Patent Publication Number: US-2021171580-A1

Title: Thermostable viral reverse transcriptase

Description:
FIELD OF THE INVENTION 
     The present invention provides novel engineered polypeptides that support both reverse transcription and DNA amplification, in manganese-independent reactions. The present invention also provides methods for amplifying template nucleic acids using such polypeptides. This invention addresses deficiencies in the current state of the art in nucleic acid amplification-based detection of template nucleic acids, especially RNA targets, including deficiencies in detection sensitivity, specificity, enzyme stability, inhibitor tolerance and time to result compared with manganese-dependent thermostable reverse transcriptases and two-enzyme solutions. 
     BACKGROUND OF THE INVENTION 
     Sensitive amplification of specific RNA sequences enables molecular detection and quantification of targets including, e.g., transcription products that may indicate disease states like cancer, RNA viruses that may be associated with infectious diseases, and rRNA that can allow extremely sensitive detection of prokaryotic and eukaryotic cells. Improvements in detection are highly valued in the areas of diagnostics, human and veterinary health care, agriculture, food safety, environmental monitoring and scientific research. 
     In the current state of the art, primary tools for detecting and quantifying RNA are variants of reverse transcription polymerase chain reaction (RT-PCR), such as quantitative RT-PCR (RT-qPCR) or real-time RT-PCR. Other variants of RT-PCR include digital RT-PCR (dRT-PCR) or digital droplet RT-PCR (ddRT-PCR). These methods are all improved by this invention. The present invention is also useful in related methods of amplifying RNA without high temperature thermal cycling, such as loop-mediated isothermal amplification (LAMP), helicase dependent amplification (HDA) and recombinase polymerase amplification (RPA). 
     These methods are further facilitated by enzymatic functionalities that allow fluorescent detection of the amplification products. 
     In the current state of the art, RT-PCR typically uses two distinct enzymes, a thermolabile reverse transcriptase (RT), often a murine Moloney leukemia virus (MMLV) RT derivative, that synthesizes complementary DNA (cDNA) based on an RNA template, and a distinct DNA polymerase, commonly Taq polymerase, for amplification of the DNA product. Commonly, a third enzymatic activity, 5′→3′ exonuclease activity, inherent in Taq DNA polymerase, facilitates fluorescent detection by amplification-dependent hydrolysis and dequenching of a fluorescent DNA probe. 
     Several RT-PCR mixes, including some One Step RT-PCR kits, are currently provided, e.g., by QIAGEN (e.g., QIAGEN OneStep RT-PCR Kit) and Thermo Fisher Scientific (e.g., TaqMan® Fast Virus 1-Step Master Mix). All of these are two enzyme systems using derivatives of a retroviral RT and Taq DNA polymerase. 
     The reliance on multiple enzymes for these different steps has an inherent consequence that reaction conditions are necessarily a compromise between those optimal for the respective enzymes. This has a negative impact on sensitivity, specificity, time-to-result, ease of use, stability in storage and other key characteristics. Further, the presence of both enzymes in a single tube may lead to direct interference between the RT and the polymerase thereby limiting the sensitivity of RNA detection (Sellner, 1992). 
     In addition to retroviral RTs,  Bacillus  PolA enzymes often have moderately thermostable inherent RT activity, but, like the retroviral RTs, none has been thermostable enough for PCR. Reported attempts to increase thermostability of retroviral RTs by mutagenesis and in vitro evolution have been unsuccessful in providing adequate thermostability to allow single enzyme RT-PCR. Some inherently thermostable DNA polymerases, e.g. Tth polymerase and Hawk Z05 (Roche), can be induced to function as reverse transcriptases by modifying the buffer to include manganese rather than the typical magnesium. Other variants of thermostable DNA polymerases, e.g. those of  Thermus  (U.S. Pat. No. 5,455,170), Thermatoga and other thermophiles, have been modified by mutagenesis and directed evolution to use RNA templates. Intron encoded RTs from various thermophilic bacteria been explored for their potential use in single enzyme RT-PCR. 
     Single enzyme magnesium-dependent RT-PCR was enabled by PyroPhage® DNA polymerase (Lucigen). A 588 amino acid sequence was submitted as GenBank Acc. No. AFN99405.1 with the patent filings, i.e. U.S. Pat. No. 8,093,030 and related patents, and presumptively comprises the PyroPhage DNA polymerase. However, it was later found that this sequence contains an error from amino acid positions 450 to 463. This error was corrected by submission of GenBank Acc. No. AGL03984, a 611 amino acid open reading frame, the carboxyterminal 588 amino acids of which comprise the correct PyroPhage polymerase sequence. The corrected 588 amino acid sequence, including mutation E51A intentionally incorporated into the PyroPhage DNA polymerase to eliminate exonuclease activity, is shown in SEQ ID NO:15. This enzyme has both thermostable reverse transcriptase and DNA polymerase activities. This enzyme, as described in patents (e.g., U.S. Pat. No. 8,093,030) and literature (Schoenfeld et al., 2013; Moser et al., 2012), proved difficult to manufacture consistently, did not have sufficient RT activity, and was not competitive with the two enzyme systems with regard to ease of use, sensitivity, versatility in target RNAs, time-to-result, functionality in detection using probes or overall reliability. 
     Overall, none of these alternative thermostable reverse transcriptase/polymerase enzymes has been sufficiently effective in RT-PCR and the two enzyme mixes continue to be the state of the art for the great majority of practitioners. 
     The polypeptides of the present invention improve on the previously described molecule of SEQ ID NO:15 in that the amino acid sequence of the polymerase domain is altered by truncation of the N terminus of this sequence, such as elimination of eleven amino terminal amino acids from the N terminus of the protein sequence. 
     Previous attempts to use this molecule failed due the inability to produce a consistent product and for that reason failed to address the needs for reliable RT-PCR. The inventors of the present invention discovered that this variability was likely due to different levels of an internal translational initiation intrinsic to the host cells that eliminates those eleven amino acids from the amino terminus during normal enzyme expression. The result is a highly variable, heterogeneous mix of full-length and truncated product. It was also found that this truncated product, and not the full-length product, actually provides the RT activity and that truncating the gene to produce the smaller product results in a homogeneous product with higher overall RT activity. 
     Further, the primary sequence of this enzyme was improved by in vitro evolution. The improvements originated from a screening of published variants of differing levels of divergence (Schoenfeld et al., 2013) for polymerases with biochemical profiles that could potentially enhance functionality. The selected variants (Parent 1, 2, 3; SEQ ID NOs: 18-20) showed either high RT activity or thermostability. 
     The inventors of the present invention found that by combining specific regions derived from the different parent molecules, i.e., a region comprising the amino acid sequence of SEQ ID NO:16 and a region comprising an amino acid sequence selected from the group consisting of SEQ ID NO:17 and SEQ ID NO:72 or amino acid sequences at least 90%, preferably at least 95%, more preferably at least 98% identical thereto, polypeptides having improved combinations of properties regarding both RT activity and thermostability could be generated. Advantageously, the polypeptides of the invention may be applied in single enzyme RT-PCR reactions or in related reactions, such as amplifying RNA without high temperature thermal cycling, in conjunction with a second DNA polymerase, e.g. Taq DNA polymerase, for two enzyme RT PCR systems or preparative uses such as cDNA synthesis for cloning or for RNA sequence analysis. 
     In addition to analytic applications, there exist preparative uses for cDNA synthesis and RT-PCR, including cDNA cloning, preparation of templates for sequence determination of messenger and noncoding RNA, and other similar applications known in the art. In contrast to analytic methods, preservation of the integrity of the nucleotide sequence is critical for these preparative applications and there is an unmet need for improved accuracy of cDNA synthesis, both in conjunction with and independent of subsequent PCR typical of RT-PCR reactions. Substantial improvements in the accuracy of synthesis and amplification using DNA templates have been realized over the past three decades since the introduction of the first thermostable proofreading DNA polymerases, e.g. Lundberg K S, et al. (1991) Gene. 108(1):1-6; however, no such proofreading reverse transcriptase has been available for high accuracy, high efficiency synthesis using RNA templates. 
     A native proofreading activity is inherent to the parent molecules used to derive the enzymes of this invention. To limit complications from this secondary activity such as degradation of primers, this proofreading exonuclease activity was disabled by mutagenesis in versions of the enzyme of this invention that are intended for analytic uses. Since this activity is beneficial in preparative use, this proofreading function was reconstituted in the best mode RT constructs by reversion of the proofreading exonuclease domain to the wildtype sequence. These constructs represent the preferred embodiment for preparative use in of the invention in high fidelity RT-PCR. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention relates to polypeptides comprising a polymerase domain comprising an amino acid sequence of SEQ ID NO:16 and an amino acid sequence selected from the group consisting of SEQ ID NO:17 and SEQ ID NO:72, or amino acid sequences at least 90%, preferably at least 95%, more preferably at least 98% identical thereto. 
     In some embodiments, the N-terminus of the polymerase domain corresponds to the sequence of positions 12-22 of the sequence of SEQ ID NO:15, or a sequence at least 90%, preferably at least 95%, more preferably at least 98% identical thereto. In some preferred embodiments, the N-terminus is an amino acid sequence of “MN(X 1 )PKPILKPQ(X 2 )KALVEPVLC(X 3 )SI(X 4 )EIPA” (SEQ ID NO:21); or variants thereof, wherein X 1 =A or T; X 2 ═P or S; X 3 ═N or D; and X 4 ═N or D. 
     In certain embodiments, the polymerase domain comprises an amino acid sequence of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:10, or SEQ ID NO:12, or an amino acid sequence at least 90%, preferably at least 95%, more preferably at least 98% identical thereto. 
     In other embodiments, the polypeptide further comprises an exonuclease domain connected to the polymerase domain, preferably via a linker. 
     In one embodiment, the polypeptide exhibits only reverse transcriptase and DNA polymerase activity. In another embodiment, the polypeptide also exhibits 5′→3′ exonuclease activity. 
     One aspect of the present invention relates to compositions comprising a polypeptide of the invention. Another aspect of the present invention relates to vectors encoding the polypeptides of the invention. In another aspect, the present invention relates to transformed host cells comprising the vectors. 
     In another aspect, the present invention refers to methods for amplifying template nucleic acids comprising contacting the template nucleic acids with a polypeptide of the invention. 
     In one embodiment, the method is RT-PCR. 
     In certain embodiments, the method comprises a) generating cDNA using a polypeptide of the invention, and b) amplifying the generated cDNA using a polypeptide of the invention. 
     In some embodiments, the same polypeptide is applied for steps a) and b). 
     In other embodiments, the reverse transcription of step a) and the amplification of step b) are performed at isothermal conditions. 
     In another aspect, the present invention relates to kits comprising the polypeptide of the invention and a buffer. 
     In other embodiments, this invention provides a proofreading function coupled to high efficiency reverse transcription and inhibitor tolerance to enable high fidelity cDNA synthesis that enables high accuracy RT PCR. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 : Truncated 3173 polymerase maintains RT activity when extensively purified.
         Panel A. SDS-PAGE images of 1) full-length 3173 polymerase lot 1 purified extensively, 2) partially purified 3173 polymerase lot 4151, 3) Extensively purified truncated 3173 polymerase, and 4) extensively purified M160 polymerase.   Panel B. Endpoint pUC19 DNA PCR demonstrates that both lots of full length 3173 polymerase possess DNA polymerase activity and are capable of thermal cycling.   Panel C. Endpoint MS2 RNA RT-PCR demonstrates that only the partially purified full-length 3173 polymerase displays RT-PCR activity, whereas the extensively purified 3173 full-length polymerase does not allow product generation in RT-PCR.   Panel D. Real-time RT-qPCR demonstrates that the truncated 577 amino acid 3173 polymerase retains RT-PCR activity despite being extensively purified. In addition, the extensively purified M160 polymerase has lower Cq values on RNA templates, indicated a higher reverse transcriptase activity compared with 3173 polymerase.       
         FIG. 2 : Motifs shared by the RT-PCR enhanced mutant enzymes.
         Panel A. The RT-PCR enhanced mutants contained the region between 400 and 472 (SEQ ID NOs: 26-34) derived from Parent 2, i.e., a region comprising an amino acid sequence corresponding to SEQ ID NO:17 or SEQ ID NO:72 or amino acid sequences at least 90%, preferably at least 95% identical thereto.   Panel B. All the RT-PCR enhanced clones contained the region between 231 and 260 (SEQ ID NOs: 35-43), i.e., a region comprising the amino acid sequence corresponding to SEQ ID NO:16 or amino acid sequences at least 90%, preferably at least 95% identical thereto derived from Parent 1 or 3, which are almost indistinguishable in that region. Based on alignment to Taq Pol (not shown) this region probably includes the H helix.       
         FIG. 3 : Reverse transcriptase activity of M160 and M160-nuc at 55° C. and 60° C. 
         FIG. 4 : Thermal activity profile of M160-nuc. 
         FIG. 5 : Sensitivity and efficiency of detection of viral RNA. 
       Panel A. Detection by M160. 
       Panel B. Detection by M160-nuc. 
         FIG. 6 : M160-nuc compatibility with dye- and probe-based qPCR reaction chemistry. 
         FIG. 7 : Comparison of M160-nuc with two-enzyme RT-PCR mix.
         Panel A. Detection of a synthetic DNA target corresponding to MS2 RNA.   Panel B. Detection MS2 RNA target.       
         FIG. 8 : Amplification of an mRNA transcript from total human RNA with M160-nuc. 
         FIG. 9 : Amplification of 16S rRNA directly from bacterial cell lysate with M160-nuc. 
         FIG. 10 : Illustrates the binding affinity of engineered polymerases to primed-template DNA using an electrophoretic mobility shift assay.
         Panel A. Shows the binding affinity of M160 polymerase.   Panel B. Illustrates the increased binding affinity of the M160-nuc polymerase.   Panel C. Illustrates the further increased binding affinity of the M502 mutant polymerase.       
         FIG. 11 : Illustrates a comparison of the biochemical activity of the M160-nuc heparin resistant mutants with M160-nuc.
         Panel A. DNA polymerase specific activity as measured using oligonucleotide-primed M13 DNA template.   Panel B. Reverse transcriptase activity as measured using an oligo(dT) 20 -primed poly(A) template.       
         FIG. 12 : Illustrates the increased salt tolerance of the M160-nuc heparin resistant mutants compared with M160-nuc by measuring DNA polymerase activity on an oligonucleotide-primed M13 DNA template and reverse transcriptase activity using an oligo(dT) 20 -primed poly(A) template. 
         FIG. 13 : Illustrates tolerance to the inhibitory effects of heparin on the detection of MS2 viral RNA using either M160-nuc, M501, M502, or M503 polymerase in one-step RT-qPCR reactions.
         Panel A. Reaction buffer lacking human serum albumin.   Panel B. Reaction buffer including 1 mg/ml human serum albumin.       
         FIG. 14 : Illustrates tolerance to the inhibitory effects of hematin on the detection of MS2 viral RNA using either M160-nuc, M501, M502, or M503 polymerase in one-step RT-qPCR reactions.
         Panel A. Reaction buffer lacking human serum albumin.   Panel B. Reaction buffer including 1 mg/ml human serum albumin.       
         FIG. 15 : Illustrates tolerance to the inhibitory effects of humic acid on the detection of MS2 viral RNA using either M160-nuc, M501, M502, or M503 polymerase in one-step RT-qPCR reactions.
         Panel A. Reaction buffer lacking human serum albumin.   Panel B. Reaction buffer including 1 mg/ml human serum albumin.       
         FIG. 16 : Illustrates tolerance to the inhibitory effects of hemoglobin on the detection of MS2 viral RNA using either M160-nuc, M501, M502, or M503 polymerase in one-step RT-qPCR reactions.
         Panel A. Reaction buffer lacking human serum albumin.   Panel B. Reaction buffer including 1 mg/ml human serum albumin.       
         FIG. 17 : Illustrates tolerance to the inhibitory effects of xylan on the detection of MS2 viral RNA using either M160-nuc, M501, M502, or M503 polymerase in one-step RT-qPCR reactions.
         Panel A. Reaction buffer lacking human serum albumin.   Panel B. Reaction buffer including 1 mg/ml human serum albumin.       
         FIG. 18 : Illustrates the detection sensitivity of the M160-nuc, M501, M502, or M503 polymerase in one-step RT-qPCR reactions.
         Panel A. Detection of MS2 viral RNA using hydrolysis probe-based chemistry.   Panel B. Detection of MS2 viral RNA using Eva Green dye-based chemistry.   Panel C. Detecti4 on of LDHA mRNA from total human RNA using hydrolysis probe-based chemistry.       
         FIG. 19 : Illustrates the improved detection of LDHA mRNA from total human RNA in probe-based one-step RT-qPCR reactions using mixtures of M503 and Taq polymerase.
         Panel A. Shows the improvement in the fluorescent signal generated using enzyme mixtures containing Taq polymerase.   Panel B. Shows the improvement in Cq values using enzyme mixtures containing Taq polymerase.       
         FIG. 20 : Illustrates the extension speed of M160-nuc, M503, and a mixture of M503 and Taq polymerase using end-point PCR amplification of MS2 viral RNA. 
         FIG. 21 : Illustrates the detection sensitivity of four target DNA sequences using a mixture of M503 and Taq polymerase in multiplex one-step qPCR reactions.
         Panel A. Each of the four target DNA sequences was present in reactions at the same copy number.   Panel B. The ACTB, IL1B, and TUBA DNA sequences were present in all reactions at 10 8  copies. The GAPDH DNA sequences were present in reactions at the indicated copy number.       
         FIG. 22 : Proofreading on a DNA-primed RNA template using the 3′→5′ nuclease-active mutants was demonstrated by comparing the efficiency of the extension of a primer with a 3′-terminal matched base pair versus the three possible 3′-terminal mismatched base pairs, as indicated. Error bars represent the standard deviation of triplicate reactions. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Definitions 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art (e.g., in cell culture, molecular genetics, nucleic acid chemistry, hybridization techniques and biochemistry). 
     In practicing the present invention, many conventional techniques in molecular biology, microbiology, and recombinant DNA may be used. These techniques are well known and are explained in, for example, Current Protocols in Molecular Biology, Volumes I, II, and III, 1997 (F. M. Ausubel ed.); Sambrook et al., 1989, Molecular Cloning: A Laboratory Manual, Second Edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.; DNA Cloning: A Practical Approach, Volumes I and II, 1985 (D. N. Glover ed.); Oligonucleotide Synthesis, 1984 (M. L. Gait ed.); Nucleic Acid Hybridization, 1985 (Hames and Higgins); Transcription and Translation, 1984 (Hames and Higgins eds.); Animal Cell Culture, 1986 (R. I. Freshney ed.); Immobilized Cells and Enzymes, 1986 (IRL Press); Perbal, 1984, A Practical Guide to Molecular Cloning; the series, Methods In Enzymology (Academic Press, Inc.); Gene Transfer Vectors for Mammalian Cells, 1987 (J. H. Miller and M. P. Calos eds., Cold Spring Harbor Laboratory); and Methods in Enzymology Vol. 154 and Vol. 155 (Wu and Grossman, and Wu, eds., respectively). 
     As used herein, the term “comprising” is to be construed as encompassing both “including” and “consisting of”, both meanings being specifically intended, and hence individually disclosed embodiments in accordance with the present invention. 
     The term “DNA” in the present invention relates to any one of viral DNA, prokaryotic DNA, archaeal DNA, and eukaryotic DNA. The DNA may also be obtained from any one of viral RNA, and mRNA from prokaryotes, archaea, and eukaryotes by generating complementary DNA (cDNA) by using a reverse transcriptase. 
     The term “PCR” refers to polymerase chain reaction, which is a standard method in molecular biology for DNA amplification. 
     “RT-PCR” relates to reverse transcription polymerase chain reaction, a variant of PCR commonly used for the detection and quantification of RNA. RT-PCR comprises two steps, synthesis of complementary DNA (cDNA) from RNA by reverse transcription and amplification of the generated cDNA by PCR. Variants of RT-PCR include quantitative RT-PCR (RT-qPCR), real-time RT-PCR, digital RT-PCR (dRT-PCR) or digital droplet RT-PCR (ddRT-PCR). 
     “Methods of amplifying RNA without high temperature thermal cycling” as referred to herein, may be isothermal nucleic acid amplification technologies, such as loop-mediated amplification (LAMP), helicase dependent amplification (HDA) and recombinase polymerase amplification (RPA). 
     “Truncate”, “truncation” or “truncated” as referred to herein includes modifications of the N-terminal sequences incorporated during synthesis of the corresponding nucleic acids encoding the proteins. Despite a common, stricter usage in the art that does not include modification of the N-terminus, as used herein, “truncate” and its derivatives “truncation” and “truncated” may encompass both reduction in molecular weight and modification of the N-terminal sequence as defined herein. 
     Polymerases/Enzymes 
     In a first aspect, the present invention provides polypeptides comprising a polymerase domain comprising an amino acid sequence of SEQ ID NO:16 and an amino acid sequence selected from the group consisting of SEQ ID NO:17 and SEQ ID NO:72, or amino acid sequences at least 90%, preferably at least 95%, more preferably at least 98% identical thereto. Preferably, the polypeptides of the present invention comprise a polymerase domain comprising an amino acid sequence of SEQ ID NO:16 and an amino acid sequence selected from the group consisting of SEQ ID NO:17 and SEQ ID NO:72. In one embodiment, the polypeptides of the present invention comprise a polymerase domain comprising the amino acid sequence of SEQ ID NO:16 and an amino acid sequence at least 90%, preferably at least 95%, more preferably at least 98% identical to the amino acid sequence of SEQ ID NO:17 or SEQ ID NO:72. In another embodiment, the polypeptides of the present invention comprise a polymerase domain comprising the amino acid sequence of SEQ ID NO:17 or SEQ ID NO:72 and an amino acid sequence at least 90%, preferably at least 95%, more preferably at least 98% identical to SEQ ID NO:16. 
     In some embodiments, the N-terminus of the polymerase domain corresponds to the sequence of positions 12-22 of the sequence of SEQ ID NO:15, ora sequence at least 90%, preferably at least 95%, more preferably at least 98% identical thereto. In other embodiments, the N-terminus of the polymerase domain corresponds to the sequence of positions 12-25, more preferably 12-27, most preferably 12-30 of SEQ ID NO:15, or a sequence at least 90%, preferably at least 95%, more preferably at least 98% identical thereto. 
     In some preferred embodiments, the N-terminus is an amino acid sequence of “MN(X 1 )PKPILKPQ(X 2 )KALVEPVLC(X 3 )SI(X 4 )EIPA” (SEQ ID NO:21); or variants thereof, wherein X 1 =A or T; X 2 ═P or S; X 3 ═N or D; and X 4 ═N or D. 
     In some preferred embodiments, the polymerase domain of the polypeptide of the present invention comprises an amino acid sequence of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:10, or SEQ ID NO:12, or an amino acid sequence at least 95% identical thereto. In some particularly preferred embodiments, the polypeptide of the invention comprises a polymerase domain having an amino acid sequence as shown in SEQ ID NO:4. 
     In some embodiments, the polypeptide of the present invention has an amino acid sequence as shown in SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:12, or SEQ ID NO:14, or an amino acid sequence at least 90%, preferably at least 95%, more preferably at least 98% identical thereto. In some particularly preferred embodiments, the polypeptide of the invention has an amino acid sequence as shown in SEQ ID NO:4. 
     In some embodiments, the proofreading 3′→5′ exonuclease activity is disabled by at least one point mutation. In other embodiments, the proofreading activity can be reconstituted by reversion of this point mutation. The native enzyme molecules from which the polypeptides of the invention were derived, e.g. SEQ ID NO:15, have inherent proofreading 3′→5′ exonuclease activity. Since this activity may interfere with certain common analytical applications, in some embodiments, e.g., in the polypeptides comprising the amino acid sequences of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:12, or SEQ ID NO:14, SEQ ID NO:18, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:49, SEQ ID NO:51, SEQ ID NO:53, SEQ ID NO:55, or an amino acid sequence at least 90%, preferably at least 95%, more preferably at least 98% identical thereto, this activity has been disabled by at least one point mutation. A preferred embodiment (for analytic uses) is a polypeptide comprising the amino acid sequence of SEQ ID NO:55, or an amino acid sequence at least 90%, preferably at least 95%, more preferably at least 98% identical thereto. 
     In other embodiments, this point mutation has been reversed so that the proofreading activity is reconstituted. One can envision certain uses, especially preparative applications, in which the increased accuracy of synthesis provided by such a proofreading activity would be advantageous. In one embodiment, the polypeptide comprising a restored proofreading activity comprises an amino acid sequence of SEQ ID NO:45, or an amino acid sequence at least 90%, preferably at least 95%, more preferably at least 98% identical thereto. In other embodiments, the polypeptide comprising a restored proofreading activity has an amino acid sequence of SEQ ID NO:45, SEQ ID NO:47, SEQ ID NO:74, SEQ ID NO:76, SEQ ID NO:78 or SEQ ID NO:80 or an amino acid sequence at least 90%, preferably at least 95%, more preferably at least 98% identical thereto. A preferred embodiment (for preparative uses) is a polypeptide comprising the amino acid sequence of SEQ ID NO:80, or an amino acid sequence at least 90%, preferably at least 95%, more preferably at least 98% identical thereto. 
     In some preferred embodiments, in addition to the polymerase domain, the polypeptide of the invention further comprises a 5′→3′ exonuclease domain connected to the polymerase domain, preferably via a linker. 
     Suitable linkers may be amino acid linkers comprising 5-15 amino acids, more preferably 7-12 amino acids, most preferably 9-11 amino acids. In a preferred embodiment, the linker has the sequence “GGGGSGGGGS” (SEQ ID NO:22). Alternatively, suitable linkers may be non-amino acid linkers. 
     In polypeptides according to the invention comprising a 5′→3′ exonuclease domain connected to the polymerase domain, for instance via a linker, the N-terminus of the polymerase domain comprises the sequence of positions 13-22, preferably of positions 13-25, more preferably of positions 13-27, most preferably of positions 13-30 of the sequence of SEQ ID NO:15. 
     Advantageously, polypeptides of the present invention comprise an additional 5′→3′ exonuclease domain to facilitate fluorescent detection of the amplification products, for instance using hydrolysis probes, such as TaqMan probes. In some embodiments, such a polypeptide comprising a polymerase domain and an additional exonuclease domain comprises an amino acid sequence of SEQ ID NO:14, SEQ ID NO:45, SEQ ID NO:47, SEQ ID NO:49, SEQ ID NO:51, SEQ ID NO:53, SEQ ID NO:55, SEQ ID NO:74, SEQ ID NO:76, SEQ ID NO:78 or SEQ ID NO:80 or an amino acid sequence at least 90%, preferably at least 95%, more preferably at least 98% identical thereto. In some preferred embodiments, a polypeptide comprising a polymerase domain and an exonuclease domain has an amino acid sequence at least 90%, preferably at least 95%, more preferably at least 98%, most preferably 100% identical to SEQ ID NO:14, SEQ ID NO:45, SEQ ID NO:47, SEQ ID NO:49, SEQ ID NO:51, SEQ ID NO:53, SEQ ID NO:55, SEQ ID NO:74, SEQ ID NO:76, SEQ ID NO:78 or SEQ ID NO:80. In some particularly preferred embodiments, such polypeptide comprises an amino acid sequence at least 90%, preferably at least 95%, more preferably at least 98%, most preferably 100% identical to SEQ ID NO:55. In other particularly preferred embodiments, such polypeptide comprises an amino acid sequence at least 90%, preferably at least 95%, more preferably at least 98%, most preferably 100% identical to SEQ ID NO:80. 
     Preferably, a polypeptide according to the invention comprises an amino acid sequence at least 90%, preferably at least 95%, more preferably at least 98% identical to SEQ ID NO:14, wherein one or more of amino acids H751, Q752, L753, W777, D781, D622, or Q627 of SEQ ID NO:14 is substituted. More preferably, a polypeptide according to the invention comprises an amino acid sequence at least 90%, preferably at least 95%, more preferably at least 98% identical to SEQ ID NO: 14, wherein SEQ ID NO:14 comprises at least one or more of the following substitutions: H751Q, Q752K, L753K, W777C, D781A, D622N, and/or Q627N. More preferably, a polypeptide according to the invention comprises an amino acid sequence at least 90%, preferably at least 95%, more preferably at least 98% identical to SEQ ID NO:14, wherein SEQ ID NO:14 comprises one of the following groups of substitutions: Q627N, H751Q, Q752K, and L753K; or H751Q, Q752K, and L753K; or W777C, D781A, D622N and Q627N. Most preferably, a polypeptide according to the invention comprises an amino acid sequence at least 90%, preferably at least 95%, more preferably at least 98% identical to SEQ ID NO:14, wherein SEQ ID NO:14 comprises the following substitutions: Q627N, H751Q, Q752K, L753K. Accordingly, most preferably, a polypeptide according to the invention comprises an amino acid sequence at least 90%, preferably at least 95%, more preferably at least 98% identical to SEQ ID NO:55 having the following substitutions: Q627N, H751Q, Q752K, L753K. Similarly, most preferably, a polypeptide according to the invention comprises an amino acid sequence at least 90%, preferably at least 95%, more preferably at least 98% identical to SEQ ID NO:80 having the following substitutions: Q627N, H751Q, Q752K, L753K. Advantageously, polypeptides having the indicated mutations exhibit beneficial properties, such as increased resistance to PCR inhibitors or salt tolerance, while retaining good polymerase activity and RT activity. 
     In some embodiments, the polypeptide of the invention exhibits reverse transcriptase activity. In other embodiments, the polypeptide of the invention exhibits 5′→3′ exonuclease activity. In some embodiments the 5′→3′ exonuclease domain can be included, but the catalytic activity can be disabled by point mutation, as is known in the art, to provide enhanced nucleic acid binding affinity while avoiding nuclease catalytic activity when it might interfere with an intended application. In still another embodiment the 5′→3′ exonuclease domain could be included for binding affinity, but disabled catalytically, while the 3′→5′ proofreading exonuclease activity can be reconstituted and active. 
     Beneficially, the activity of the polypeptides of the invention does not require the presence of manganese so that the polypeptides of the inventions may be used in conventional magnesium containing buffers. This compatibility with magnesium provides practical advantages in simplicity of reaction formulation and accuracy of synthesis, as is known in the art. 
     In one aspect, the polypeptides according to the invention are used in a method of the invention. In another aspect, the invention relates to compositions comprising a polypeptide of the invention. 
     Another aspect of the invention refers to vectors encoding a polypeptide of the invention. In some embodiments, the vector comprises a nucleic acid sequence as shown in SEQ ID NO: 1, SEQ ID NO:3, SEQ ID NO:5, SEQ ID NO:7, SEQ ID NO:9, SEQ ID NO:11, or SEQ ID NO:13. In a particularly preferred embodiment, the vector coding for a polypeptide of the invention comprises a nucleic acid sequence as shown in SEQ ID NO:3. Alternatively, the vector comprises a nucleic acid sequence as shown in SEQ ID NO:13, more preferably the vector comprise a nucleic acid sequence as shown in any of SEQ ID NO:50, SEQ ID NO:52, SEQ ID NO:54, SEQ ID NO:73, SEQ ID NO:75, SEQ ID NO:77 or SEQ ID NO:79. In a particularly preferred embodiment, the vector comprises a nucleic acid sequence as shown in SEQ ID NO:54. In another particularly preferred embodiment, the vector comprises a nucleic acid sequence as shown in SEQ ID NO:79. 
     Another aspect of the invention relates to transformed host cells comprising such vector, such as  E. coli  or other suitable host cells. 
     Methods 
     In another aspect, the present invention refers to methods for amplifying template nucleic acids comprising contacting the template nucleic acids with a polypeptide according to the invention. 
     Template nucleic acids according to the present invention may be any type of nucleic acids, such as RNA, DNA, or RNA:DNA hybrids. Template nucleic acids may either be artificially produced (e.g. by molecular or enzymatic manipulations or by synthesis) or may be a naturally occurring DNA or RNA. In some preferred embodiments, the template nucleic acids are RNA sequences, such as transcription products, RNA viruses, or rRNA. 
     Advantageously, the method of the invention also enables amplification and detection/quantification of template nucleic acids, such as specific RNA target sequences, out of a complex mixture of target and non-target background RNA. For instance, the method of the invention allows amplification of an mRNA transcript from total human RNA or amplification of rRNA directly from bacterial cell lysate. 
     In some embodiments, the method referred to herein is RT-PCR. RT-PCR may be quantitative RT-PCR (RT-qPCR), real-time RT-PCR, digital RT-PCR (dRT-PCR) or digital droplet RT-PCR (ddRT-PCR). 
     In other embodiments, the method referred to herein is a method of amplifying RNA without high temperature thermal cycling, such as loop-mediated isothermal amplification (LAMP), helicase dependent amplification (HDA) and recombinase polymerase amplification (RPA). 
     In some preferred embodiments, the method of the invention comprises the steps of
         a) generating cDNA using a polypeptide of the invention; and   b) amplifying the generated cDNA using a polypeptide of the invention.       

     In some embodiments, the method of the invention further comprises detecting and/or quantifying the amplified nucleic acids. Quantification/detection of amplified nucleic acids may be performed, e.g., using non-sequence-specific fluorescent dyes (e.g., SYBR® Green, EvaGreen®) that intercalate into double-stranded DNA molecules in a sequence non-specific manner, or sequence-specific DNA probes (e.g., oligonucleotides labelled with fluorescent reporters) that permit detection only after hybridization with the DNA targets, synthesis-dependent hydrolysis or after incorporation into PCR products. 
     In some preferred embodiments, in the method of the invention, the same polypeptide is applied for generating cDNA in step a) and for amplifying the generated cDNA in step b). Advantageously, in the method of the invention, reverse transcription and subsequent amplification of the generated cDNA may be performed in a single enzyme format. In other particularly preferred embodiments, the generation of cDNA in step a) and the amplification of the generated cDNA in step b) are performed at isothermal conditions. Suitable temperatures may, for instance, be between 30-96° C., preferably 55-95° C., more preferably 55-75° C., most preferably 55-65° C. 
     In some embodiments, in the method of the invention, a polypeptide of the invention is used in combination with Taq DNA polymerase. In other embodiments, human serum albumin is added during amplification, preferably at a concentration of 1 mg/ml. 
     Kits 
     Reagents necessary to perform the method of the invention may be comprised in kits. 
     In some embodiments, the invention relates to kits for amplifying template nucleic acids, wherein the kit comprises a polypeptide of the invention and a buffer. Optionally, the kit additionally comprises Taq DNA polymerase and/or serum albumin. Buffers comprised in the kit may be conventional buffers containing magnesium. Suitable buffer solutions do not need to contain manganese. 
     EXAMPLES 
     The invention is illustrated in the following examples. 
     Example 1: Expression of Truncated DNA Polymerases 
     The 588 amino acid sequence shown in GenBank Acc. No. AFN99405.1, presumptively comprising the PyroPhage polymerase (Lucigen, Middleton, Wis.), contains a sequencing error from amino acid positions 450 to 463. This error was corrected by submission of GenBank Acc. No. AGL03984, a 611 amino acid open reading frame, the carboxyterminal 588 amino acids of which comprise the correct PyroPhage polymerase sequence. The corrected 588 amino acid sequence, including mutation E51A intentionally incorporated to eliminate exonuclease activity, is shown in SEQ ID NO:15. This enzyme was purified numerous times and the performance of the enzyme preparations in RT-PCR was highly variable. In two representative examples (Lots 1 and 4151), this molecule was purified to varying degrees of homogeneity by iterative rounds of affinity and ion exchange column chromatography as is well known in the art, and the molecular weights of the generated products were determined by SDS PAGE. The Lot 1 preparation ( FIG. 1 , Panel A, Lane 2) shows a homogeneous enzyme estimated to comprise the full-length 588 amino acid molecule of SEQ ID NO:15. Lot 4151 ( FIG. 1 , Panel A, Lane 2) was less completely purified, as evidenced by spurious bands of lower molecular weight. A close examination of the SDS PAGE data ( FIG. 1 , Panel A, Lane 2) reveals that the apparent major band is actually two bands estimated to correspond to 588 (SEQ ID NO:15) and 577 (SEQ ID NO:18) amino acids. 
     Lots 1 and 4151 were tested in quadruplicate reactions for their ability to PCR amplify a 860 bp DNA target from the pUC19 beta-lactamase gene. Equivalent quantities of enzyme were thermal cycled under conditions described for a control PCR in the PyroPhage® 3173 DNA polymerase, Exo-product manual (MA 100 v. 1.0, Lucigen Corp.). DNA products were analyzed by agarose gel electrophoresis ( FIG. 1 , Panel B, Lanes 2 to 5 and Lanes 6 to 9). The DNA product was present in all lanes indicating that both enzyme preparations were fully capable of PCR amplifying from DNA templates. 
     Lots 1 ( FIG. 1 , Panel C, Lanes 2 to 3) and 4151 ( FIG. 1 , Panel C, Lanes 4 to 5) were tested in duplicate reactions for their ability to RT PCR amplify MS2 phage RNA. Using the control RT-PCR conditions described in the PyroScript™ RT-PCR 2× Master Mix Kit manual (MA 102, Lucigen Corp.), equivalent units of enzyme were thermal cycled and the products were analyzed by agarose gel electrophoreses. In this case, only the less purified Lot 4151 generated the expected 160 bp product, indicating successful amplification from the RNA template and suggesting the RT-PCR capacity is associated with a lower molecular weight product seen in  FIG. 1 , Panel A, Lane 2. 
     Examination of the corrected sequence corresponding to GenBank Acc. No. AFN99405.1 (SEQ ID NO:15) shows two methionine residues at positions 11 and 12. It was hypothesized that the reverse transcription activity was associated with a 577 amino acid translation product generated by spurious internal translational initiation or, alternatively, proteolysis to generate a product initiating at the position 12 methionine. The variability in the generation of this product is believed to result in a mix of 588 and 577 amino acid products (SEQ ID NOs: 15 and 18) and, therefore, the variability or ineffectiveness of some preparations in RT-PCR. Purification to homogeneity of the 588 amino acid product, as in Lot 1, results in an enzyme preparation that fails to reverse transcribe RNA targets prior to PCR amplification. 
     To test the hypothesis that the 577 amino acid enzyme is the active form of the enzyme responsible for reverse transcriptase activity, a gene construct that encodes the 577 amino acid protein truncated at the N terminus by 11 amino acids, but otherwise identical to SEQ ID NO:15, was used to produce homogenous 577 amino acid product, the sequence of which is shown in SEQ ID NO:18. 
     The 577 amino acid product was tested ( FIG. 1 , Panel D) in RT-PCR in 20 μl reactions containing 50 mM Tris, pH 8.7, 75 mM KCl, 4 mM MgCl 2 , 0.3 mM dNTPs, 0.04 mg/ml human serum albumin, 0.2 M trehalose, 0.2× EvaGreen dye (Biotium), 0.3 μM forward and reverse primer (25 nucleotides each in size), 300 ng polymerase, and either 1×10 7  copies of MS2 phage RNA (Roche) or 1×10 7  copies of a synthetic double-stranded DNA gene block (IDT) with sequence corresponding to MS2 RNA. The 243 bp amplicon corresponded to position 472 to 714 of the MS2 genome (GenBank Acc. No. V00642.1; SEQ ID NO:23) and the 362 bp amplicon corresponded to position 353 to 714 of the MS2 genome. Reactions were thermal cycled in a StepOnePlus (Thermo Fisher) as follows: 94° C. 30 sec (×1), 94° C. 3 sec, 64° C. 1 minute (×40). 
     As hypothesized and in contrast to the full length 588 amino acid product in Lot 1, this 577 amino acid truncated enzyme had reproducible performance in RT-PCR ( FIG. 1 , Panel D). This 577 amino acid enzyme truncation product of SEQ ID NO:15 was used as Parent 1 (SEQ ID NO:18) in subsequent work described below. 
     Example 2: Testing of Parent Molecules 
     Six viral DNA polymerase genes ranging from 100% to 44% compared to Parent 1 (SEQ ID NO:18) were identified in a published source (Schoenfeld, 2013). Truncated derivatives of each were mutagenized to eliminate 3′-5′ exonuclease activity as described (Moser et al, 2008), expressed and tested for RT activity and thermostability. Three of the six constructs were chosen for shuffling based on enhanced thermostability (Parents 1 and 3, SEQ ID NOs: 18 and 20) or high reverse transcriptase activity (Parent 2, SEQ ID NO:19). 
     Example 3: Generation and Screening of Clone Libraries 
     Clones were generated by dividing each of the genes encoding Parent enzymes 1, 2 and 3 (SEQ ID NO:18, SEQ ID NO:19, SEQ ID NO:20) into nine homologous segments and randomly shuffling the sequences with each other. The DNA segments for each of these regions were synthesized based on the sequences of Parents 1, 2 and 3, respectively and shuffled using the RepliQa™ Assembly Mix (Quantabio) according to the manufacturer&#39;s recommendation. Over 400 clones were expressed in  E. coli  and screened for performance in RT-PCR by measuring the ability of crude heat-treated lysate to amplify the 243 bp segment of MS2 phage RNA. Whereas most of the clones were completely nonfunctional or had diminished performance in RT-PCR, six of the mutant enzymes (M66, M160, M180, M295, M384, and M392) had enhanced performance in RT-PCR compared to both the full length enzyme (GenBank Acc. No. AFN99405.1; SEQ ID NO:15) and truncated Parent 1 (SEQ ID NO:18) as evidenced by lower Cq values (Table 1). 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 High-efficiency polymerase variants. 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                 Amino acid 
                   
               
               
                   
                   
                   
                 conservation 
                 Cycle threshold 
               
               
                   
                 Nucleic acid 
                 Amino acid 
                 compared to 
                 for detection of 
               
               
                 Variant 
                 sequence 
                 sequence 
                 parent 1 
                 MS2by RT-qPCR 
               
               
                   
               
               
                 Parent 1 
                   
                 Truncated sequence 
                 100%  
                 17.3 
               
               
                   
                   
                 derived from 
               
               
                   
                   
                 SEQ ID NO: 15 
               
               
                   
                   
                 (SEQ ID NO: 18) 
               
               
                 Parent 2 
                   
                 Truncated sequence 
                 84% 
                 None detected 
               
               
                   
                   
                 derived from the 
               
               
                   
                   
                 sequence of 
               
               
                   
                   
                 GenBank AGL03983 
               
               
                   
                   
                 (SEQ ID NO: 19) 
               
               
                 Parent 3 
                   
                 Truncated sequence 
                 93% 
                 26.4 
               
               
                   
                   
                 derived from the 
               
               
                   
                   
                 sequence of 
               
               
                   
                   
                 GenBank AGL03985 
               
               
                   
                   
                 (SEQ ID NO: 20) 
               
               
                 M66 
                 SEQ ID NO: 1 
                 SEQ ID NO: 2 
                 92% 
                 15.3 
               
               
                 M160 
                 SEQ ID NO: 3 
                 SEQ ID NO: 4 
                 89% 
                 12.1 
               
               
                 M180 
                 SEQ ID NO: 5 
                 SEQ ID NO: 6 
                 94% 
                 14.4 
               
               
                 M295 
                 SEQ ID NO: 7 
                 SEQ ID NO: 8 
                 94% 
                 12.9 
               
               
                 M384 
                 SEQ ID NO: 9 
                 SEQ ID NO: 10 
                 95% 
                 14.6 
               
               
                 M392 
                 SEQ ID NO: 11 
                 SEQ ID NO: 12 
                 95% 
                 14.5 
               
               
                 M160-nuc 
                 SEQ ID NO: 13 
                 SEQ ID NO: 14 
                 89% 
                 not determined 
               
               
                   
               
            
           
         
       
     
     Example 4. Bioinformatic Analysis of the RT-PCR Competent Clones 
     In the original analysis, Parent 2 (SEQ ID NO:19) had higher RT activity, but inadequate thermostability for RT-PCR. In contrast, Parents 1 (SEQ ID NO:18) and 3 (SEQ ID NO:20) had higher thermostability, but lower RT activity. Presumably the sequences comprising enhanced RT-PCR clones are combinations of the regions of the parents that confer the optimal combination of these functions, i.e., a region comprising the amino acid sequence of SEQ ID NO:16 and a region comprising an amino acid sequence selected from the group consisting of SEQ ID NO:17 and SEQ ID NO:72, or amino acid sequences at least 90%, preferably at least 95%, more preferably 98% identical thereto. The sequences of the enhanced RT-PCR clones were compared to those of the ineffective RT-PCR clones to identify common features conserved in the enhanced RT-PCR enzyme constructs but not the ineffective enzymes. Although there were numerous positions that varied among the enhanced RT-PCR clones, this analysis identified a region (amino acids 400 to 472) between motifs B and C (Delarue, 1990) that was fully conserved and apparently derived from Parent 2 (SEQ ID NO:19;  FIG. 2 , Panel A). In better characterized Family A DNA polymerases, this inter-motif region is characterized by two alpha helices, 0 Helix and P Helix and Beta Sheets 10 and 11, known to be in close contact with the template (Li, 1998). This proximity to the template is very consistent with the improved utilization of the non-natural RNA template. In all the positive clones, the bulk of the sequence outside this inter-motif region is derived from Parents 1 and 3 (SEQ ID NO:18 and SEQ ID NO:20) and the residues conserved in these Parents are more distributed. However, the region between residues 231 to 260 of all the enhanced RT-PCR clones are conserved and appear derived from Parent 1 or 3, which are almost identical in this region ( FIG. 2 , Panel B). This region includes H helix, which appears to be critical to binding the phosphate backbone of the template in both the open and closed forms (Li, 1998). 
     Example 5: High-Efficiency Polymerase Variants 
     Of the six RT PCR enhanced variants, M160 provided the shortest cycle threshold and was used for further development. This enzyme was further improved by fusing to its N-terminus a domain from the Taq DNA polymerase enzyme that conferred 5′→3′ exonuclease activity and consequently the ability to utilize hydrolyzable probes such as TaqMan (Roche) probes. 
     As shown in the following examples, the fusion construct M160-nuc had the additional advantage of improving reverse transcriptase activity at elevated temperatures ( FIGS. 3 and 4 ), and RNA detection sensitivity ( FIG. 5 ). The M160-nuc can detect amplification by dye-based chemistry or hydrolyzable probes ( FIG. 6 ) and can detect viral RNA ( FIG. 7 ), mRNA transcripts ( FIG. 8 ) and bacterial rRNA ( FIG. 9 ) with high sensitivity and fast time to result compared to alternative two enzyme RT-PCR systems. 
     Example 6: Reverse Transcriptase Activity 
     Reverse transcriptase activities of the purified variant M160 and the purified fusion construct M160-nuc, in which the 5′→3′ nuclease domain from Taq polymerase was fused to the N-terminus of M160 via a 10-amino acid flexible linker, were assessed at different temperatures and the activities were compared. Reactions (20 μl) containing 50 mM Tris, pH 8.3, 75 mM NaCl, 5 mM MgCl 2 , 1 mM DTT, 0.01% Tween-20, 2% trehalose, 0.4× EvaGreen dye (Biotium), 0.8 mM dTTP, 0.01 ug/μl Poly(A), 0.1 μM olgo(dT)20 primer, and 0-20 ng polymerase were incubated at the indicated temperature and fluorescence readings were taken every 15 seconds. The initial slopes of fluorescence curves were calculated and compared for each polymerase. 
       FIG. 3  shows relative reverse transcriptase activities of M160 and M160-nuc at different temperatures (55° C. and 60° C.). In addition to demonstrating that the presence of the 5′→3′ nuclease domain does not interfere with M160 reverse transcriptase activity at high temperature, the results indicate that the nuclease domain improves activity, presumably by increasing affinity of the enzyme for the nucleic acid template. 
     Example 7: Measurement of Thermal Activity Profile 
     DNA polymerase activities of M160-nuc were measured by determining the relative rates of nucleotide incorporation ( FIG. 4 ) using either a primed M13 template or activated calf thymus DNA, each of which is an effective substrate over different temperature ranges. Both types of reaction contained 20 mM Tris, pH 8.8, 10 mM (NH 4 ) 2 SO 4 , 10 mM KCl, 2 mM MgSO 4 , and 0.1% Triton X-100. The M13-based reactions (20 μl) also contained 200 uM dNTPs, 1×SYBR Green I (Thermo Fisher), 7.5 μg/ml M13mp18 DNA, 0.25 mM each of a mixture of three primers 24-33 nt in size, and 0.03-1 ng of M160-nuc enzyme. Reactions were incubated at the indicated temperature, fluorescence readings were taken every 15 seconds, and fluorescence initial slope values were calculated and compared. For the calf thymus DNA-based reactions, reactions (50 μl) also contained 4 μg activated calf thymus DNA, 100 μM dNTPs, 7.5 μCi/ml  3 H-dTTP, and 0.8-25 ng M13-nuc polymerase. Reactions were incubated at the indicated temperatures, then the TCA-insoluble radioactive counts were measured. The slopes of the initial rates of nucleotide incorporation were then determined and compared. In both cases the temperature at which the activity was highest was set at 100% activity and other values were plotted relative to this number. As shown in  FIG. 4 , the M160-nuc construct displays peak activity from 65-80° C. 
     Example 8: Presence of 5′ Nuclease Domain Improves Sensitivity and Efficiency of Detection of Viral RNA 
     The M160 ( FIG. 5 , Panel A) or M160-nuc ( FIG. 5 , Panel B) constructs were tested in RT-qPCR amplification using serial dilutions of MS2 RNA template. In both cases amplifications were performed without a pre-incubation step prior to thermal cycling. 
     Reactions (20 μl) contained 50 mM Tris, pH 8.7, 75 mM KCl, 4 mM MgCl 2 , 0.3 mM dNTPs, 0.04 mg/ml human serum albumin, 0.2 M trehalose, 0.225× EvaGreen dye (Biotium), the indicated number of copies of MS2 phage RNA, 0.3 μM forward and reverse primer (25 nucleotides each in size), and 300 ng polymerase. The amplicon was 531 bp in size and corresponded to position 184 to 714 of the MS2 genome (GenBank Acc. No. V00642.1; SEQ ID NO:23). Reactions were thermal cycled in a StepOnePlus (Thermo Fisher) as follows: 94° C. 30 sec (×1), 94° C. 3 sec, 64° C. 1 minute (×40). Compared with M160 alone ( FIG. 5 , Panel A), the M160-nuc ( FIG. 5 , Panel B) polymerase displays significantly improved detection sensitivity and amplification at lower cycle numbers, indicated by lower Cq values and higher efficiency amplification. 
     Example 9: M160-Nuc Compatibility with Dye- and Probe-Based qPCR Reaction Chemistry 
     To test capacity of M160-nuc to support detection by hydrolysable probes, RT-qPCR reactions were performed using either EvaGreen-based detection chemistry or by using a dual-quenched FAM-labeled hydrolysis probe for amplification detection ( FIG. 6 ). Reactions (20 μl) contained 50 mM Tris, pH 8.75, 75 mM KCl, 3 mM MgCl 2 , 0.3 mM dNTPs, 0.04 mg/ml human serum albumin, 0.2 M trehalose, the indicated number of copies of MS2 phage RNA, 0.3 μM forward and reverse primer (25 nucleotides each in size), and 100 ng of M160-nuc polymerase. The amplicon was 362 bp in size and corresponded to position 353 to 714 of the MS2 genome (GenBank Acc. No. V00642.1; SEQ ID NO:23). Dye-based reactions contained 0.225× Eva Green (Biotium) and probe-based reactions contained 0.2 μM of a 5′-FAM/internal ZEN/3′-Iowa Black quenched 22 nt oligonucleotide (MS2 position 650-671). Reactions were thermal cycled in a QuantStudio system (Thermo Fisher) as follows: 94° C. 30 sec (×1), 94° C. 3 sec, 72° C. 1 minute (×40). In both cases, serially diluted MS2 RNA was used as template and the resulting Cq values were assessed. The equivalent Cq values indicate compatibility of the M160-nuc polymerase with both detection chemistries in terms of sensitivity and efficiency. 
     Example 10: Comparison of M160-Nuc with Two-Enzyme RT-PCR Mix 
     Hydrolysis probe-based qPCR reactions were performed with dilutions of either a synthetic double-stranded DNA molecule corresponding to a portion of the MS2 phage genomic RNA sequence ( FIG. 7 , Panel A) or using single-stranded MS2 phage RNA ( FIG. 7 , Panel B). The 25 nt primers generate a 531 bp product and corresponded to position 184 to 714 of the MS2 genome (GenBank Acc. No. V00642.1; SEQ ID NO:23). M160-nuc reactions were thermal cycled at 94° C. 30 sec (1 cycle), 94° C. 3 sec, 72° C. 1 minute (40 cycles) and reactions with the Taq/MMLV RNase H-enzyme mixture (ZipScript, QIAGEN) were additionally pre-incubated at 50° C. for 15 min. 
     Whereas reactions with the Taq/MMLV RNase H-enzyme mixture required a pre-incubation step (50° C., 15 min) in the RNA reactions for cDNA conversion because the MMLV enzyme is thermolabile and denatures during the cycling phase, the M160-nuc polymerase does not require a pre-incubation phase because it is highly active at the temperatures used for DNA extension during cycling conditions (72° C.). 
     In addition, the Cq values for the Taq/MMLV RNase H-mixture were approximately 5.5 cycles higher than with the M160-nuc polymerase, indicating that the M160-nuc polymerase is significantly more efficient at reverse transcription of the highly structured MS2 RNA genome during the PCR cycling phase (72° C.) compared with the MMLV RNase H-enzyme during the pre-incubation phase (50° C.). 
     Example 11: Amplification of an mRNA Transcript from Total Human RNA 
     To test the capacity of the M160-nuc enzyme to detect mRNA transcripts, the M160-nuc polymerase was used to amplify a 145 bp region of the LDHA mRNA from total human RNA using a FAM probe-based RT-qPCR assay ( FIG. 8 ). Reactions (20 μl) contained 50 mM Tris, pH 8.75, 75 mM KCl, 3 mM MgCl 2 , 0.3 mM dNTPs, 0.04 mg/ml human serum albumin, 0.2 M trehalose, the indicated quantity of total human RNA, 0.3 μM forward and reverse primer (40 nt and 26 nt, respectively), 0.2 uM probe, and 100 ng of M160-nuc polymerase. The amplicon corresponded to position 1428 to 1572 of the LDHA transcript (GenBank Acc. No. NM_005566.3; SEQ ID NO:24). The probe was 34 nt in size, corresponded to position 1509-1542 and contained 5′-FAM/internal ZEN/3′-Iowa Black modifications. Reactions were thermal cycled in a QuantStudio system (Thermo Fisher) as follows: 94° C. 30 sec (×1), 94° C. 3 sec, 72° C. 45 sec (×45). Detection sensitivity was demonstrated to be approximately 200 fg, which corresponds to approximately 5 copies as determined by digital PCR quantification. This demonstrates a high degree of sensitivity and specificity for the M160-nuc polymerase for mRNA detection in the presence of a complex mixture of target and non-target background RNA. 
     Example 12: Amplification of 16S rRNA Directly from Bacterial Cell Lysate 
     The capacity of the M160-nuc in detection of a highly structured ribosomal RNA target directly from cell lysate without processing was tested in a RT PCR reaction ( FIG. 9 ). From serial dilutions of total cell lysate, M160-nuc polymerase was used in FAM probe-based RT-qPCR reactions to directly amplify a variable portion of the 16S rRNA.  Vibrio natriegens  cells were grown to early log phase in 2× YT media and the cell number was quantified by plating serial dilutions of cells to LB-agar and growing overnight at 30° C. Cells resuspended in 200 μl of a buffer containing 10 mM Tris, pH 7.5, 0.5 mM EDTA, 100 mM NaCl, 0.1% Triton X-100 and were lysed by addition of 1 μl of Ready-Lyse™ Lysozyme solution (Lucigen) and incubating for 15 minutes at room temperature. The lysate was briefly vortexed and serial dilutions were made using 0.01% Tween-20. Finally, 2 μl of this lysate was used directly in RT-PCR reactions (20 μl) containing 50 mM Tris, pH 8.75, 75 mM KCl, 3 mM MgCl 2 , 0.3 mM dNTPs, 0.04 mg/ml human serum albumin, 0.2 M trehalose, 0.3 μM forward and reverse primer (25 nt and 26 nt, respectively), 0.2 μM probe, and 100 ng of M160-nuc polymerase. The amplicon (159 nt) corresponded to position 56 to 214 of the  Vibrio natriegens  strain ATCC 14048 16S ribosomal RNA gene (GenBank Acc. No. NR_117890.1; SEQ ID NO:25). The probe was 34 nt in size, corresponded to position 145-178 and contained 5′-FAM/internal ZEN/3′-Iowa Black modifications. Reactions were thermal cycled in a QuantStudio system (Thermo Fisher) as follows: 94° C. 30 sec (1 cycle), 94° C. 3 sec, 72° C. 30 sec (45 cycles). rRNA is present at copy numbers as high as 10,000 per cell. The detection limit by RT-PCR was significantly lower than the extinction limit based on serial plating of cells, demonstrating the efficiency of detection of the structured rRNA in the presence of cell lysate components and the capacity of this method to allow detection of cells at limits of detection well below single cell. 
     Example 13: Presence of the 5′ Nuclease Domain Increases Binding Affinity to Primed-Template DNA 
     To determine whether the increased activity and improved performance characteristics of M160-nuc were correlated with increased binding affinity to primed-template nucleic acid, the enzymes were tested with target substrates using an electrophoretic mobility shift assay ( FIG. 10 ). The sequence and preparation of the primed template oligonucleotides was as previously described (Yamagami et al., 2014). Reactions (30 μl) containing 20 mM Tris, pH 8.8, 10 mM NaCl, 5 mM MgCl 2 , 1 mM DTT, 0.05 mg/ml BSA, 10% glycerol, and 5 nM unlabeled DNA substrate were incubated with polymerase at 37° C. for 10 minutes to allow equilibrium to be reached, then components were fractionated by native 3-12% polyacrylamide gel electrophoresis. Gels were stained with 2×SYBR Gold (Invitrogen) and the band intensities were quantified. The fraction bound was determined by dividing the intensity of the shifted band by the total DNA signal. Binding affinity to primed-template DNA for M160-nuc (Kd=78.9 nM) increased 12-fold compared with the M160 polymerase (Kd=946 nM) lacking the exonuclease domain. 
     Example 14: Biochemical Characterization of M160-Nuc Exonuclease Derivatives 
     While not essential for RT PCR-based nucleic acid detection, high nucleotide incorporation fidelity of the reverse transcriptase would be beneficial for preparative applications, e.g. cDNA cloning and RNA-seq methods, where sequence accuracy is important. As noted above, M160-nuc had its proofreading activity eliminated by mutagenesis. The error rate of M160-nuc, measured using a standard blue-white screen of sequence errors in PCR-amplified lacl repressor, was 1.91×10 −4  (Table 2), similar to the error rates measured for retroviral reverse transcriptases and for a variant KOD polymerase with RT activity (Ellefson et al., 2013; Yasukawa et al., 2016). In contrast, in preparations of altered versions of the M160-nuc enzyme in which the 3′→5′ proofreading nuclease activity was reactivated with an A339E reversion (Table 2, M401, SEQ ID NO:45), the measured error rate was reduced by nearly two orders of magnitude, resulting in an error rate similar to KOD polymerase, a prototypical proofreading PCR enzyme. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Biochemical characterization of M160-nuc exonuclease derivatives. 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Nucleic 
                 Amino 
                   
                   
                 Relative 
                 Relative RT 
                 Relative 
                   
               
               
                   
                 acid 
                 acid 
                 5′→3′ 
                 3′→5′ 
                 pol 
                 activity, 
                 ssExo 
                 Error 
               
               
                 Enzyme 
                 sequence 
                 sequence 
                 exo 
                 exo 
                 activity 
                 50° C. 
                 activity 
                 rate 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 M160- 
                 SEQ ID 
                 SEQ ID 
                 + 
                 − 
                 1 
                 1 
                 Not 
                  1.91 × 10 −4  +/− 0.196 
               
               
                 nuc 
                 NO 13 
                 NO 14 
                   
                   
                   
                   
                 detected 
               
               
                 M401 
                 SEQ ID 
                 SEQ ID 
                 + 
                 + 
                 0.91 
                 1.02 
                 0.76 
                 2.22 × 10 −6  +/− 0.02 
               
               
                   
                 NO 44 
                 NO 45 
               
               
                 M402 
                 SEQ ID 
                 SEQ ID 
                 − 
                 + 
                 1.05 
                 0.98 
                 0.89 
                 2.49 × 10 −6  +/− 0.33 
               
               
                   
                 NO 46 
                 NO 47 
               
               
                 M403 
                 SEQ ID 
                 SEQ ID 
                 − 
                 − 
                 0.72 
                 1.03 
                 Not 
                 Not 
               
               
                   
                 NO 48 
                 NO 49 
                   
                   
                   
                   
                 tested 
                 tested 
               
               
                   
               
            
           
         
       
     
     To discern whether the RT-PCR performance improvement of M160-nuc compared to M160 was due to nuclease activity or simply the presence of the nuclease domain providing enhanced template binding affinity, two additional constructs were generated. In the first of these, M402, the 5′→3′ nuclease domain was present but inactivated by the G46D mutation. In the second, M403, both the 5′→3′ and the 3′→5′ activities were inactivated by mutagenesis. To test for exonuclease activity, reactions (50 μl) containing 50 mM Tris, pH 8.7, 75 mM KCl, 4 mM MgCl 2 , 0.3 mM dNTPs, 0.04 mg/ml human serum albumin, 0.2 M trehalose, 50 nM  3 H-dTTP end-labeled single-stranded 59-mer oligonucleotide and 0.39-50 ng polymerase were incubated at 37° C. for 60 minutes. 
     Reactions were stopped by addition of salmon sperm carrier DNA and TCA-soluble radioactive counts were measured. Exonuclease activity measurements were made relative to Pfu polymerase. Elimination of the 5′→3′ nuclease did not have a measurable impact on the RT activity, regardless of the associated 3′→5′ exonuclease activity (Table 2), suggesting the improvement of RT-PCR function was dependent on biochemical attributes other than nucleolytic activity, presumably modification of the binding affinity provided by the domain. In addition, the presence or absence of a 5′→3′ nuclease activity did not substantially affect fidelity. 
     Example 15: Focused Mutagenesis of M160-Nuc for Increased Inhibitor Resistance 
     The sensitivity and specificity of nucleic acid amplification-based detection methods are often hindered by the presence of biological, chemical, and environmental inhibitors in target samples. These inhibitors include blood components, blood preservation chemicals, fabrics, plant and soil components, excess salts, detergents, and nucleic acid extraction chemicals. Methods that tolerate nucleic acid amplification inhibitors are therefore highly desirable and there remains a need for polymerases and polymerase formulations that increase resistance to inhibitors. 
     Heparin, a branched polymer of variable molecular weight and variably sulfated repeating disaccharide units, is commonly used as an anticoagulant and can copurify with nucleic acid samples derived from blood. With its high negative charge density, heparin can bind to DNA-interacting proteins such as reverse transcriptases and DNA polymerases, competing with nucleic acid template binding and interfering with activity. To engineer mutants of M160-nuc with increased heparin resistance, mutagenesis efforts targeted regions of the molecule predicted to associate with template nucleic acid. Mutations in the molecule that increase specificity of binding to nucleic acid template by increasing primed-template binding affinity or by decreasing heparin affinity should confer increased heparin resistance in RT-qPCR. Mutagenesis of M160-nuc focused on three regions of the polymerase, chosen based on sequence alignment with better characterized family A polymerases. The first region mutated was M160-nuc amino acid residues 750-753, predicted to correspond to a region of helix P, an exterior alpha helix in the fingers domain adjacent to template. The next round of mutagenesis targeted amino acids 776-783, predicted to correspond to helix Q, a region running parallel to the DNA template strand in the palm at the base of the fingers domain that faces DNA template and participates in binding to the minor groove. The final round targeted amino acids 622-627, predicted to correspond to motif 2, a region at the base of the fingers and thumb domain involved in binding primer-template duplex through minor groove and sugar phosphate interactions (Loh and Loeb, 2005). 
     Random and semi-random mutant libraries of M160-nuc sequences were prepared by assembling a partially degenerate oligonucleotide containing 25 nucleotide terminal overlaps with an inverse PCR-generated expression plasmid lacking the region to be mutagenized. Assembly was done using the RepliQa Assembly Mix™ (Quantabio) according to the manufacturer&#39;s recommendation. Approximately 128 clones from each mutagenized segment were expressed in  E. coli  and screened for performance in RT-PCR by measuring the ability of crude heat-treated lysate to amplify the 243 bp segment of phage MS2 RNA in the presence of heparin. In the helix P library, four distinct mutants (Helix P-62, 63, 69, and 88) were identified that showed enhanced performance in the presence of 10 ng/μl heparin compared with M160-nuc as evidenced by lower Cq values (Table 3). In the helix Q library, four distinct heparin-resistant mutants were also identified (Helix Q-9, 69, 87, 88). Of these, Helix Q-69 showed the most heparin resistance, resulting in a Cq of 9.1 in the presence of 10 ng/μl heparin, which is comparable to that of the parent M160-nuc in the absence of heparin (Cq=8.1). To identify mutants with even further increased heparin resistance, the next round of mutagenesis targeted Helix Q-69 at motif 2 and used a screen based on RT-PCR activity in the presence of 40 ng/μl heparin. Six distinct mutants (Motif 2-11, 25, 41, 108, 120, and 121) showed Cq values lower than the Helix Q-69 mutant. Of these, the Motif 2-108 mutant showed the highest heparin resistance and was able to amplify MS2 RNA in the presence of 40 ng/μl heparin with equal efficiency (Cq=7.9) as the parent M160-nuc in the absence of heparin (Cq=8.1). 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Primary screen of M160-nuc heparin resistant mutants 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Heparin 
                 Cycle threshold 
               
               
                   
                   
                 quantity 
                 for detection of 
               
               
                 Mutant 
                 Amino acid changes 
                 (ng/μl) 
                 MS2 by RT-qPCR 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Unmodified 
                 None 
                 0 
                 8.1 
               
               
                 M160-nuc 
               
               
                 Unmodified 
                 None 
                 10 
                 22.8-28.3 
               
               
                 M160-nuc 
               
               
                 Helix P-62 
                 Q750W, H751Q, 
                 10 
                 21.0 
               
               
                   
                 Q752K, L753K 
               
               
                 Helix P-63 
                 H751Q, Q752K, 
                 10 
                 14.9 
               
               
                   
                 L753K 
               
               
                 Helix P-65 
                 H751L, Q752K 
                 10 
                 19.5 
               
               
                 Helix P-89 
                 Q750W, Q752K, 
                 10 
                 18.1 
               
               
                   
                 L753Q 
               
               
                 Helix Q-9 
                 W777G, D781H 
                 10 
                 9.6 
               
               
                 Helix Q-69 
                 W777C, D781A 
                 10 
                 9.1 
               
               
                 Helix Q-87 
                 W777Y, D781A 
                 10 
                 12.3 
               
               
                 Helix Q-88 
                 W777Y, D781R 
                 10 
                 21.5 
               
               
                 Unmodified 
                 None 
                 40 
                 No amplification 
               
               
                 M160-nuc 
               
               
                 Helix Q-69 
                 W777C, D781A 
                 40 
                 18.7-19.3 
               
               
                 Motif2-11 
                 W777C, D781A, 
                 40 
                 12.5 
               
               
                   
                 D622N, I623L, 
               
               
                   
                 Q627N 
               
               
                 Motif2-25 
                 W777C, D781A, 
                 40 
                 11.5 
               
               
                   
                 D622S, Q627N 
               
               
                 Motif2-41 
                 W777C, D781A, 
                 40 
                 14.1 
               
               
                   
                 D622G, Q627S 
               
               
                 Motif2-108 
                 W777C, D781A, 
                 40 
                 7.9 
               
               
                   
                 D622N, Q627N 
               
               
                 Motif2-120 
                 W777C, D781A, 
                 40 
                 9.9 
               
               
                   
                 D622N, I623L, 
               
               
                   
                 Q627S 
               
               
                 Motif2-121 
                 W777C, D781A, 
                 40 
                 13.5 
               
               
                   
                 Q627N 
               
               
                   
               
            
           
         
       
     
     Example 16: Secondary Screening of Heparin Resistant Mutants 
     To downselect from the group of identified heparin-resistant mutants, a secondary screen assessed performance in RT-qPCR by measuring heparin resistance and MS2 RNA detection sensitivity (Table 4). Two heparin-resistant mutants from each structural domain library were expressed in  E. coli  and purified by strong cation exchange and heparin spin-column chromatography as is known in the art. In addition, we constructed and purified three hybrid mutants (Hyb-1, Hyb-2, and Hyb-3) that contained mutations combined from different structural domains or subsets of the previously identified mutations. The quantity of enzyme to be used per RT-qPCR reaction was determined as the smallest quantity that showed no increase in the Cq value and the heparin resistance was defined as the highest quantity that increased the Cq value by &lt;3 compared with reactions without heparin. The results of the hybrid mutant analysis of Hyb-2 and Hyb-3 showed that whereas the D622N and Q627N mutations enhanced the heparin resistance of the Q-69 mutant, the mutations on their own conferred no heparin resistance and so were excluded from further analysis. In addition, some mutants such as Q-69 appeared to show compromised detection sensitivity and therefore were also excluded. However, based on the results of the RT-qPCR analyses, three mutants showed both significant heparin resistance and high MS2 detection sensitivity and were chosen for further analysis (Table 5). 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Secondary screen of M160-nuc heparin resistant mutants for RNA 
               
               
                 detection sensitivity in addition to heparin resistance. 
               
            
           
           
               
               
               
            
               
                   
                 Quantity 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 enzyme 
                   
                 MS2 RNA 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 tested in 
                 Heparin 
                 detection 
               
               
                 Mutant 
                 Amino acid changes 
                 RT-qPCR 
                 resistance 
                 sensitivity 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Unmodified 
                 None 
                 50 
                 ng 
                 &lt;2.5 
                 ng/μl 
                 20 
                 copies 
               
               
                 M160-nuc 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Helix P-62 
                 Q750W, H751Q, 
                 12.5 
                 ng 
                 &lt;2.5 
                 ng/μl 
                 Not tested 
               
               
                   
                 Q752K, L753K 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Helix P-63 
                 H751Q, Q752K, 
                 12.5 
                 ng 
                 10 
                 ng/μl 
                 20 
                 copies 
               
               
                   
                 L753K 
               
               
                 Helix Q-69 
                 W777C, D781A 
                 18 
                 ng 
                 40 
                 ng/μl 
                 2000 
                 copies 
               
               
                 Helix Q-88 
                 W777Y, D781R 
                 12.5 
                 ng 
                 10 
                 ng/μl 
                 200 
                 copies 
               
               
                 Motif2-108 
                 W777C, D781A, 
                 5 
                 ng 
                 &gt;80 
                 ng/μl 
                 200 
                 copies 
               
               
                   
                 D622N, Q627N 
               
               
                 Motif2-121 
                 W777C, D781A, 
                 12.5 
                 ng 
                 &gt;80 
                 ng/μl 
                 200 
                 copies 
               
               
                   
                 Q627N 
               
               
                 Hyb-1 
                 Q627N, H751Q, 
                 12.5 
                 ng 
                 10 
                 ng/μl 
                 20 
                 copies 
               
               
                   
                 Q752K, L753K 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Hyb-2 
                 D622N, Q627N 
                 25 
                 ng 
                 &lt;2.5 
                 ng/μl 
                 Not tested 
               
               
                 Hyb-3 
                 Q627N 
                 25 
                 ng 
                 &lt;2.5 
                 ng/μl 
                 Not tested 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Inhibitor resistant mutant sequences 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Nucleic acid 
                 Amino acid 
               
               
                 Enzyme 
                 Mutations 
                 sequence 
                 sequence 
               
               
                   
               
               
                 M501 
                 H751Q, Q752K, 
                 SEQ ID NO: 50 
                 SEQ ID NO: 51 
               
               
                   
                 L753K 
               
               
                 M502 
                 W777C, D781A, 
                 SEQ ID NO: 52 
                 SEQ ID NO: 53 
               
               
                   
                 D622N, Q627N 
               
               
                 M503 
                 Q627N, H751Q, 
                 SEQ ID NO: 54 
                 SEQ ID NO: 55 
               
               
                   
                 Q752K, L753K 
               
               
                   
               
            
           
         
       
     
     Example 17: Polymerase and Reverse Transcriptase Activity of Heparin Resistant Mutants 
     For further characterization of the biochemical properties of the M501, M502, and M503 mutants, the genes were overexpressed in  E. coli  and the polymerases were purified by iterative rounds of affinity and ion exchange column chromatography. To measure DNA polymerase activity, the relative rates of nucleotide incorporation were determined using a primed M13 template. Reactions (20 μl) containing 20 mM Tris, pH 8.8, 10 mM (NH 4 ) 2 SO 4 , 10 mM KCl, 2 mM MgSO 4 , 0.1% Triton X-100, 200 uM dNTPs, 1×SYBR Green I (Thermo Fisher), 7.5 μg/ml M13mp18 DNA, 0.25 mM each of a mixture of three primers 24-33 nt in size, and 0-10 ng of enzyme were incubated at 72° C. To measure reverse transcriptase activity, reactions (20 μl) containing 50 mM Tris, pH 8.3, 75 mM NaCl, 5 mM MgCl 2 , 1 mM DTT, 0.01% Tween-20, 2% trehalose, 0.4× EvaGreen dye (Biotium), 0.8 mM dTTP, 0.01 ug/μl Poly(A), 0.1 μM olgo(dT) 20  primer, and 0-20 ng polymerase were incubated at 55° C. For both assays, fluorescence was measured at 15 second intervals and the initial slopes of fluorescence curves were calculated and compared for each polymerase. Despite the increased heparin resistance of the M501, M502, and M503 mutants and the high activity in RT-qPCR as shown in Table 4, neither the polymerase specific activity ( FIG. 11 , Panel A) nor the reverse transcriptase activity ( FIG. 11 , Panel B) of the mutants were significantly altered compared with the parental M160-nuc polymerase. In contrast, the increased binding affinity to primed-template DNA ( FIG. 10 , Panel B and Panel C), suggests improved discrimination between template and heparin binding. For the M502 mutant, the measured affinity to primed template was at the sensitivity limit of the binding assay (Kd&lt;6.1 nM), at least a 12-fold improvement compared with the parental M160-nuc polymerase (Kd=78.9). 
     Increased ionic strength due to the presence of elevated salt in nucleic acid samples has the potential to affect the binding between polymerase and DNA template. Elevated salt tolerance is correlated in DNA polymerases with processivity, which affects performance in PCR. To test whether the altered template binding effects produced by the mutations in the M501, M502, and M503 mutants also had the effect of improving salt tolerance, DNA polymerase activity assays were performed in the presence of between 2.5 and 100 mM KCl and reverse transcriptase activity assays were performed in the presence of between 10 and 200 mM NaCl ( FIG. 12 ). Activity was measured by calculating the initial slopes of the fluorescent curves and the salt tolerance was determined as the quantity that reduced the maximum activity to 50% activity. For both DNA polymerase and reverse transcriptase activities, all three mutants additionally showed improved salt tolerance compared with the parental M160-nuc polymerase. 
     Example 18: Resistance to Additional PCR Inhibitors 
     Although the M501, M502, and M503 mutants were isolated from a biochemical screen designed to improve heparin resistance, they were further tested to determine possible resistance to additional PCR inhibitors ( FIGS. 13-17 ). RT-qPCR reactions were performed using viral MS2 RNA as template and a dual-quenched FAM-labeled hydrolysis probe for amplification detection. Reactions (20 μl) contained 50 mM Tris, pH 8.75, 75 mM KCl, 3 mM MgCl 2 , 0.3 mM dNTPs, 0.2 M trehalose, 0.025% Tween-20, 0.75 M betaine, 10 6  copies of MS2 phage RNA, 0.3 μM forward and reverse primer (25 nucleotides each in size), 0.2 μM probe, and polymerase (100 ng of M160-nuc polymerase, 50 ng of M501, 25 ng of M502, or 50 ng of M503). The amplicon was 243 bp in size and corresponded to position 472 to 714 of the MS2 genome (GenBank Acc. No. V00642.1; SEQ ID NO:23). The probe was 22 nt in size, corresponded to position 650-671 and contained 5′-FAM/internal ZEN/3′-Iowa Black modifications. Each polymerase was tested with the following inhibitor concentrations: 0 to 50 ng/μl heparin, 0 to 4 μM hematin, 0 to 8 ng/μl humic acid, 0 to 800 ng/μl hemoglobin, or 0 to 80 ng/μl xylan. Reactions were thermal cycled in a QuantStudio system (Thermo Fisher) as follows: 94° C. 30 sec (1 cycle), 94° C. 5 sec, 72° C. 30 sec (40 cycles). Whereas all three mutants displayed improved resistance to heparin as expected ( FIG. 13 , Panel A), M503 also displayed slightly improved hematin resistance ( FIG. 14 , Panel A), hemoglobin resistance ( FIG. 16 , Panel A), and xylan resistance ( FIG. 17 , Panel A). 
     It has been previously shown that addition of serum albumin protein to PCR reactions improves tolerance to several inhibitors including FeCl 3 , hemin, fulvic acids, humic acids, tannic acids, and fecal extracts (Kreader, 1996). However, the addition of 1 mg/ml human serum albumin to RT-qPCR reactions catalyzed by M160-nuc polymerase resulted in amplification inhibition, even in the absence of inhibitors (not shown). In contrast, the addition of 1 mg/ml human serum albumin to RT-qPCR reactions catalyzed by M501 and M503 provided additional tolerance to hematin ( FIG. 14 , Panel B), humic acid ( FIG. 15 , Panel B), hemoglobin ( FIG. 16 , Panel B), and xylan ( FIG. 17 , Panel B). 
     Example 19: RNA Detection Sensitivity in One-Step RT-qPCR Reactions 
     To test sensitivity of M501, M502 and M503 in detection of viral MS2 RNA, RT-qPCR reactions were performed using either a dual-quenched FAM-labeled hydrolysis probe for amplification detection ( FIG. 18 , Panel A) or using EvaGreen-based detection chemistry ( FIG. 18 , Panel B). Reactions (20 μl) contained 50 mM Tris, pH 8.75, 75 mM KCl, 3 mM MgCl 2 , 0.3 mM dNTPs, 0.2 M trehalose, 0.025% tween 20, 0.75M betaine, the indicated number of copies of MS2 phage RNA, 0.3 μM forward and reverse primer (25 nucleotides each in size), and polymerase (100 ng of M160-nuc, 25 ng of M501, 12.5 ng of M502, or 25 ng of M503). The amplicon was 243 bp in size and corresponded to position 472 to 714 of the MS2 genome (GenBank Acc. No. V00642.1; SEQ ID NO:23). 
     Dye-based reactions contained 0.225× Eva Green (Biotium) and probe-based reactions contained 0.2 μM of a 5′-FAM/internal ZEN/3′-Iowa Black quenched 22 nt oligonucleotide (MS2 position 650-671). Reactions were thermal cycled in a QuantStudio system (Thermo Fisher) as follows: 94° C. 30 s (1 cycle), 94° C. 5 sec, 72° C. 30 sec (40 cycles). In both cases, the resulting Cq values were assessed. The results indicate compatibility of the M501, M502 and M503 mutants with both probe- and dye-based detection chemistries and that the presence of the mutations did not reduce detection sensitivity. The slightly lower Cq values for M501, M502 and M503 compared to M160-nuc indicate improved reverse transcription under these reaction conditions. 
     The M501, M502 and M503 mutants were tested for detection of mRNA transcripts in reactions designed to amplify a 145 bp region of the LDHA mRNA from total human RNA using a FAM probe-based RT-qPCR assay ( FIG. 18 , Panel C). Reactions (20 μl) contained 50 mM Tris, pH 8.75, 75 mM KCl, 3 mM MgCl 2 , 0.3 mM dNTPs, 0.2 M trehalose, 0.025% Tween-20, 0.75 M betaine, total human RNA (Agilent), 0.3 μM forward and reverse primer (40 nt and 26 nt, respectively), 0.2 uM probe, and polymerase (100 ng of M160-nuc, 25 ng of M501, 12.5 ng of M502, or 25 ng of M503). The amplicon corresponded to position 1428 to 1572 of the LDHA transcript (GenBank Acc. No. NM_005566.3; SEQ ID NO:24). The probe was 34 nt in size, corresponded to position 1509-1542 and contained 5′-FAM/internal ZEN/3′-Iowa Black modifications. 
     Reactions were thermal cycled in a QuantStudio system (Thermo Fisher) as follows: 94° C. 30 sec (1 cycle), 94° C. 5 sec, 72° C. 30 sec (45 cycles). The LDHA copy number was determined in the total human RNA by digital PCR quantification. We found that in reactions catalyzed by the M502 mutant, only as few as 10,000 copies of the LDHA mRNA were detected, compared with as few as 10 copies for M160-nuc. This indicates a negative effect of the M502 mutations in amplification reactions using this complex template, likely associated with reduced template specificity. However, in reactions catalyzed by the M501 and M503 mutants, as few as 10 copies were detected, indicating a high degree of sensitivity and specificity in the presence of a complex mixture of target and non-target background RNA. 
     Example 20: Improved Hydrolysis Probe-Based Fluorescent Signal Generation Using Polymerase Mixtures Containing Taq DNA Polymerase 
     Although the hybrid and mutant polymerases described in this invention comprising fusions with the 5′→3′ nuclease domain of Taq polymerase are able to efficiently utilize hydrolysis probe-based detection chemistry in qPCR reactions, it is possible that the nuclease and polymerase domains are not in an optimal configuration for maximum fluorescent signal generation for all probe sequences and templates. Taq polymerase and its derivatives are commonly used in qPCR mixtures for probe-based detection, so its inclusion in the enzyme mixture may be advantageous for signal generation. To test whether fluorescent probe-based signal could be improved in one-step RT-qPCR detection of LDHA, mRNA from total human RNA, 20 μl amplification reactions were compared to M503 polymerase alone with mixtures of M503 and Taq polymerase ( FIG. 19 ). The addition of either 2 U or 4 U of Taq polymerase to the M503 mutant did increase the maximum normalized relative fluorescence units (RFU) for all quantities of template tested, up to a 2.8-fold increase in reactions with the fewest copies of template ( FIG. 19 , Panel A). In addition, the increase in fluorescent signal in reactions containing Taq polymerase in the enzyme mixture allowed for earlier detection and lower Cq values ( FIG. 19 , Panel B). 
     Example 21: Inhibitor Resistance and Amplification Speed Using Enzyme Mixtures Containing Taq DNA Polymerase 
     To test the upper limits of inhibitor resistance of the M503 mutant in amplification reactions containing both HSA and Taq polymerase, the following concentration ranges of inhibitory components were tested: 0 to 50 ng/μl heparin, 0 to 100 μM hematin, 0 to 80 ng/μl humic acid, 0 to 5 μg/μl hemoglobin, and 0 to 1 μg/μl xylan. Reactions (20 μl) contained 50 mM Tris, pH 8.75, 75 mM KCl, 3 mM MgCl 2 , 0.3 mM dNTPs, 0.2 M trehalose, 0.025% Tween-20, 0.75 M betaine, 10 6  copies of MS2 phage RNA, 0.3 μM forward and reverse primer (25 nucleotides each in size), 0.2 μM probe, and polymerase (100 ng of M160-nuc polymerase or a mixture of 50 ng M503 and 2 U Taq polymerase). Reactions containing M503 also contained 1 mg/ml HSA. The amplicon was 243 bp in size and corresponded to position 472 to 714 of the MS2 genome (GenBank Acc. No. V00642.1; SEQ ID NO:23). The probe was 22 nt in size, corresponding to position 650-671, and contained 5′-FAM/internal ZEN/3′-Iowa Black modifications. Reactions were thermal cycled in a QuantStudio system (Thermo Fisher) as follows: 94° C. 30 sec (1 cycle), 94° C. 5 sec, 72° C. 30 sec (40 cycles). For these reactions, the resistance was defined as the highest inhibitor quantity that increased the Cq value by &lt;3 compared with reactions without inhibitor. The formulation containing HSA and the mixture of M503 and Taq polymerase showed resistance to high levels of all inhibitors tested, especially compared with the unmodified M160-nuc polymerase alone in a formulation lacking HSA (Table 6). 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Inhibitor resistance of an M503 and Taq polymerase mixture in the 
               
               
                 presence of human serum albumin compared with the M160-nuc polymerase 
               
               
                 without human serum albumin in one-step RT-qPCR reactions. 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Heparin 
                 Hematin 
                 Humic acid 
                 Hemogloban 
                 Xylan 
               
               
                 Polymerase 
                 resistance 
                 resistance 
                 resistance 
                 resistance 
                 resistance 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 M160-nuc 
                 &lt;0.78 
                 ng/μl 
                 &lt;1.6 
                 μM 
                 &lt;1.25 
                 ng/μl 
                 0.1 μg/μl 
                 &lt;0.016 
                 μg/ul 
               
               
                 no HSA 
               
               
                 M503 + 
                 12.5 
                 ng/μl 
                 &gt;100 
                 μM 
                 20 
                 ng/μl 
                 2.5 μg/μl 
                 &gt;1 
                 μg/ul 
               
               
                 Taq with 
               
               
                 1 mg/ml HSA 
               
               
                   
               
            
           
         
       
     
     High polymerase extension speed is desirable in PCR-based nucleic acid detection reactions because it allows for reduced cycle times, thereby reducing the overall time-to-result. PCR extension speed was measured in end-point reactions in which the combined anneal and extension time was varied to determine the minimum time required to efficiently amplify a 243-nucleotide region of the MS2 viral genome ( FIG. 20 ). Reactions (20 μl), contained 50 mM Tris, pH 8.75, 75 mM KCl, 3 mM MgCl 2 , 0.3 mM dNTPs, 0.2 M trehalose, 0.025% Tween-20, 0.75 M betaine, 10 7  copies of MS2 phage RNA, 0.3 μM forward and reverse primer, 1 mg/ml HSA (except for reactions using M160-nuc), and polymerase (100 ng of M160-nuc polymerase, 50 ng M503, or a mixture of 50 ng M503 and 2 U Taq polymerase). After preparing each composition, reactions were thermal cycled as follows: 94° C. 30 sec (1 cycle), 94° C. 5 sec, 72° C. for the indicated time (30 cycles), then products were analyzed by 1% agarose gel electrophoresis, stained with ethidium bromide, and visualized using ultraviolet light ( FIG. 20 ). For each of the three polymerase compositions tested (M160-nuc, M503 and M503/Taq), efficient amplification of the 243 bp product was seen with an extension time as short as 5 seconds. 
     Example 22: Amplification of Four DNA Sequences with M503 in Multiplex qPCR Reactions 
     The capacity of the mixture of M503 and Taq polymerase to catalyze the simultaneous detection of four target genes was tested in multiplex qPCR reactions using probe-based chemistry in which each of the four probes is labeled with a different fluorophore ( FIG. 21 ). The template pool consisted of a mixture of DNA plasmids containing either ACTB (SEQ ID NO:56), GAPDH (SEQ ID NO:57), IL1 B (SEQ ID NO:58), or TUBA (SEQ ID NO:59) gene sequences and were present in reactions at a quantity of 10 8  to 10 1  copies as indicated. Reactions (20 μl) containing 50 mM Tris, pH 8.75, 75 mM KCl, 3 mM MgCl 2 , 0.3 mM dNTPs, 0.2 M trehalose, 0.025% Tween-20, 0.75 M betaine, 1 mg/ml HSA, 50 ng M503, 2 U Taq polymerase, 0.2 μM forward and reverse primer and 0.3 uM probe (Table 7) were thermal cycled as follows: 94° C. 3 minutes (1 cycle), 94° C. 10 sec, 58° C. 1.5 minutes (45 cycles). In reactions containing equal quantities of each of the four target DNA sequences ( FIG. 21 , Panel A), each amplicon was detected successfully using different spectral emission filters from a starting template quantity as low as 10 copies. In addition, the GAPDH gene sequence was detected successfully from starting quantity as few as 10 copies, even in the presence of 10 8  copies of the other three target sequences ( FIG. 21 , Panel B). Together, these indicate high detection sensitivity and dynamic range, and compatibility with multiple fluorophores in probe-based detection chemistry using this formulation. 
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 Oligonucleotide sequences used in multiplex qPCR assays. 
               
            
           
           
               
               
               
               
            
               
                 Oligo name 
                 Nucleic acid sequence 
                 5′-Label 
                 3′-Quencher 
               
               
                   
               
               
                 GAPDH Fwd 
                 SEQ ID NO: 60 
                   
                   
               
               
                 GAPDH Rev 
                 SEQ ID NO: 61 
               
               
                 GAPDH Probe 
                 SEQ ID NO: 62 
                 6-FAM 
                 BHQ1 
               
               
                 ACTB Fwd 
                 SEQ ID NO: 63 
               
               
                 ACTB Rev 
                 SEQ ID NO: 64 
               
               
                 ACTB Probe 
                 SEQ ID NO: 65 
                 CAL Orange 560 
                 BHQ1 
               
               
                 IL1-B Fwd 
                 SEQ ID NO: 66 
               
               
                 IL1-B Rev 
                 SEQ ID NO: 67 
               
               
                 IL1-B Probe 
                 SEQ ID NO: 68 
                 CAL Red 610 
                 BHQ2 
               
               
                 TUBA Fwd 
                 SEQ ID NO: 69 
               
               
                 TUBA Rev 
                 SEQ ID NO: 70 
               
               
                 TUBA Probe 
                 SEQ ID NO: 71 
                 Quasar 670 
                 BHQ2 
               
               
                   
               
            
           
         
       
     
     Example 23: Activating 3′→5′ Nuclease Activity Enables Reverse Transcription Proofreading on an RNA Template 
     Enzyme constructs that combine the inhibitor tolerant properties of the mutants M502 and M503 with the proofreading properties of the exonuclease derivative mutants (Table 2), i.e. mutants M601, M602, M603, and M604, were constructed by introducing the G46D and A339E mutations into the M502 and M503 parent sequences (Table 8), expressing the recombinant proteins in  E. coli , and purifying the mutant polymerases. 
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 Inhibitor resistant and proofreading mutant sequences. 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 Amino 
                 Nucleic 
                 Amino 
                   
                   
               
               
                   
                   
                 acid 
                 acid 
                 acid 
                 3′→5′ 
                 5′→3′ 
               
               
                 Enzyme 
                 Parent 
                 changes 
                 sequence 
                 sequence 
                 nuclease 
                 nuclease 
               
               
                   
               
               
                 M601 
                 M502 
                 A339E 
                 SEQ ID 
                 SEQ ID 
                 + 
                 + 
               
               
                   
                   
                   
                 NO: 73 
                 NO: 74 
               
               
                 M602 
                 M502 
                 G46D, 
                 SEQ ID 
                 SEQ ID 
                 + 
                 − 
               
               
                   
                   
                 A339E 
                 NO: 75 
                 NO: 76 
               
               
                 M603 
                 M503 
                 A339E 
                 SEQ ID 
                 SEQ ID 
                 + 
                 + 
               
               
                   
                   
                   
                 NO: 77 
                 NO: 78 
               
               
                 M604 
                 M503 
                 G46D, 
                 SEQ ID 
                 SEQ ID 
                 + 
                 − 
               
               
                   
                   
                 A339E 
                 NO: 79 
                 NO: 80 
               
               
                   
               
            
           
         
       
     
     Proofreading reverse transcriptase activity was demonstrated using a modified version of the DPE-PCR assay (Zweitzig et al., 2012). Substrates were constructed by annealing a template RNA strand (SEQ ID NO:81) to a DNA primer strand containing either a 3′-terminal nucleotide match (SEQ ID NO:82), a 3′-terminal dC mismatch (SEQ ID NO:83), a 3′-terminal dA mismatch (SEQ ID NO:84), or a 3′-terminal dT mismatch (SEQ ID NO:85) opposite the RNA cytosine base. Extension reactions (50 μl) containing 20 mM Tris, pH 8.8, 10 mM NaCl, 10 mM (NH 4 ) 2 SO 4 , 2 mM MgSO 4 , 0.1% Triton X-100, 0.2 mM dNTPs, 0.001 μM annealed substrate, and a quantity of polymerase normalized for reverse transcriptase activity were incubated at 65° C. for 10 minutes and then the polymerases were heat inactivated by incubating at 95° C. for 3 minutes. The extent of reverse transcription extension was then measured in quantitative PCR reactions (20 μl) containing 1× Phoenix Hot Start buffer (QIAGEN), 0.2 mM dNTPs, 333 nM forward primer (SEQ ID NO:86), 333 nM reverse primer (SEQ ID NO:87), 166 nM probe (SEQ ID NO:88), 2 U RNase H (QIAGEN), 0.4 U Phoenix Hot Start Taq polymerase (QIAGEN), and 2 μl extension reaction product. Reactions were incubated at 37° C. for 10 minutes, 50° C. for 10 minutes, then 95° C. for 3 minutes; followed by 40 cycles of 95° C. for 5 s and 65° C. for 20 s with fluorescence data collection during the anneal/extension step. Compared with a fully matched primed RNA template, reactions with the 3′→5′ exo-M502 and M503 polymerases displayed higher Cq values using the terminal mismatched templates ( FIG. 22 ), indicating inefficient reverse transcription extension of primers terminating in mismatched bases. In contrast, the 3′→5′ exo+M601, M603, and M604 polymerases showed equivalent reverse transcription extension efficiency from both the matched and all terminal mismatched templates, indicating an efficient ability to excise and correct the mismatched terminal base, i.e. proofread.