results in the accumulation of the non-phosphorylated nucleosides and a loss of usable NMP substrate, thus reducing synthetic RNA yield. Dephosphorylation of NMPs, NDPs, or NTPs following RNA depolymerization results in futile energy cycles (energy cycles that produce a low yield of synthetic RNA) during which NMPs are phosphorylated to NDPs and NTPs, which in turn are dephosphorylated back to their NMP or nucleoside starting point. Futile cycles reduce the yield of RNA product per unit energy input (e.g., polyphosphate, ATP, or other sources of high energy phosphate). In some embodiments, the enzymatic activities are eliminated by removal from the host genome. In some embodiments, the enzymatic activities are eliminated by heat inactivation. In some embodiments, the enzymatic activities are eliminated by protease targeting. In some embodiments the enzymatic activities are eliminated through the use of chemical inhibitors. A combination of any of the foregoing approaches may also be used.
Enzymes deleterious to the biosynthesis of RNA, as provided herein, may be deleted from the host cell genome during the process of engineering the host cell, provided the enzymes are not essential for host cell (e.g., bacterial cell) survival and/or growth. Deletion of enzymes or enzyme activities may be achieved, for example, by deleting or modifying in the host cell genome a gene encoding the essential enzyme. An enzyme is “essential for host cell survival” if the enzyme is necessary for the survival of the host cell. That is, if a host cell cannot survive without expression and/or activity of a particular enzyme, then that enzyme is considered essential for host cell survival. Similarly, an enzyme is “essential for host cell growth” if the enzyme is necessary for the growth of the host cell. That is, if a host cell cannot divide and/or grow without expression and/or activity of a particular enzyme, then that enzyme is considered essential for host cell growth.
If enzymes deleterious to the biosynthesis of RNA are essential for host cell survival and/or growth, then it may not be possible to delete or modify the genes encoding the enzymes. In such instances, the enzymes may be heat inactivated. “Heat inactivation” refers to the process of heating a cell lysate to a temperature sufficient to inactivate (or at least partially inactivate) endogenous nucleases and phosphatases. Generally, the process of heat inactivation involves denaturation of (unfolding of) the deleterious enzyme. The temperature at which endogenous cellular proteins denature varies among organisms. In E. coli, for example, endogenous cellular enzymes generally denature at temperatures above 41° C. The denaturation temperature may be higher or lower than 41° C. for other organisms. Enzymes of a cell lysate, as provide here, may be heat inactivated at a temperature of 40° C.-95° C., or higher. In some embodiments, enzymes of a cell lysate may be heat inactivated at a temperature of 40-90° C., 40-80° C., 40-70° C., 40-60° C., 40-50° C., 50-80° C., 50-70° C., 50-60° C., 60-80° C., 60-70° C., or 70-80° C. For example, enzymes of a cell lysate may be heat inactivated at a temperature of 40° C., 42° C., 45° C., 50° C., 55° C., 60° C., 65° C., 70° C., 75° C., 80° C., 85° C., 90° C., or 95° C. In some embodiments, enzymes of a cell lysate may be heat inactivated at a temperature of 50-80° C. In some embodiments, enzymes of a cell lysate may be heat inactivated at a temperature of 70° C. In some embodiments, enzymes of a cell lysate may be heat inactivated at a temperature of 60° C. It may also be possible to introduce chemical inhibitors of deleterious enzymes. Such inhibitors may include, but are not limited to, sodium orthovanadate (inhibitor of protein phosphotyrosyl phosphatases), sodium fluoride (inhibitor of phosphoseryl and phosphothreonyl phosphatases), sodium pyrophosphate (phosphatase inhibitor), sodium phosphate, and/or potassium phosphate.
The period of time during which a cell lysate is incubated at elevated temperatures to achieve heat inactivation of endogenous enzymes may vary, depending, for example, on the volume of the cell lysate and the organism from which the cell lysate was prepared. In some embodiments, a cell lysate is incubated at a temperature of 35° C.-80° C. for 2 minutes (min) to 48 hours (hr). For example, a cell lysate may be incubated at a temperature of 35° C.-80° C. for 2 min, 4 min, 5 min, 10 min, 15 min, 30 min, 45 min, or 1 hr. In some embodiments, a cell lysate is incubated at a temperature of 35° C.-80° C. for 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 24, 36, 42, or 48 hr.
In some embodiments, enzymes are heat inactivated at a temperature of 60-80° C. for 10-20 min. In some embodiments, enzymes are heat inactivated at a temperature of 70° C. for 15 min.
In some embodiments, enzymes that depolymerize endogenous RNA comprise one or more modifications (e.g., mutations) that render the enzymes more sensitive to heat. These enzymes are referred to as “heat-sensitive enzymes.” Heat-sensitive enzymes denature and become inactivated at temperatures lower than that of their wild-type counterparts, and/or the period of time required to reduce the activity of the heat-sensitive enzymes is shorter than that of their wild-type counterparts.
It should be understood that enzymes that are heat inactivated may, in some instances, retain some degree of activity. For example, the activity level of a heat-inactivated enzyme may be less than 50% of the activity level of the same enzyme that has not been heat inactivated. In some embodiments, the activity level of a heat-inactivated enzyme is less than 40%, less than 30%, less than 20%, less than 10%, less than 5%, less than 1%, or less than 0.1% of the activity level of the same enzyme that has not been heat inactivated.
Thus, an enzyme's activity may be completely eliminated or reduced. An enzyme is considered completely inactive if the denatured (heat inactivated) form of the enzyme no longer catalyzes a reaction catalyzed by the enzyme in its native form. A heat-inactivated, denatured enzyme is considered “inactivated” when activity of the heat-inactivated enzyme is reduced by at least 50% relative to activity of the enzyme that is not heated (e.g., in its native environment). In some embodiments, activity of a heat-inactivated enzyme is reduced by 50-100% relative to the activity of the enzyme that is not heated. For example, activity of a heat-inactivated enzyme is reduced by 50-90%, 50-85%, 50-80%, 50-75%, 50-70%, 50-65%, 50-60%, or 50-55% relative to activity of the enzyme that is not heated. In some embodiments, the activity of a heat-inactivated enzyme is reduced by 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98%, 99%, or 100% relative to the activity of the enzyme that is not heated.
Examples of enzymes that may be heat inactivated, or deleted from the genome of a host cell, include, without limitation, nucleases (e.g., RNase III, RNase I, RNase R, PNPase, RNase II, and RNase T), phosphatases (e.g., nucleoside monophosphatase, nucleoside diphosphatase, nucleoside triphosphatase), and other enzymes that depolymerize RNA or dephosphorylate nucleotides. Enzymes that depolymerize RNA include any enzyme that is able to cleave, partially hydrolyze, or completely hydrolyze a RNA molecule. Table 2 provides a list of non-limiting examples of nucleases that may be heat inactivated or, in some instances, deleted from an engineered host cell. Table 3 provides a list of non-limiting examples of phosphatases that may be heat inactivated or, in some instances, deleted from an engineered host cell. Heat inactivation of these and other nucleases and phosphatases is encompassed by the present disclosure.
E. coli RNase III preferentially cleaves dsRNA as well as some single-stranded mRNA molecules. The presence of RNase III in cell lysate may limit the accumulation of high concentrations of synthetic RNA (e.g., dsRNA), because the synthetic RNA is readily cleaved. Neither RNase III nor the gene encoding RNase III, mc, is essential for cell viability, thus, in some embodiments, mc is deleted or mutated in engineered host cells. In other embodiments, RNase III is heat inactivated following depolymerization of endogenous RNA.
E. coli RNase I localizes to the periplasmic space in intact cells and catalyzes depolymerization of a wide range of RNA molecules, including rRNA, mRNA, and tRNA. Under physiological conditions the periplasmic localization of this enzyme means that the enzyme has little impact on RNA stability within the cell; however, mixing of the periplasm and cytoplasm in cell lysates permits RNase I access to cellular RNA. The presence of RNaseI in a cell lysate may reduce the yield of synthetic RNA through RNA degradation. Neither RNase I nor the gene encoding RNase I, ma, is essential for cell viability, thus, in some embodiments, ma is deleted or mutated in engineered host cells. In other embodiments, RNase I is heat inactivated following depolymerization of endogenous RNA.
E. coli RNase R and RNase T catalyze the depolymerization of dsRNA, rRNA, tRNA, and mRNA, as well as small unstructured RNA molecules. Neither the enzymes nor the genes encoding the enzymes, rnr and rnt, respectively, are essential for cell viability, thus, in some embodiments, rnr and/or rnt are deleted or mutated in engineered host cells (e.g., E. coli host cells). In other embodiments, RNase R and/or RNase T are heat inactivated following the depolymerization of endogenous RNA.
E. coli RNase E and PNPase are components of the degradasome, which is responsible for mRNA turnover in cells. RNase E is thought to function together with PNPase and RNase II to turn over cellular mRNA pools. Disruption of the gene encoding RNase E, me, is lethal in E. coli. Thus, in some embodiments, RNase E is heat inactivated following depolymerization of endogenous RNA. Neither PNPase nor the gene encoding PNPase, pnp, is essential for cell viability, thus, in some embodiments, pnp is deleted or mutated in engineered host cells (e.g., E. coli host cells). In other embodiments, PNPase is heat inactivated following depolymerization of endogenous RNA.
E. coli RNase II depolymerizes both mRNA and tRNA in a 3′ 4 5′ direction. Neither RNase II nor the gene encoding RNase II, rnb, is essential for cell viability, thus, in some embodiments, rnb is deleted or mutated in engineered host cells. In other embodiments, RNase II is heat inactivated following depolymerization of endogenous RNA.
While neither pnp nor rnb is essential to host cell survival, disruption of both simultaneously may be lethal. Thus, in some embodiments, both PNPase and RNase II are heat inactivated.
Phosphorylation of Nucleoside Monophosphates to Nucleoside TriphosphatesFollowing conversion of endogenous RNA to its monomeric form, and following heat inactivation of endogenous nucleases and phosphatases, the resulting nucleoside monophosphates (NMPs) in a cell lysate are phosphorylated before they are polymerized to form a desired synthetic RNA, such as a double-stranded RNA or single-stranded RNA (e.g., mRNA or antisense RNA). This process is highly energy dependent, thus, this process requires an energy source. The phosphates are typically donated from a high-energy phosphate source, such as, for example, phosphoenolpyruvate, ATP, or polyphosphate.
In some embodiments, the energy source is ATP that is directly added to a cell lysate. In other embodiments, the energy source is provided using an ATP regeneration system. For example, polyphosphate and polyphosphate kinase may be used to produce ATP. Other examples included the use of acetyl-phosphate and acetate kinase to produce ATP; phospho-creatine and creatine kinase to produce ATP; and phosphoenolpyruvate and pyruvate kinase to produce ATP. Other ATP (or other energy) regeneration systems may be used. In some embodiments, at least one component of the energy source is added to a cell lysate or cell lysate mixture. A “component” of an energy source includes the substrate(s) and enzyme(s) required to produce energy (e.g., ATP). Non-limiting examples of these components include polyphosphate, polyphosphate kinase, acetyl-phosphate, acetate kinase, phospho-creatine, creatine kinase, phosphoenolpyruvate, and pyruvate kinase.
A kinase is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules, such as ATP, to specific substrates/molecules. This process is referred to as phosphorylation, where the substrate gains a phosphate group and the high-energy ATP molecule donates a phosphate group. This transesterification produces a phosphorylated substrate and ADP. The kinases of the present disclosure, in some embodiments, convert the NMPs to NDPs and NDPs to NTPs.
In some embodiments, a kinase is a nucleoside monophosphate kinase, which catalyzes the transfer of a high-energy phosphate from ATP to an NMP, resulting in ADP and NDP. Non-limiting examples of nucleoside monophosphate kinases are provided in Tables 4 and 5. As discussed below, thermostable variants of the enzymes listed in Tables 4 and 5 are encompassed by the present disclosure. In some embodiments, a cell lysate comprises one or more (or all) of the following four nucleoside monophosphate kinases: thermostable uridylate kinase, thermostable cytidylate kinase, thermostable guanylate kinase and thermostable adenylate kinase. In some embodiments, UMP kinase is obtained from Pyrococcus furiosus (e.g., SEQ ID NO:3 or a variant comprising an amino acid sequence that is at least 70% identical to the amino acid sequence identified by SEQ ID NO:3). In some embodiments, CMP kinase is obtained from Thermus thermophilus (e.g., SEQ ID NO:4 or a variant comprising an amino acid sequence that is at least 70% identical to the amino acid sequence identified by SEQ ID NO:4). In some embodiments, GMP kinase is obtained from Thermotoga maritima (e.g., SEQ ID NO:5 or a variant comprising an amino acid sequence that is at least 70% identical to the amino acid sequence identified by SEQ ID NO:5). In some embodiments, AMP kinase is obtained from Thermus thermophilus (e.g., SEQ ID NO:6 or a variant comprising an amino acid sequence that is at least 70% identical to the amino acid sequence identified by SEQ ID NO:6).
Thus, in some embodiments, a NMP kinase has an amino acid sequence identified by the amino acid sequence of any one of SEQ ID NO: 3-6. In some embodiments, the NMP kinase has an amino acid sequence that is at least 70% identical to the amino acid sequence of any one of SEQ ID NO: 3-6. For example, the NMP kinase may have an amino acid sequence that is at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical to the amino acid sequence identified by any one of SEQ ID NO: 3-6.
It should be understood that the present disclosure encompasses the use of any one or more of the enzymes described herein as well as variants of the enzymes (e.g., “PPK2 variants”). Variant enzymes may share a certain degree of sequence identity with the reference enzyme. The term “identity” refers to a relationship between the sequences of two or more polypeptides or polynucleotides, as determined by comparing the sequences. Identity measures the percent of identical matches between the smaller of two or more sequences with gap alignments (if any) addressed by a particular mathematical model or computer program (e.g., “algorithms”). Identity of related molecules can be readily calculated by known methods. “Percent (%) identity” as it applies to amino acid or nucleic acid sequences is defined as the percentage of residues (amino acid residues or nucleic acid residues) in the candidate amino acid or nucleic acid sequence that are identical with the residues in the amino acid sequence or nucleic acid sequence of a second sequence after aligning the sequences and introducing gaps, if necessary, to achieve the maximum percent identity. Identity depends on a calculation of percent identity but may differ in value due to gaps and penalties introduced in the calculation. Variants of a particular sequence may have at least 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% but less than 100% sequence identity to that particular reference sequence, as determined by sequence alignment programs and parameters described herein and known to those skilled in the art.
The comparison of sequences and determination of percent identity between two sequences can be accomplished using a mathematical algorithm. Techniques for determining identity are codified in publicly available computer programs. Exemplary computer software to determine homology between two sequences include, but are not limited to, GCG program package (Devereux, J. et al. Nucleic Acids Research, 12(1): 387, 1984), the BLAST suite (Altschul, S. F. et al. Nucleic Acids Res. 25: 3389, 1997), and FASTA (Altschul, S. F. et al. J. Molec. Biol. 215: 403, 1990). Other techniques include: the Smith-Waterman algorithm (Smith, T. F. et al. J. Mol. Biol. 147: 195, 1981; the Needleman-Wunsch algorithm (Needleman, S. B. et al. J. Mol. Biol. 48: 443, 1970; and the Fast Optimal Global Sequence Alignment Algorithm (FOGSAA) (Chakraborty, A. et al. Sci Rep. 3: 1746, 2013).
In some embodiments, a kinase is a nucleoside diphosphate kinase, which transfers a phosphoryl group to NDP, resulting in NTP. The donor of the phosphoryl group may be, without limitation, ATP, polyphosphate polymer, or phosphoenolpyruvate. Non-limiting examples of kinases that convert NDP to NTP include nucleoside diphosphate kinase, polyphosphate kinase, and pyruvate kinase. As discussed below, thermostable variants of the foregoing enzymes are encompassed by the present disclosure. In some embodiments, the NDP kinase(s) is/are obtained from Aquifex aeolicus (e.g., SEQ ID NO: 9 or a variant comprising an amino acid sequence that is at least 70% identical to the amino acid sequence identified by SEQ ID NO:9). In some embodiments, the NDP kinase has an amino acid sequence that is at least 70% identical to the amino acid sequence identified by SEQ ID NO: 9. For example, the NDP kinase may have an amino acid sequence that is at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical to the amino acid sequence identified by SEQ ID NO: 9.
Phosphorylation of NMPs to NTPs occurs, in some embodiments, through the polyphosphate-dependent kinase pathway (
In some embodiments, the polyphosphate kinase belongs to the polyphosphate kinase 2 (PPK2) family. In some embodiments, the polyphosphate kinase belongs to a Class I PPK2 family, which transfers high-energy phosphate from polyphosphate to NDPs to form NTPs. ATP produced by the system is used as a high-energy phosphate donor to convert NMPs to NDPs. In some embodiments, the polyphosphate kinase belongs to a Class III PPK2 family, which transfers high-energy phosphate from polyphosphate to NMPs and NDPs to form NTPs. In some embodiments, Class III PPK2 is used alone to produce NTPs from NMPs. In other embodiments, Class III PPK2 is used in combination with other kinases. Class III PPK2 produces ATP from ADP, AMP, and polyphosphate, which is subsequently used by NMP and NDP kinases to convert NMPs to NTPs.
Non-limiting examples of PPK2 enzymes for use as provided herein are listed in Table 6 (SEQ ID NO: 8-18). Thus, in some embodiments, the PPK2 enzymes are thermostable. For example, the PPK2 enzymes may be thermostable Class III PPK2 enzymes, which favor ATP synthesis over polyphosphate polymerization, and convert both ADP and AMP to ATP. In some embodiments, the PPK2 enzymes are used to convert a polyphosphate, such as hexametaphosphate to ATP, at rates ranging, for example, from 10 to 800 mM per hour (e.g., 10, 15, 20, 25, 50, 75, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, or 800 mM per hour).
In some embodiments, the RNA biosynthesis methods of the present disclosure utilize a PPK2 enzyme that comprises an amino acid sequence identical to the amino acid sequence identified by any one of SEQ ID NO: 8-18. In some embodiments, the PPK2 enzyme comprises an amino acid sequence that is at least 70% identical to the amino acid sequence identified by any one of SEQ ID NO: 8-18. For example, the PPK2 enzyme may comprise an amino acid sequence that is at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical to the amino acid sequence identified by any one of SEQ ID NO: 8-18.
The present disclosure also encompasses fusion enzymes. Fusion enzymes may exhibit multiple activities, each corresponding to the activity of a different enzyme. For example, rather than using an independent nucleoside monophosphate kinase and an independent nucleoside diphosphate kinase, a fusion enzyme (or any other enzyme) having both nucleoside monophosphate kinase activity and nucleoside diphosphate kinase activity may be used.
The final step in the biosynthesis of a RNA of interest is the polymerization of NTPs to the RNA (e.g., dsRNA or ssRNA) end product using, for example, a DNA-dependent RNA polymerase. In this step of the process, a DNA designed to encode the RNA of interest serves as the template for the synthesis of the RNA of interest. The DNA template may be engineered, in some instances, to have a transcriptional promoter that selectively drives transcription of the RNA of interest. An example DNA template is shown in
Polymerization of RNA requires NTPs, a DNA template comprising a transcriptional promoter, and a polymerase (RNA polymerase) specific to the transcriptional promoter. Typically, a polymerase for use as provided herein is a single subunit polymerase, is highly selective for its cognate transcriptional promoters, has high fidelity, and is highly efficient. Examples of polymerases include, without limitation, T7 RNA polymerase, T3 RNA polymerase, and SP6 RNA polymerase. Bacteriophage T7 RNA polymerase is a DNA-dependent RNA polymerase that is highly specific for the T7 phage promoters. The 99 KD enzyme catalyzes in vitro RNA synthesis from a cloned DNA sequence under control of the T7 promoter. Bacteriophage T3 RNA polymerase is a DNA-dependent RNA polymerase that is highly specific for the T3 phage promoters. The 99 KD enzyme catalyzes in vitro RNA synthesis from a cloned DNA sequence under the T3 promoter. Bacteriophage SP6 RNA polymerase is a DNA-dependent RNA polymerase that is highly specific for the SP6 phage promoter. The 98.5 KD polymerase catalyzes in vitro RNA synthesis from a cloned DNA template under the SP6 promoter. Each of T7, T3, and SP6 polymerase are optimally active at 37-40° C. In some embodiments, thermostable variants of T7, T3, and SP6 polymerase are used. Thermostable variant polymerases are typically optimally active at temperatures above 40° C. (or about 50-60° C.).
“Conditions that result in production of nucleoside triphosphates and polymerization of the nucleoside triphosphates,” also referred to as “conditions for the biosynthesis of RNA,” may be determined by one of ordinary skill in the art, taking into consideration, for example, optimal conditions for polymerase activity, including pH, temperature, length of time, and salt concentration of the cell lysate as well as any exogenous cofactors.
The pH of a cell lysate during the biosynthesis of RNA may have a value of 3.0 to 8.0. In some embodiments, the pH value of a cell lysate is 3.0-8.0, 4.0-8.0, 5.0-8.0, 6.0-8.0, 7.0-8.0, 3.0-7.0, 4.0-7.0, 5.0-7.0, 6.0-7.0, 3.0-6.0, 4.0-6.0, 5.0-6.0, 3.0-5.0, 3.0-4.0, or 4.0-5.0. In some embodiments, the pH value of a cell lysate is 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, or 8.0. In some embodiments, the pH value of a cell lysate during biosynthesis of RNA is 7.0.
The temperature of a cell lysate during biosynthesis of RNA may be 15° C. to 70° C. In some embodiments, the temperature of a cell lysate during biosynthesis of RNA is 15-60° C., 15-50° C., 15-40° C., 15-30° C., 25-70° C., 25-60° C., 25-50° C., 25-40° C., 30-70° C., 30-60° C., 30-50° C., 40-70° C., 40-60° C., 40-50° C., 50-70° C., or 50-60° C. In some embodiments, the temperature of a cell lysate during biosynthesis of RNA is 15° C., 25° C., 32° C., 37° C., 42° C., 45° C., 55° C., 56° C., 57° C., 58° C., 59° C., 60° C., 61° C., 62° C., 63° C., 64° C., 65° C., 66° C., 67° C., 68° C., 69° C., or 70° C. In some embodiments, the temperature of a cell lysate during biosynthesis of RNA is 50° C.
A cell lysate during biosynthesis of RNA may be incubated for 15 minutes (min) to 72 hours (hrs). In some embodiments, a cell lysate during biosynthesis of RNA is incubated for 30 min-48 hrs. For example, a cell lysate during biosynthesis of RNA may be incubated for 30 min, 45 min, 1 hr, 2 hrs, 3 hrs, 4 hrs, 5 hrs, 6 hrs, 7 hrs, 8 hrs, 9 hrs, 10 hrs, 11 hrs, 12 hrs, 18 hrs, 24 hrs, 30 hrs, 36 hrs, 42 hours, or 48 hours. In some embodiments, a cell lysate during biosynthesis of RNA is incubated for 3 hours. In some embodiments, a cell lysate during biosynthesis of RNA is incubated for 24 hours at a temperature of 37° C.
In some embodiments, a cell lysate during biosynthesis of RNA is incubated at a pH of 7.0 for 2-4 hours at a temperature of 50° C.
Some polymerase activities may require the presence of metal ions. Thus, in some embodiments, metal ions are added to a cell lysate. Non-limiting examples of metal ions include Mg2+, Li+, Na+, K+, Ni2+, Ca2+, Cu2+, and Mn2+. Other metal ions may be used. In some embodiments, more than one metal ion may be used. The concentration of a metal ion in a cell lysate may be 0.1 mM to 100 mM, or 10 mM to 50 mM. In some embodiments, the concentration of a metal ion in a cell lysate is 0.1, 0.2, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5., 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, 10.0, 20.0, 25.0, 30.0, 35.0, 40.0, 45.0, 50.0, 60.0, 70.0, 80.0, 90.0, or 100.0 mM.
In some embodiments, salt is added to a cell lysate, for example, to prevent enzyme aggregation. For example, sodium chloride, potassium chloride, sodium acetate, potassium acetate, or a combination thereof, may be added to a cell lysate. The concentration of salt in a cell lysate during a RNA depolymerization reaction may be 5 mM to 1 M. In some embodiments, the concentration of salt in a cell lysate during a RNA depolymerization reaction 5 mM, 10 mM, 15 mM, 20 mM, 25 mM, 50 mM, 100 mM, 150 mM, 200 mM, 250 mM, 500 mM, 750 mM, or 1 M.
Thermostable EnzymesOne advantage of the cell-free RNA-biosynthesis methods of the present disclosure is that all of the enzymes needed to convert endogenous RNA to synthetic double-stranded RNA, for example, may be (but need not be) expressed in a single engineered cell. For example, a clonal population of the engineered cell is cultured to a desired cell density, the cells are lysed, incubated under conditions that result in depolymerization of endogenous RNA to its monomer form (e.g., at a temperature of 30-37° C.), subjected to temperatures sufficient to inactivate endogenous nucleases and phosphatases (e.g., 40-90° C.), and incubated under conditions that result in the polymerization of RNA (e.g., dsRNA or ssRNA) (e.g., 30-50° C.). In order to proceed to end product synthetic RNA, the enzymes required for conversion of NMPs to NDPs (e.g., nucleoside monophosphate kinases and/or polyphosphate kinases), from NDPs to NTPs (e.g., nucleoside diphosphate kinases and/or polyphosphate kinase), and from NTPs to RNA (e.g., polymerase) should be thermostable to avoid denaturation during heat inactivation of the endogenous nuclease (and/or exogenous nucleases) and phosphatases. Thermostability refers to the quality of enzymes to resist denaturation at relatively high temperature. For example, if an enzyme is denatured (inactivated) at a temperature of 42° C., an enzyme having similar activity (e.g., kinase activity) is considered “thermostable” if it does not denature at 42° C.
An enzyme (e.g., kinase or polymerase) is considered thermostable if the enzyme (a) retains activity after temporary exposure to high temperatures that denature other native enzymes or (b) functions at a high rate after temporary exposure to a medium to high temperature where native enzymes function at low rates.
In some embodiments, a thermostable enzyme retains greater than 50% activity following temporary exposure to relatively high temperature (e.g., higher than 41° C. for kinases obtained from E. coli, higher than 37° C. for many RNA polymerases) that would otherwise denature a similar (non-thermostable) native enzyme. In some embodiments, a thermostable enzyme retains 50-100% activity following temporary exposure to relatively high temperature that would otherwise denature a similar (non-thermostable) native enzyme. For example, a thermostable enzyme may retain 50-90%, 50-85%, 50-80%, 50-75%, 50-70%, 50-65%, 50-60%, or 50-55% activity following temporary exposure to relatively high temperature that would otherwise denature a similar (non-thermostable) native enzyme. In some embodiments, a thermostable enzyme retains 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98%, 99%, or 100% activity following temporary exposure to relatively high temperature that would otherwise denature a similar (non-thermostable) native enzyme.
In some embodiments, the activity of a thermostable enzyme after temporary exposure to medium to high temperature (e.g., 42-80° C.) is greater than (e.g., 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98%, 99%, or 100% greater than) the activity of a similar (non-thermostable) native enzyme.
The activity of a thermostable kinase, for example, may be measured by the amount of NMP or NDP the kinase is able to phosphorylate. Thus, in some embodiments, a thermostable kinase, at relatively high temperature (e.g., 42° C.) converts greater than 50% of NMP to NDP, or greater than 50% of NDP to NTP, in the same amount of time required to complete a similar conversion at 37° C. In some embodiments, a thermostable kinase, at relatively high temperature (e.g., 42° C.) converts greater than 60% of NMP to NDP, or greater than 60% of NDP to NTP, in the same amount of time required to complete a similar conversion at 37° C. In some embodiments, a thermostable kinase, at relatively high temperature (e.g., 42° C.) converts greater than 70% of NMP to NDP, or greater than 70% of NDP to NTP, in the same amount of time required to complete a similar conversion at 37° C. In some embodiments, a thermostable kinase, at relatively high temperature (e.g., 42° C.) converts greater than 80% of NMP to NDP, or greater than 80% of NDP to NTP, in the same amount of time required to complete a similar conversion at 37° C. In some embodiments, a thermostable kinase, at relatively high temperature (e.g., 42° C.) converts greater than 90% of NMP to NDP, or greater than 90% of NDP to NTP, in the same amount of time required to complete a similar conversion at 37° C.
The activity of a thermostable polymerase, for example, is assessed based on fidelity and polymerization kinetics (e.g., rate of polymerization). Thus, one unit of a thermostable T7 polymerase, for example, may incorporate 10 nmoles of NTP into acid insoluble material in 30 minutes at temperatures above 37° C. (e.g., at 50° C.).
Thermostable enzymes (e.g., kinases or polymerases) may remain active (able to catalyze a reaction) at a temperature of 42° C. to 80° C., or higher. In some embodiments, thermostable enzymes remain active at a temperature of 42-80° C., 42-70° C., 42-60° C., 42-50° C., 50-80° C., 50-70° C., 50-60° C., 60-80° C., 60-70° C., or 70-80° C. For example, thermostable enzymes may remain active at a temperature of 42° C., 43° C., 44° C., 45° C., 46° C., 47° C., 48° C., 49° C., 50° C., 51° C., 52° C., 53° C., 54° C., 55° C., 55° C., 56° C., 57° C., 58° C., 59° C., 60° C., 61° C., 62° C., 63° C., 64° C., 65° C., 66° C., 67° C., 68° C., 69° C., 70° C., 71° C., 72° C., 73° C., 74° C., 75° C., 76° C., 77° C., 78° C., 79° C., or 80° C. Thermostable enzymes may remain active at relatively high temperatures for 15 minutes to 48 hours, or longer. For example, thermostable enzymes may remain active at relatively high temperatures for 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 24, 36, 42, or 48 hours.
Non-limiting examples of thermostable NMP kinases are listed in Tables 5 and 7. Other thermostable kinases include thermostable nucleoside diphosphate kinases, thermostable pyruvate kinases, and thermostable polyphosphate kinases (see, e.g., Table 6). Other thermostable kinases are encompassed by the present disclosure.
Non-limiting examples of RNA polymerases are listed in Table 8. Other RNA polymerases, including thermostable RNA polymerases, are encompassed by the present disclosure.
Thermostable RNA polymerases may be prepared by modifying wild-type enzymes. Such modifications (e.g., mutations) are known. For example, variant thermostable T7 RNA polymerases may include one or more of the following point mutations: V426L, A702V, V795I, S430P, F849I, S633I, F880Y, C510R, and S767G (EP2377928 and EP1261696A1, each of which is incorporated herein by reference). In some embodiments, a variant thermostable T7 RNA polymerase includes V426L, A702V, and V795I mutations. In some embodiments, a variant thermostable T7 RNA polymerase includes S430P, F849I, S633I, and F880Y mutations. In some embodiments, a variant thermostable T7 RNA polymerase includes F880Y, S430P, F849I, S633I, C510R, and S767G mutations. In some embodiments, a variant thermostable T7 RNA polymerase includes Y639V, H784G, E593G, and V685A mutations. In some embodiments, a variant thermostable T7 RNA polymerase includes S430P, N433T, S633P, F849I, and F880Y mutations. Other variant and recombinant thermostable polymerases are encompassed by the present disclosure.
In some embodiments, a thermostable T7 polymerase is used to produce a RNA of interest. For example, a thermostable T7 polymerase (e.g., incubated at a temperature of 40-60° C.) having a concentration of 1-2% total protein may be used to synthesize RNA of interest at a rate of greater than 2 g/L/hr (or, e.g., 2 g/L/hr-10 g/L/hr). As another example, a thermostable T7 polymerase (e.g., incubated at a temperature of 40-60° C.) having a concentration of 3-5% total protein may be used to synthesize RNA of interest at a rate of greater than 10 g/L/hr (or, e.g., 10 g/L/hr-20 g/L/hr).
It should be understood that while many embodiments of the present disclosure describe the use of thermostable polymerases/enzymes, other enzymes/polymerases may be used. In some embodiments, polymerase may be exogenously added to heat-inactivated cell lysates, for example, to compensate for any reduction or loss of activity of the thermostable enzyme(s).
RNA of InterestMethods of the present disclosure are used to biosynthesize a RNA of interest. The RNA may be single-stranded or double-stranded. In some embodiments, the RNA is a double-stranded RNA interference molecule. For example, a RNA of interest may be an siRNA or a hairpin RNA interference molecule. As discussed above, a RNA of interest is encoded by a DNA template, examples of which are shown in
A double-stranded hairpin stem domain is formed by the binding of two complementary nucleic acid domains (e.g., discrete nucleotide sequences) to each other. Nucleic acid domains are “complementary” if they bind (base pair via Watson-Crick interactions, hybridize) to each other to form a double-stranded nucleic acid. The complementary domains of a DNA template encoding a RNA of interest may vary, depending, for example, on the desired end product. Complementary domains may have a length of, for example, 4 to 1000 nucleotides, or longer. For example, complementary domains may have a length of 4 to 10, 4 to 20, 4 to 30, 4 to 50, 4 to 60, 4 to 70, 4 to 80, 4 to 90, 4 to 100, 4 to 200, 4 to 300, 4 to 400, or 4 to 500, or 4 to 1000 nucleotides. In some embodiments complementary domains have a length of 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 nucleotides. In some embodiments, complementary domains have a length of 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100 nucleotides.
A hairpin loop domain is also formed by binding of two complementary nucleic acid domains. A hairpin loop domain is the intervening sequence between two complementary domains. Typically, a hairpin loop domain is non-specific, meaning that it is not designed to bind intramolecularly or to another nucleic acid. A hairpin loop domain forms a loop-like structure upon binding of the complementary domains to form a double-stranded hairpin stem domain. In some embodiments, a hairpin loop domain has a length of 4 to 500 nucleotides, or more. For example, a hairpin loop domain may have a length of 4 to 10, 4 to 20, 4 to 30, 4 to 50, 4 to 60, 4 to 70, 4 to 80, 4 to 90, 4 to 100, 4 to 200, 4 to 300, 4 to 400, or 4 to 500 nucleotides. In some embodiments, a hairpin loop domain has a length of 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100 nucleotides.
A “double-stranded RNA” of the present disclosure encompasses wholly double-stranded molecules, which do not contain a single-stranded region (e.g., a loop or overhang), as well as partially double-stranded molecules, which contain a double-stranded region and a single-stranded region (e.g., a loop or overhang). The dsRNA product depicted at the bottom of
Examples of “single-stranded RNA” of interest include messenger RNA (mRNA) and antisense RNA. Thus, provided herein are methods of synthesizing mRNA and other single-stranded RNA molecules.
These methods may comprise (a) lysing cultured engineered cells that comprise RNA, an enzyme that depolymerizes RNA, thermostable kinases, a thermostable RNA polymerase, thereby producing a cell lysate, (b) incubating the cell lysate produced in step (a) under conditions that result in depolymerization of RNA, thereby producing a cell lysate that comprises nucleoside monophosphates, (c) heating the cell lysate produced in step (b) to a temperature that inactivates endogenous nucleases and phosphatases without inactivating the thermostable kinases and thermostable RNA polymerase, thereby producing a cell lysate that comprises heat-inactivated nucleases and phosphatases, and (d) incubating the cell lysate produced in (c) in the presence of an energy source and an engineered DNA template containing a promoter operably linked to a nucleotide sequence encoding a RNA of interest, under conditions that result in production of nucleoside triphosphates and polymerization of the nucleoside triphosphates, thereby producing a cell lysate that comprises the mRNA of interest.
Alternatively, such methods may comprise (a) combining cell lysates obtained from engineered cells that comprise endogenous, polymeric RNA, an enzyme that depolymerizes RNA, thermostable nucleoside monophosphate (NMP) kinases, thermostable nucleoside diphosphate (NDP) kinases, a thermostable PPK2 kinase, and/or a polyphosphate, to produce a cell lysate mixture, (b) incubating the cell lysate mixture produced in step (a) under conditions that result in depolymerization of RNA, thereby producing a cell lysate that comprises nucleoside monophosphates, (c) heating the cell lysate produced in step (b) to a temperature that inactivates phosphatases and RNases (and any other activities that may be detrimental to RNA stability or polymerization fidelity, such as native RNA polymerase, NMP reductases, and/or nucleosidases) without inactivating the thermostable kinase and thermostable RNA polymerase, thereby producing a cell lysate that comprises heat-inactivated phosphatases and RNases (and other deleterious cellular activities), and (d) incubating the cell lysate produced in step (c) in the presence of an energy source and an engineered DNA template containing a promoter operably linked to a nucleotide sequence encoding a RNA of interest, under conditions that result in production of nucleoside triphosphates and polymerization of the nucleoside triphosphates, thereby producing a cell lysate that comprises mRNA.
In some embodiments, the DNA template encoding the RNA containing a single target domain is transcribed using a DNA-dependent RNA polymerase, such as, for example, a T7 RNA polymerase, and the resulting RNA transcript serves as a template for a RNA-dependent RNA polymerase, such as, for example, the phage ϕ6 RdRP, to synthesize a complementary RNA molecule, yielding a dsRNA. See, e.g.,
In some embodiments, the engineered cells comprise a DNA template encoding the RNA of interest. A DNA template encoding the RNA may be integrated into the genomic DNA of the engineered cells, or a DNA template may be introduced into the engineered cells on a plasmid. In other embodiments, the DNA template is added to the cell lysate during biosynthesis of the RNA of interest (e.g., following a heat inactivation step). In some embodiments, the concentration of the DNA template in a cell lysate is 0.05-1 μg/μl. In some embodiments, the concentration of the DNA template in a cell lysate is 0.05 μg/μl, 0.1 μg/μl, 0.5 μg/μl, 1.0 μg/μl.
As discussed above, other examples of RNA end products of interest include messenger RNA (mRNA) and short/small-interfering RNA (siRNA) (a synthetic RNA duplex designed to specifically target a particular mRNA for degradation).
In some embodiments, the concentration of RNA end product (biosynthesized RNA of interest) is at least 1 g/L to 50 g/L of cell lysate. For example, the concentration of RNA end product may be 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, or 50 g/L, or more.
In some embodiments, a RNA of interest is designed to bind to a target nucleic acid of interest and is used, for example, as a therapeutic, prophylactic, or diagnostic agent.
Protease TargetingEngineered cells of the present disclosure may express (e.g., endogenously express) enzymes necessary for the health of the cells that may have a negative impact on the production of nucleic acids, such as RNA. Such enzymes are referred to herein as “target enzymes.” For example, target enzymes expressed by engineered cells may compete for substrates or cofactors with an enzyme that increases the rate of precursor supplied to a RNA biosynthetic pathway. As another example, target enzymes expressed by the engineered cells may compete for substrates or cofactors with an enzyme that is a key pathway entry enzyme of a RNA biosynthetic pathway. As yet another example, target enzymes expressed by the engineered cells may compete for substrates or cofactors with an enzyme that supplies a substrate or cofactor of a RNA biosynthetic pathway.
To negate, or reduce, this negative impact, target enzymes can be modified to include a site-specific protease-recognition sequence in their protein sequence such that the target enzyme may be “targeted” and cleaved for inactivation during RNA production (see, e.g., U.S. Publication No. 2012/0052547 A1, published on Mar. 1, 2012; and International Publication No. WO 2015/021058 A2, published Feb. 12, 2015, each of which is incorporated by reference herein).
Cleavage of a target enzyme containing a site-specific protease-recognition sequence results from contact with a cognate site-specific protease is sequestered in the periplasm of cell (separate from the target enzyme) during the cell growth phase (e.g., as engineered cells are cultured) and is brought into contact with the target enzyme during the RNA production phase (e.g., following cell lysis to produce a cell lysate). Thus, engineered cells of the present disclosure comprise, in some embodiments, (i) an engineered nucleic acid encoding a target enzyme that negatively impacts the rate of RNA production and includes a site-specific protease-recognition sequence in the protein sequence of the target enzyme, and (ii) an engineered nucleic acid encoding a site-specific protease that cleaves the site-specific protease-recognition sequence of the target enzyme and includes a periplasmic-targeting sequence. This periplasmic-targeting sequence is responsible for sequestering the site-specific protease to the periplasmic space of the cell until the cell is lysed. Examples of periplasmic-targeting sequences are provided below.
Examples of proteases that may be used in accordance with the present disclosure include, without limitation, alanine carboxypeptidase, proteases obtained from Armillaria mellea, astacin, bacterial leucyl aminopeptidase, cancer procoagulant, cathepsin B, clostripain, cytosol alanyl aminopeptidase, elastase, endoproteinase Brg-C, enterokinase, gastricsin, gelatinase, Gly-X carboxypeptidase, glycyl endopeptidase, human rhinovirus 3C protease, hypodermin C, Iga-specific serine endopeptidase, leucyl aminopeptidase, leucyl endopeptidase, lysC, lysosomal pro-X carboxypeptidase, lysyl aminopeptidase, methionyl aminopeptidase, myxobacter, nardilysin, pancreatic endopeptidase E, picornain 2B, picornain 3C, proendopeptidase, prolyl aminopeptidase, proprotein convertase I, proprotein convertase II, russellysin, saccharopepsin, semenogelase, T-plasminogen activator, thrombin, tissue kallikrein, proteases obtained from tobacco etch virus (TEV), togavirin, tryptophanyl aminopeptidase, U-plasminogen activator, V8, venombin B, venombin BB and Xaa-pro aminopeptidase.
Periplasmic TargetingEnzymes of a nucleic acid (e.g., RNA) biosynthetic pathway may include at least one enzyme that has a negative impact on the health (e.g., viability) of a cell. To negate or reduce this negative impact, an enzyme can be modified to include a relocation sequence such that the enzyme is relocated to a cellular or extra-cellular compartment where it is not naturally located and where the enzyme does not negatively impact the health of the cell (see, e.g., Publication No. US-2011-0275116-A1, published on Nov. 10, 2011, incorporated by reference herein). For example, an enzyme of a biosynthetic pathway may be relocated to the periplasmic space of a cell.
Thus, in some embodiments, engineered cells of the present disclosure comprise at least one enzyme of a nucleic acid (e.g., RNA) biosynthetic pathway that is linked to a periplasmic-targeting sequence. A “periplasmic-targeting sequence” is an amino acid sequence that targets to the periplasm of a cell the protein to which it is linked. A protein that is linked to a periplasmic-targeting sequence will be sequestered in the periplasm of the cell in which the protein is expressed.
Periplasmic-targeting sequences may be derived from the N-terminus of bacterial secretory protein, for example. The sequences vary in length from about 15 to about 70 amino acids. The primary amino acid sequences of periplasmic-targeting sequences vary, but generally have a common structure, including the following components: (i) the N-terminal part has a variable length and generally carries a net positive charge; (ii) following is a central hydrophobic core of about 6 to about 15 amino acids; and (iii) the final component includes four to six amino acids which define the cleavage site for signal peptidases.
Periplasmic-targeting sequences of the present disclosure, in some embodiments, may be derived from a protein that is secreted in a Gram negative bacterium. The secreted protein may be encoded by the bacterium, or by a bacteriophage that infects the bacterium. Examples of Gram negative bacterial sources of secreted proteins include, without limitation, members of the genera Escherichia, Pseudomonas, Klebsiella, Salmonella, Caulobacter, Methylomonas, Acetobacter, Achromobacter, Acinetobacter, Aeromonas, Agrobacterium, Alcaligenes, Azotobacter, Burkholderia, Citrobacter, Comamonas, Enterobacter, Erwinia, Rhizobium, Vibrio, and Xanthomonas.
Examples of periplasmic-targeting sequences for use in accordance with the present disclosure include, without limitation, sequences selected from the group consisting of: MKIKTGARILALSALTTMMFSASALA (SEQ ID NO: 19); MKQSTIALALLPLLFTPVTKA (SEQ ID NO: 20); MMITLRKLPLAVAVAAGVMSAQAMA (SEQ ID NO: 21); MNKKVLTLSAVMASMLFGAAAHA (SEQ ID NO: 22); MKYLLPTAAAGLLLLAAQPAMA (SEQ ID NO: 23); MKKIWLALAGLVLAFSASA (SEQ ID NO: 24); MMTKIKLLMLIIFYLIISASAHA (SEQ ID NO: 25); MKQALRVAFGFLILWASVLHA (SEQ ID NO: 26); MRVLLFLLLSLFMLPAFS (SEQ ID NO: 27); and MANNDLFQASRRRFLAQLGGLTVAGMLGPSLLTPRRATA (SEQ ID NO: 28).
Engineered CellsEngineered cells of the present disclosure typically comprise at least one, most, or all, of the enzymatic activities required to biosynthesize RNA. “Engineered cells” are cells that comprise at least one engineered (e.g., recombinant or synthetic) nucleic acid, or are otherwise modified such that they are structurally and/or functionally distinct from their naturally-occurring counterparts. Thus, a cell that contains an engineered nucleic acid is considered an “engineered cell.”
Engineered cells of the present disclosure, in some embodiments, comprise RNA, enzymes that depolymerizes RNA, thermostable kinases, and/or thermostable polymerases. In some embodiments, the engineered cells further comprise a DNA template containing a promoter operably linked to a nucleotide sequence encoding a RNA of interest.
Engineered cells, in some embodiments, express selectable markers. Selectable markers are typically used to select engineered cells that have taken up and expressed an engineered nucleic acid following transfection of the cell (or following other procedure used to introduce foreign nucleic acid into the cell). Thus, a nucleic acid encoding product may also encode a selectable marker. Examples of selectable markers include, without limitation, genes encoding proteins that increase or decrease either resistance or sensitivity to antibiotics (e.g., ampicillin resistance genes, kanamycin resistance genes, neomycin resistance genes, tetracycline resistance genes and chloramphenicol resistance genes) or other compounds. Additional examples of selectable markers include, without limitation, genes encoding proteins that enable the cell to grow in media deficient in an otherwise essential nutrient (auxotrophic markers). Other selectable markers may be used in accordance with the present disclosure.
An engineered cell “expresses” a product if the product, encoded by a nucleic acid (e.g., an engineered nucleic acid), is produced in the cell. It is known in the art that gene expression refers to the process by which genetic instructions in the form of a nucleic acid are used to synthesize a product, such as a protein (e.g., an enzyme).
Engineered cells may be prokaryotic cells or eukaryotic cells. In some embodiments, engineered cells are bacterial cells, yeast cells, insect cells, mammalian cells, or other types of cells.
Engineered bacterial cells of the present disclosure include, without limitation, engineered Escherichia spp., Streptomyces spp., Zymomonas spp., Acetobacter spp., Citrobacter spp., Synechocystis spp., Rhizobium spp., Clostridium spp., Corynebacterium spp., Streptococcus spp., Xanthomonas spp., Lactobacillus spp., Lactococcus spp., Bacillus spp., Alcaligenes spp., Pseudomonas spp., Aeromonas spp., Azotobacter spp., Comamonas spp., Mycobacterium spp., Rhodococcus spp., Gluconobacter spp., Ralstonia spp., Acidithiobacillus spp., Microlunatus spp., Geobacter spp., Geobacillus spp., Arthrobacter spp., Flavobacterium spp., Serratia spp., Saccharopolyspora spp., Thermus spp., Stenotrophomonas spp., Chromobacterium spp., Sinorhizobium spp., Saccharopolyspora spp., Agrobacterium spp., and Pantoea spp.
Engineered yeast cells of the present disclosure include, without limitation, engineered Saccharomyces spp., Schizosaccharomyces, Hansenula, Candida, Kluyveromyces, Yarrowia and Pichia.
In some embodiments, engineered cells of the present disclosure are engineered Escherichia coli cells, Bacillus subtilis cells, Pseudomonas putida cells, Saccharomyces cerevisae cells, or Lactobacillus brevis cells. In some embodiments, engineered cells of the present disclosure are engineered Escherichia coli cells.
Engineered Nucleic AcidsA “nucleic acid” is at least two nucleotides covalently linked together, and in some instances, may contain phosphodiester bonds (e.g., a phosphodiester “backbone”). Nucleic acids (e.g., components, or portions, of nucleic acids) may be naturally occurring or engineered. “Naturally occurring” nucleic acids are present in a cell that exists in nature in the absence of human intervention. “Engineered nucleic acids” include recombinant nucleic acids and synthetic nucleic acids. A “recombinant nucleic acid” refers to a molecule that is constructed by joining nucleic acid molecules (e.g., from the same species or from different species) and, typically, can replicate in a living cell. A “synthetic nucleic acid” refers to a molecule that is biologically synthesized, chemically synthesized, or by other means synthesized or amplified. A synthetic nucleic acid includes nucleic acids that are chemically modified or otherwise modified but can base pair with naturally-occurring nucleic acid molecules. Recombinant and synthetic nucleic acids also include those molecules that result from the replication of either of the foregoing. Engineered nucleic acids may contain portions of nucleic acids that are naturally occurring, but as a whole, engineered nucleic acids do not occur naturally and require human intervention. In some embodiments, a nucleic acid encoding a product of the present disclosure is a recombinant nucleic acid or a synthetic nucleic acid. In other embodiments, a nucleic acid encoding a product is naturally occurring.
An engineered nucleic acid encoding RNA, as provided herein, may be operably linked to a “promoter,” which is a control region of a nucleic acid at which initiation and rate of transcription of the remainder of a nucleic acid are controlled. A promoter drives expression or drives transcription of the nucleic acid that it regulates.
A promoter may be one naturally associated with a gene or sequence, as may be obtained by isolating the 5′ non-coding sequences located upstream of the coding segment of a given gene or sequence. Such a promoter can be referred to as “endogenous.”
In some embodiments, a coding nucleic acid sequence may be positioned under the control of a recombinant or heterologous promoter, which refers to a promoter that is not normally associated with the encoded sequence in its natural environment. Such promoters may include promoters of other genes; promoters isolated from any other cell; and synthetic promoters or enhancers that are not “naturally occurring” such as, for example, those that contain different elements of different transcriptional regulatory regions and/or mutations that alter expression through methods of genetic engineering that are known in the art. In addition to producing nucleic acid sequences of promoters and enhancers synthetically, sequences may be produced using recombinant cloning and/or nucleic acid amplification technology, including polymerase chain reaction (PCR).
A promoter is considered to be “operably linked” when it is in a correct functional location and orientation in relation to the nucleic acid it regulates to control (“drive”) transcriptional initiation and/or expression of that nucleic acid.
Engineered nucleic acids of the present disclosure may contain a constitutive promoter or an inducible promoter. A “constitutive promoter” refers to a promoter that is constantly active in a cell. An “inducible promoter” refers to a promoter that initiates or enhances transcriptional activity when in the presence of, influenced by, or contacted by an inducer or inducing agent, or activated in the absence of a factor that causes repression. Inducible promoters for use in accordance with the present disclosure include any inducible promoter described herein or known to one of ordinary skill in the art. Examples of inducible promoters include, without limitation, chemically/biochemically-regulated and physically-regulated promoters such as alcohol-regulated promoters, tetracycline-regulated promoters, steroid-regulated promoters, metal-regulated promoters, pathogenesis-regulated promoters, temperature/heat-inducible, phosphate-regulated (e.g., PhoA), and light-regulated promoters.
An inducer or inducing agent may be endogenous or a normally exogenous condition (e.g., light), compound (e.g., chemical or non-chemical compound) or protein that contacts an inducible promoter in such a way as to be active in regulating transcriptional activity from the inducible promoter. Thus, a “signal that regulates transcription” of a nucleic acid refers to an inducer signal that acts on an inducible promoter. A signal that regulates transcription may activate or inactivate transcription, depending on the regulatory system used. Activation of transcription may involve directly acting on a promoter to drive transcription or indirectly acting on a promoter by inactivation a repressor that is preventing the promoter from driving transcription. Conversely, deactivation of transcription may involve directly acting on a promoter to prevent transcription or indirectly acting on a promoter by activating a repressor that then acts on the promoter.
Engineered nucleic acids may be introduced into host cells using any means known in the art, including, without limitation, transformation, transfection (e.g., chemical (e.g., calcium phosphate, cationic polymers, or liposomes) or non-chemical (e.g., electroporation, sonoporation, impalefection, optical transfection, hydrodynamic transfection)), and transduction (e.g., viral transduction).
Enzymes or other proteins encoded by a naturally-occurring, intracellular nucleic acid may be referred to as “endogenous enzymes” or “endogenous proteins.”
Cell Cultures and Cell LysatesTypically, engineered cells are cultured. “Culturing” refers to the process by which cells are grown under controlled conditions, typically outside of their natural environment. For example, engineered cells, such as engineered bacterial cells, may be grown as a cell suspension in liquid nutrient broth, also referred to as liquid “culture medium.”
Examples of commonly used bacterial Escherichia coli growth media include, without limitation, LB (Lysogeny Broth) Miller broth (1% NaCl): 1% peptone, 0.5% yeast extract, and 1% NaCl; LB (Lysogeny Broth) Lennox Broth (0.5% NaCl): 1% peptone, 0.5% yeast extract, and 0.5% NaCl; SOB medium (Super Optimal Broth): 2% peptone, 0.5% Yeast extract, 10 mM NaCl, 2.5 mM KCl, 10 mM MgCl2, 10 mM MgSO4; SOC medium (Super Optimal broth with Catabolic repressor): SOB+20 mM glucose; 2×YT broth (2× Yeast extract and Tryptone): 1.6% peptone, 1% yeast extract, and 0.5% NaCl; TB (Terrific Broth) medium: 1.2% peptone, 2.4% yeast extract, 72 mM K2HPO4, 17 mM KH2PO4 and 0.4% glycerol; and SB (Super Broth) medium: 3.2% peptone, 2% yeast extract, and 0.5% NaCl and or Korz medium (Korz, D J et al. 1995).
Examples of high density bacterial Escherichia coli growth media include, but are not limited to, DNAGro™ medium, ProGro™ medium, AutoX™ medium, DetoX™ medium, InduX™ medium, and SecPro™ medium.
In some embodiments, engineered cells are cultured under conditions that result in expression of enzymes or nucleic acids. Such culture conditions may depend on the particular product being expressed and the desired amount of the product.
In some embodiments, engineered cells are cultured at a temperature of 30° C. to 40° C. For example, engineered cells may be cultured at a temperature of 30° C., 31° C., 32° C., 33° C., 34° C., 35° C., 36° C., 37° C., 38° C., 39° C. or 40° C. Typically, engineered cells, such as engineered E. coli cells, are cultured at a temperature of 37° C.
In some embodiments, engineered cells are cultured for a period of time of 12 hours to 72 hours, or more. For example, engineered cells may be cultured for a period of time of 12, 18, 24, 30, 36, 42, 48, 54, 60, 66, or 72 hours. Typically, engineered cells, such as engineered bacterial cells, are cultured for a period of time of 12 to 24 hours. In some embodiments, engineered cells are cultured for 12 to 24 hours at a temperature of 37° C.
In some embodiments, engineered cells are cultured (e.g., in liquid cell culture medium) to an optical density, measured at a wavelength of 600 nm (OD600), of 5 to 200. In some embodiments, engineered cells are cultured to an OD600 of 5, 10, 15, 20, 25, 50, 75, 100, 150, or 200.
In some embodiments, engineered cells are cultured to a density of 1×108 (OD<1) to 2×1011 (OD ˜200) viable cells/ml cell culture medium. In some embodiments, engineered cells are cultured to a density of 1×108, 2×108, 3×108, 4×108, 5×108, 6×108, 7×108, 8×108, 9×108, 1×109, 2×109, 3×109, 4×109, 5×109, 6×109, 7×109, 8×109, 9×109 1×1010, 2×1010, 3×1010, 4×1010, 5×1010, 6×1010, 7×1010, 8×1010, 9×1010, 1×1011, or 2×1011 viable cells/ml. (Conversion factor: OD 1=8×108 cells/ml).
In some embodiments, engineered cells are cultured in a bioreactor. A bioreactor refers simply to a container in which cells are cultured, such as a culture flask, a dish, or a bag that may be single-use (disposable), autoclavable, or sterilizable. The bioreactor may be made of glass, or it may be polymer-based, or it may be made of other materials.
Examples of bioreactors include, without limitation, stirred tank (e.g., well mixed) bioreactors and tubular (e.g., plug flow) bioreactors, airlift bioreactors, membrane stirred tanks, spin filter stirred tanks, vibromixers, fluidized bed reactors, and membrane bioreactors. The mode of operating the bioreactor may be a batch or continuous processes and will depend on the engineered cells being cultured. A bioreactor is continuous when the feed and product streams are continuously being fed and withdrawn from the system. A batch bioreactor may have a continuous recirculating flow, but no continuous feeding of nutrient or product harvest. For intermittent-harvest and fed-batch (or batch fed) cultures, cells are inoculated at a lower viable cell density in a medium that is similar in composition to a batch medium. Cells are allowed to grow exponentially with essentially no external manipulation until nutrients are somewhat depleted and cells are approaching stationary growth phase. At this point, for an intermittent harvest batch-fed process, a portion of the cells and product may be harvested, and the removed culture medium is replenished with fresh medium. This process may be repeated several times. For production of recombinant proteins and antibodies, a fed-batch process may be used. While cells are growing exponentially, but nutrients are becoming depleted, concentrated feed medium (e.g., 10-15 times concentrated basal medium) is added either continuously or intermittently to supply additional nutrients, allowing for further increase in cell concentration and the length of the production phase. Fresh medium may be added proportionally to cell concentration without removal of culture medium (broth). To accommodate the addition of medium, a fedbatch culture is started in a volume much lower that the full capacity of the bioreactor (e.g., approximately 40% to 50% of the maximum volume).
Some methods of the present disclosure are directed to large-scale production of RNA (e.g., ssRNA or dsRNA). For large-scale production methods, engineered cells may be grown in liquid culture medium in a volume of 5 liters (L) to 250,000 L, or more. In some embodiments, engineered cells may be grown in liquid culture medium in a volume of greater than (or equal to) 10 L, 100 L, 1000 L, 10000 L, or 100000 L. In some embodiments, engineered cells are grown in liquid culture medium in a volume of 5 L, 10 L, 15 L, 20 L, 25 L, 30 L, 35 L, 40 L, 45 L, 50 L, 100 L, 500 L, 1000 L, 5000 L, 10000 L, 100000 L, 150000 L, 200000 L, 250000 L, or more. In some embodiments, engineered cells may be grown in liquid culture medium in a volume of 5 L to 10 L, 5 L to 15 L, 5 L to 20 L, 5 L to 25 L, 5 L to 30 L, 5 L to 35 L, 5 L to 40 L, 5 L to 45 L, 10 L to 15 L, 10 L to 20 L, 10 L to 25 L, 20 L to 30 L, 10 L to 35 L, 10 L to 40 L, 10 L to 45 L, 10 L to 50 L, 15 L to 20 L, 15 L to 25 L, 15 L to 30 L, 15 L to 35 L, 15 L to 40 L, 15 L to 45 L, or 15 to 50 L. In some embodiments, engineered cells may be grown in liquid culture medium in a volume of 100 L to 300000 L, 100 L to 200000 L, or 100 L to 100000 L.
Typically, culturing of engineered cells is followed by lysing the cells. “Lysing” refers to the process by which cells are broken down, for example, by viral, enzymatic, mechanical, or osmotic mechanisms. A “cell lysate” refers to a fluid containing the contents of lysed cells (e.g., lysed engineered cells), including, for example, organelles, membrane lipids, proteins, nucleic acids and inverted membrane vesicles. Cell lysates of the present disclosure may be produced by lysing any population of engineered cells, as provided herein.
Methods of cell lysis, referred to as “lysing,” are known in the art, any of which may be used in accordance with the present disclosure. Such cell lysis methods include, without limitation, physical lysis such as homogenization.
Cell lysis can disturb carefully controlled cellular environments, resulting in protein degradation and modification by unregulated endogenous proteases and phosphatases. Thus, in some embodiments, protease inhibitors and/or phosphatase inhibitors may be added to the cell lysate or cells before lysis, or these activities may be removed by heat inactivation, gene inactivation, or protease targeting.
Cell lysates, in some embodiments, may be combined with at least one nutrient. For example, cell lysates may be combined with Na2HPO4, KH2PO4, NH4Cl, NaCl, MgSO4, CaCl2. Examples of other nutrients include, without limitation, magnesium sulfate, magnesium chloride, magnesium orotate, magnesium citrate, potassium phosphate monobasic, potassium phosphate dibasic, potassium phosphate tribasic, sodium phosphate monobasic, sodium phosphate dibasic, sodium phosphate tribasic, ammonium phosphate monobasic, ammonium phosphate dibasic, ammonium sulfate, ammonium chloride, and ammonium hydroxide.
Cell lysates, in some embodiments, may be combined with at least one cofactor. For example, cell lysates may be combined with adenosine diphosphate (ADP), adenosine triphosphate (ATP), nicotinamide adenine dinucleotide (NAD+), or other non-protein chemical compounds required for activity of an enzyme (e.g., inorganic ions and coenzymes).
In some embodiments, cell lysates are incubated under conditions that result in RNA depolymerization. In some embodiments, cell lysates are incubated under conditions that result in production of ssRNA or dsRNA.
The volume of cell lysate used for a single reaction may vary. In some embodiments, the volume of a cell lysate is 0.001 to 250 m3. For example, the volume of a cell lysate may be 0.001 m3, 0.01 m3, 0.1 m3, 1 m3, 5 m3, 10 m3, 15 m3, 20 m3, 25 m3, 30 m3, 35 m3, 40 m3, 45 m3, 50 m3, 55 m3, 60 m3, 65 m3, 70 m3, 75 m3, 80 m3, 85 m3, 90 m3, 95 m, 100 m3, 105 m3, 110 m3, 115 m3, 120 m3, 125 m3, 130 m3, 135 m3, 140 m3, 145 m3, 150 m, 155 m3, 160 m3, 165 m3, 170 m3, 175 m3, 180 m3, 185 m3, 190 m3, 195 m3, 200 m3, 205 m, 210 m3, 215 m3, 220 m3, 225 m3, 230 m3, 235 m3, 240 m3, 245 m3, or 250 m3. In some embodiments, the volume of a cell lysate is 25 m3 to 250 m3, 50 m3 to 250 m3, or 100 m3 to 250 m3.
Downstream ProcessingThe methods and systems provided herein, in some embodiments, yield RNA (e.g., dsRNA, ssRNA) product at a concentration of 1-50 g/L (e.g., 30, 35, 40, 45, or 50 g/L). Downstream processing increases purity to as much as 99% (e.g., 75, 80, 85, 90, 95, 96, 97, 98, or 99%) dsRNA by weight. An example of downstream processing is shown in
Additional embodiments of the present disclosure are encompassed by the following numbered paragraphs 1-46:
1. A cell-free method of biosynthesizing ribonucleic acid (RNA), the method comprising:
(a) lysing cultured engineered cells that comprise RNA, an enzyme that depolymerizes RNA, thermostable kinases, a thermostable RNA polymerase, thereby producing a cell lysate;
(b) incubating the cell lysate produced in step (a) under conditions that result in depolymerization of RNA, thereby producing a cell lysate that comprises nucleoside monophosphates;
(c) heating the cell lysate produced in step (b) to a temperature that inactivates endogenous nucleases and phosphatases without inactivating the thermostable kinases and thermostable RNA polymerase, thereby producing a cell lysate that comprises heat-inactivated nucleases and phosphatases; and
(d) incubating the cell lysate produced in (c) in the presence of an energy source and an engineered DNA template containing a promoter operably linked to a nucleotide sequence encoding a RNA of interest, under conditions that result in production of nucleoside triphosphates and polymerization of the nucleoside triphosphates, thereby producing a cell lysate that comprises the RNA of interest.
2. The method of paragraph 1, wherein the energy source is polyphosphate, polyphosphate kinase, or both polyphosphate and polyphosphate kinase.
3. The method of paragraph 1 or 2, wherein the cultured engineered cells comprise the engineered DNA template.
4. The method of paragraph 1 or 2, wherein the engineered DNA template is added to the cell lysate of step (d).
5. The method of any one of paragraphs 1-4, wherein an ATP regeneration system is added to the cell lysate of step (d).
6. The method of paragraph 1, wherein the cultured engineered cells further comprise a thermostable polyphosphate kinase.
7. The method of any one of paragraphs 1-6, wherein the RNA of the engineered cells of step (a) is endogenous RNA.
8. The method of any one of paragraphs 1-7, wherein the RNA comprises ribosomal RNA, messenger RNA, transfer RNA, or a combination thereof.
9. The method of any one of paragraphs 1-8, wherein the cultured engineered cells comprise at least two enzymes that depolymerize RNA.
10. The method of any one of paragraphs 1-9, wherein the enzyme that depolymerizes RNA is selected from the group consisting of: S1 nuclease, Nuclease P1, RNase II, RNase III, RNase R, RNase JI, NucA, PNPase, RNase T, RNase E, RNaseG and combinations thereof.
11. The method of paragraph 10, wherein the enzyme that depolymerizes RNA is Nuclease P1.
12. The method of any one of paragraphs 1-11, wherein the cell lysate of step (b) comprises a Mg2+− chelating agent.
13. The method of paragraph 12, wherein the Mg2+− chelating agent is ethylenediaminetetraacetic acid (EDTA).
14. The method of paragraph 13, wherein the concentration of the EDTA is 0.1 mM to 25 mM.
15. The method of paragraph 14, wherein the concentration of the EDTA is 8 mM.
16. The method of any one of paragraphs 1-15, wherein the thermostable kinases comprise thermostable nucleoside monophosphate kinases.
17. The method of paragraph 16, wherein the thermostable nucleoside monophosphate kinases are selected from the group consisting of thermostable uridylate kinases, thermostable cytidylate kinases, thermostable guanylate kinases, and thermostable adenylate kinases.
18. The method of paragraph 17, wherein the stable nucleoside monophosphate kinases a selected from the group consisting of a thermostable Pyrococcus furiosus uridylate kinase encoded by a pyrH gene (PfPyrH), a thermostable Thermus thermophilus adenylate kinase encoded by a adk gene (TthAdk), a thermostable Thermus thermophilus cytidylate kinase encoded by a cmk gene (TthCmk), and a thermostable Pyrococcus furiosus guanylate kinase encoded by a gmk gene (PfGmk).
19. The method of any one of paragraphs 1-18, wherein the thermostable kinases comprise thermostable nucleoside diphosphate kinases.
20. The method of paragraph 19, wherein the thermostable nucleoside diphosphate kinases are selected from the group consisting of thermostable nucleoside phosphate kinases, thermostable pyruvate kinases, and thermostable polyphosphate kinases.
21. The method of paragraph 20, wherein at least one of the thermostable nucleoside diphosphate kinases is a thermostable Aquifex aeolicus enzyme encoded by a ndk gene.
22. The method of any one of paragraphs 1-21, wherein the cells comprise a thermostable nucleoside monophosphate kinase and a thermostable nucleoside diphosphate kinase.
23. The method of any one of paragraphs 1-22, wherein the cultured engineered cells comprise thermostable uridylate kinase, thermostable cytidylate kinase, thermostable guanylate kinase, thermostable adenylate kinase, and thermostable polyphosphate kinase.
24. The method of any one of paragraphs 1-23, wherein the thermostable RNA polymerase is a thermostable DNA-dependent RNA polymerase.
25. The method of paragraph 24, wherein the DNA-dependent RNA polymerase is selected from the group consisting of thermostable T7 RNA polymerases, thermostable SP6 RNA polymerases, and thermostable T3 RNA polymerases.
26. The method of paragraph 25, wherein the DNA-dependent RNA polymerase is a thermostable T7 RNA polymerase.
27. The method of any one of paragraphs 1-26, wherein the temperature in step (c) is at least 50° C.
28. The method of paragraph 27, wherein the temperature in step (c) is at 50° C.-80° C.
29. The method of any one of paragraphs 1-28, wherein step (c) comprises heating the cell lysate for at least 15 minutes.
30. The method of any one of paragraphs 1-29, wherein step (c) comprises heating the cell lysate to a temperature of at least 65° C. for 15 minutes.
31. The method of any one of paragraphs 1-30, wherein the nucleoside triphosphates in step (d) are produced at a rate of 15-30 mM/hour.
32. The method of any one of paragraphs 1-31, wherein the RNA of interest produced in step (d) is double-stranded RNA.
33. The method of any one of paragraphs 1-32, wherein the RNA of interest is produced in step (d) a RNA interference molecule.
34. The method of any one of paragraphs 1-33, wherein the RNA of interest produced in step (d) is an mRNA containing complementary domains linked by a hinged domain.
35. The method of any one of paragraphs 1-34, wherein the RNA of interest produced in step (d) is produced at a concentration of at least 4 g/L, at least 6 g/L, at least 6 g/L, or at least 10 g/L.
36. The method of paragraph 35 further comprising purifying the double-stranded RNA.
37. The method of paragraph 36, wherein the purifying step comprises combining the cell lysate of step (d) with a protein precipitating agent and removing precipitated protein, lipids, and DNA.
38. A cell lysate produced by the method of any one of paragraphs 1-37.
39. An engineered cell comprising RNA, an enzyme that depolymerizes RNA, a thermostable kinase and a thermostable RNA polymerase.
40. The engineered cell of paragraph 39 further comprising an engineered DNA template containing a promoter operably linked to a nucleotide sequence encoding a RNA of interest.
41. A population of engineered cells of paragraph 39 or 40.
42. A method, comprising:
maintaining in cell culture media engineered cells of paragraph 39.
43. The method of paragraph 42 further comprising lysing the cultured engineered cells to produce a cell lysate.
44. The method of paragraph 43 further comprising incubating the cell lysate under conditions that result in depolymerization of RNA to produce a cell lysate that comprises nucleoside monophosphates.
45. The method of paragraph 44 further comprising heating the cell lysate to a temperature that inactivates endogenous nucleases and phosphatases without inactivating the thermostable kinases and thermostable RNA polymerase to produce a cell lysate that comprises heat-inactivated nucleases and phosphatases.
46. The method of paragraph 45 further comprising incubating the cell lysate that comprises heat-inactivated nucleases and phosphatases in the presence of an energy source and an engineered DNA template containing a promoter operably linked to a nucleotide sequence encoding a RNA of interest, under conditions that result in production of nucleoside triphosphates and polymerization of the nucleoside triphosphates, to produce a cell lysate that comprises the RNA of interest.
To identify the optimal nuclease(s) for digesting lysate RNA, a series of screening experiments were performed using commercially-available enzymes chosen based on their ability to generate 5′-NMPs or oligonucleotides. The activity of these enzymes was first determined using purified E. coli RNA and reaction conditions recommended by the manufacturer, where RNA depolymerization was monitored by the release of acid-soluble nucleotides. Under these conditions, four nucleases demonstrated depolymerization activity over background. The endonucleases Benzonase and RNase A, which served as positive controls, yielded immediate conversion of RNA to acid-soluble nucleotides (
RNase R was then tested for its ability to depolymerize endogenous RNA in bacterial lysates. In these experiments, purified RNase R (0.5 mg/mL final concentration) was added to lysates (50% final concentration), and free nucleotides were quantified by UPLC. A representative experiment is shown in
To assess the toxicity of RNase R expression, two bacterial strains were constructed. One strain included the base strain (GL16-170) transformed with an empty protein expression vector, and the other included GL16-170 transformed with the same protein expression vector encoding RNase R. Both strains were grown under batch conditions in 1 L bioreactors, induced at OD600=20, and harvested before glucose exhaustion. Induction yielded strong expression of RNase R (
Next, the activity of overexpressed RNase R was assessed in high-density lysates. Mg2+, which is known to stabilize ribosome structure and protect rRNA from nucleases, is also required (in low amounts) for RNase R activity. Therefore, depolymerization rates were measured in the presence of varying concentrations of EDTA (
After RNA depolymerization, the resulting NMP pool is progressively phosphorylated to NTPs before polymerization into dsRNA. Deleterious enzymatic activities, such as NMP degradation into nucleosides and subsequent hydrolysis into sugars and bases, negatively impact dsRNA yields. Therefore, the stability of individual NMPs was assessed in lysates. Stability assessments were performed by adding isotopically-labeled “heavy” NMPs (hAMP, hCMP, hUMP, and hGMP) to lysates, and quantifying abundance over time using LC-MS (Figured 9A-9D, solid lines). In contrast to hAMP, which is relatively stable, hCMP, hUMP, and hGMP are actively degraded by the lysate, with approximate half-lives (t1/2) of 1 hour, 30 minutes, and 20 minutes, respectively.
One pathway for metabolism of NMPs is dephosphorylation into nucleosides. To assess whether dephosphorylation was contributing to NMP degradation, stability assessments were repeated with the addition of inexpensive phosphatase inhibitors. Increased concentrations of phosphate (PO4, 150 mM), as well as the structural mimic orthovandate (VO4, 10 mM) were pre-incubated with lysates before hNMP addition. Increasing phosphate concentration stabilized hUMP (t1/2≈60 minutes), while minimally affecting hCMP and hGMP (
To stabilize NMPs, as well as NDPs, NTPs, and dsRNA, a heat inactivation protocol was developed. The objective was to identify the lowest temperature and shortest incubation time that would eliminate nucleotide and RNA degradation activities in lysates. To assess the efficacy of heat inactivation, NTP consumption rates (at 37° C.) were compared across heat-inactivated lysates by LC-MS, where the time and temperature of heat inactivation was varied. Before heat inactivation, lysates consumed NTPs at approximately 120 mM/hr (
Next, these conditions were evaluated for their ability to stabilize NMPs and dsRNA in lysates. To evaluate the effects of heat inactivation on NMP degradation, lysates were treated with exogenous RNase R to release RNA, then subjected to heat inactivation at 70° C. Post-inactivation, the temperature was lowered to 37° C., and the reaction incubated for an additional 60 minutes. As shown in
Finally, these conditions were evaluated for their ability to stabilize the reactants and products of an in vitro transcription reaction (including NTPs and dsRNA) in lysates. First, lysates were pre-incubated at elevated temperature for 15 minutes. Then, the temperature was lowered to 37° C. and transcription reactants (including exogenous NTPs, DNA template, and purified T7 RNA polymerase) were added. As shown in
Taken together, these results suggest that a 70° C. incubation for 15 minutes is sufficient to stabilize NMPs, NTPs, and dsRNA in cell-free reactions.
Selection and Evaluation of Thermostable KinasesAfter heat-inactivation, a series of kinase activities are required to sequentially phosphorylate 5′-NMPs liberated from RNA to NTPs that can be polymerized to form dsRNA. These kinases, which use high-energy phosphate groups from ATP to phosphorylate NMPs and NDPs, must be sufficiently thermostable to remain active following high-temperature incubation, as well as sufficiently active to produce NTPs at high rates (21 mM/hr NTPs for 1 g dsRNA/L/hr). Enzymes from thermophilic organisms were chosen for evaluation (Table 9) based on literature reports of successful expression in E. coli and biochemical characterization of the recombinant enzymes.
To evaluate the suitability of these enzymes for cell-free production of dsRNA, enzymes were cloned into an E. coli protein expression vector with an N-terminal hexahistidine tag, overexpressed, and purified using immobilized metal affinity chromatography (IMAC). Activities of the purified enzymes were first quantified using luciferase-coupled assays, where the consumption of ATP served as a proxy for NMP and NDP phosphorylation. Assays were performed at a range of incubation temperatures to determine the optimal reaction temperature for each enzyme.
Expression of UMP kinases (encoded by the pyrH gene) from T. thermophilus and E. coli yielded insoluble protein under the tested induction and purification conditions. Purified PyrH from P. furiosus exhibited a specific activity of approximately 2 μmol/min/mg protein that was largely temperature-independent (
Expression of AMP kinases (encoded by the adk gene) from E. coli and T. thermophilus yielded soluble recombinant protein, while the Thermosynechococcus enzyme was insoluble under the tested expression and purification conditions. The purified E. coli enzyme was active at 37° C. and 50° C., but exhibited no detectable activity at higher temperatures (
Expression of CMP kinases (encoded by the cmk gene) from E. coli and P. furiosus yielded insoluble protein, while expression of the T. thermophilus enzyme yielded soluble protein under the tested conditions. T. thermophilus CMP kinase exhibited activity largely independent of temperature, although enzyme activity decreased slightly at temperatures above 60° C. (
In contrast to the tested CMP kinases, expression of GMP kinases from E. coli, T. thermophilus, and T. maritima yielded soluble recombinant protein. The E. coli enzyme had the highest tested specific activity at 37° C., but was less active at higher temperatures (
Unlike the NMP kinases, which are largely specific for a single substrate, NDP kinase phosphorylates ADP, CDP, UDP, and GDP. To compare NDP kinases, enzymes from the thermophiles T. thermophilus and A. aeolicus were cloned and expressed in E. coli. While the T. thermophilus enzyme was insoluble under the tested conditions, expression of the A. aeolicus enzyme yielded soluble protein. Activity measurements in a luciferase assay using ATP and GDP as substrates revealed that AaNdk is highly active across a broad range of temperatures, with a temperature optimum of 50° C. (
Polyphosphate kinase (Ppk) reversibly transfers high-energy phosphate groups between polymeric phosphate chains and adenosine nucleotides. Multiple Ppk enzymes, belonging to the Type I family of polyphosphate kinases, were evaluated for activity, including enzymes from E. coli as well as the thermophilic organisms T. thermophilus and Thermosynechococcus elongatus. These enzymes were selected for testing as they belonged to the well-characterized Type I family or had previously been shown to be active. Expression and purification of Ppk enzymes yielded soluble protein, which was then tested for activity in a luciferase assay system using ADP and sodium hexametaphosphate as substrates. As shown in
After evaluating each enzymatic activity in a purified system, enzymes were tested for activity in heat-inactivated lysates. Lysates expressing individual kinases were pre-incubated at 70° C. for 15 minutes before substrates were added. As in the purified reactions, ATP consumption (for NMP and NDP kinases) or ATP production (for polyphosphate kinase) were quantified using a luciferase assay kit. As shown in Table 16 below, rates of NMP and NDP phosphorylation were well in excess of targets.
After confirming that individual kinases were sufficiently active in lysates (with the exception of Ppk), kinases were evaluated in a multi-enzyme system for their ability to convert NMPs to NTPs. In these studies, equal volumes of lysates expressing individual kinases (5) were combined, heat-inactivated, then assayed for ATP-dependent production of NTPs from an equimolar mix of NMPs by LC-MS. As shown in Table 17, overall NTP production rates exceeded 24 mM/hr for UTP, CTP, and GTP, suggesting that a simple mixture of lysates, without any optimization of reaction conditions, could provide NTPs at sufficient rates to support synthesis of 1 g/L/hr dsRNA, in the presence of adequate ATP.
After depolymerization of RNA into NMPs and phosphorylation of NMPs to their corresponding NTPs, a RNA polymerase is required to convert NTPs into the dsRNA product. RNA polymerase from the bacteriophage T7 is an attractive enzyme for use in a recombinant system for several reasons. T7 RNA polymerase includes a single subunit (unlike many RNA polymerases from Bacteria and Eukarya) and has been extensively characterized by biochemical and molecular biology studies. Additionally, multiple T7 RNA polymerase mutants have been described that confer improved thermostability (see Table 18).
First, the activities of commercially-available T7 RNA polymerases were evaluated using duplex DNA template (e.g.
Next, the LVI mutant of T7 RNA polymerase was cloned with an N-terminal hexahistidine tag, expressed in E. coli, and purified using IMAC.
Next, activities of the MegaScript and ThermoT7 polymerases were tested alongside the LVI mutant polymerase in dilute heat-inactivated lysates (35% final lysate concentration) under standardized reaction conditions (
After confirming the activity of ThermoT7 at 50° C. in dilute heat-inactivated lysate, the activity of ThermoT7 was investigated under conditions representative of cell-free dsRNA production at scale. In addition to increased concentrations of lysate, reactions at scale may include precipitated lysate components (e.g., protein) that arise from the heat inactivation process. To assess the performance of ThermoT7 under these conditions, RNA polymerization was quantified in heat-inactivated high-density lysates (68% final lysate concentration) with and without clarification to remove precipitated proteins after heat inactivation (
Finally, the thermostability of ThermoT7 was tested at higher temperatures to evaluate compatibility with the heat inactivation conditions established earlier in this program. In these studies, ThermoT7 enzyme was pre-incubated at elevated temperature (50-70° C.) for varying lengths of time (0-15 minutes), and the remaining activity quantified at 37° C. As shown in
Analysis of nucleotide monophosphates (AMP, CMP, UMP, and GMP) was performed by liquid chromatography coupled with mass spectrometry (LC-MS). Samples were separated using an Agilent 1100 series HPLC equipped with a ZIC-cHILIC column (2.1×20 mm, 3 μm i.d.) (Merck) at room temperature with a flow rate of 0.5 mL/min and a 2 injection volume. Mobile phases consisted of 20 mM ammonium acetate (A) and 20 mM ammonium acetate in 90% acetonitrile (B). The separation method consisted of a gradient from 15-50% (B) for 3.5 minutes, followed by 50% (B) for 1.5 minutes, then 15% B for 3 minutes. Quantification was performed on an ABSciex API 3200 mass spectrometer using electrospray ionization (capillary voltage: −3000V, temperature: 600° C., desolvation gas: 20 psi) in multiple reaction monitoring (MRM) mode. Analysis of nucleotide monophosphate, diphosphate, and triphosphate species (NMPs, NDPs, and NTPs) used the method described above with the following separation gradient: 15% B to 50% B for 3.5 minutes, followed by 50% B for 2.5 min, then 15% B for 4 minutes. Peak areas were compared to standard curves consisting of purified compounds (Sigma-Aldrich). For analysis of samples in lysates, standard curves were prepared in lysate backgrounds that had been acid-quenched, clarified, pH-neutralized, and filtered as in the sample preparation steps described below.
Analysis of 2′-, 3′-, and 5′-NMPs was performed by liquid chromatography using an ACQUITY H-Class UPLC (Waters) equipped with an ACQUITY CSH fluoro-phenyl column (2.1×150 mm, 1.7 μm i.d.) (Waters) at 40° C. with a flow rate of 0.5 mL/min and a 0.5 μL injection volume. Mobile phases consisted of 10 mM ammonium acetate in 0.2% formic acid (A) and 10 mM ammonium acetate in 95% acetonitrile, 0.2% formic acid (B). The separation method consisted of 1% B for 2.8 minutes, followed by a gradient from 1%-30% B for 2.2 minutes, followed by 100% B for 7 minutes, then 1% B for 3 minutes. Quantification was performed using an ACQUITY UPLC PDA (Waters) at 260, 254, and 210 nm. Peak areas were compared to standard curves consisting of purified compounds (purchased from Sigma-Aldrich except for 2′ and 3′ CMP, UMP, and GMP which were purchased from Biolog Life Science Institute). For analysis of samples in lysates, standard curves were prepared in lysate backgrounds that had been acid-quenched, clarified, pH-neutralized, and filtered as in the sample preparation steps described below.
Extraction and Purification of E. coli RNA
RNA was extracted and purified from high-density E. coli lysates (protein concentration: 40-50 mg/mL) according to established protocols (Mohanty, B. K., Giladi, H., Maples, V. F., & Kushner, S. R. (2008). Analysis of RNA decay, processing, and polyadenylation in Escherichia coli and other prokaryotes. Methods in enzymology, 447, 3-29). For every 400 μL of frozen E. coli lysate, 67 μL of 20 mM acetic acid was added to reduce RNase activity. Samples were thawed in a bead bath at 37° C. Immediately upon thawing, 400 μL of a 10% (w/v) solution of trimethyl(tetradecyl)ammonium bromide (Sigma-Aldrich) was added. The resulting suspensions were then clarified by centrifugation at 10,000×g in a microcentrifuge at 4° C., and the supernatant removed. Pellets were resuspended in 1 mL of a 2M solution of lithium chloride (Sigma-Aldrich) in 35% ethanol. The suspensions were incubated at room temperature for 5 minutes, then clarified by centrifugation at 15,000×g for 6 minutes at 4° C. and the supernatants removed. Pellets were then resuspended in 1 mL of a 2M solution of lithium chloride in water, and incubated at room temperature for 2 minutes before clarification at 15,000×g for 6 minutes. Supernatants were then removed and the remaining pellets washed by resuspending in 70% ethanol and centrifuging at maximum speed (21,000×g) for 5 minutes at 4° C. Supernatants were then removed and the pellets were air-dried for 15 minutes at room temperature. Pellets were then resuspended in 200 μL nuclease-free water, and incubated overnight at 4° C. to solubilize RNA. RNA solutions were clarified by centrifugation (maximum speed for 5 minutes at 4° C.) and supernatants containing soluble RNA were transferred to sterile RNase-free tubes and stored at −20° C.
Nuclease DownselectionNucleases were obtained from commercial sources as follows: Benzonase and Nuclease P1 were obtained from Sigma-Aldrich, RNase R, Terminator exonuclease, and RNase III were obtained from Epicentre, RNase A was obtained from Thermo Fisher, and Exonuclease T was obtained from New England BioLabs. For screening assays, 1 μL of each enzyme solution was added to 100 μL of 2× assay buffer (100 mM potassium phosphate pH 7.4, 10 mM magnesium chloride, 1 mM zinc chloride), then combined with an equal volume of 1 mM RNA solution (˜340 ng/μL) at time t=0 and mixed well. Reactions were incubated at 37° C. and periodically sampled by transferring 20 μL to acid quench solution (180 μL of 0.2M sulfuric acid) on ice. After completion of the time course, quenched samples were clarified by centrifugation at 3,000×g for 5 minutes at 4° C. 170 μL of supernatant from each sample was then transferred to a UV-transparent 96-well half area plate (Corning) and acid-soluble nucleotides were quantified by absorbance at 260 nm using a microplate reader and an extinction coefficient of 10665 M−1 cm−1, estimated by averaging individual extinction coefficients for each mononucleotide. For subsequent analysis by LC-MS, 45 μL clarified supernatant was pH-neutralized with 5 μL of 2.5 M potassium hydroxide. The total nucleotide pool (i.e. 100% depolymerization) was determined by alkaline hydrolysis of RNA: RNA was combined with an equal volume of 0.2M potassium hydroxide, then heated to 99° C. for 20 minutes. Alkaline-hydrolyzed samples were then quenched and analyzed as described above.
Protein Expression and PurificationRecombinant proteins were cloned from synthetic DNA encoding the relevant gene along with a hexahistidine tag into pETDuet-1 (Novagen). Plasmids were transformed into E. coli T7Express (New England Biolabs), then grown in 1 L cultures using ZYM-505 media (Studier, F. W. (2005). Protein production by auto-induction in high-density shaking cultures. Protein expression and purification, 41(1), 207-234) supplemented with 50 μg/mL carbenicillin. Expression was induced at A600=0.6. For RNase R and kinases, expression was induced with 0.1 mM IPTG, the temperature lowered to 16° C., and the culture grown for 24 hours at 16° C. For T7 RNA polymerase, expression was induced with 0.8 mM IPTG and the culture grown for 3 hours at 37° C. Biomass was harvested by centrifugation and the supernatant decanted before storing the cell pellets at −80° C. Cell pellets were thawed and lysed by resuspension into 4 volumes B-PER Complete (Thermo Fisher Scientific) supplemented with Benzonase (0.04 μL/mL) and incubation with gentle agitation for 15 minutes at room temperature. Lysates were then clarified by centrifugation at 16,000×g for 1 hour at 4° C. Proteins were purified by immobilized metal affinity chromatography using His GraviTrap columns (GE Healthcare) or HisTrap HP columns connected to an AKTAPrime Plus FPLC system (GE Healthcare). For both purification methods, columns were equilibrated in Equilibration/Wash buffer (50 mM phosphate buffer pH 7.4, 500 mM NaCl, 20 mM imidazole), loaded with lysate, and then washed with 30 column volumes Equilibration/Wash Buffer. Proteins were eluted with Elution Buffer (50 mM phosphate buffer pH 7.4, 500 mM NaCl, 500 mM imidazole). For purification of kinases, Equilibration/Wash and Elution buffers used 50 mM Tris-HCl pH 7.5 instead of phosphate buffer. Elution fractions were analyzed by SDS-PAGE and protein content quantified by BCA (Thermo Fisher Scientific). Fractions were then combined and buffer exchanged by dialysis into 1000 volumes 2× Storage Buffer. For RNase R, 2× Storage Buffer consisted of 2×PBS supplemented with an additional 500 mM NaCl. For T7 RNA polymerase, 2× Storage Buffer consisted of 2×PBS. For kinases, 2× Storage Buffer consisted of 100 mM Tris-HCl pH 7.0 with 100 mM NaCl. After dialysis, proteins were mixed with an equal volume of 100% glycerol (50% final concentration) and stored at −20° C.
Cell Lysate PreparationE. coli strains GL16-170 (BL21(DE3).t526pgi.Δedd.ΔtktB.ΔtolC_wt-7-E1.ΔmgsA*-F3.ΔappA*.Δamn*-F1.nagD(keio)::zeoR-1.ΔphoA*.t352BAA1644.ΔushA*-C4.rna::tolC-B04) and GL14-322 (BL21(DE3).t526pgi.Δedd.ΔtktB.ΔtolC_wt-7-E1.ΔmgsA*-F3.ΔappA*.Δamn*-F1.nagD(keio)::zeoR-1.ΔphoA*.t352BAA1644.ΔushA::tolC-A01) were grown in Korz media in 10 L bioreactors until the end of batch phase, then harvested by centrifugation and frozen at −80° C. Pellets were resuspended to 10% dry cell weight in 58.8 mM potassium phosphate dibasic and lysed using 2 passes through a PandaPLUS homogenizer (GEA Niro Soavi) cooled to 4° C. at 15,000 psi. Lysates were clarified by centrifugation at 16,000×g for 1 hour at 4° C. and protein content was analyzed by BCA assay (Thermo Fisher) before storage at −80° C.
Depolymerization of Lysate RNA with Exogenous RNase R
GL16-170 lysate (protein content 34.5 mg/mL) and RNase R solution (1 mg/mL in 300 mM potassium phosphate buffer pH 7.4, 200 mM KCl, 2 mM MgCl2) were pre-equilibrated at 2° C. before initiating the reaction. At time t=0, 50 μL of E. coli lysate and 50 μL RNase R solution were mixed and the reaction initiated by transferring to a preheated 37° C. block. Reactions including deoxycholate were assembled as described above, except that lysates were premixed with 0.2 volumes of 5× sodium deoxycholate solutions in water and incubated at 2° C. for 15 minutes before initiation. After initiation, reactions were incubated at 37° C. and periodically sampled by transferring 10 μL to acid quench solution (90 of 0.2M sulfuric acid) on ice. After completion of the time course, quenched samples were clarified by centrifugation at 3,200×g for 10 minutes at 2° C. Depolymerization was first quantified by absorbance of acid-soluble nucleotides: 10 μL of quenched and clarified reactions was added to 160 μL of 0.2M sulfuric acid in a UV-transparent 96-well half area plate (Corning). Acid-soluble nucleotides were quantified by absorbance at 260 nm using a microplate reader (see above). Depolymerization was also quantified by UPLC analysis of 5′, 2′, and 3′ NMPs: 30 μL of each acid-quenched sample was pH-neutralized by adding 10 of 1M KOH, then passed through a 0.2 μm filter before UPLC analysis. The total nucleotide pool (i.e. 100% depolymerization) was determined by alkaline hydrolysis of lysate RNA: 50 lysate was combined with 150 μL of 0.2M potassium hydroxide, then heated to 99° C. for 20 minutes. Alkaline-hydrolyzed samples were then quenched and analyzed as described above.
Depolymerization of RNA in Lysates with Overexpressed RNase R
E. coli strain GL16-170 was transformed with pETDuet-1 encoding the E. coli rnr gene with a C-terminal hexahistidine tag. This strain, alongside GL16-170 transformed with empty pETDuet-1, was grown in batch phase in Korz medium supplemented with 50 mg/L carbenicillin. Cultures were induced with 0.8 mM IPTG at A600=20 and supplemented with an additional 10 g/L glucose at induction. One hour following induction, biomass was harvested by centrifugation and frozen. Lysates were prepared from frozen biomass as described above (Protein concentrations: 36.6 mg/mL for GL16-170 biomass with empty pETDuet-1; 53.2 mg/mL for GL16-170 with pETDuet-1 carrying cloned RNase R). Depolymerization in dilute lysates was assessed as described above with 50% final lysate concentration in the reaction. Depolymerization in concentrated lysates was assessed by pre-incubating 9 volumes lysate with 1 volume 10×EDTA solution for 5 minutes at 2° C. Reactions were then initiated by transferring to a preheated 37° C. block and sampling as described above.
NMP Stability AssessmentFour volumes of GL14-322 lysate (protein concentration: 50.5 mg/mL) were combined with one volume of phosphatase inhibitor solution (final concentrations of 50 mM potassium phosphate pH 7.4, 150 mM potassium phosphate pH 7.4, or 10 mM sodium orthovanadate) on ice. An equimolar solution of isotopically labeled NMPs (Adenosine-13C10, 15N5 5′-monophosphate, Cytidine-15N3-5′-monophosphate, monophosphate, and Guanosine-15N5-5′-monophosphate [Sigma-Aldrich], 25 mM each) was prepared in water. Lysates and NMPs were equilibrated to 37° C. for 10 minutes before reactions were initiated. To initiate reactions, 90 μL lysate solution was added to 10 μL NMP solution, and the reactions mixed well. Reactions were monitored by sampling at the indicated time points. During sampling, 12 μL of reaction mixture were transferred to 108 of 0.2M sulfuric acid on ice. Quenched reactions were then clarified by centrifugation, pH-neutralized, and filtered for LC-MS analysis as described above.
Development of Heat InactivationGL14-322 lysate was aliquoted into 5 microcentrifuge tubes on ice, then transferred to a heat block equilibrated at the desired heat inactivation temperature. At the indicated times, tubes were cooled on ice, then clarified by centrifugation (21,000×g for 5 minutes) and the supernatants harvested. Supernatants from heat-inactivated lysates, along with an equimolar mixture of NTPs (Sigma-Aldrich, 25 mM each) were equilibrated at 37° C. for 10 minutes. At time t=0, 9 volumes of heat-inactivated lysate supernatant were combined with 1 volume of NTP solution, and the reaction monitored by sampling into acid quench solution, pH-neutralized, filtered, and analyzed by LC-MS as described above. For transcription reactions in lysates, 10 μL reactions were performed using the MegaScript T7 Transcription Kit (Thermo Fisher) following kit instructions, except for a reduced amount of enzyme mix (5% of final reaction volume), and including heat-inactivated lysate supernatant (40% of final reaction volume). Positive control reactions were performed in MegaScript reaction buffer alone, while negative control reactions included lysate but omitted enzyme mix. Reactions were analyzed by agarose gel electrophoresis stained with SYBR Safe (Invitrogen) alongside the 1 kb DNA ladder (New England Biolabs).
Nucleotide Kinase Activity AssaysNucleotide kinases were assayed at varying temperatures (37° C., 50° C., 60° C., 70° C., and 80° C.) in a buffer consisting of 50 mM Tris-HCl pH 7.0, 4 mM MgSO4, 4 mM ATP, and 4 mM of the corresponding NMP or NDP. Reaction buffer (1.2× concentrate) and enzyme solution (0.5 mg/mL) were pre-equilibrated at reaction temperature for 1 minute before reactions were initiated. At time t=0, reactions were initiated by mixing 80 reaction buffer with 20 μL enzyme. Reactions were monitored by sampling at the indicated time points. During sampling, 15 μL of reaction mixture were transferred to 135 μL of 0.2M sulfuric acid on ice. After completion of the reaction, samples were pH-neutralized with 1M KOH as described above, then diluted 1:10 in ice-cold water. ATP was quantified in each sample using the ATP Bioluminescent Assay Kit (Sigma-Aldrich), following kit instructions. For reactions in lysates, the above protocol was modified as follows: lysates were aliquoted into individual reaction tubes, then heat-inactivated by incubating at 70° C. for 15 minutes. Reaction buffer (2× concentrate) and heat-inactivated lysates were pre-equilibrated at reaction temperature, and the reactions initiated by combining equal volumes of lysate and reaction buffer. Reactions were sampled by quenching individual reaction tubes with 9 volumes acid quench solution, then analyzed as described above.
To assay the combined activity of kinases in lysates (i.e. from NMPs to NTPs), lysates individually expressing each kinase were mixed in a 1:1 ratio, divided into 10 aliquots, then heat-inactivated as described above. Kinase activity was analyzed in a buffer consisting of 50 mM Tris-HCl pH 7.0, 16 mM MgSO4, 2 mM each nucleotide monophosphate (AMP, CMP, UMP, and GMP), and 16 mM ATP. Reaction buffer (2× concentrate) pre-equilibrated at reaction temperature was combined with an equal volume of lysate to initiate the reaction. Reactions were performed at 70° C. and sampled by quenching individual reaction tubes with 9 volumes acid quench solution, then analyzed as described above.
Polyphosphate Kinase Activity AssaysPolyphosphate kinases were assayed at varying temperatures (37° C., 50° C., 60° C., 70° C., and 80° C.) in a buffer consisting of 50 mM Tris-HCl pH 7.0, 4 mM MgSO4, 25 mM (NH4)2SO4, 1 mM ADP, and 1 mM sodium hexametaphosphate. Reaction buffer (1.2× concentrate) and enzyme solution (0.25 mg/mL) were pre-equilibrated at reaction temperature for 1 minute before reactions were initiated. At time t=0, reactions were initiated by mixing 80 μL reaction buffer with 20 μL enzyme. Reactions were monitored by sampling at the indicated time points. During sampling, 15 μL of reaction mixture were transferred to 135 μL of 0.2M sulfuric acid on ice. After completion of the reaction, samples were pH-neutralized with 1M KOH as described above, then diluted 1:10 in ice-cold water. ATP was quantified in each sample using the ATP Bioluminescent Assay Kit (Sigma-Aldrich), following kit instructions. For reactions in lysates, the above protocol was modified as follows: lysates were aliquoted into individual reaction tubes, then heat-inactivated by incubating at 70° C. for 15 minutes. Reaction buffer (2× concentrate) and heat-inactivated lysates were pre-equilibrated at reaction temperature, and the reactions initiated by combining equal volumes of lysate and reaction buffer. Reactions were sampled by quenching individual reaction tubes with 9 volumes acid quench solution, then analyzed as described above. Reaction rates in lysates were calculated by subtracting signal from a control lysate (without overexpressed polyphosphate kinase) under the same reaction conditions.
Generation of Transcription TemplatesDuplex DNA template was prepared by PCR amplification of synthetic gBlock DNA (Integrated DNA Technologies). Reactions were purified and concentrated by isopropanol precipitation.
RNA Polymerase DownselectionCommercially-available RNA polymerases were compared using conditions recommended by each manufacturer. Each 50 μL reaction consisted of 10× reaction buffer (supplied by the manufacturer), NTPs, DNA template (0.5 μs), and enzyme. For the NEB T7 RNA polymerase, reactions included 0.5 mM each NTP, 5 mM DTT, and 100 U enzyme. Reactions with ThermoT7 polymerase were identical, except that DTT was omitted. Reactions with MegaScript T7 included 7.5 mM each NTP and 5 μL enzyme mix. Enzyme concentrations were determined by BCA assay (Thermo Fisher). Reactions were monitored by sampling at the indicated time points. During sampling, 10 μL of reaction mixture were transferred to 90 μL of RNA quench solution (10 mM Tris-HCl pH 8.0, 5 mM EDTA) and stored on ice. RNA samples in quench solution were quantified using the Quant-iT RNA Broad Range Assay Kit (Thermo Fisher), following kit instructions. Serial dilutions of purified dsRNA, prepared using the MegaScript Kit and purified following kit instructions, were used to construct standard curves for quantitation. Reactions were qualitatively analyzed by agarose gel electrophoresis.
RNA Polymerase Evaluation in LysatesGL14-322 lysates were heat-inactivated and clarified by centrifugation as described previously. Each 20 μL reaction consisted of clarified lysate (7 μL), 10× cofactor solution (300 mM MgCl2, 20 mM spermidine), NTPs (7.5 mM each, prepared from pH-neutralized stock solutions), DNA template (0.6 μg), and enzyme (1 μL). Reactions were incubated for 1 hour at 37° C. or 50° C., then quenched by adding 9 volumes RNA quench solution. Quenched reactions were further diluted 10-fold in quench solution (final dilution: 100-fold). Diluted reactions were then quantified using the Quant-iT kit (see above). RNA produced by the reaction was calculated by subtracting RNA quantified in a control reaction (omitting RNA polymerase).
RNA polymerase assays in high-density lysates were performed as described above, with the following modifications. Each 100 μL reaction consisted of lysate (67.5 μL), 10× cofactor solution (300 mM MgCl2, 20 mM spermidine), NTPs (7.5 mM each, prepared from pH-neutralized stock solutions), DNA template (3 μg), and enzyme (10 μL). For reactions performed in unclarified reaction matrix, GL14-322 lysates (67.5 μL) were aliquoted into individual reaction tubes, then heat-inactivated as described previously. Additional reactants were added to heat-inactivated matrix, then mixed well by vortexing until homogenous. For reactions performed in buffer, the 10× cofactor solution consisted of 300 mM MgCl2, 20 mM spermidine, and 400 mM DTT. In addition, 50 mM potassium phosphate pH 7.4 was included in buffer-only reactions. All reactions were incubated for 2 hours at 50° C. Samples from each reaction were quenched by adding 9 volumes RNA quench solution, then clarified by centrifugation for 1 minute at maximum speed (21,000×g). Supernatants from these reactions were further diluted 40-fold in quench solution (final dilution: 400-fold), then quantified using the Quant-iT kit (see above).
Example 2Thermostable PPK2 enzymes were codon-optimized for expression in E. coli, synthesized, and cloned into pETDuet-1 (Novagen). Plasmids were then transformed into GL16-170. To generate the Control strain, empty pETDuet-1 plasmid was transformed into GL16-170. After overnight preculture in 5 mL Lysogeny Broth (LB), strains were cultivated in 1 L LB at 37° C. until cell densities reached an OD600 of approximately 0.5. Cultures were then briefly chilled on ice, and PPK2 expression was induced by adding isopropyl thiogalactopyranoside (IPTG) to a final concentration of 0.25 mM. Post-induction, cultures were grown at 20° C. for approximately 16 hours. Cultures were harvested by centrifugation, and the cell pellets stored at −80° C. Lysates were produced by thawing frozen pellets, resuspending in 2 pellet volumes 150 mM MOPS-NaOH pH 7, and homogenizing using 4 passes through an EmulsiFlex C3 homogenizer (Avestin) at 15,000 psi. Lysates were then clarified by centrifugation at 15,000×g for 1 hour at 4° C. Aliquots of lysates were frozen at −80° C. before use.
Thermostable PPK2 activity was then measured in heat-inactivated lysates. Thawed lysates expressing PPK2 enzymes were first diluted 1:100 into lysates prepared from the Control strain, except for the D. geothermalis PPK2 lysate, which was diluted 1:10. Pre-chilled solutions of manganese chloride (MnCl2) and sodium hexametaphosphate (HMP) were added to final concentrations of 10 mM and 1 mM, respectively. Lysates were then heat-inactivated by incubation in a pre-heated 70° C. thermocycler for 15 minutes. Reactions were then initiated by mixing heat-inactivated lysates with an equal volume of 2× Reaction Buffer, consisting of 10 mM MnCl2, 2 mM adenosine diphosphate (ADP) or adenosine monophosphate (AMP), and 9 mM HMP. Reactions were incubated at 70° C., and time points were taken by removing an aliquot of reaction mixture and diluting with 9 parts Quench Solution (200 mM H2SO4) on ice. The initial timepoint (t0) was taken by directly mixing lysate with Quench Solution, storing the quenched lysate on ice for 15 minutes, then adding 2× reaction buffer. At the conclusion of the assay, quenched timepoint solutions were clarified by centrifugation at 3,200×g for 10 minutes. Supernatants from the quenched reactions were then pH neutralized by mixing 3 parts quenched reaction solution with 1 part Neutralization Solution (1M KOH). Quenched and neutralized samples were then diluted 1:10 with water before quantitation using the Adenosine 5′-triphosphate (ATP) Bioluminescent Assay Kit (Sigma-Aldrich cat #: FLAA), following kit instructions. Initial reaction rates were calculated based on the accumulation of ATP in PPK2-containing reactions, subtracting ATP concentrations from the Control lysate.
Five Class III PPK2 enzymes exhibited soluble expression, thermostability, and high reaction rates in lysates at 70° C. Representative expression and activity data is shown in
A summary of expression and kinetic data for each tested enzyme is shown in Table 17. The C. aerophila, Roseiflexus, A. thermophila, and R. castenholzii enzymes rapidly converted AMP and ADP to ATP using HMP as a substrate. The D. geothermalis enzyme exhibited conversion rates roughly 20× lower than other tested PPK2s for both AMP and ADP substrates.
Thermostable C. aerophila PPK2 was then used to supply ATP for cell-free production of dsRNA from NMPs, ADP, and HMP. Cell-free dsRNA synthesis reactions were performed with a mixture of six E. coli lysates individually overexpressing the kinases detailed in Table 18.
First, lysates detailed in Table 18 were combined in equal volumes on ice. Pre-chilled solutions of manganese chloride (MnCl2), magnesium sulfate (MgSO4), and sodium hexametaphosphate (HMP) were added to final concentrations of 0-2.5 mM, 30 mM, and 1 mM, respectively. The lysate mixture was then then heat-inactivated by incubation in a pre-heated 70° C. thermocycler for 15 minutes. To initiate the reactions, heat-inactivated lysates were combined with the following components: an equimolar mixture of nucleotide monophosphates (adenosine 5′-monophosphate, cytidine 5′-monophosphate, uridine 5′-monophosphate, and guanosine 5′-monophosphate, 2 mM each), 50 mM Tris pH 7.0, 30 mM MgSO4, 0-2.5 mM MnCl2, 1 mM adenosine 5′-diphosphate, 2 mM spermidine, 1.5 μg plasmid DNA template, and 3 μg thermostable T7 RNA polymerase (S430P, F849I, F880Y) in a total volume of 20 μL. As a control, dsRNA was synthesized from an equimolar mixture of 2 mM NTPs (with lysates, ADP, and HMP omitted). As an additional control, dsRNA was synthesized from an equimolar mixture of 2 mM NMPs (with PPK2-expressing lysate, ADP, and HMP omitted, but including 8 mM ATP as an energy source). As negative controls, duplicate reactions were performed omitting polymerase. All reactions were incubated at 50° C. for 2 hours, then terminated by the addition of 9 volumes TE+ buffer (10 mM Tri s-HCl pH 8.0, 5 mM EDTA). Samples were mixed with an equal volume of 2×RNA Loading Dye (New England Biolabs) and heated to 70° C. for 10 minutes, followed by agarose/TAE gel electrophoresis.
As shown in
As shown in
In the claims articles such as “a,” “an,” and “the” may mean one or more than one unless indicated to the contrary or otherwise evident from the context. Claims or descriptions that include “or” between one or more members of a group are considered satisfied if one, more than one, or all of the group members are present in, employed in, or otherwise relevant to a given product or process unless indicated to the contrary or otherwise evident from the context. The invention includes embodiments in which exactly one member of the group is present in, employed in, or otherwise relevant to a given product or process. The invention includes embodiments in which more than one, or all of the group members are present in, employed in, or otherwise relevant to a given product or process.
Furthermore, the invention encompasses all variations, combinations, and permutations in which one or more limitations, elements, clauses, and descriptive terms from one or more of the listed claims is introduced into another claim. For example, any claim that is dependent on another claim can be modified to include one or more limitations found in any other claim that is dependent on the same base claim. Where elements are presented as lists, e.g., in Markush group format, each subgroup of the elements is also disclosed, and any element(s) can be removed from the group. It should it be understood that, in general, where the invention, or aspects of the invention, is/are referred to as comprising particular elements and/or features, certain embodiments of the invention or aspects of the invention consist, or consist essentially of, such elements and/or features. For purposes of simplicity, those embodiments have not been specifically set forth in haec verba herein. It is also noted that the terms “comprising” and “containing” are intended to be open and permits the inclusion of additional elements or steps. Where ranges are given, endpoints are included. Furthermore, unless otherwise indicated or otherwise evident from the context and understanding of one of ordinary skill in the art, values that are expressed as ranges can assume any specific value or sub-range within the stated ranges in different embodiments of the invention, to the tenth of the unit of the lower limit of the range, unless the context clearly dictates otherwise.
This application refers to various issued patents, published patent applications, journal articles, and other publications, all of which are incorporated herein by reference. If there is a conflict between any of the incorporated references and the instant specification, the specification shall control. In addition, any particular embodiment of the present invention that falls within the prior art may be explicitly excluded from any one or more of the claims. Because such embodiments are deemed to be known to one of ordinary skill in the art, they may be excluded even if the exclusion is not set forth explicitly herein. Any particular embodiment of the invention can be excluded from any claim, for any reason, whether or not related to the existence of prior art.
Those skilled in the art will recognize or be able to ascertain using no more than routine experimentation many equivalents to the specific embodiments described herein. The scope of the present embodiments described herein is not intended to be limited to the above Description, but rather is as set forth in the appended claims. Those of ordinary skill in the art will appreciate that various changes and modifications to this description may be made without departing from the spirit or scope of the present invention, as defined in the following claims.
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Claims
1. A cell-free method of biosynthesizing ribonucleic acid (RNA), the method comprising:
- (a) incubating a cell lysate mixture that comprises (i) cellular RNA and (ii) an enzyme that depolymerizes RNA, and (iii) a thermostable kinase and/or a thermostable RNA polymerase; and producing, a cell lysate mixture that comprises nucleoside monophosphates;
- (b) heating the cell lysate mixture produced in step (a) to a temperature that inactivates or partially inactivates the enzymes that depolymerize RNA and enzymes that degrade nucleotides and/or nucleic acids without completely inactivating the thermostable kinase activities and/or thermostable RNA polymerase activities, and producing a cell lysate mixture that comprises nucleoside monophosphates, and the thermostable kinase and/or the RNA polymerase; and
- (c) incubating the cell lysate mixture produced in (b) in the presence of an energy source and a deoxyribonucleic acid (DNA) template encoding a RNA of interest, producing nucleoside triphosphates, and producing a cell lysate mixture that comprises the RNA of interest.
2.-3. (canceled)
4. The method of claim 1, wherein the cellular RNA of step (a) is messenger RNA (mRNA), transfer RNA (tRNA), or ribosomal RNA (rRNA).
5. The method of claim 1, wherein the enzyme that depolymerizes RNA comprises a ribonuclease.
6. The method of claim 5, wherein the at least one ribonuclease is selected from the group consisting of S1 nuclease, Nuclease P1, RNase II, RNase III, RNase R, RNase JI, NucA, PNPase, RNase T, RNase E, and RNaseG.
7. (canceled)
8. The method of claim 1, wherein the thermostable kinase is selected from the group consisting of thermostable nucleoside monophosphate kinases, thermostable nucleoside diphosphate kinases, and thermostable polyphosphate kinases.
9. The method of claim 8, wherein the thermostable nucleoside monophosphate kinases are selected from the group consisting of thermostable uridylate kinases, thermostable cytidylate kinases, thermostable guanylate kinases, and thermostable adenylate kinases.
10. (canceled)
11. The method of claim 8, wherein the thermostable nucleoside diphosphate kinases are selected from the group consisting of thermostable nucleoside diphosphate kinases encoded by a Aquifex aeolicus ndk gene.
12. The method of claim 1, wherein the thermostable polyphosphate kinases are selected from the group consisting of thermostable polyphosphate kinase 1 (PPK1) enzymes and thermostable polyphosphate kinase 2 (PPK2) enzymes.
13.-18. (canceled)
19. The method of claim 1, wherein the at least one thermostable RNA polymerase is selected from the group consisting of thermostable DNA-dependent RNA polymerases.
20. The method of claim 19, wherein the thermostable DNA-dependent RNA polymerases are selected from the group consisting of thermostable T7 RNA polymerases, thermostable SP6 RNA polymerases, and thermostable T3 RNA polymerases.
21. (canceled)
22. The method of claim 1, wherein the energy source is adenosine triphosphate (ATP) or an ATP regeneration system.
23.-27. (canceled)
28. The method of claim 1, wherein the cell lysate mixture of step (a) comprises the DNA template encoding the RNA of interest.
29.-38. (canceled)
39. The method of claim 1, wherein the temperature of step (b) is 50° C.-80° C.
40.-43. (canceled)
44. The method of claim 1, wherein the cells are bacterial cells or yeast cells.
45.-50. (canceled)
51. An engineered cell comprising an enzyme that depolymerizes RNA, at least one thermostable kinase, and a thermostable RNA polymerase.
52. The cell of claim 51 further comprising an engineered DNA template containing a promoter operably linked to a nucleotide sequence encoding a RNA of interest.
53. A cell lysate produced by lysing the cultured engineered cell of claim 51.
54. The cell lysate of claim 53 further comprising an energy source and nucleoside monophosphates.
55.-88. (canceled)
89. The method of claim 1, the method comprising:
- (a) incubating a cell lysate mixture that comprises cellular RNA, an enzyme-that depolymerizes RNA, and a thermostable kinase and producing a cell lysate mixture that comprises nucleoside monophosphates;
- (b) heating the cell lysate mixture produced in step (a) to a temperature that inactivates or partially inactivates endogenous nucleases and phosphatases without completely inactivating the thermostable kinase, and producing a cell lysate mixture that comprises nucleoside monophosphates, and the thermostable kinase; and
- (c) incubating the cell lysate mixture produced in (b) in the presence of an RNA polymerase, an energy source and a deoxyribonucleic acid (DNA) template encoding a RNA of interest, producing nucleoside triphosphates, and producing a cell lysate mixture that comprises the RNA of interest.
90. The method of claim 1, the method comprising:
- (a) incubating a cell lysate mixture that comprises cellular RNA, an enzyme-that depolymerizes RNA, and a thermostable RNA polymerase and producing a cell lysate mixture that comprises nucleoside monophosphates;
- (b) heating the cell lysate mixture produced in step (a) to a temperature that inactivates or partially inactivates endogenous nucleases and phosphatases without completely inactivating the thermostable RNA polymerase, and producing a cell lysate mixture that comprises nucleoside monophosphates, and the thermostable RNA polymerase; and
- (c) incubating the cell lysate mixture produced in (b) in the presence of a kinase, an energy source and a deoxyribonucleic acid (DNA) template encoding a RNA of interest, producing nucleoside triphosphates, and producing a cell lysate mixture that comprises the RNA of interest.
Type: Application
Filed: Mar 15, 2021
Publication Date: Mar 3, 2022
Applicant: GreenLight Biosciences, Inc. (Medford, MA)
Inventors: William Jeremy Blake (Winchester, MA), Drew S. Cunningham (Winchester, MA), Daniel MacEachran (Medford, MA), Mehak Gupta (Medford, MA), James Robbins Abshire (Cambridge, MA)
Application Number: 17/202,029
