Abstract:
Thermostable DNA polymerases both in native form and having single amino acid substitutions and optionally N-terminal deletions are disclosed. These polymerases exhibit a substantial improvement of DNA sequencing performance compared to Taq DNA polymerase. The instant DNA polymerases also possess improved salt tolerance.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The instant disclosure pertains to thermostable DNA polymerases which exhibit improved robustness and efficiency. In particular, the DNA polymerases have been shown to result in a substantial improvements in desirable properties as compared to the native DNA polymerases when used in DNA sequencing reactions.  
           [0003]    2. Background  
           [0004]    DNA polymerases are enzymes which are useful in many recombinant DNA techniques such as nucleic acid amplification by the polymerase chain reaction (“PCR”), self-sustained sequence replication (“3 SR”), and DNA sequencing. Thermostable DNA polymerases are particularly useful. Because heat does not destroy the polymerase activity, there is no need to add additional polymerase after every denaturation step.  
           [0005]    Naturally occurring DNA polymerases preferentially incorporate unlabeled nucleotides over corresponding labeled nucleotides into polynucleotides. This ability of DNA polymerases to discriminate against fluorescently labeled nucleotides had an undesirable effect on many molecular biology procedures that require the enzymatic addition of labeled nucleotides, e.g., labeled dideoxy terminator sequencing. Ambiguous sequencing determinations often result from the disproportionate number of labeled and unlabeled dideoxy terminators and nucleotides. On an electropherogram obtained from a capillary electrophoresis sequencing unit, this phenomena shows up as uneven peaks. Large signals due to a larger amount of incorporated labeled ddNTP can obscure smaller signals and lead to ambiguous sequence determinations. Additionally, many of the enzymes presently available are sensitive to high salt environments.  
           [0006]    Thus, a need continues to exist for an improved DNA polymerase having improved discrimination properties (and thus resulting in improved signal uniformity) and increased tolerance to high salt conditions. These and other concerns are addressed in greater detail below.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    The instant disclosure teaches purified recombinant thermostable DNA polymerases comprising the amino acid sequences set forth in FIGS. 2, 5 and  6 . The instant disclosure also teaches an isolated nucleic acid sequences that encodes such thermostable DNA polymerases, wherein said nucleic acid sequences consist of or contain of the nucleotide sequences set forth in FIG. 9, as well as a recombinant DNA vector that comprises the nucleic acid sequence, and a recombinant host cell transformed with the recombinant vector. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0008]    [0008]FIG. 1 depicts the amino acid sequence of Tth DNA polymerase (Genbank accession P52028).  
         [0009]    [0009]FIG. 2 depicts the amino acid sequence deduced from the DNA sequence of Tsp JS1 DNA polymerase.  
         [0010]    [0010]FIG. 3 depicts the amino acid sequence of Taq DNA polymerase (Genbank accession 1TAU A).  
         [0011]    [0011]FIG. 4 depicts a phylogenetic comparison based on amino acid sequence alignment (FIG. 17) of DNA polymerases from various Thermus species with the polymerase of Tsp JS1.  
         [0012]    [0012]FIG. 5 depicts the amino acid sequence of Tsp JS1 Δ271/F667Y DNA polymerase.  
         [0013]    [0013]FIG. 6 depicts the amino acid sequence of Tsp JS1 Δ271/F667Y/E681R DNA polymerase.  
         [0014]    [0014]FIG. 7 depicts the amino acid sequence of Tth D18A/F667Y/E681 R DNA polymerase.  
         [0015]    [0015]FIG. 8 depicts the amino acid sequence of Tth Δ271/F667Y/E681R DNA polymerase.  
         [0016]    [0016]FIG. 9 depicts the nucleotide sequence of the gene for Tsp JS1 DNA polymerase.  
         [0017]    [0017]FIG. 10 depicts the stability of Tsp Δ271/F667Y and Tsp Δ271/F667Y/E681R DNA polymerases at 95° C. The DNA polymerase (approximately 2 units/μl) was incubated at 95° C. in 50 mM Tris pH 8.0, 10% glycerol, 35 mM KC1, and 1 mM MgCl 2  for the times indicated. Polymerase activity was then determined using the standard assay.  
         [0018]    FIGS.  11 - 16  DNA sequencing experiments using polymerases from Tsp JS1 were performed using the DYEnamic ET terminator dye-labeled dideoxynucleotides (Amersham Biosciences). All nucleotides were used at standard concentrations, and polymerase was present at a concentration of 0.5-1.0 units/μl. Reactions were cycled 25 times at 95° C. for 30 sec., 45° C. for 15 sec. and 60° C. for 240 sec.  
         [0019]    [0019]FIG. 11 depicts the result of a sequencing experiment using DYEnamic ET terminators (Amersham Biosciences) and bacteriophage M13mp18 template DNA using Tsp Δ271/F667Y DNA polymerase. The extension reactions were run in buffer that included 50 mM Tris pH 9.5, and 5 mM MgCl 2 .  
         [0020]    [0020]FIG. 12 depicts the result of a sequencing experiment using DYEnamic ET terminators (Amersham Biosciences) and bacteriophage M13mp18 template DNA using Tsp Δ271/F667Y/E681R DNA polymerase. The extension reactions were run in buffer that included 50 mM Tris pH 9.5, and 5 mM MgCl 2 .  
         [0021]    [0021]FIG. 13 depicts the result of a sequencing experiment using DYEnamic ET terminators (Amersham Biosciences) and bacteriophage M13mp18 template DNA using Tsp Δ271/F667Y DNA polymerase. The extension reactions were run in buffer that included 50 mM Tris pH 9.5, 5 mM MgCl 2 .  
         [0022]    [0022]FIG. 14 depicts the result of a sequencing experiment using DYEnamic ET terminators (Amersham Biosciences) and bacteriophage M13mp18 template DNA using Tsp Δ271/F667Y/E681R DNA polymerase. The extension reactions were run in buffer that included 50 mM Tris pH 9.5, 5 mM MgCl 2 .  
         [0023]    [0023]FIG. 15 depicts the result of a sequencing experiment using DYEnamic ET terminators (Amersham Biosciences) and bacteriophage M13mp18 template DNA using Tsp Δ271/F667Y DNA polymerase. The extension reactions were run in buffer that included 50 mM Tris pH 8.0, 5 mM MgCl 2  and 17.5% (v/v) glycerol.  
         [0024]    [0024]FIG. 16 depicts the result of a sequencing experiment using DYEnamic ET terminators (Amersham Biosciences) and bacteriophage M13mp18 template DNA using Tsp Δ271/F667Y/E681R DNA polymerase. The extension reactions were run in buffer that included 50 mM Tris pH 8.0, 5 mM MgCl 2  and 17.5% (v/v) glycerol.  
         [0025]    [0025]FIG. 17 depicts an alignment of the amino acid sequences of DNA polymerases from various  Thermus  species. The alignments were made using the Clustal algorithm and the PAM 100 similarity matrix. The polymerases included were: Tth ( T. thermophilus  Genbank accession P52028), Tca ( T. caldophilus  Genbank accession P80194), Taq ( T. aquaticus  Genbank accession 1TAU A), Tfl ( T. flavus  Genbank accession P30313), Tfi ( T. filiformis  Genbank accession O52225), Tos (Tsp SPS17, now called  T. oshimai  Genbank accession AAA94380), and the polymerase from Tsp JS1. 
     
    
     DETAILED DESCRIPTION  
       [0026]    One objective of the instant disclosure is to increase the uniformity of dye-terminator incorporation in fluorescent dye DNA sequencing. One important DNA polymerase is Tth DNA polymerase isolated from the thermophilic bacterium  Thermus thermophilus,  the amino acid sequence for which is shown at FIG. 1. Another DNA polymerase was found in an uncharacterized thermophylic bacterium we have designated Tsp JS1. This DNA polymerase was found to have the sequence shown in FIG. 2. To eliminate 5′ to 3′ exonuclease activity and to provide a polypeptide more stable to proteolysis and heat treatment the N-terminus of the polymerases can be truncated, removing approximately 271 amino acids. One such truncated enzyme Taq Δ271/F272M/F667Y DNA polymerase, which is commercially available from Amersham Biosciences is known as Thermo Sequenase® DNA polymerase. Position 1 (amino acid Met) in Taq Δ271/F272M/F667Y DNA polymerase corresponds to position 272 in full length Taq polymerase. It should be noted that the numbering used in the instant disclosure is that for full-length Taq polymerase, the sequence of which is shown in FIG. 3.  
         [0027]    Single amino acid substitutions were introduced into full-length or truncated polymerases as described (Davis, Fuller, Mamone &amp; Huang WO 99/65938 incorporated herein by reference). These substitutions are designated as D18A, F667Y, E681R, E681M, E681H or E681W to describe the amino acid substitutions using the numbering corresponding to positions in Taq polymerase. Each of the substituted polymerases was expressed, purified, and analyzed for uniformity of dye-terminator incorporation in fluorescent sequencing studies, as assayed by signal uniformity (Davis, Nelson, Kumar, Finn, Nampalli, Flick WO 01/14568). The E681R substitution was found to result in a substantial improvement of signal uniformity compared to the native DNA polymerases. This, combined with the high stability make this polymerase an excellent choice for DNA sequencing purposes.  
         [0028]    The polymerases may be used to generate fluorescently labeled polynucleotides by using primed templates, which templates may be used in chain termination sequencing or PCR as well understood by those skilled in the art and are described in WO 99/65938 previously incorporated herein by reference and U.S. Pat. No. 5,210,036, incorporated herein by reference.  
         [0029]    The following examples are for illustration purposes only and should not be used in any way to limit the appended claims.  
       EXAMPLES  
       [0030]    The following examples illustrate certain preferred embodiments of the invention but are not intended to be illustrative of all embodiments. 1)  
       Example 1  
       [0031]    Thermostability at 95° C.:  
         [0032]    The thermostability of Tsp JS1 Δ271//F667Y and Tsp JS1 Δ271//F667Y/E410R was assayed as follows. First, a 95° C. heating step was performed in a buffer containing 50 mM Tris-HCl pH 8.0, 1 mM MgCl 2 , 35 mM KCl and 10% glycerol with polymerase at a concentration of 2 units/μl. At various times after the start of heating (0, 2, 5, 10 and 20 minutes), aliquots (20 μl each) were removed and immediately placed on ice. Next, dilutions were made and diluted samples were assayed for polymerase activity using a standard polymerase assay method (Davis, Fuller, Mamone &amp; Huang WO 99/65938). FIG. 10 shows the results of the assays. The half-life of both Tsp JS1 Δ271//F667Y and Tsp JS1 Δ271//F667Y/E410R polymerases is apparently about 1 hour under these conditions at 95° C. Under the same conditions, the half-life of Taq polymerase, and that of FY7 (Davis, Fuller, Mamone &amp; Huang WO 99/65938) was found to be less than 10 minutes. 2)  
       Example 2  
       [0033]    Uniform termination events for uniform band intensities:  
         [0034]    The new polymerases also result in highly uniform termination events during sequencing reactions containing dye-labeled dideoxynucleotide terminators. This results uniform in electropherogram band intensities for determining long, accurate sequences. For example, as shown in FIGS.  11 - 16 , the average variation of band intensity using the new polymerases averages less than about 25% deviation compared with the 20 closest bands.  
         [0035]    It is apparent that many modifications and variations of the invention as hereinabove set forth may be made without departing from the spirit and scope thereof The specific embodiments described are given by way of example only, and the invention is limited only by the terms of the appended claims.