Patent Description:
In a first aspect of the invention, a hybrid reverse transcriptase is provided comprising a finger domain, a palm domain, a thumb domain, a connection domain and an RNase H domain, wherein.

In some embodiments of (I) above, the portion of the MLVRT comprises SEQ ID NO: <NUM> and the portion of the FLVRT comprises SEQ ID NO:<NUM>. In some embodiments of (I) above, the portion of the FLVRT comprises SEQ ID NO:<NUM> or SEQ ID NO:<NUM>. In some embodiments of (I) above, the portion of the FLVRT comprises SEQ ID NO:<NUM>, SEQ ID NO:<NUM>, SEQ ID NO:<NUM>, or SEQ ID NO:<NUM>.

In some embodiments of (I) above, the portion of the MLVRT comprises SEQ ID NO:<NUM>. In some embodiments of (I) above, the portion of the MLVRT comprises SEQ ID NO:<NUM> or SEQ ID NO:<NUM>.

In some embodiments of (I) above, the portion of the MLVRT comprises SEQ ID NO:<NUM> and the portion of the FLVRT comprises SEQ ID NO:<NUM>; or.

In some embodiments of (I) above, the hybrid reverse transcriptase comprises a sequence at least <NUM>% identical to SEQ ID NO: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>.

In some embodiment of (II) above s, the hybrid reverse transcriptase comprises a sequence at least <NUM>% identical to SEQ ID NO:<NUM>.

In some embodiments of (III) above, the hybrid reverse transcriptase comprises a sequence at least <NUM>% identical to SEQ ID NO:<NUM>.

In some embodiments, the hybrid reverse transcriptase as described above has at least one mutation corresponding to L139, D200, N479, D522, F526, H592, L601, E605, and H632 in SEQ ID NO:<NUM> that improves thermostability.

As a second aspect, provided is a nucleic acid comprising a polynucleotide encoding the hybrid reverse transcriptase as described in the first aspect above. The nucleic acid may further comprise a (optionally heterologous) promoter operably linked to the polynucleotide.

As a third aspect, provided is a reaction mixture comprising: an RNA or DNA template; and the hybrid reverse transcriptase as described in the first aspect above. The reaction mixture may further comprise at least one oligonucleotide primer and/or deoxynucleotides.

As a fourth aspect, provided is a method of performing reverse transcription the method comprises contacting the hybrid reverse transcriptase as described in the first aspect above or elsewhere herein in a reaction mixture with a template RNA and a primer that hybridizes to the template RNA under conditions such that the hybrid reverse transcriptase extends the primer in a template RNA-dependent manner to form a cDNA. The conditions may comprise an extension step between <NUM>-<NUM>.

Unless defined otherwise, all technical and scientific terms used herein generally have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Generally, the nomenclature used herein and the laboratory procedures in cell culture, molecular genetics, organic chemistry, and nucleic acid chemistry and hybridization described below are those well-known and commonly employed in the art. Standard techniques are used for nucleic acid and peptide synthesis. The techniques and procedures are generally performed according to conventional methods in the art and various general references (see generally, <NPL>. ), which are provided throughout this document. The nomenclature used herein and the laboratory procedures in analytical chemistry, and organic synthetic described below are those well-known and commonly employed in the art. Standard techniques, or modifications thereof, are used for chemical syntheses and chemical analyses.

"Heterologous", when used with reference to portions of a protein, indicates that the protein comprises two or more domains that are not found in the same relationship to each other in nature. Such a protein, e.g., a fusion protein such as the hybrid RTs described herein, contains two or more sequences covalently linked via a peptide bond or peptide linker sequence arranged to make a new functional protein.

A "primer" refers to a polynucleotide sequence that hybridizes to a sequence on a target nucleic acid and serves as a point of initiation of nucleic acid synthesis. Primers can be of a variety of lengths and are often less than <NUM> nucleotides in length, for example <NUM>-<NUM> nucleotides, in length. The length and sequences of primers for use in PCR can be designed based on principles known to those of skill in the art, see, e.g., Innis et al.

"Polymerase" refers to an enzyme that performs template-directed synthesis of polynucleotides. The term encompasses both the full- length polypeptide and a domain that has polymerase activity.

A "template" refers to a polynucleotide sequence that comprises the polynucleotide to be amplified, optionally flanked by one or two primer hybridization sites.

As used herein, "nucleic acid" means DNA, RNA, single-stranded, double-stranded, or more highly aggregated hybridization motifs, and any chemical modifications thereof. Modifications include those providing chemical groups that incorporate additional charge, polarizability, hydrogen bonding, electrostatic interaction, points of attachment and functionality to the nucleic acid ligand bases or to the nucleic acid ligand as a whole. Such modifications include peptide nucleic acids (PNAs), phosphodiester group modifications (e.g., phosphorothioates, methylphosphonates), <NUM>'-position sugar modifications, <NUM>-position pyrimidine modifications, <NUM>-position purine modifications, modifications at exocyclic amines, substitution of <NUM>-thiouridine, substitution of <NUM>-bromo or <NUM>-iodo-uracil; backbone modifications, methylations, and unusual base-pairing combinations such as the isobases, isocytidine and isoguanidine. Nucleic acids can also include non-natural bases, such as, for example, nitroindole. Modifications can also include <NUM>' and <NUM>' modifications such as capping with a fluorophore (e.g., quantum dot) or another moiety.

The terms "polypeptide," "peptide" and "protein" are used interchangeably herein to refer to a polymer of amino acid residues. The terms apply to amino acid polymers in which one or more amino acid residue is an artificial chemical mimetic of a corresponding naturally occurring amino acid, as well as to naturally occurring amino acid polymers and non-naturally occurring amino acid polymers.

The term "amino acid" refers to naturally occurring and synthetic amino acids, as well as amino acid analogs and amino acid mimetics that function in a manner similar to the naturally occurring amino acids. Naturally occurring amino acids are those encoded by the genetic code, as well as those amino acids that are later modified, e.g., hydroxyproline, gamma-carboxyglutamate, and O-phosphoserine. Amino acid analogs refers to compounds that have the same basic chemical structure as a naturally occurring amino acid, i.e., a carbon atom that is bound to a hydrogen atom, a carboxyl group, an amino group, and an R group, e.g., homoserine, norleucine, methionine sulfoxide, methionine methyl sulfonium. Such analogs have modified R groups (e.g., norleucine) or modified peptide backbones, but retain the same basic chemical structure as a naturally occurring amino acid. Amino acid mimetics refers to chemical compounds that have a structure that is different from the general chemical structure of an amino acid, but that functions in a manner similar to a naturally occurring amino acid.

Amino acids may be referred to herein by either their commonly known three letter symbols or by the one-letter symbols recommended by the IUPAC-IUB Biochemical Nomenclature Commission. Nucleotides, likewise, may be referred to by their commonly accepted single-letter codes.

The term "promoter" refers to regions or sequence located upstream and/or downstream from the start of transcription and which are involved in recognition and binding of RNA polymerase and other proteins to initiate transcription.

A "vector" refers to a polynucleotide, which when independent of the host chromosome, is capable replication in a host organism. Preferred vectors include plasmids and typically have an origin of replication. Vectors can comprise, e.g., transcription and translation terminators, transcription and translation initiation sequences, and promoters useful for regulation of the expression of the particular nucleic acid.

Two nucleic acid sequences or polypeptides are said to be "identical" if the sequence of nucleotides or amino acid residues, respectively, in the two sequences is the same when aligned for maximum correspondence as described below. The terms "identical" or percent "identity," in the context of two or more nucleic acids or polypeptide sequences, refer to two or more sequences or subsequences that are the same or have a specified percentage of amino acid residues or nucleotides that are the same, when compared and aligned for maximum correspondence over a comparison window, as measured using one of the following sequence comparison algorithms or by manual alignment and visual inspection. When percentage of sequence identity is used in reference to proteins or peptides, it is recognized that residue positions that are not identical often differ by conservative amino acid substitutions, where amino acids residues are substituted for other amino acid residues with similar chemical properties (e.g., charge or hydrophobicity) and therefore do not change the functional properties of the molecule. Where sequences differ in conservative substitutions, the percent sequence identity may be adjusted upwards to correct for the conservative nature of the substitution. Means for making this adjustment are well known to those of skill in the art. Typically this involves scoring a conservative substitution as a partial rather than a full mismatch, thereby increasing the percentage sequence identity. Thus, for example, where an identical amino acid is given a score of <NUM> and a non-conservative substitution is given a score of zero, a conservative substitution is given a score between zero and <NUM>. The scoring of conservative substitutions is calculated according to, e.g., the algorithm of <NPL>) e.g., as implemented in the program PC/GENE (Intelligenetics, Mountain View, Calif.

Sequences are "substantially identical" to each other if they have a specified percentage of nucleotides or amino acid residues that are the same (e.g., at least <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, or <NUM>% identity over a specified region or when not specified the whole sequence (SEQ ID NO)), when compared and aligned for maximum correspondence over a comparison window, or designated region as measured using one of the following sequence comparison algorithms or by manual alignment and visual inspection.

For sequence comparison, typically one sequence acts as a reference sequence, to which test sequences are compared. When using a sequence comparison algorithm, test and reference sequences are entered into a computer, subsequence coordinates are designated, if necessary, and sequence algorithm program parameters are designated. Default program parameters can be used, or alternative parameters can be designated. The sequence comparison algorithm then calculates the percent sequence identities for the test sequences relative to the reference sequence, based on the program parameters.

A "comparison window", as used herein, includes reference to a segment of any one of the numbers of contiguous positions selected from the group consisting of from <NUM> to <NUM>, usually about <NUM> to about <NUM>, more usually about <NUM> to about <NUM> in which a sequence may be compared to a reference sequence of the same number of contiguous positions after the two sequences are optimally aligned. Methods of alignment of sequences for comparison are well-known in the art. Optimal alignment of sequences for comparison can be conducted, e.g., by the local homology algorithm of <NPL>), by the homology alignment algorithm of <NPL>), by the search for similarity method of <NPL>), by computerized implementations of these algorithms (GAP, BESTFIT, FASTA, and TFASTA in the Wisconsin Genetics Software Package, Accelrys), or by manual alignment and visual inspection.

Percent sequence identity and sequence similarity is determined using the BLAST algorithm, which is described in <NPL>). Software for performing BLAST analyses is publicly available through the National Center for Biotechnology Information (http://www. This algorithm involves first identifying high scoring sequence pairs (HSPs) by identifying short words of length W in the query sequence, which either match or satisfy some positive-valued threshold score T when aligned with a word of the same length in a database sequence. T is referred to as the neighborhood word score threshold (Altschul et al, supra). These initial neighborhood word hits act as seeds for initiating searches to find longer HSPs containing them. The word hits are extended in both directions along each sequence for as far as the cumulative alignment score can be increased. Extension of the word hits in each direction are halted when: the cumulative alignment score falls off by the quantity X from its maximum achieved value; the cumulative score goes to zero or below, due to the accumulation of one or more negative-scoring residue alignments; or the end of either sequence is reached. The BLAST algorithm parameters W, T, and X determine the sensitivity and speed of the alignment. The BLAST program uses as defaults a word length (W) of <NUM>, the BLOSUM62 scoring matrix (see <NPL>)) alignments (B) of <NUM>, expectation (E) of <NUM>, M=<NUM>, N=-<NUM>, and a comparison of both strands.

<FIG> depicts an exemplary soluble fraction SDS-PAGE gel following RT mutant expression.

The inventors have discovered that reverse transcriptase (RT) hybrids formed from a mouse leukemia virus reverse transcriptase (MLVRT) and feline leukemia virus reverse transcriptase (FLVRT) have improved solubility compared to FLVRT enzymes. The hybrids described herein are also expected to have improved stability, expression, or a combination thereof compared to at least one of non-hybrid MLVRT or FLVRT enzymes. For example, the inventors have generated hybrid reverse transcriptases comprising a finger domain, a palm domain, a thumb domain, a connection domain and an RNase H domain, wherein at least one of said domains is a mouse leukemia virus reverse transcriptase (MLVRT) and other of said domains are from feline leukemia virus reverse transcriptase (FLVRT). As discussed in more detail, the inventors have generated hybrids in which the finger and palm domains are either FLVRT or MLVRT sequences with at least some of the remainder being from the alternative enzyme.

Provided herein are hybrid reverse transcriptases (RTs) that have the five RT domains (from amino to carboxyl: finger, palm, thumb, connection, and RNase H domains) where at least one (e.g., <NUM>, <NUM>, <NUM>, or <NUM>) of those domains are from MLVRT and the remaining domain(s) are from FLVRT. The structure of MLVRT and finger, palm, thumb, connection, and RNase H domains are described in, e.g., <NPL>). The resulting hybrid RTs have improved expression (e.g., in E. coli) compared to an RT where all of the domains are from FLVRT (i.e., wildtype FLVRT) while in some embodiments having improved accuracy compared to MLVRT.

The hybrid RT may comprise the finger and palm domains of MLVRT linked to the thumb, connection, and RNase H domains of FLVRT. Exemplary portions of MLVRT that comprise finger and palm domains include, for example, SEQ ID NO:<NUM> or a substantially identical sequence thereof. The portion of MLVRT that comprises finger and palm domains may comprise SEQ ID NO:<NUM> or a sequence substantially identical thereto. The above-described MLVRT portion can be linked to a portion of FLVRT that comprises the thumb, connection, and RNase H domains. An exemplary portion of FLVRT that comprises the thumb, connection, and RNase H domains is SEQ ID NO: <NUM> or a substantially identical sequence thereof. The portion of FLVRT that comprises the thumb, connection, and RNase H domains may be SEQ ID NO: <NUM> or a substantially identical sequence thereof or SEQ ID NO: <NUM> or a substantially identical sequence thereof. Exemplary hybrid RTs can comprise, for example SEQ ID NO: <NUM> or SEQ ID NO:<NUM> any of SEQ ID NO:<NUM>, SEQ ID NO:<NUM>, or SEQ ID NO: <NUM>. The hybrid RT may comprise one of SEQ ID NOs: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM> or a substantially identical sequence thereof.

The hybrid RT may comprise at least the finger and palm domains of FLVRT linked to the thumb and connection domains of either FLVRT or MLVRT, which in turn is linked to the RNase H domains of MLVRT. The hybrid RT may comprise finger and palm domains of FLVRT and thumb, connection, and RNase H domains of MLVRT. Exemplary portions of FLVRT that comprise finger and palm domains include, for example, SEQ ID NO:<NUM> or a substantially identical sequence thereof. Exemplary portions of MLVRT that comprise thumb, connection, and RNase H domains include, for example, SEQ ID NO:<NUM> or a substantially identical sequence thereof.

The hybrid RT may comprise finger, palm, thumb, and connection domains of FLVRT and RNase H domain of MLVRT. Exemplary portions of FLVRT that comprise finger, palm, thumb, and connection domains include, for example, SEQ ID NO:<NUM> or a substantially identical sequence thereof. Exemplary portions of MLVRT that comprise the RNase H domain include, for example, SEQ ID NO:<NUM> or a substantially identical sequence thereof. The hybrid RT may comprise one of SEQ ID NO: <NUM> or <NUM>, or a substantially identical sequence thereof.

Any of the hybrid RTs described herein can include further amino acids at the amino or carboxyl terminus. Exemplary additional amino acid sequences can include, for example, epitope tags or other tags that allow for purification of the proteins or whose underlying codons allow for cloning sites. Such tags can be fused at either end of the hybrid RT as most convenient for purification. Examples of such tags include poly-histidine sequences or FLAG-tag. Various linker sequences can also be included to link such tags or other sequences to the hybrid RT sequence. Linkers can include, for example glycine, serine or other amino acids that do not significantly interfere with protein folding such that the activity of the hybrid RT is not harmed. The linker sequences can also include protease cleavage sequences such that the tag can be removed by a protease or other cleavage mechanism, optionally once the hybrid RT has been purified (e.g., using the tag). The hybrid RT may include one or more (e.g., <NUM>-<NUM>, <NUM>-<NUM>, e.g., <NUM>) alanines at the carboxyl terminus.

As noted herein, any hybrid RTs as described herein can include one or more mutation that improves the thermostability (i.e., ability to remain active during or after exposure to temperatures over <NUM> ° C, e.g., <NUM>-<NUM> ° C) of the enzyme. Exemplary mutations include one or more (e.g., <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or more) mutation at a position corresponding to L139 (including L139P), D200 (including D200N), N479 (including N479D), D522 (including D522G, D522N, or D522A), F526 (including F526I), H592 (including H592K), L601 (including L601W), E605 (including E605K), and H632 (including H632Y) in SEQ ID NO:<NUM>. It should be understood that such position designations do not indicate the number of amino acids in the claimed molecule per se, but indicate where in the claimed molecule the residue occurs when the claimed molecule sequence is maximally aligned with SEQ ID NO:<NUM>.

Two portions of different RTs as described herein as described can be joined via a linker by methods well known to those of skill in the art. These methods can include both recombinant and chemical methods.

Linking portions of different RTs may also comprise a peptide bond formed between moieties that are separately synthesized by standard peptide synthesis chemistry or recombinant methods. Alternatively, in some embodiments, the coding sequences of each portion in the hybrid RT are directly joined and expressed as a fusion protein. Alternatively, an amino acid linker sequence may also be encoded in the polypeptide coding sequence and employed to separate the first and second RT portions by a distance sufficient to ensure that each polypeptide folds into its secondary and tertiary structures. Such an amino acid linker sequence is incorporated into the fusion protein using recombinant techniques well known in the art. Suitable peptide linker sequences may be chosen based on the following factors: (<NUM>) their ability to adopt a flexible extended conformation; (<NUM>) their inability to adopt a secondary structure that could interact with functional epitopes on the first and second polypeptides; and (<NUM>) the lack of hydrophobic or charged residues that might react with the polypeptide functional epitopes. Typical peptide linker sequences contain Gly, Ser, Val and Thr residues. Other near neutral amino acids, such as Ala can also be used in the linker sequence. Amino acid sequences which may be usefully employed as linkers include those disclosed in <NPL>; <NPL>; <CIT> and <CIT>. The linker sequence may generally be from <NUM> to about <NUM> amino acids in length, e.g., <NUM>, <NUM>, <NUM>, or <NUM> amino acids in length, but can be <NUM> or <NUM> amino acids in length. Linker sequences are not necessarily required.

Chemical linking can be performed, for example, as described in <NPL>). Joining can include, for example, derivatization for the purpose of linking the two proteins to each other, either directly or through a linking compound, by methods that are well known in the art of protein chemistry. For example, in one chemical conjugation embodiment, the means of linking the catalytic domain and the nucleic acid binding domain comprises a heterobifunctional-coupling reagent which ultimately contributes to formation of an intermolecular disulfide bond between the two moieties. Other types of coupling reagents that are useful in this capacity for the present invention are described, for example, in <CIT>. Alternatively, an intermolecular disulfide may conveniently be formed between cysteines in each moiety, which occur naturally or are inserted by genetic engineering. The means of linking moieties may also use thioether linkages between heterobifunctional crosslinking reagents or specific low pH cleavable crosslinkers or specific protease cleavable linkers or other cleavable or noncleavable chemical linkages. Other chemical linkers include carbohydrate linkers, lipid linkers, fatty acid linkers, polyether linkers, e.g., PEG. For example, polyethylene glycol) linkers are available from Shearwater Polymers, Inc. Huntsville, Alabama. These linkers optionally have amide linkages, sulfhydryl linkages, or heterobifunctional linkages. The linking group can be a chemical crosslinking agent, including, for example, succinimidyl-(N-maleimidomethyl)-cyclohexane-<NUM>-carboxylate (SMCC). The linking group can also be an additional amino acid sequence(s), including for example, a polyalanine, polyglycine or similarly, linking group.

Nonclassical amino acids or chemical amino acid analogs can be introduced as a substitution or addition into the sequence. Non-classical amino acids include the D-isomers of the common amino acids, α-amino isobutyric acid, <NUM>-aminobutyric acid, Abu, <NUM>-amino butyric acid, γ-Abu, ε-Ahx, <NUM>-amino hexanoic acid, Aib, <NUM>-amino isobutyric acid, <NUM>-amino propionic acid, ornithine, norleucine, norvaline, hydroxy-proline, sarcosine, citrulline, cysteic acid, t-butylglycine, t-butylalanine, phenylglycine, cyclohexylalanine, β- alanine, fluoro-amino acids, designer amino acids such as β-methyl amino acids, Cα-methyl amino acids, N -methyl amino acids, and amino acid analogs in general. Furthermore, the amino acid can be D (dextrorotary) or L (levorotary).

Nucleic acids encoding the hybrid RTs can be obtained using routine techniques in the field of recombinant genetics. Basic texts disclosing the general methods of use in this invention include <NPL>); <NPL>); and <NPL>). Such nucleic acids may also be obtained through in vitro amplification methods such as those described herein and in Berger, Sambrook, and Ausubel, as well as <CIT>; <NPL>is); <NPL>; <NPL>; <NPL>; <NPL>; <NPL>; <NPL>; <NPL>; <NPL>; and <NPL>.

One of skill will recognize that modifications can additionally be made to the hybrid RTs without diminishing their biological activity. Some modifications may be made to facilitate the cloning, expression, or incorporation of a domain into a fusion protein. Such modifications are well known to those of skill in the art and include, for example, the addition of codons at either terminus of the polynucleotide that encodes the binding domain to provide, for example, a methionine added at the amino terminus to provide an initiation site, or additional amino acids (e.g., poly His) placed on either terminus to create conveniently located restriction sites or termination codons or purification sequences.

The hybrid RT polypeptides as described herein can be expressed in a variety of host cells, including E. coli, other bacterial hosts, yeasts, filamentous fungi, and various higher eukaryotic cells such as the COS, CHO and HeLa cells lines and myeloma cell lines. Techniques for gene expression in microorganisms are described in, for example, <NPL>. Examples of bacteria that are useful for expression include Escherichia, Enterobacter, Azotobacter, Erwinia, Bacillus, Pseudomonas, Klebsielia, Proteus, Salmonella, Serratia, Shigella, Rhizobia, Vitreoscilla, and Paracoccus. Filamentous fungi that are useful as expression hosts include, for example, the following genera: Aspergillus, Trichoderma, Neurospora, Penicillium, Cephalosporium, Achlya, Podospora, Mucor, Cochliobolus, and Pyricularia. See, e.g., <CIT> and <NPL>. Synthesis of heterologous proteins in yeast is well known and described in the literature. Methods in <NPL>) is a well-recognized work describing the various methods available to produce the enzymes in yeast.

There are many expression systems for producing the polypeptides that are well known to those of ordinary skill in the art. (See, e.g., <NPL>; Sambrook and Russell, supra; and Ausubel et al, supra. ) Typically, the polynucleotide that encodes the polypeptide is placed under the control of a promoter that is functional in the desired host cell. Many different promoters are available and known to one of skill in the art, and can be used in the expression vectors of the invention, depending on the particular application. Ordinarily, the promoter selected depends upon the cell in which the promoter is to be active. Other expression control sequences such as ribosome binding sites, and transcription termination sites are also optionally included. Constructs that include one or more of these control sequences are termed "expression cassettes. " Accordingly, the nucleic acids that encode the joined polypeptides are incorporated for high level expression in a desired host cell.

Expression control sequences that are suitable for use in a particular host cell are often obtained by cloning a gene that is expressed in that cell. Commonly used prokaryotic control sequences, which are defined herein to include promoters for transcription initiation, optionally with an operator, along with ribosome binding site sequences, include such commonly used promoters as the beta-lactamase (penicillinase) and lactose (lac) promoter systems (<NPL>), the tryptophan (trp) promoter system (<NPL>), the tac promoter (<NPL>); and the lambda-derived PL promoter and N-gene ribosome binding site (<NPL>). The particular promoter system is not critical; any available promoter that functions in prokaryotes and provides the desired level of activity can be used. Standard bacterial expression vectors include plasmids such as pBR322-based plasmids, e.g., pBLUESCRIPT™, pSKF, pET23D, lambda-phage derived vectors, and fusion expression systems such as GST and LacZ. Epitope tags can also be added to recombinant proteins to provide convenient methods of isolation, e.g., c-myc, HA-tag, <NUM>-His tag, maltose binding protein, VSV-G tag, anti-DYKDDDDK (SEQ ID NO:<NUM>) tag, or any such tag, a large number of which are well known to those of skill in the art.

The polypeptides described herein can be expressed intracellularly, or can be secreted from the cell. Intracellular expression often results in high yields. If necessary, the amount of soluble, active fusion polypeptide may be increased by performing refolding procedures (see, e.g., Sambrook et al. ; <NPL>; <NPL>). Polypeptides can be expressed in a variety of host cells, including E. coli, other bacterial hosts, yeast, and various higher eukaryotic cells such as the COS, CHO and HeLa cells lines and myeloma cell lines. The host cells can be mammalian cells, insect cells, or microorganisms, such as, for example, yeast cells, bacterial cells, or fungal cells.

Once expressed, the polypeptides can be purified according to standard procedures of the art, including ammonium sulfate precipitation, affinity columns, column chromatography, and gel electrophoresis (see, generally, <NPL>), <NPL>)). Substantially pure compositions of at least about <NUM> to <NUM>% homogeneity are preferred, and <NUM> to <NUM>% or more homogeneity are most preferred. Once purified, partially or to homogeneity as desired, the polypeptides may then be used (e.g., as immunogens for antibody production).

To facilitate purification of the polypeptides, the nucleic acids that encode the polypeptides can also include a coding sequence for an epitope or "tag" for which an affinity binding reagent is available. Examples of suitable epitopes include the myc and V-<NUM> reporter genes; expression vectors useful for recombinant production of fusion polypeptides having these epitopes are commercially available (e.g., Invitrogen (Carlsbad Calif. ) vectors pcDNA3. <NUM>/Myc-His and pcDNA3. <NUM>/V5-His are suitable for expression in mammalian cells). Additional expression vectors suitable for attaching a tag to the fusion proteins of the invention, and corresponding detection systems are known to those of skill in the art, and several are commercially available (e.g., "FLAG" (Kodak, Rochester N. Another example of a suitable tag is a polyhistidine sequence, which is capable of binding to metal chelate affinity ligands. Typically, six adjacent histidines are used, although one can use more or less than six. Suitable metal chelate affinity ligands that can serve as the binding moiety for a polyhistidine tag include nitrilo-tri-acetic acid (NTA) (<NPL>. ; commercially available from Qiagen (Santa Clarita, Calif.

After biological expression or purification, the hybrid RT polypeptide(s) may possess a conformation substantially different than the native conformations of the constituent polypeptides. In this case, it may be necessary or desirable to denature and reduce the polypeptide and then to cause the polypeptide to re-fold into the preferred conformation. Methods of reducing and denaturing proteins and inducing re-folding are well known to those of skill in the art (See, <NPL>; <NPL>; and<NPL>). Debinski et al. , for example, describe the denaturation and reduction of inclusion body proteins in guanidine-DTE. The protein is then refolded in a redox buffer containing oxidized glutathione and L-arginine.

Reverse transcription (RT) is an amplification method that copies RNA into DNA. RT reactions can be performed with reaction mixtures as described herein. For example, the invention provides for reverse transcribing one or more RNA (including for example, all RNA in a cell, e.g., to make a cDNA library) under conditions to allow for reverse transcription using a hybrid RT as described herein and generation of a first and optionally second strand cDNA. The RT reaction can be primed with a random primer, an oligo dT, or an RNA-specific primer. Components and conditions for RT reactions are generally known.

If desired, the reactions can further comprise RT-PCR. Standard techniques for performing PCR assays are known in the art (<NPL>); <NPL>); <NPL>); <NPL>); <NPL>)). RT and PCR reactions are often used in the same assay and are referred to as RT-PCR. RT-PCR combines reverse transcription of RNA into DNA and subsequent DNA amplification reactions in a single reaction. Optimal reverse transcription, hybridization, and amplification conditions will vary depending upon the sequence composition and length(s) of the primers and target(s) employed, and the experimental method selected by the practitioner. Various guidelines may be used to select appropriate primer sequences and hybridization conditions (see, e.g., <NPL>); <NPL>); <NPL>).

Hybrid RTs described herein are used in a reverse transcriptase reaction at a higher temperature than would ordinarily be used. Thus, in embodiments, some hybrid RTs described herein can be used at, <NUM> ° C or <NUM> ° C, or a temperature greater than <NUM> ° C, for example, between <NUM> °-<NUM> °, <NUM> °-<NUM> ° C, <NUM> °-<NUM> ° C, <NUM> °-<NUM> ° C. Higher temperature RT reactions are particularly helpful in situations where the template RNA forms secondary structure at normal RT temperatures (e.g., <NUM> ° or <NUM> ° C) that partially or completely inhibit reverse transcription.

Reaction mixtures comprising the hybrid RT polypeptides described herein are provided. The reaction mixtures can comprise, for example, a target nucleic acid, e.g., an RNA target where reverse transcription is to take place. The reaction mixtures can comprise appropriate nucleotides (e.g., deoxynucleotides (dNTPs) or dideoxynucleotides) and in some embodiments, at least one buffer. Exemplary buffers can include, for example Tris, HEPES, ACES, PIPES, MOPSO, BES, MOPS, TES, TAPSO, POPSO, BICINE, TAPS, or AMPSO. The reaction mixtures can optionally comprise one or more oligonucleotides that function as a primer for template-dependent nucleic acid extension, one or more oligonucleotides that function as a probe (e.g., linked to a label such as a quencher, fluorescent dye. ), and/or a double stranded DNA binding dye (e.g., SYBRGREEN). The reaction mixture may further comprise a separate DNA-dependent DNA polymerase, and/or the reaction mixture may further comprise magnesium (Mg++).

Kits for conducting reverse transcription (and optionally cyclic amplification, e.g., such as PCR) reactions are also described herein. Such kits include a hybrid RT as described herein, and optionally dNTPs, and at least one buffer. Such kits may also include stabilizers and other additives to increase the efficiency of the amplification reactions. Such kits may also include one or more primers (e.g. poly-T, random hexamers, or specific primers) as well as instructions for conducting reverse transcription reactions using the components of the kits. The kits may further comprise a separate DNA-dependent DNA polymerase.

In order to increase the recombinant FLV RT solubility in E. coli cells, hybrid RTs were constructed with part of the RT polypeptide sequence from MLV RT, and part of the RT sequence from FLV RT. Hybrid RT constructs made exhibited improved solubility in E. coli cells as demonstrated in <FIG>.

The hybrid RT FM1 (<NUM>/<NUM> FLV-<NUM>/<NUM> MLV RT) included a N-terminal sequence of FLV RT from amino acid <NUM>-<NUM>, which includes the finger and palm domains, and a C-terminal sequence of MLV RT from amino acid <NUM>-<NUM>, which includes the thumb, connection and RNase H domain.

The hybrid RT FM2 (<NUM>/<NUM> FLV-<NUM>/<NUM> MLV RT) included a N-terminal sequence of FLV RT from amino acid <NUM>-<NUM>, which includes the finger, palm, thumb, and connection domains, and a C-terminal sequence of MLV RT from amino acid <NUM>-<NUM>, which includes the RNase H domain.

The hybrid RT MF includes a N-terminal sequence of MLV RT from amino acid <NUM>-<NUM>, which includes the finger and palm domains, and a C-terminal sequence of FLV RT from amino acid <NUM>-<NUM>, which includes the thumb, connection, and RNase H domains.

The hybrid RT MF(P) includes a N-terminal sequence of MLV RT from amino acid <NUM>-<NUM>, which includes the finger and palm domains, and a C-terminal sequence of FLV RT from amino acid <NUM>-<NUM>, which includes the thumb, connection, and RNase H domains.

Point mutations were introduced into MF and MF(P) hybrid RTs in order to improve the enzyme performance.

Fresh LB broth was inoculated with overnight culture of BL21 cells containing expression plasmids in a ratio of <NUM>:<NUM>. The cultures were grown at <NUM> for about <NUM> or until OD600 nm = <NUM>-<NUM>. IPTG was added to <NUM>, and grown O/N for <NUM> at <NUM>. Cells were harvested by collecting the pellet after centrifugation. Cells were resuspended in <NUM> lysis buffer and lysed by sonication. The cell debris was spun down and <NUM>µl of supernatant was collected. <NUM>µl sample and <NUM>µl loading buffer were combined in a PCR strip and heated at <NUM> for <NUM>. <NUM>µl of the samples were loaded onto an SDS-PAGE gel for analysis. Exemplary results are shown in <FIG>.

The hybrid and mutant proteins were tested for a number of characteristics, which is summarized in part in the following table (blanks indicate the activity was not tested):.

A listing of exemplary mutant and hybrid sequences is provided below:.

Based on FP(M)-TCH(F), which has finger and palm domains from MLV RT and thumb, connection and RNase H domain from FLV RT. Amino acids at the following positions were mutated: L139P(<NUM>), D200N(<NUM>), D522A(<NUM>), F526I(<NUM>), H632Y(<NUM>). Italics indicate sequence from MLV; bolded text indicates sequence from FLV; the mutated amino acids are underlined.

Based on FP(M)-TCH(F), which has finger and palm domains from MLV RT and thumb, connection and RNase H domain from FLV RT with an extended C-terminal sequence. Amino acids at the following positions were mutated: L139P(<NUM>), D200N(<NUM>), D522G(<NUM>), L601W(<NUM>), E605K(<NUM>). Italics indicate sequence from MLV; bolded text indicates sequence from FLV; the mutated amino acids are underlined.

Based on FP(M)-TCH(F), which has finger and palm domains from MLV RT and thumb, connection and RNase H domain from FLV RT without an extended C-terminal sequence. Amino acids at the following positions were mutated: L139P(<NUM>), D200N(<NUM>), D522G(<NUM>), L601W(<NUM>), E605K(<NUM>). Italics indicate sequence from MLV; bolded text indicates sequence from FLV; the mutated amino acids are underlined.

Based on FP(M)PstI-TCH(F), which has a shorter finger and palm domains sequence from MLV RT compared to MF4G. The thumb, connection and RNase H domains from FLV RT without an extended C-terminal sequence. Amino acids at the following positions were mutated: L139P(<NUM>), D200N(<NUM>), D522G(<NUM>), L601W(<NUM>), E605K(<NUM>). Italics indicate sequence from MLV; bolded text indicates sequence from FLV; the mutated amino acids are underlined.

Based on FP(M)PstI-TCH(F), which has a shorter finger and palm domains sequence from MLV RT compared to MF4G. The thumb, connection and RNase H domains from FLV RT without an extended C-terminal sequence. Amino acids at the following positions were mutated: D200N(<NUM>), N479D(<NUM>), H592K(<NUM>), L601W(<NUM>). Italics indicate sequence from MLV; bolded text indicates sequence from FLV; the mutated amino acids are underlined. <IMG>
<IMG>.

Based on FP(M)PstI-TCH(F), which has a shorter finger and palm domains sequence from MLV RT compared to MF4G. The thumb, connection and RNase H domains from FLV RT without an extended C-terminal sequence. Amino acids at the following positions were mutated: D200N(<NUM>), D522G(<NUM>), L601W(<NUM>), D651N(<NUM>). Italics indicate sequence from MLV; bolded text indicates sequence from FLV; the mutated amino acids are underlined.

Based on FP(M)-TCH(F), which has finger and palm domains from MLV RT and thumb, connection and RNase H domain from FLV RT with an extended C-terminal sequence. Amino acids at the following positions were mutated: L139P(<NUM>), D200N(<NUM>), D522A(<NUM>), F526I(<NUM>), H632Y(<NUM>). Italics indicate sequence from MLV; bolded text indicates sequence from FLV; the mutated amino acids are underlined.

Based on FP(M)PstI-TCH(F), which has a shorter finger and palm domains sequence from MLV RT compared to MF4G. The thumb, connection and RNase H domains from FLV RT with an extended C-terminal sequence. Amino acids at the following positions were mutated:
L139P(<NUM>), D200N(<NUM>), D522G(<NUM>), L601W(<NUM>), E605K(<NUM>). Italics indicate sequence from MLV; bolded text indicates sequence from FLV; the mutated amino acids are underlined.

A hybrid RT with <NUM>/<NUM> of the N-terminal sequence from FLV RT (finger and palm domains)and the rest of <NUM>/<NUM> of the C-terminal sequence from MLV RT (Thumb, connection, RNase H domains). Italics indicate sequence from MLV; bolded text indicates sequence from FLV.

A hybrid RT with <NUM>/<NUM> of the N-terminal sequence from FLV RT (finger, palm, thumb, connection domains) and the rest of <NUM>/<NUM> of the C-terminal sequence from MLV RT (RNase H domain only). Italics indicate sequence from MLV; bolded text indicates sequence from FLV.

Claim 1:
A hybrid reverse transcriptase comprising a finger domain, a palm domain, a thumb domain, a connection domain and an RNase H domain, wherein
(I) a portion of mouse leukemia virus reverse transcriptase (MLVRT) comprising the finger and palm domains is linked to a portion of feline leukemia virus reverse transcriptase (FLVRT) comprising the thumb, connection and RNase H domains, wherein the portion of the MLVRT is at least <NUM>% identical to SEQ ID NO:<NUM> and the portion of the FLVRT is at least <NUM>% identical to SEQ ID NO:<NUM>; or
(II) a portion of feline leukemia virus reverse transcriptase (FLVRT) comprising the finger and palm domains is linked to a portion of mouse leukemia virus reverse transcriptase (MLVRT) comprising the thumb, connection and RNase H domains, wherein the portion of the FLVRT is at least <NUM>% identical to SEQ ID NO:<NUM> and the portion of the MLVRT is at least <NUM>% identical to SEQ ID NO:<NUM>; or
(III) a portion of feline leukemia virus reverse transcriptase (FLVRT) comprising the finger, palm, thumb and connection domains is linked to a portion of mouse leukemia virus reverse transcriptase (MLVRT) comprising the RNase H domain, wherein the portion of the FLVRT is at least <NUM>% identical to SEQ ID NO:<NUM> and the portion of the MLVRT is at least <NUM>% identical to SEQ ID NO:<NUM>.