THERMOSTABLE NUCLEOSIDE DIPHOSPHATE KINASE FOR NUCLEIC ACID DETECTION

Abstract: A thermostable nucleoside diphosphate kinase (NDPK) enzyme useful in a process for the detection of nucleic acid is disclosed. The enzyme, its variants and analogs exhibit higher NDPK activity at a temperature of about 50 DEG C to about 90 DEG C relative to NDPK activity at 37 DEG C. Methods of obtaining, preparing and using the enzyme are also disclosed.

Description: Description THERMOSTABLE NUCLEOSIDE DIPHOSPHATE KINASE FOR NUCLEICACIDDETECTION Technical Field The invention relates to a thermostable enzyme. More specifically, the invention relates to a thermostable nucleoside diphosphate kinase (NDPK) enzyme useful in a process for the detection of nucleic acid. Background of the Invention Nucleoside diphosphate kinase (NDPK) is an enzyme found in most organisms that catalytically transfers a terminal 5 ' phosphate group from a deoxynucleoside triphosphate (dNTP) molecule or a nucleoside triphosphate (NTP) molecule to an ADP molecule to form ATP according to the following reaction (1) : NDPK dNTP* + ADP → dNDP + ATP* (1) wherein P* is the terminal 5' phosphate transferred. NDPK from yeast and from bovine liver are commercially available from a number of suppliers such as Sigma Chemical Co., St. Louis, MO and ICN Biomedicals, Inc., Costa Mesa, CA. These materials typically exhibit maximal activities at a temperature of about 20°C to about 40°C, and degrade at temperatures greater than about 50°C. For example, as illustrated hereinafter, yeast NDPK exhibits a half- life at a temperature of 70°C of about 0.6 minutes. The sequences of various nucleoside diphosphate kinase enzymes are known. A yeast nucleoside diphosphate kinase is published at Genbank Accession No. D13562, Saccharomyces cerevisiae . Bovine nucleoside diphosphate kinases are published at Genbank Accession No. X92957 and X92956, Bos taurus . T. Fukuchi et al . describe the overexpression of a bovine gene encoding a nucleoside diphosphate kinase at Gene, 129 : 141-146 (1993). The biological sequence for a protein exhibiting nucleoside diphosphate kinase activity from a thermostable organism is known for Pyrococcus abysii (Genbank Accession No. NC_00868) . The nucleotide sequence for the entire genome of the thermostable microbe Pyrococcus horikoshii was published on the National Center for Biotechnology website, www.ncbi.nlm.nih.gov, Genbank Accession No. AP000003. All the putative protein sequences with lengths 100 codons or more were submitted to a homology search against known proteins using Smith- Waterman algorithm against GenBank and GenPept release 103, EMBL release 52.0; SwissProt release 34.0; PIR-Protein release 54.0 and OWL release 29.5. Within the Genbank Accession No. AP000003, a putative protein gene at locus PH0698 was found to have some homology to nucleoside diphosphate kinase proteins, and was thus identified as a 160 amino acid long hypothetical nucleoside diphosphate kinase protein (SEQ ID NO: 2) . The sequence for the entire genome of Pyrococcus furiosus is published at www.genome.Utah.edu. An open reading frame analysis of the nucleotide sequence was conducted, and putative polypeptide sequences were identified. The nucleotide sequence Pf_894645 and polypeptide sequence Pf_894645 were identified therein. That publication did not include any suggestion of the activity of that putative protein, nor did it disclose any results of a homology search for that putative peptide Pf_894645. The predicted amino acid Pf_894645 sequence is identical to SEQ ID NOs : 3 and 14, and the nucleotide sequence differs in one residue from that of SEQ ID NO : 6 determined independently during the development of the present invention. Brief Summary of the Invention An embodiment of the invention contemplates an isolated and purified nucleoside diphosphate kinase (NDPK) enzyme that exhibits higher NDPK activity at a temperature of about 50°C to about 90°C relative to the NDPK activity at 37°C. A contemplated NDPK enzyme is at least 80 percent, preferably 90 percent, more preferably 95 percent, and most preferably 100 percent identical with the amino acid residue sequence of SEQ ID NO : 3 (PfuNDPK-2) or the truncated amino acid residue sequence SEQ ID NO:14 (PfuNDPK-1) . Preferred contemplated NDPK enzymes are encoded by the nucleotide having the sequence of SEQ ID NO:6 (PfuNDPK-2) , SEQ ID NO:26 or the truncated DNA coding sequence of SEQ ID NO:13 (PfuNDPK-1) , that begins at base 16 of SEQ ID NO : 6. Another embodiment of the invention contemplates a method for producing an NDPK polypeptide that exhibits higher NDPK activity at a temperature of about 50 degrees C to about 90 degrees C relative to the NDPK activity at 37 degrees C. Such a method comprises the steps of providing a host cell culture wherein the host cell harbors an expression vector comprising the nucleotide sequence of SEQ ID NO: 6, 13, 26, 27 or 28 or a DNA variant thereof. The DNA variant has at least 80 percent, preferably 90 percent, more preferably 95 percent, and most preferably 100 percent identity to the NDPK sequence of SEQ ID NO: 6, 13, 26, 27, or 28. Such a DNA variant hybridizes with said NDPK sequence under moderate stringency conditions comprising hybridization at a temperature of about 50°C to about 65°C in 0.2 to 0.3 M NaCl , followed by washing at about 50°C to about 55°C in 0.2X SSC, 0.1% SDS . An NDPK polypeptide is expressed from the expression vector. The NDPK polypeptide is isolated and purified from the host cell culture. In another embodiment, the invention contemplates an isolated and purified polynucleotide comprising the nucleotide sequence of SEQ ID NOs : 6 , 13, 26, 27 or 28 or DNA variant thereof. The DNA variant is as discussed above. The DNA polynucleotide sequence (SEQ ID NO: 6, 13, 26) encodes an NDPK enzyme of the invention. The complementary sequence of SEQ ID NOs 6 and 13 are SEQ ID NOs 27 and 28 respectively. The NDPK enzyme uses NTPs or dNTPs as a phosphate source to convert ADP to ATP with a higher NDPK activity at a temperature of about 50°C to about 90°C than the NDPK activity at 37°C. In a further embodiment, the invention contemplates an isolated and purified NDPK enzyme that is encoded by a DNA molecule comprising a BseR l/Dra I restriction fragment of about 240 bases and a Dra I /Pst I restriction fragment of about 35 bases. Preferably, the DNA molecule comprises restriction sites BseR I, Dra I, and Pst I as shown in Table 1. In an alternative embodiment, the DNA molecule further comprises a PpuM I/BseR I restriction fragment of about 180 bases, preferably comprising a restriction map as shown in Table 1. In another alternative preferred embodiment, the restriction fragments hybridize with the NDPK sequences of SEQ ID Nos : 6 , 13, 26, 27 or 28 under moderate stringency conditions as described above. In a still further embodiment, the invention contemplates an isolated and purified analog nucleic acid sequence that encodes an amino acid residue sequence that is at least 80 percent, preferably 90 percent, more preferably 95 percent, and most preferably 100 percent identical to the sequence of an NDPK of SEQ ID NO: 3 or 14. The nucleic acid sequence upon suitable transfection and expression in a host, provides an enzyme that (1) uses NTPs or dNTPs as a phosphate source to convert ADP to ATP, and (2) exhibits higher NDPK activity at a temperature of about 50°C to about 90°C than at 37°C. Preferably, the analog nucleic acid sequence encodes an amino acid residue sequence that is at least 90 percent, most preferably 95 percent, and particularly preferably 100 percent identical to the sequence of an NDPK of SEQ ID NO : 3 or 14. In still another embodiment, the invention contemplates a composition comprising an aqueous solution containing an isolated and purified polynucleotide comprising a nucleotide sequence having at least 80 percent, preferably 90 percent, more preferably 95 percent, and most preferably 100 percent identity with the sequence of SEQ ID NOs : 6 , 13, 26, 27 or 28. A contemplated composition is useful for determining the presence or absence of a predetermined nucleic acid target sequence in a nucleic acid sample comprising an aqueous solution. Such a composition contains: (A) a purified and isolated enzyme whose activity is to release one or more nucleotides from the 3' terminus of a hybridized nucleic acid probe; (B) at least one nucleic acid probe, said nucleic acid probe being complementary to said predetermined nucleic acid target sequence; and (C) a purified and isolated nucleoside diphosphate kinase (NDPK) enzyme that comprises an amino acid residue sequence that exhibits higher NDPK activity at a temperature of about 50°C to about 90°C relative to NDPK activity at 37°C. Preferably, the NDPK enzyme comprises an amino acid residue sequence at least 80 percent, preferably 90 percent, more preferably 95 percent, and most preferably 100 percent identical with the sequence of SEQ ID NO : 3 or 14. Another contemplated composition is useful for determining the presence or absence of at least one predetermined nucleic acid target sequence in a nucleic acid sample. Such a composition comprises an aqueous solution that contains: (A) a purified and isolated enzyme whose activity in the presence of pyrophosphate is to release identifier nucleotide as a nucleoside triphosphate from the 3 ' end of a nucleic acid probe hybridized to said nucleic acid target sequence; (B) adenosine 5' diphosphate; (C) pyrophosphate; (D) a purified and isolated nucleoside diphosphate kinase (NDPK) enzyme that exhibits higher NDPK activity at a temperature of about 50 to about 90 degrees C relative to NDPK activity at 37 degrees C; and (E) at least one nucleic acid probe, said nucleic acid probe being complementary to said predetermined nucleic acid target sequence. Preferably, the purified and isolated enzyme whose activity in the presence of pyrophosphate is to release identifier nucleotides is selected from the group consisting of the Tne triple mutant DNA polymerase, Bst DNA polymerase, Ath DNA polymerase, Tag DNA polymerase and Tvu DNA polymerase. Also preferably, the purified and isolated NDPK enzyme comprises an amino acid residue sequence at least 80 percent, preferably 90 percent, more preferably 95 percent, and most preferably 100 percent identical with the sequence of SEQ ID NO: 3 or 14. In another embodiment of the invention, a contemplated recombinant DNA molecule comprises a vector operatively linked to an exogenous DNA segment that contains at least 486 base pairs that define a gene for the Pyrococcus furiosus NDPK enzyme or a DNA variant that has at least 80 percent, preferably 90 percent, more preferably 95 percent, and most preferably 100 percent identity to the NDPK sequence of SEQ ID NOs: 6, 13, 26, 27 or 28 and hybridizes with said NDPK sequence under moderate stringency conditions (described above) . The nucleotide segment encodes an enzyme that uses NTPs or dNTPs as a source of phosphate to convert ADP to ATP with a higher activity at a temperature of about 50°C to about 90°C than the NDPK activity at 37°C, and a promoter for driving the expression of said enzyme in host organism cells. Preferably wherein the exogenous DNA segment comprises the nucleotide sequence of SEQ ID NO: 13 or 28. Also preferably wherein the promoter is inducible by an exogenously supplied agent, most preferably the promoter is induced by exogenously supplied IPTG. Also preferably, the recombinant DNA molecule is present in a host organism. Also contemplated is a recombinant DNA molecule that comprises a vector operatively linked to a promoter for driving the expression of the enzyme in host organism cells and a DNA segment that is an analog nucleic acid sequence that encodes an amino acid residue sequence that is at least 80 percent, preferably 90 percent, more preferably 95 percent, and most preferably 100 percent identical to the sequence of a Pyrococcus furiosus NDPK of SEQ ID NOs : 3 or 14, wherein said recombinant DNA molecule, upon suitable transfection and expression in a host, provides an enzyme that (1) uses NTPs or dNTPs as a source of phosphate to convert ADP to ATP and (2) exhibits higher NDPK activity at a temperature of about 50°C to about 90°C than at 37°C. Preferably, the recombinant DNA molecule is present in a host organism. Also contemplated is a host organism containing a polynucleotide that comprises a sequence that is at least 80 percent, preferably 90 percent, more preferably 95 percent, and most preferably 100 percent identical to the sequence of SEQ ID NO: 3, 16, 26, 27 or 28. The present invention has many benefits and advantages, several of which are listed below. One benefit of the invention is that the thermostable NDPK enzyme can be used for processes of nucleic acid hybrid detection with very high levels of sensitivity without the need for radiochemicals or electrophoresis . An advantage of the invention is that the enzyme can be used in conjunction with high temperature amplification without substantial loss of NDPK activity. A further advantage of the invention is that the enzyme can be used in high throughput robotically-manipulated procedures because greater enzymatic stability is retained at room temperature. Still further benefits and advantages will be apparent to the worker of ordinary skill from the disclosure that follows. Brief Description of the Drawings In the drawings forming a portion of this disclosure, Fig. 1 illustrates the single letter code alignment of the amino acid residue sequences of NDPK enzymes of Pyrococcus abysii (SEQ ID NO:l; Genbank Accession No. NC_000868) and Pyrococcus horikoshii (SEQ ID NO: 2; GenBank Accession No. AP000003, putative polypeptide PH0698) with Pyrococcus furiosus (PfuNDPK-2; SEQ ID NO : 3 ) , as well as a consensus sequence of identical residues present in all three enzymes, wherein dashes in a sequence represent residues absent from a Pyrococcus horikoshii or Pyrococcus furiosus sequence that are present in the Pyrococcus furiosus sequence. Fig. 2 illustrates on two sheets as Fig. 2A and Fig. 2B the nucleotide alignment of the coding sequences of NDPK enzymes of Pyrococcus abysii (SEQ ID NO:4; Genbank Accession No. NC_000868) and Pyrococcus horikoshii (SEQ ID NO : 5 ; GenBank Accession No. AP000003) with Pyrococcus furiosus (PfuNDPK-2; SEQ ID NO: 6) that are aligned for maximal identity of amino acid residue sequences, as well as a consensus sequence of identical bases present in all three enzymes, wherein dashes in a sequence represent bases absent from a Pyrococcus horikoshii or Pyrococcus furiosus sequence that are present in the Pyrococcus furiosus sequence. Fig. 3 is an illustration similar to that of Fig. 1 showing a visual alignment of the amino acid residue sequences of two NDPK enzymes of Bos taurus (two sequences; SEQ ID NO: 7 and SEQ ID NO : 8 ; GenBank Accession Nos. X92967 and X92956) and Saccharomyces cerevisiae (SEQ ID NO : 9 ; Genbank Accession No. D13562) with Pyrococcus furiosus (PfuNDPK-2; SEQ ID NO : 3 ) , as well as a consensus sequence of identical residues present in all three enzymes, wherein dashes in a sequence represent residues absent from a Bos taurus or Saccharomyces cerevisiae sequence that are present in the Pyrococcus furiosus sequence. Definitions To facilitate understanding of the invention, a number of terms are defined below. "Nucleoside", as used herein, refers to a compound consisting of a purine [guanine (G) or adenine (A) ] or pyrimidine [thymine (T) , uridine (U) or cytidine (C) ] base covalently linked to a pentose, whereas "nucleotide" refers to a nucleoside phosphorylated at one of its pentose hydroxyl groups. "XTP", "XDP" and "XMP" are generic designations for ribonucleotides and deoxyribonucleotides, wherein the "TP" stands for triphosphate, "DP" stands for diphosphate, and "MP" stands for monophosphate, in conformity with standard usage in the art. Subgeneric designations for ribonucleotides are "NMP", "NDP" or "NTP", and subgeneric designations for deoxyribonucleotides are "dNMP", "dNDP" or "dNTP". Also included as "nucleoside", as used herein, are materials that are commonly used as substitutes for the nucleosides above such as modified forms of these bases (e.g. methyl guanine) or synthetic materials well known in such uses in the art, such as inosine. A "nucleic acid, " as used herein, is a covalently linked sequence of nucleotides in which the 3 ' position of the pentose of one nucleotide is joined by a phosphodiester group to the 5' position of the pentose of the next, and in which the nucleotide residues (bases) are linked in specific sequence; i.e., a linear order of nucleotides. A "polynucleotide, " as used herein, is a nucleic acid containing a sequence that is greater than about 100 nucleotides in length. An "oligonucleotide, " as used herein, is a short polynucleotide or a portion of a polynucleotide. An oligonucleotide typically contains a sequence of about two to about one hundred bases. The word "oligo" is sometimes used in place of the word "oligonucleotide" . A base "position" as used herein refers to the location of a given base or nucleotide residue within a nucleic acid. Nucleic acid molecules are said to have a "5 ' -terminus" (5' end) and a "3 ' -terminus" (3' end) because nucleic acid phosphodiester linkages occur to the 5 ' carbon and 3 ' carbon of the pentose ring of the substituent mononucleotides . The end of a polynucleotide at which a new linkage would be to a 5' carbon is its 5' terminal nucleotide. The end of a polynucleotide at which a new linkage would be to a 3' carbon is its 3' terminal nucleotide. A terminal nucleotide, as used herein, is the nucleotide at the end position of the 3'- or 5 '-terminus. As used herein, a nucleic acid sequence, even if internal to a larger oligonucleotide or polynucleotide, also can be said to have 5'- and 3'- ends. For example, a gene sequence located within a larger chromosome sequence can still be said to have a 5'- and 3' -end. Polypeptide molecules are said to have an "amino terminus" (N-terminus) and a "carboxy terminus" (C-terminus) because peptide linkages occur between the backbone amino group of a first amino acid residue and the backbone carboxyl group of a second amino acid residue. Typically, the terminus of a polypeptide at which a new linkage would be to the carboxy-terminus of the growing polypeptide chain, and polypeptide sequences are written from left to right beginning at the amino terminus . As used herein, "near" the N- or C-terminus refers to within about 10 amino acid residues of the terminus. In either a linear or circular nucleic acid molecule, discrete elements are referred to as being "upstream" or "5'" relative to an element if they are bonded or would be bonded to the 5 ' -end of that element. Similarly, discrete elements are "downstream" or "3'" relative to an element if they are or would be bonded to the 3 ' -end of that element. Transcription proceeds in a 5 ' to 3 ' manner along the DNA strand. This means that RNA is made by the sequential addition of ribonucleotide-5 ' - triphosphates to the 3 ' -terminus of the growing chain (with the elimination of pyrophosphate) . As used herein, the term "target nucleic acid" or "nucleic acid target" refers to a particular nucleic acid sequence of interest. Thus, the "target" can exist in the presence of other nucleic acid molecules or within a larger nucleic acid molecule . Nucleic acids are known to contain different types of mutations. A "point" mutation refers to an alteration in the sequence of a nucleotide at a single base position from the wild type sequence. A "lesion" is a site within a nucleic acid where one or more bases are mutated by deletion or insertion, so that the nucleic acid sequence differs from the wild-type sequence. In vitro manipulations, resulting in the insertion or deletion of one or more codons relative to a wild type nucleic acid sequence also lead to functional polypeptide products. A "single nucleotide polymorphism" or SNP is a variation from the most frequently occurring base of a wild type sequence at a particular nucleic acid position. As used herein, the term "hybridization" is used in reference to the pairing of complementary nucleic acid strands. Hybridization and the strength of hybridization (i.e., the strength of the association between nucleic acid strands) is impacted by many factors well known in the art including the degree of complementarity between the nucleic acids, stringency of the conditions involved affected by such conditions as the concentration of salts, the T m (melting temperature) of the formed hybrid, the presence of other components { e . g. , the presence or absence of polyethylene glycol) , the molarity of the hybridizing strands and the G:C content of the nucleic acid strands. As used herein, the term "nucleic acid probe" refers to an oligonucleotide or polynucleotide that is capable of hybridizing to another nucleic acid of interest. A nucleic acid probe can occur naturally as in a purified restriction digest or be produced synthetically, recombinantly or by PCR amplification. As used herein, the term "nucleic acid probe" refers to the oligonucleotide or polynucleotide used in a method discussed herein. That same oligonucleotide could also be used, for example, in a PCR method as a primer for polymerization, but as used herein, that oligonucleotide would then be referred to as a "primer" . Herein, oligonucleotides or polynucleotides may contain a phosphorothioate bond. As used herein, the term "stringency" is used in reference to the conditions of temperature, ionic strength, and the presence of other compounds, under which nucleic acid hybridizations are conducted. With "high stringency" conditions, nucleic acid base pairing will occur only between nucleic acid fragments that have a high frequency of complementary base sequences. Thus, conditions of "weak" or "low" stringency are often required when it is desired that nucleic acids which are not completely complementary to one another be hybridized or annealed together. The art knows well that numerous equivalent conditions can be employed to comprise low stringency conditions. The choice of hybridization conditions is generally evident to one skilled in the art and is usually be guided by the purpose of the hybridization, the type of hybridization (DNA-DNA, or DNA-RNA) , and the level of desired relatedness between the sequences (Sambrook et al . , 1989. Nucleic Acid Hybridization, A Practical Approach, IRL Press, Washington D.C., 1985, which is incorporated by reference herein) . The stability of nucleic acid duplexes is known to decrease with an increased number of mismatched bases, and further to be decreased to a greater or lesser degree depending on the relative positions of mismatches in the hybrid duplexes. Thus, the stringency of hybridization can be used to maximize or minimize stability of such duplexes. Hybridization stringency can be altered by: adjusting the temperature of hybridization; adjusting the percentage of helix destabilizing agents, such as formamide, in the hybridization mix; and adjusting the temperature and/or salt concentration of the wash solutions. For filter hybridizations, the final stringency of hybridizations often is determined by the salt concentration and/or temperature used for the post-hybridization washes. In general, the stringency of hybridization reaction itself can be reduced by reducing the percentage of formamide in the hybridization solution. High stringency conditions, for example, utilize high temperature hybridization (e.g., 65°C to 70°C) in aqueous solution containing 4X to 6X SSC (IX SSC = 0.15 M NaCl, 0.015 M sodium citrate) or 40 to 45°C in 50% formamide combined with washes at high temperature (e.g. 5°C to 25°C below the T m ) , in a solution having a low salt concentration (e.g., 0. IX SSC) . Moderate stringency conditions typically utilize hybridization at a temperature about 50°C to about 65°C in 0.2 to 0.3 M NaCl , and washes at about 50°C to about 55°C in 0.2X SSC, 0.1% SDS . Low stringency conditions can utilize lower hybridization temperature (e.g. 35°C to 45°C in 20% to 50% formamide) with washes conducted at a low intermediate temperature (e.g. 40 to 55°C) and in a wash solution having a higher salt concentration (e.g. 2X to 6X SSC) . Moderate stringency conditions are preferred for use in conjunction with the disclosed polynucleotide molecules as probes to identify clones encoding nucleoside diphosphate kinases of the invention. As used herein, the term "T m " is used in reference to the "melting temperature" . The melting temperature is the temperature at which 50% of a population of double-stranded nucleic acid molecules becomes dissociated into single strands. The equation for calculating the T m of nucleic acids is well-known in the art. The T m of a hybrid nucleic acid is often estimated using a formula adopted from hybridization assays in 1 M salt, and commonly used for calculating T m for PCR primers: [(number of A + T) x 2°C + (number of G + C) x 4°C] . C.R. Newton et al. PCR, 2 nd Ed., Springer-Verlag (New York: 1997), p. 24. This formula was found to be inaccurate for primers longer that 20 nucleotides. Id . Other more sophisticated computations exist in the art which take structural as well as sequence characteristics into account for the calculation of T m . A calculated T m is merely an estimate; the optimum temperature is commonly determined empirically. The term "isolated" when used in relation to a nucleic acid or protein, refers to a nucleic acid sequence or protein that is identified and separated from at least one contaminant (nucleic acid or protein, respectively) with which it is ordinarily associated in its natural source. Isolated nucleic acid or protein is present in a form or setting that is different from that in which it is found in nature. In contrast, non-isolated nucleic acids or proteins are found in the state they exist in nature. As used herein, the term "purified" or "to purify" means the result of any process that removes some contaminants from the component of interest, such as a protein or nucleic acid. The percent of a purified component is thereby increased in the sample . As used herein, the term "operatively linked" means that a gene is covalently bonded in correct reading frame to another DNA (or RNA as appropriate) segment, such as to an expression vector or a promoter so that the gene is under the control of the expression vector. As used herein, the term "promoter" means a recognition site on a DNA sequence or group of DNA sequences that provide an expression control element for a gene and to which RNA polymerase specifically binds and initiates RNA synthesis (transcription) of that gene . As used herein, the term "recombinant DNA molecule" means a hybrid DNA sequence comprising at least two nucleotide sequences not normally found together in nature . As used herein, the term "gene" means a double-stranded DNA sequence that is expressed as a polypeptide; i.e., an amino acid residue sequence. As used herein, the term "vector" means a DNA molecule capable of replication in a cell and/or to which another DNA segment can be operatively linked so as to bring about replication of the attached segment. A plasmid is an exemplary vector. The term "NDPK activity" or "nucleoside diphosphate kinase activity" means the catalysis of the transfer of a phosphoryl group to a nucleoside diphosphate to form a nucleoside triphosphate. Herein, the nucleoside triphosphate product of interest is typically ATP, thus the substrate nucleoside diphosphate is typically ADP. An "NDPK enzyme" is an enzyme that exhibits NDPK activity. An "NDPK polypeptide" is a polypeptide that has the same amino acid sequence as an NDPK enzyme. If an NDPK polypeptide is folded properly and under appropriate conditions to maintain NDPK activity, then an NDPK polypeptide is an NDPK enzyme . All amino acid residues identified herein are in the natural L-configuration. In keeping with standard polypeptide nomenclature, J. Biol . Chem . , 243 :3557-59, (1969), abbreviations for amino acid residues are as shown in the following Table of Correspondence : TABLE OF CORRESPONDENCE SYMBOL 1-Letter 3 -Letter AMINO ACID Y Tyr L-tyrosine G Gly glycine F Phe L-phenylalanine M Met L-methionine A Ala L-alanine S Ser L-serine I lie L-isoleucine L Leu L-leucine T Thr L-threonine V Val L-valine P Pro L-proline K Lys L-lysine H His L-histidine Q Gin L-glutamine E Glu L-glutamic acid W Trp L-tryptophan R Arg L-arginine D Asp L-aspartic acid N Asn L-asparagine C Cys L-cysteine Detailed Description of the Invention As is illustrated in the Examples that follow, it can be beneficial to carry out a contemplated method at elevated temperatures, e.g., about 50°C to about 90°C. An above-mentioned yeast ( Saccharomyces cerevisiae) or bovine (Bos taurus) NDPK has a very short half-life at these temperatures, e.g., less than about one minute, and it is preferred to use a thermostable NDPK that exhibits the activity of a contemplated enzyme for reactions at these elevated temperatures. In addition, it can be desirable to utilize an enzyme that exhibits relatively high NDPK activity at an elevated temperature range and a relatively low NDPK activity at a lower temperature range. In addition, it can be desirable to use a thermostable NDPK enzyme for high throughput applications because of its increased stability at room temperature. A nucleoside diphosphate kinase reaction is the transfer of a phosphoryl group to a nucleoside diphosphate to form a nucleoside triphosphate . Enzymes capable of catalyzing this reaction exhibit NDPK activity. Preferably, the nucleoside triphosphate product of interest is ATP, thus the preferred nucleoside diphosphate substrate is ADP. There are a wide variety of useful phosphoryl donors for nucleoside diphosphate kinases, including NTPs, dNTPs, and analogs thereof. Preferred phosphoryl donors are NTPs or dNTPs . Methods to ascertain NDPK activity are known in the art. Example 3 provides exemplary NDPK activity assays, including reagents and conditions. A contemplated thermostable NDPK enzyme exhibits a higher NDPK activity at a temperature of about 50°C to about 90°C relative to the NDPK activity at 37°C. This means that when the NDPK activity is compared at two temperatures (37°C and 50°C-90°C) for the same amount (e.g. by a standard Bradford assay) of the same NDPK enzyme, the enzyme is able to convert more ADP to ATP at the elevated temperature than it is able to convert at 37°C. Thus, the contemplated thermostable enzyme has a higher activity at the elevated temperature than its activity at 37°C. For a comparison of activity, it is best to monitor the activity in a linear range, typically for 0.002 to 0.0002 units (usually 0.012 to 1.2 ng protein) in a 10 -minute assay. A convenient assay measures the amount of luminescence light output from an ATP-dependent luciferase/luciferin reaction. Ten μM ADP and 100 μM dCTP are current optimum reagent concentrations using the above amount of protein with a luciferase/luciferin reaction and a luminometer. For example, looking at the data in Example 3, at 10 minutes time, the light output from yeast NDPK at 37°C is 7503, while the light output from yeast NDPK at 70°C is 6687. The activity is lower for yeast NDPK at the elevated temperature, and it is not a contemplated thermostable NDPK. On the other hand, both fractions of the Pfu NDPK gave higher light output and thus, higher activity at 70°C than at 37°C. Those are contemplated thermostable NDPK enzymes. Fig. 1 shows the amino acid residue sequence of three NDPK enzymes from three thermophilic archaebacteria [Pyrococcus abysii (SEQ ID NO: 1 / Pab; Genbank Accession No. NC_000868) / Pyrococcus horikoshii (SEQ ID NO: 2; Pho; GenBank Accession No. AP000003, putative polypeptide PH0698J and Pyrococcus furiosis (SEQ ID NO: 3; Pfu; PfuNDPK- 2)] aligned for maximal identity, as well as a consensus sequence that lists the identical residues present in each of the three sequences. The Pfu enzyme exhibits about 71 percent identity with the Pab enzyme sequence (118/166 residues) , about 82 percent identity with the Pho sequence (136/166 residues) , and about 69 percent with the consensus sequence (115/166 residues) . The Pfu sequence is also four residues longer at the carboxy-terminus (C- terminus) than either Pab or Pho, and two residues of the Pfu sequence are absent from near the amino- terminus (N-terminus) of the Pho sequence. Fig. 3 shows a similar alignment of the Pfu (PfuNDPK-2) amino acid residue sequence with two published sequences of mesophilic bovine NDPK [Bos taurus (Genbank Accession Nos. X92957 and X92956; SEQ ID NO: 7 and 8)] and yeast NDPK [Saccharomyces cerevisiae (Watanabe, DDBJ/EMBL/GenBank Accession No. D13562; SEQ ID NO: 9)] . Examination of these four sequences shows that the Pfu amino acid residue sequence is longer at both termini than either of the other three sequences, and that those three sequences also require additional residues shown by dashes about three-quarters of the way to the C-terminus to provide maximal identity with the Pfu sequence. Visual inspection also shows that the sequences of the three mesophiles are more similar to each other than is any single sequence to the sequence of Pfu . The Pfu sequence exhibits only about 31 percent identity with the consensus sequence (51/166 residues) for the non-thermophilic NDPK sequences. Thus, examination of Figs. 1 and 3 shows that the amino acid residue sequence of Pfu NDPK is quite dissimilar from that of the bovine or yeast NDPK sequences. That examination also shows a significant difference between the Pfu amino acid sequence and that of either of the other thermophiles, Pab and Pho . A particularly preferred thermostable NDPK was obtained by cloning the appropriate DNA of the thermophilic archaebacteria Pyrococcus furiosis ( Pfu) . As is illustrated elsewhere, Pyrococcus furiosis DNA can encode two NDPK enzymes, one of which is five amino acid residues shorter than the other at the amino-terminus of the molecule. The shorter of the two enzymes is referred to herein as PfuNDPK-1, whereas the longer is referred to as PfuNDPK-2. These NDPK enzymes are collectively referred to herein in the singular as NDPK Pfu or Pfu NDPK, or as a "contemplated NDPK" . Where one or the other of the two enzymes is specifically intended, that enzyme is referred to as either PfuNDPK-1 or PfuNDPK-2, or by reference to a SEQ ID NO. As discussed in Example 3, yeast NDPK produces a light output that increases over time at elevated temperature, and then levels off, suggesting heat inactivation of the yeast NDPK. On the other hand, the light output from Pfu NDPK continues to increase over time at elevated temperature, remaining active. Thus, Pfu NDPK retains higher activity-after maintenance at a temperature of about 50°C to about 90°C, such as 70°C, for a time period of 5 minutes than did yeast NDPK, and was found to have a half- life at a temperature of 70°C of about 10 minutes as compared to yeast NDPK that had a half-life at that temperature of about 0.6 minutes. In addition, the Pfu NDPK enzyme exhibits higher NDPK activity than the yeast NDPK at a temperature of about 50°C. A contemplated Pfu NDPK-2 can contain 166 amino acid residues and is encoded by a recombinant DNA containing 498 base pairs (bp) from the starting Met residue to the C-terminal Cys . Three added base pairs for the stop codon provide a total of 501 bp from start through stop codons (Fig. 2) . One preferred recombinant enzyme is truncated by five residues at the amino-terminus and contains 161 amino acid residues and is encoded by a recombinant DNA containing 483 bp . The amino acid sequence of that truncated enzyme (PfuNDPK-1) is provided as sequence Pf5 (SEQ ID NO: 14), hereinafter, beginning at residue 6 of SEQ ID NO : 3 with a corresponding truncated DNA sense strand sequence as Pf4 (SEQ ID NO: 13) beginning at nucleotide position 16 in SEQ ID NO : 6. Fig. 2 shows the aligned coding DNA sequences of the NDPK enzymes from Pab (Genbank Accession No. NC_000868) , Pho and Pfu, along with a consensus DNA that shows the identical bases for all three. Examination of this Figure shows that the coding DNA sequences for these enzymes are even more different than are their amino acid residue sequences. For example, the nucleic acid sequences that encode Pfu and Pab NDPK share only about 70 percent identical bases when aligned for maximal identity of amino acid residue sequences, whereas nucleic acid sequences that encode Pfu and Pho NDPK share about 75 percent identical bases. The Pfu sequence shares only about 63 percent identical bases with the consensus sequence. Still further, the Pab and Pho sequences start with the codon GTG, whereas the Pfu sequence starts with ATG. Similarly, the Pab and Pho sequences both use TAA as the stop codon, whereas the Pfu sequence utilizes TGA. The Pfu gene also contains restriction sites that are unique to that sequence that are not present in either of the Pab or Pho DNA sequences. Some exemplary restriction endonuclease sites unique to the Pfu NDPK and their positions in the coding DNA strand of SEQ ID NO: 6 are shown in Table 1, below. Table 1 Unique Restriction Sites Restriction Spans SEQ ID NO : 6 Enzyme Nucleotides PpuM I 5 - 11 BseR I 172 - 188 Dra I 424 - 429 Pst I 458 - 463 Approximate fragment sizes: PpuM I to BseR I 180 bases BseR I to Dra I 240 bases Dra I to Pst I 35 bases A gene that encodes a contemplated Pfu NDPK enzyme preferably contains three of the four unique endonuclease restriction sites of Table 1 when that DNA is aligned with the DNA sequence of SEQ ID NO : 6. Those three restriction sites are preferably (1) the BseR I site that spans bases I 84-97, (2) the Dra I site that spans bases 424-429, and (3) the Pst I site that spans bases 458-463 of SEQ ID NO: 6. In other preferred embodiments, the DNA also contains a site (4) that is the PpuM I site that spans bases 5-11 of SEQ ID NO: 6. Isoschizomers are enzymes that recognize and cuts the same nucleotide sequence, and are known in the art. Enzymes that are isoschizomers of those listed in Table 1 are equivalent in Table 1 and elsewhere herein. Aha III is a known isoschizomer of Dra I; Psp5 II and PspPP I are known isoschizomers of PpuM I. Thus, where a N-terminal -truncated NDPK is prepared, such that of SEQ ID NO: 14 that is encoded by the DNA of SEQ ID NO: 13, the fourth, PpuM I site, is not present. However, when a preferred DNA encoding that enzyme such as that of SEQ ID NO: 13 is aligned with SEQ ID NO: 6, the other three restriction sites are present in that preferred DNA at positions that correlate to the stated positions in the nucleic acid of SEQ ID NO : 6. Thus, in one embodiment, a contemplated gene comprises a PpuM 1 /BseR I restriction fragment of about 180 bases, a BseR l/Dra I restriction fragment of about 240 bases and a Dra I /Pst I restriction fragment of about 35 bases. In another embodiment, only the BseR l/Dra I restriction fragment of about 240 bases and the Dra l /Pst I restriction fragment of about 30 bases are present. Both embodiments comprise a BseR l /Pst I restriction fragment of about 290 bases. A ten nucleotide length variance from these approximate sizes is contemplated. Of course, restriction fragments using different pairs of restriction endonucleases can be prepared. A contemplated NDPK of the present invention need not be identical in amino acid residue sequence to that of Pfu NDPK of SEQ ID NOs : 3 or 14 so long as the enzyme includes at least 80 percent, and more preferably 90% of the amino acid residues of SEQ ID NOs : 3 or 14, and exhibits higher NDPK activity at a temperature of about 50°C to about 90°C than at 37°C. An even more preferred NDPK variant is one having at least 95% amino acid sequence identity to an NDPK disclosed herein (SEQ ID NOs : 3 or 14) . Therefore, a contemplated NDPK can be subject to various changes such as insertions and deletions. For example, the N-terminal five amino acid residues of the enzyme of SEQ ID NO : 3 can be deleted, thereby providing an enzyme with an amino acid sequence of SEQ ID NO: 14. Also contemplated are conservative substitutions of one amino acid for another. Conservative substitutions are those in which one amino acid residue is replaced by another, biologically similar residue. Examples of conservative substitutions include the substitution of one hydrophobic residue such as isoleucine, valine, leucine or methionine for one another or one of tyrosine, phenylalanine or tryptophan for one another, or the substitution of one polar residue for another as between arginine and lysine, between glutamic and aspartic acids or between glutamine and asparagine . More rarely, a variant includes "nonconservative" changes, e.g., replacement of a glycine with a tryptophan. Analogous minor variations can also include amino acid deletions or insertions, or both. Guidance in determining which amino acid residues can be substituted, inserted, or deleted without abolishing biological activity can be found using computer programs well known in the art, for example LASERGENE software (DNASTAR Inc., Madison, Wis . ) When a NDPK enzyme of the present invention has an amino acid residue sequence that is not identical to that of SEQ ID NO: 3 or 14 because one or more conservative substitutions has been made, it is preferred that no more than 20 percent, and more preferably no more than 10 percent, and most preferably no more than 5 percent of the amino acid residues are substituted as compared to SEQ ID NO : 3 or 14. A contemplated NDPK can therefore contain up to about 33 residues that are different from those of SEQ ID NO: 3 and up to 32 residues that are different from those of SEQ ID NO: 14, and preferably about 17 and 16 residues, respectively. More preferably, up to 8 residues can be different from either sequence (SEQ ID NO: 3 or 14) . A contemplated NDPK can also have a length shorter than that of a NDPK of SEQ ID NO : 3. Thus, about 5 residues can be deleted from the N-terminus as in an enzyme of SEQ ID NO: 14. NDPK enzymes that contain substitutions or deletions as discussed above are referred to as NDPK variants . Such variants exhibit NDPK activity as discussed elsewhere herein and exhibit higher NDPK activity at a temperature of about 50°C to about 90°C than at 37°C. An isolated, purified DNA segment comprising a nucleotide sequence of at least 486 base pairs that define a gene for the enzyme Pfu NDPK is also contemplated. This gene is double stranded and it typically contains about 486 bp of the truncated sequence (SEQ ID NO: 13 and its complement) , but can also contain 501 bp (SEQ ID NO: 6 and its complement) of the native sequence or a lesser number as discussed elsewhere. An isolated and purified DNA (gene) of this invention includes a double stranded sequence plus variants wherein single strands thereof hybridize non-randomly with a DNA of SEQ ID NO: 6, 13 or 26 (or their complements, SEQ ID NO: 27 or 28 or the complement of SEQ ID NO: 26) under at least moderate stringency conditions described hereinafter. Such a contemplated gene includes a recited non-randomly hybridizable variant DNA sequence, encodes NDPK and also produces biologically active molecules of the encoded NDPK enzyme that exhibit higher NDPK activity at a temperature of about 50°C to about 90°C than at 37°C when suitably transfected into and expressed in an appropriate host . Nucleic acid hybridization is a function of sequence identity (homology) , G+C content of the sequence, buffer salt content, sequence length and duplex melt temperature (T m ) among other variables. See, Maniatis et al . , Molecular Cloning, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. (1982) , page 388. High stringency conditions, for example, utilize high temperature hybridization (e.g., 65°C to 70°C) in aqueous solution containing 4X to 6X SSC (IX SSC = 0.15 M NaCl, 0.015 M sodium citrate) or 40 to 45°C in 50% formamide combined with washes at high temperature (e.g. 5°C to 25°C below the T m ) , in a solution having a low salt concentration (e.g., 0. IX SSC) . Moderate stringency conditions typically utilize hybridization at a temperature about 50°C to about 65°C in 0.2 to 0.3 M NaCl, and washes at about 50°C to about 55°C in 0.2X SSC, 0.1% SDS . Low stringency conditions can utilize lower hybridization temperature (e.g. 35°C to 45°C in 20% to 50% formamide) with washes conducted at a low intermediate temperature (e.g. 40 to 55°C) and in a wash solution having a higher salt concentration (e.g. 2X to 6X SSC) . Moderate stringency conditions are preferred for use in conjunction with the disclosed polynucleotide molecules as probes to identify clones encoding nucleoside diphosphate kinases of the invention. Also contemplated herein is a variant of an illustrated sequence that is an isolated and purified DNA or RNA segment (nucleic acid) that contains a nucleotide sequence that is at least 85 percent, and more preferably at least 90 percent identical, and most preferably at least 95 percent identical to a DNA sequence for NDPK shown in SEQ ID NOs : 6 , 13, 26, 27 or 28. Such a nucleotide sequence hybridizes non- randomly under moderate stringency conditions to a nucleic acid of SEQ ID Nos : 6 , 13, 26, 27 or 28 and expresses biologically active NDPK as discussed before when present in a host cell as part of a plasmid or integrated into the host genome. In living organisms, the amino acid residue sequence of a protein or polypeptide is directly related via the genetic code to the deoxyribonucleic acid (DNA) sequence of the gene that codes for the protein. Thus, through the well-known degeneracy of the genetic code additional DNAs and corresponding RNA sequences can be prepared that encode the same NDPK amino acid residue sequences, but are sufficiently different from a before-discussed gene sequence that the two sequences do not hybridize at high stringency, but do hybridize at moderate stringency. A DNA sequence or an RNA sequence that (1) itself encodes, or its complement encodes, an enzyme molecule exhibiting the biological activity of a NDPK molecule expressed by a DNA sequence of SEQ ID NOs: 6, 13 or 26 and (2) hybridizes with a DNA sequence of SEQ ID NO: 6, 13, 26, 27 or 28; at least at moderate stringency and (3) shares at least 80 percent, and more preferably at least 90 percent, and even more preferably at least 95 percent, and most preferable 100 percent identity with a DNA sequence of SEQ ID NOs: 6, 13, 26, 27 or 28, is defined as a DNA variant sequence. As is well-known, a nucleic acid sequence such as a contemplated nucleic acid sequence is expressed when operatively linked to an appropriate promoter in an appropriate expression system as discussed hereinafter. An analog or analogous nucleic acid (DNA or RNA) sequence that encodes the above enzyme is also contemplated as part of this invention. An analog or its complementary nucleic acid sequence encodes an amino acid residue sequence that is at least 80 percent, and more preferably at least 90 percent, and most preferably at least 95 percent identical to that of an NDPK shown in SEQ ID NOs : 3 or 14 or its complement. This DNA or RNA is referred to herein as an "analog of" or "analogous to" a sequence of a nucleic acid of SEQ ID NOs : 3 or 14, and hybridizes with the nucleic acid sequence of SEQ ID NOs : 6 , 13, 26 or their complements herein under moderate stringency hybridization conditions. A nucleic acid that encodes an analogous sequence with the necessary regulatory sequences, upon suitable transfection and expression, also produces biologically active NDPK; i.e., an enzyme that (1) uses NTPs or dNTPs as a phosphate source to convert ADP to ATP and (2) exhibits higher NDPK activity at a temperature of about 50°C to about 90°C than at 37°C. In an exemplary situation, an analog of a DNA of SEQ ID NO : 6 and its complement could contain 501 base pairs that encode the exact 167 amino acid residue sequence of SEQ ID NO: 3, and have most residues encoded by a codon (three base nucleic acid sequence) different from those shown in SEQ ID NO : 6 , just because of the well-known degenerate nature of the genetic code often referred to as wobble in the third position of the codon. That different DNA molecule could have less than about 85 percent identity with the sequence of SEQ ID NO : 6 and still encode a contemplated NDPK molecule. Different hosts often have preferences for a particular codon to be used for encoding a particular amino acid residue. Such codon preferences are well known and a DNA sequence encoding a desired NDPK sequence can be altered, using in vi tro mutagenesis for example, so that host- preferred codons are utilized for a particular host in which the enzyme is to be expressed. In addition, one can also use the degeneracy of the genetic code to encode a sequence of SEQ ID Nos : 3 or 14 that avoids substantial identity with SEQ ID Nos : 6 or 13. Thus, a useful analogous DNA sequence need not hybridize with the nucleotide sequences of SEQ ID Nos: 6, 13, 26, 27 or 28 under conditions of moderate stringency, but it likely would hybridize under moderate conditions. That a DNA sequence variant or analog encodes a "biologically active" enzyme or an enzyme having "the biological activity" is determined by whether the variant or analog DNA sequence expresses an enzyme that has the biochemical functions of a naturally occurring NDPK molecule; i.e. the expressed enzyme converts ADP to ATP using dNTPs or NTPs as the phosphate source as discussed herein. Thus, a DNA analog or variant sequence that expresses a NDPK molecule that converts provided ADP into ATP via NTPs or dNTPs as in Reaction 1 is defined as biologically active. Expression of biologically active NDPK from a variant or analog DNA sequence can be assayed by the production of ATP or removal of the phosphate from the NTP or dNTP donor. An isolated and purified DNA segment of the invention thus includes a NDPK nucleotide sequence of SEQ ID NOs: 6, 13, or 26 or their complements (including SEQ ID NOs 27 and 28) , and DNA variants or analogs thereof . A recombinant DNA molecule, comprising a vector operatively linked to an exogenous DNA segment defining a gene that can express biologically active Pfu NDPK, as discussed above, and a promoter suitable for driving the expression of the gene in a compatible host organism, is also contemplated in this invention. More particularly, also contemplated is a recombinant DNA molecule that comprises a vector comprising a promoter for driving the expression of the enzyme in host organism cells operatively linked to a DNA segment that contains at least 486 base pairs (501 bp from start through stop codons) that define a gene for the Pyrococcus furiosis enzyme NDPK or a DNA variant that has at least 90 percent identity to the NDPK gene of SEQ ID NOs : 6 or 13 and hybridizes with that gene under moderate stringency conditions comprising hybridization at a temperature of about 50°C to about 65°C in 0.2 to 0.3 M NaCl, followed by washing at about 50°C to about 55°C in 0.2X SSC, 0.1% SDS. The nucleotide segment thus encodes an enzyme that uses NTPs or dNTPs as a phosphate source to convert ADP to ATP with a higher activity at a temperature of about 50°C to about 90°C than the NDPK activity at 37°C. A particularly preferred DNA segment is the at least 486 bp segment of SEQ ID NO: 13 and its complement, SEQ ID NO: 28. Further contemplated is a recombinant DNA molecule that comprises a vector containing a promoter for driving the expression of a NDPK enzyme in host organism cells operatively linked to a DNA segment that is an analog nucleic acid sequence that encodes an amino acid residue sequence that is at least 80 percent identical, more preferably 90 percent identical, and most preferably 95 percent identical to the sequence of a Pyrococcus furiosis NDPK of SEQ ID NOs : 3 or 14. That recombinant DNA molecule, upon suitable transfection and expression a host, provides an enzyme that (1) uses NTPs or dNTPs to convert ADP to ATP and (2) exhibits higher NDPK activity at a temperature of about 50°C to about 90°C than at 37°C. The previously described DNA segments are noted as having a minimal length, as well as total overall lengths. That minimal length defines the length of a DNA segment having a sequence that encodes the enzyme. Inasmuch as the coding sequences for the gene disclosed herein is illustrated m SEQ ID NOs: 6, 13, 26, 27 or 28, isolated DNA segments, variants and analogs thereof can be prepared by in vi tro mutagenesis, as is well known m the art and discussed m Current Protocols In Molecular Biology, Ausabel et al . eds . , John Wiley & Sons (New York: 1987) p. 8.1.1-8.1.6, that begin at the initial ATG codon for a gene and end at or ust downstream of the stop codon for each gene. Thus, a desired restriction site can be engineered at or upstream of the initiation codon, and at or downstream of the stop codon so that other genes can be prepared, excised and isolated. As is well known in the art, so long as the required DNA sequence is present, (including start and stop signals) , additional base pairs can usually be present at either end of the segment and that segment can still be utilized to express the protein. This, of course, presumes the absence m the segment of an operatively linked DNA sequence that represses expression, expresses a further product that consumes the enzyme desired to be expressed, expresses a product that consumes a wanted reaction product produced by that desired enzyme, or otherwise interferes with expression of the gene of the DNA segment . Thus, so long as the DNA segment is free of such interfering DNA sequences, a DNA segment of the invention can be about 500 to about 15,000 base pairs in length. The maximum size of a recombinant DNA molecule, particularly an expression vector, is governed mostly by convenience and the vector size that can be accommodated by a host cell, once all of the minimal DNA sequences required for replication and expression, when desired, are present. Minimal vector sizes are well known. Such long DNA segments are not preferred, but can be used. DNA segments that encode the before- described enzyme can be synthesized by chemical techniques, for example, the phosphotriester method of Matteucci et al . , J " . Am. Chem . Soc , 103 : 3185 (1981) . (The disclosures of the art cited herein are incorporated herein by reference.) Of course, by chemically synthesizing the coding sequence, any desired modifications can be made simply by substituting the appropriate bases for those encoding the native amino acid residue sequence. However, DNA segments including sequences discussed previously are preferred. Furthermore, DNA segments containing a gene encoding the enzyme can be obtained from recombinant DNA molecules (plasmid vectors) containing that gene. Vectors capable of directing the expression of a NDPK gene is referred to herein as an "expression vector" . An expression vector contains expression control elements for the transcription and translation of the inserted coding region, including the promoter. The enzyme-coding gene is operatively linked to the expression vector to permit the promoter sequence to direct RNA polymerase binding and expression of the NDPK gene. Useful m expressing the polypeptide coding gene are promoters which are mducible, viral, synthetic, constitutive as described by Poszkowski et al . , EMBO J. , 3:2719 (1989) and Odell et al . , Na ture, 313:810 (1985), or temporally regulated, spatially regulated, or spatiotemporally regulated as given m Chua et al . , Science, 244 : 174-181 (1989) . A preferred promoter is mducible by an exogenously supplied agent. Methods which are well known to those skilled m the art can be used to construct expression vectors containing sequences encoding NDPK and appropriate transcriptional and translational control elements. These methods include in vi tro recombinant DNA techniques, synthetic techniques, and m vivo genetic recombination. Such techniques are described J. Sambrook et al . , Molecular Cloning, A Laboratory Manual , Cold Spring Harbor Press (Plamview, NY. : 1989) . A variety of expression vector/host systems can be utilized to contain and express sequences encoding NDPK. These include, but are not limited to, microorganisms such as bacteria transformed with recombinant bacteriophage, plasmid, or cosmid DNA expression vectors; yeast transformed with yeast expression vectors; insect cell systems infected with virus expression vectors (e.g. baculovirus) ; plant cell systems transformed with virus expression vectors (e.g. cauliflower mosaic virus; tobacco mosaic virus) or with bacterial expression vectors (e.g., Ti or pBR322 plasmids) ; or animal cell systems . The invention is not limited by the host cell employed. In one preferred embodiment, a vector includes a prokaryotic replicon; i.e., a DNA sequence having the ability to direct autonomous replication and maintenance of the recombinant DNA molecule extrachromosomally in a prokaryotic host cell transformed therewith. Such replicons are well known in the art . Those vectors that include a prokaryotic replicon can also include a prokaryotic promoter region capable of directing the expression of a contemplated NDPK gene in a host cell, such as E. coli , transformed therewith. Promoter sequences compatible with bacterial hosts are typically provided in plasmid vectors containing one or more convenient restriction sites for insertion of a DNA segment of the present invention. Representative of such vectors are pUC8, pUC9, and pBR329 available from Biorad Laboratories, (Richmond, Calif.) and pPL and pKK223-3 available from Pharmacia, Piscataway, NJ. One preferred promoter for use in prokaryotic cells such as E. coli is the Rec 7 promoter that is inducible by exogenously supplied nalidixic acid. A more preferred promoter is present in plasmid vector JHEX25 (Promega Corp.) or a tac promoter, either are inducible by exogenously supplied isopropyl-β-D-thiogalacto-pyranoside (IPTG) . pGEX vectors (Promega Corp.) can also be used to express foreign polypeptides as fusion proteins with glutathione S-transferase (GST) . In general, such fusion proteins are soluble and can easily be purified from lysed cells by adsorption to glutathione-agarose beads followed by elution in the presence of free glutathione. Proteins made such systems can be designed to include heparm, thrombm, or factor XA protease cleavage sites so that the cloned polypeptide of interest can be released from the GST moiety if desired. Expression vectors compatible with eukaryotic cells, preferably those compatible with yeast cells or more preferably those compatible with cells of higher plants or mammals, are also contemplated herein. In the yeast, Saccharomyces cerevisiae, a number of vectors containing constitutive or mducible promoters such as alpha factor, alcohol oxidase, and PGH can be used. For review see Grant et al . , Methods Enzymology, 153 : 516- 544 (1987) . In cases where plant expression vectors are used, the expression of sequences encoding NDPK can be driven by any of a number of promoters . For example, viral promoters such as the 35S and 19S promoters of Cauliflower Mosaic Virus can be used alone or m combination with the omega leader sequence from Tobacco Mosaic Virus (Takamatsu, N. (1987) EMBO J. , 6:307-311). Alternatively, plant promoters such as the small subunit of RUBISCO or heat shock promoters may be used. G. Coruzzi, et al., EMBO J. , 3:1671-1680 (1984); and J. Winter et al., Resul ts Probl . Cell Differ . , 17:85-105 (1991) . These constructs can be introduced into plant cells by direct DNA transformation or pathogen-mediated transfection. Such techniques are described m the review by S . Hobbs m McGraw Hill Yearbook of Science and Technology, McGraw Hill (New York, NY: 1992) pp. 191-196. Eukaryotic cell expression vectors are well known m the art and are available from several commercial sources. Normally, such vectors contain one or more convenient restriction sites for insertion of the desired DNA segment and promoter sequences. Optionally, such vectors contain a selectable marker specific for use in eukaryotic cells. The choice of which expression vector and ultimately to which promoter a NDPK-coding gene is operatively linked depends directly on the functional properties desired, e.g. the location and timing of protein expression, and the host cell to be transformed. These are well known limitations inherent in the art of constructing recombinant DNA molecules. However, a vector useful in practicing the present invention can direct the replication, and preferably also the expression (for an expression vector) of the NDPK gene included in the DNA segment to which it is operatively linked. An insect system can also be used to express NDPK. For example, in one such system Autographa calif ornica nuclear polyhedrosis virus (AcNPV) is used as a vector to express foreign genes in Spodoptera frugiperda cells or in Trichoplusia larvae. The sequences encoding NDPK can be cloned into a non-essential region of the virus, such as the polyhedrin gene, and placed under control of the polyhedrin promoter. Successful insertion of NDPK renders the polyhedrin gene inactive and produces recombinant virus lacking coat protein. The recombinant viruses can then be used to infect, for example, S . Frugiperda cells or Trichoplusia larvae in which NDPK may be expressed. E. Engelhard et al . , Proc . Na tl . Acad . Sci . , USA, 91:3224-3227 (1994) . In mammalian host cells, a number of viral - based expression systems can be utilized such as cytomegalovirus promoter, SV40 or RSV promoters. Exemplary systems are the pCI and pCI-neo vectors (Promega Corp., Madison, Wis.) . The use of retroviral expression vectors to form the recombinant DNAs of the present invention is also contemplated. As used herein, the term "retroviral expression vector" refers to a DNA molecule that includes a promoter sequence derived from the long terminal repeat (LTR) region of a retrovirus genome. The construction and use of retroviral vectors has been described by Verma, PCT Publication No. W087/00551, and Cocking et al , Sci ence, 236:1259-62 (1987). Specific initiation signals can also be used to achieve more efficient translation of sequences encoding NDPK. Such signals include the ATG initiation codon and adjacent sequences. In cases where sequences encoding NDPK, its initiation codon, and upstream sequences are inserted into the appropriate expression vector, no additional transcriptional or translational control signals may be needed. However, m cases where only coding sequence, or a fragment thereof, is inserted, exogenous translational control signals including the ATG initiation codon should be provided. Furthermore, the initiation codon should be m the correct reading frame to ensure translation of the entire and correct insert. Exogenous translational elements and initiation codons can be of various origins, both natural and synthetic. The efficiency of expression can be enhanced by the inclusion of enhancers that are appropriate for the particular cell system which is used. D. Scharf, et al . , Resul ts Probl . Cell Di ffer . 20:125-162 (1994) . In addition, a eukaryotic host cell strain can be chosen for its ability to modulate the expression for the inserted sequences or to process the expressed protein in a desired manner. Different host cells which have specific cellular machinery and characteristic mechanisms for post translational activity (e.g. CHO, HeLa, MDCK, HEK293, and W138) , are available from the American Type Culture Collection (ATCC; Bethesda, Md.) and can be chosen to ensure correct modification and processing of the foreign protein. A contemplated NDPK, such as Pfu NDPK itself, is advantageously utilized in a so-called one-step or one-pot pyrophosphorolysis method of depolymerization. Here, a treated sample that may contain the predetermined nucleic acid target sequence hybridized with a nucleic acid probe that includes an identifier nucleotide in the 3 ' -terminal region is admixed with (i) a depolymerizing amount of an enzyme whose activity in the presence of pyrophosphate is to release identifier nucleotides as nucleoside triphosphates (XTPs) from the 3 ' -terminus of hybridized nucleic acid probe, (ii) adenosine 5' diphosphate (ADP) , (iii) pyrophosphate and (iv) NDPK to form a treated reaction mixture. The treated reaction mixture so formed is maintained for a time period sufficient to permit the enzyme to depolymerize the probe to form XTP molecules and to permit NDPK to transfer the phosphate of the XTP present to added ADP and form ATP as shown in Reaction 1. The amount of ATP formed is determined by the production of an analytical output, with that output providing the indication of the presence or absence of the presence of the target nucleic acid sequence. Although yeast, bovine or another NDPK can be used in these reactions, it is preferred to utilize a contemplated thermostable Pfu NDPK along with a thermostable depolymerizing enzyme such as the Tne triple mutant DNA polymerase (discussed below) , Bst DNA polymerase, Ath DNA polymerase, Tag DNA polymerase and Tvu DNA polymerase along with a reaction temperature of about 50°C to about 90°C. The use of these thermostable enzymes at an above temperature can provide an easier assay procedure by permitting both enzymes to be present together in a single reaction vessel while double-stranded DNA is denatured at such an elevated temperature to hybridize with a nucleic acid probe, and enhance the sensitivity of the method. The Trie triple mutant DNA polymerase is described in detail in WO 96/41014, whose disclosures are incorporated by reference, and its 610 residue amino acid sequence is provided as SEQ ID NO: 35 of that document. That enzyme is referred to in WO 96/41014 as Tne M284 (D323A, D389A) . Briefly, that enzyme is a triple mutant of the polymerase encoded by the thermophilic eubacterium Ther otoga neapoli tana (ATCC 49049). The amino-terminal 283 residues of the native sequence are deleted and the aspartic acid residues at positions 323 and 389 of the native sequence are replaced by alanine residues in this recombinant enzyme. This recombinant enzyme is thus a deletion and replacement mutant of the native enzyme. Deletion of the amino-terminal sequence removes the 5' exonuclease activity of the native enzyme, whereas replacement of the two aspartic acid residues removes a magnesium binding site whose presence facilitates exonuclease activity, and this triple mutant also exhibited no 3 ' exonuclease activity relative to the recombinant native enzyme. This triple mutant enzyme exhibited a half-life at 97.5°C of 66 minutes as compared to the full length recombinant enzyme that exhibited a half-life of only 5 minutes at that temperature . A reaction utilizing NDPK typically contains about 0.01 to 0.50 μM ADP, preferably about 0.05 μM ADP. Various useful buffers and other reaction components are set forth elsewhere. NDPK is itself present in an amount sufficient to catalyze the desired conversion of ADP to ATP. In a typical assay starting from a 20 μL depolymerization reaction, about 0.1 U of NDPK are used. Where larger volumes of reactants are used, with the target and probe concentrations being approximately proportionately larger, the amount of NDPK or the other enzymes discussed herein can be used in a similar larger proportion relative to the amount discussed for the 20 μL reaction. Indeed, a 20 μL reaction has been successfully scaled down about two fold and scaled upwardly by a factor of about 20. In some preferred embodiments, the pyrophosphorolysis reaction producing dNTP and the NDPK catalyzed reaction in which the NTPs or dNTPs are converted to ATP are performed in a single pot reaction in the nucleic acid polymerase buffer in these embodiments. NDPK activity is sufficient to convert dNTP to ATP, even though the polymerase buffer conditions are suboptimal for NDPK activity. The polymerase enzyme and NDPK can both be present initially in the reaction, or the NDPK can be added directly to the reaction after an incubation period sufficient for the production of NTP or dNTP. Alternatively, a nucleic acid polymerase and NDPK can be provided m the same vessel or mixture for use m the reactions described above. The mixture preferably contains the nucleic ac d polymerase and NDPK m a concentration sufficient to catalyze the production of ATP when m the presence of a nucleic acid, pyrophosphate and ADP. Preferably, the polymerase is provided m a concentration of about 0.1 to 100 U/reaction (i.e., where "U" is units) most preferably at about 1 U/reaction. Preferably, the NDPK is provided m a concentration of 0.1 to 100 U/reaction, most preferably at about 0.1 U/reaction. In further preferred embodiments, the mixture is substantially free of contaminating ATP. Example 1 : Cloning and Expression of a Gene Encoding a NDPK Enzyme from Thermophilic Bacteria Pyrococcus furiosis The cloning and expression of a gene from the thermophilic bacteria Pyrococcus furiosis [Pfu; Pfu-Vcl (DSM 3638)] is described in this Example. This gene encodes a nucleoside diphosphate kinase (NDPK) enzyme . The protein originates from a thermophile and remains active at elevated temperatures for a longer period of time than the corresponding protein from a mesophilic organism. The protein also remains stable at room temperature longer than the corresponding mesophilic enzyme. A protein that is stable at elevated temperature, can function in combination with a thermostable polymerase in a pyrophosphorylation reaction, thereby eliminating the need to carry out separate pyrophosphorylation and phosphate transfer steps as needed for the NDPK derived from yeast . The known amino acid sequences of NDPKs ( Gene, 129:141-146, [1993]; and a putative NDPK from Pyrococcus horikoshii [www.ncbi.nlm.nih.gov/Entrez/ Genome/org .html] ) were compared and segments of high amino acid homology identified. Two degenerate DNA primers were designed that permit the DNA between them to be amplified. These primers, Pfl (SEQ ID NO: 10) and Pf2 (SEQ ID N0:11), are shown below, and were dissolved in TE buffer (10 mM Tris, 1 mM EDTA pH 8.0) . A "6" in the primer sequence indicates an inosine that can hybridize to any base. Chromosomal DNA from Pfu was isolated by resuspending frozen cell paste in 1 mL TE buffer (10 mM Tris, 1 mM EDTA) , lysing the cells by beating with zircon beads, followed by two phenol extractions and a chloroform extraction. The DNA in the supernatant was then ethanol -precipitated, dried, and resuspended m TE buffer overnight (about 18 hours) . The resuspended DNA was treated with 20 units of RNasel, reprecipitated and resuspended m TE buffer. The Pfu genomic DNA was used m the following DNA amplification reaction. 2 μL 1.5 μg Pfu DNA (pre-denatured for 5 minutes at 99°C, then placed on ice) 5 5 μ μLL PCR buffer 4 μL 25 mM MgCl 2 2 μL each primer Pfl and Pf2 (200 picomoles each) 2 255 μuLL water 1 μL Taq (5 units) 10 μL 5M betame Different extension temperatures m the range from 41°C to 55°C were examined m the following PCR profile: 94°C, 2 minutes ; (94°C, 15 or 40 seconds; 45°C to 55°C, 45 or 90 seconds; 72°C, 1 or 2 minutes) x 20; 72°C, 2 minutes. The profile varied for the different extension temperatures, with 41°C and 43°C extension temperatures having the lesser times, and the remaining extension temperatures having the longer times. The reaction products were analyzed by gel electrophoresis on a 1.2% TBE agarose gel. The products of the reaction were detected by staining the gel with ethidium bromide and photographing the gel under UV light. A 300 bp DNA fragment was identified as the product of the reaction and was present to a greater extent when using extension temperatures from 41°C to 47°C. The 300 bp fragment was gel purified (Promega, A7170) and cloned into pGEM-T vector (Promega, A3600) . The sequence of the insert was determined and found to encode an open reading frame . The translated amino acid sequence of this open reading frame matched the protein sequence of the Pyrococcus horikoshii NDPK gene with 94% homology. A hybridization probe, Pf3 (SEQ ID NO:12), was designed from the sequence obtained. This probe was 32 P labeled and used to identify the size of the DNA fragments encoding the corresponding gene in chromosomal digests of the DNA from Pfu using standard Southern blot hybridization methods. From this analysis, an EcoR I fragment about 2 kb in size was identified as a target for additional cloning. A size-specific EcoR I library of DNA fragments from Pfu was produced by digesting Pfu chromosomal DNA with EcoR I, fractionating the DNA fragments using agarose gel electrophoresis, identifying the segment of the fractionated DNA that corresponded to the 2 Kb EcoR I fragment identified as containing the desired gene and isolating the DNA from the gel. The isolated DNA was cloned into plasmid pZER02 (Invitrogen) , and the resulting library was transformed into E. coli JM109 (Promega Corp., L2001) (Invitrogen). The transformants were probed using the same probe employed during Southern hybridization and two clones were identified as potential candidate clones. The sequences of the two candidate clones were found to contain the exact sequence present in the 300 bp DNA segments sequenced earlier in addition to DNA sequences both 5' and 3' to that sequence. The open reading frame identified earlier was found to extend significantly beyond the limits of the 300 bp segment sequenced earlier. The additional segments of the open reading frame again showed good homology with the published Pho NDPK nucleic acid sequence. The Pfu NDPK nucleotide sequence is identified as Sequence Pf4 (SEQ ID NO: 13) and the corresponding amino acid sequence is identified as Pf5 (SEQ ID NO: 14) . The protein codes for 161 amino acid residues. The coding segments of the gene were amplified using primers Pf6 (SEQ ID NO: 15) and Pf7 (SEQ ID NO:16), and placed into a high protein expression vector JHEX25 (Promega Corp.) for E. coli . The vector contains an IPTG inducible promoter system. Primer Pf6 contains a Bsp HI restriction site (underlined below) that is also compatible with Nco I, whereas primer Pf7 contains a Xba I site (underlined below) . The amplified DNA was cut with BspH I and Xba I, whereas the vector was cut with Nco I and Xba I, and the DNA was ligated into the cut vector. The vector was transfected into E. coli JM109 (Promega Corp., L2001) using standard procedures. Bacterial transformants were grown in LB media and induced for protein expression. Samples of the induced bacterial cultures were boiled in 2X SDS Sample buffer and loaded onto a SDS gel . After running, the gel was stained with Coomassie Blue. After destaining in 1% acetic acid and 10% methanol, the lanes containing extracts from cells with the open reading frame were found to contain a large amount of a protein of about 14 Kd, the expected size of the gene product from the insert. Then, a comparison of the open reading frame to the published sequence of the Pfu genome [www.genome.Utah.edu] was performed and the open reading frame was found to exactly match a region of the genome of this organism. Pfl 5' AT6AT6AA(AG)CC6GA(TC)G(GC) 6GT 3' SEQ ID NO: 10 Pf2 5' AA(AG) TC6CC6C (TG) 6AT6GT6CC6GG 3' SEQ ID NO: 11 Pf3 5' GAGAAGCACTATGAGGAGCAC 3' SEQ ID NO: 12 Pf4 5 ' ATGAACGAAGTTGAAAGAACATTGGTAATCATAAAGCCCGACGCAGTAGTT AGGGGTCTAATAGGTGAAATTATAAGCAGGTTTGAGAAGAAAGGCCTCAAGAT TGTTGGAATGAAGATGATCTGGATAGACAGGGAGTTGGCTGAGAAGCACTATG AGGAGCACAAAGGAAAGCCCTTCTTTGAGGCTCTCATAGATTACATAACGAAA GCTCCAGTAGTTGTTATGGTGGTTGAGGGAAGGTATGCAGTAGAAGTAGTT AGAAAGATGGCTGGAGCTACTGATCCAAAGGACGCAGCACCTGGGACAATTAG GGGAGATTATGGACTTGACATAGGAGATGCAATCTACAACGTGATTCATGC CAGTGATTCAAAGGAAAGTGCGGAGAGGGAAATAAGCCTGTACTTTAAACCTG AAGAAATTTATGAATACTGCAAAGCTGCAGATTGGTTTTACAGGGAAAAGAAG CAGGCTAAATGCTGA 3' SEQ ID NO: 13 Pf5 MNEVERTLVIIKPDAWRGLIGEIISRFEKKGLKIVGMKMIWIDRELAEKHYE EHKGKPFFEALIDYI TKAPVWMWEGRYAVEWRKMAGATDPKDAAPGTIRG DYGLDIGDAIYNVIHASDSKESAEREI SLYFKPEEIYEYCKAADWFYREKKQA KC SEQ ID NO : 14 Pf 6 5 ' GGGTGCTTTTCATGAACGAAGTTGA 3 ' SEQ ID NO : 15 Pf7 5' AAGGGCAAAAATTCTAGAGTTCAGCAT 3' SEQ ID NO: 16 Example 2 : Purification of Cloned Pfu NDPK Protein from E. coli An initial fermentation of JM109 (Promega Corp., L2001) E. coli cells expressing the Pfu NDPK protein, as described in Example 1 yielded about 10 g of wet cell paste. The protein purification scheme was essentially that as described in S. Kim et al . , Molecules and Cells, 7:630 (1997). One gram of cell paste was resuspended in 10 mL of 20 mM Tris-acetate pH 7.4/1 mM EDTA/2 μg/mL aprotinin/0.1 mg/mL lysozyme and incubated at room temperature for 10 minutes. The suspension was then sonicated for 2 minutes at 50% cycle, held on ice for 5 minutes, then sonicated an additional 2 minutes. The suspension was centrifuged at 15,000 x g for 20 minutes at 4°C and the supernatant transferred to a new tube. The supernatant was heated to 80°C for 20 minutes to denature non-thermostable proteins. Precipitant was pelleted by centrifugation at 14,000xg for 20 minutes at 4°C and supernatant was transferred to a new tube. Ten milliliters of supernatant were applied to a 5 mL ATP-sepharose (Sigma, A-9264) affinity column equilibrated with Buffer A (20 mM Tris-acetate pH 7.4/20 mM NaCl/0.1 mM EDTA/3 mM MgCl 2 /l5 mM BME) . The flow through was collected by gravity. The column was washed with 6 column volumes of Buffer B (Buffer A containing 500 mM NaCl) . The bound protein was eluted in two steps: 5 mL Buffer B + 1 mM dCTP (Promega, U122A) followed by 5 mL of Buffer B + 1 mM ATP (Sigma, A-7699) . SDS-PAGE analysis of the purification fractions showed a large loss of total protein following the heat denaturation step, with the NDPK being the major band loaded on the column. About 50% of the loaded NDPK was in the flow-through fraction. Eluted NDPK appeared in both the dCTP and ATP elutions at greater than 80% purity. Example 3 : Thermostable NDPK Activity Assays Activity Assay The activity assay for NDPK measures ATP created following phosphate transfer from dCTP to ADP. A linear range for the amount of enzyme was determined using yeast NDPK in a 10 minute assay at 37°C and was found to be 0.002 - 0.0002 units, or 0.012 - 1.2 ng of protein. The optimal concentrations of ADP and dCTP in the assay were found to be 100 nM and 10 μM respectively, in order to give Turner ® TD20/20 luminometer readings within a readable scale without further dilution. The optimal concentrations of ADP and dCTP if diluting the reaction prior to luminometer reading are 10 μM ADP and 100 μM dCTP . The Pfu NDPK activity in the dCTP and ATP eluted fractions, as described in Example 2, was examined after extensive dialysis of the fractions to remove nucleotides. Both the dCTP and ATP elutions of the purified Pfu NDPK were also passed over a De- Salt™ column (Pierce, 43230) according to manufacturer's recommendations to further remove excess nucleotides. Activity of Pfu NDPK was measured in a 10 minute assay at both 37°C and 70°C. Activity was observed at both temperatures. If full enzymatic activity is presumed at the 70°C optimum, then about 40% of that activity was seen at the lower temperature. The estimated unit activity for the fractions was determined by comparison of the light output, resulting from ATP formation, of yeast NDPK at 37°C with the light output of the Pfu NDPK at 70°C. For example: if 0.0002 units of yeast NDPK provides 7000 relative light units after 10 minutes at 37°C, then 0.0002 units of Pfu NDPK is presumed to provide 7000 relative light units after 10 minutes at 70°C. Using this unit equivalency based on light output, the dCTP and ATP Pfu NDPK fractions were assigned a unit activity of 0.5 units/μL. Activity Assay at two temperatures The activity levels of the Pfu NDPK from both the ATP- and the dCTP-eluted fractions were compared to the activity level of the yeast NDPK at both 70°C and 37°C. A series of 10-fold serial dilutions of the three enzyme solutions was made in Nanopure water to a final dilution of 1:10,000. The following master mix was prepared: 889 μL Nanopure water (Promega AA399) 100 μL 10X DNAP Buffer (Promega M195A) 10 μL 10 μM ADP (Promega, A-5285) 1 μL 10 μM dCTP (Promega, U128B) Into each reaction tube were placed 180 μL of master mix preheated to either 37°C or 70°C and 20 μL of each 1:10,000 NDPK dilution, and tubes were incubated at the indicated temperature. Twenty microliter samples were removed at various time points, added to 100 μL of L/L reagent (Promega, F202A) and light units read in a TMDE™ luminometer. The t=0 time point was not incubated at elevated temperatures and was placed on ice. The following data were obtained: Time (minutes) 70°C 37°C Yeast Zero 4896 NDPK 1.0 5126 2.5 5163 5 6946 10 6687 7503 15 6735 20 6806 25 7298 Pfu 0 327 NDPK 1.0 749 (dCTP) 2.5 1772 5 2794 10 3191 111 15 4364 20 5025 25 5830 Pfu 0 1255 NDPK 1.0 2235 (ATP) 2.5 4410 5 5925 10 6039 973 15 6828 20 7747 25 10026 No NDPK 0 56 The Pfu NDPK is more active at 70°C than at 37°C. This is evident by comparing relative light units at 10 minutes activity at 37°C and 70°C (1:10,000 dilution). There is about 10-fold more yeast protein in the 70°C reaction than Pfu protein as determined by a standard Bradford assay. Therefore, the Pfu NDPK enzyme appears to have a higher specific activity. For the 37°C assay, the yeast NDPK was further diluted to 1:100,000 and produced 247 light units at this dilution. The light units increased slightly and then leveled off for the yeast enzyme, suggesting thermal inactivation of the enzyme at 70°C. The Pfu NDPK light output increased over time. Thermostability of Pfu NDPK and Yeast NDPK Yeast NDPK stock at 1 unit/μL was serially diluted in Nanopure water to a 1:100,000 final dilution. The dCTP- and ATP-eluted Pfu NDPK stock were serially diluted in Nanopure water to a 1:10,000 final dilution. This equalized the amount of protein present in the Pfu NDPK and Yeast NDPK final dilutions, as determined by standard Bradford protein assay and SDS-PAGE analysis. Two microliters of the diluted enzymes were added to 18 μL of master mix in chilled tubes and placed on ice. These are the t=0 time points. The remainder of the diluted NDPK solutions were pre- warmed at 70°C. For each time point, a 2 μL aliquot of the enzyme dilution was added to 18 μL master mix and then placed on ice. After the t=10 minutes time point , all tubes were incubated at 37°C for 10 minutes. Then, 100 μL of L/L reagent were added to each reaction and the relative light units measured on a TMDE™ luminometer. The following results were obtained. Relative Light Units Time Pfu NDPK Pfu NDPK (minutes) Yeast (dCTP) (ATP) 0 1877 446 615 1 263 358 400 2 47 299 472 3 42 319 446 4 40 296 432 5 38 315 353 7.5 36 241 339 10 37 239 307 The yeast NDPK appears to be thermolabile, whereas the Pfu NDPK is relatively thermostable. The purified Pfu NDPK had a half-life of about 10 minutes, whereas the yeast NDPK had a half -life of about 0.6 minutes . Example 4: One Step Interrogation on β-globin PCR Targets: Comparison of 70°C Reaction Using Tne Triple Mutant Polymerase and Pfu NDPK to 37°C Reaction Using Klenow exo- Polymerase and Yeast NDPK An assay for native and mutant sequences of β-globin was carried out using a thermostable polymerase ( Tne triple mutant) and thermostable NDPK (Pfu) . The results of that assay were compared to those obtained using a more thermally labile polymerase (Klenow exo-) and NDPK (yeast) . These methods were carried out at 70°C and 37°C. As will be seen from the results that follow, use of the thermostable enzymes permit a one step (or one-pot) reaction in which depolymerization and ATP formation are carried out at an elevated temperature. The DNA interrogation probes used were 9994 (SEQ ID NO:17), 9995 (SEQ ID NO:18), 10665 (SEQ ID NO:19), and 11472 (SEQ ID NO:20). Probes 9994 and 9995 interrogate the TCTT site. Probes 10665 and 11472 interrogate the 17 (A to T) site. PCR probes used were Probe 9992 (SEQ ID NO: 21) and 9993 (SEQ ID NO: 22) . One microliter of the PCR amplified dsDNA sample to be assayed for the presence of the first or second target sequence was admixed with 1 μL of a nucleic acid probe and 18 μL of nanopure water to form separate hybridization compositions. Controls had 1 μL of the PCR amplified dsDNA sample and 19 μL of nanopure water. Four microliters of target solution were used for the set of 37°C reactions, 8 μL target solutions were used for the set of 70°C reactions. The assembled reactions were heated to 95°C for 5 minutes and then cooled to room temperature for 10 minutes to form separate treated samples. Twenty microliters of the 2X master mix (below) were then added. 2X Master 2X Master Mix for 37°C Mix for 70°C 10X DNAP buffer 60 μL (Promega, 100 μL (Promega M195A) M190A) Klenow exo- 3.75 units Tne triple mutant 25 units 40 mM NaPPi 7.5 μL 25 μL yeast NDPK 3 units Pfu NDPK 2.5 units 10 μM ADP 6.0 μL 10 μL Nanopure water 219.75 μL 259.4 μL The 37°C reaction set was incubated at 37°C for 15 minutes, the 70°C reaction set was incubated at 70°C for 5 minutes. Four microliters of each reaction were added to 100 μL of L/L reagent (Promega, F202A) and light output (relative light units; rlu) was immediately measured on a TMDE™ luminometer. Interrogation Oligo rlu rlu Reaction Target 1 μl/1 μg (70°C) (37°C) 1 -- -- 41 5 2 WT* -- 165 94 3 -- WT 9994 51 12 4 -- Mut 9995* 42 6 5 -- Mut 11472 44 6 7 WT WT 9994 645* 360* 8 WT Mut 9995 199* 109* 9 WT WT 10665 525* 233* 10 WT Mut 11472 169* 121* *WT = wild typ<s; Mut = =mutant; Average of two values . The high temperature interrogation conditions improve the discrimination ratios between wild type and mutant for the 17 (A to T) site primarily by reducing the background signal from the mismatch. Discrimination ratios at the TCTT site are essentially the same between the two temperatures. Interrogation Probes: 9994 5' CCCTTGGACCCAGAGGTTCT 3' SEQ ID NO: 17 9995 5' CCCTTGGACCCAGAGGTTGA 3' SEQ ID NO: 18 10665 5' CTTCATCCACGTTCACCTTG 3' SEQ ID NO: 19 11472 5' CTTCATCCACGTTCACCTAG 3' SEQ ID NO: 20 PCR Target Probes : 9992 5' GTACGGCTGTCATCACTTAGACCTCA 3' SEQ ID NO: 21 9993 5' TGCAGCTTGTCACAAGTGCAGCTCACT 3' SEQ ID NO: 22 Example 5: NDPK Transformation of ADP To ATP Using Deoxynucleotides Luciferase can detect ATP at much lower concentrations than dATP or other nucleotides. By using dNTPs to generate ATP, an increase in sensitivity results. In this experiment, the ability of enzymes to transfer the terminal phosphate of dNTPs to ADP, forming ATP and dNDPs, was analyzed. Reactions were assembled which contained 100 μL LAR Buffer, 10 ng luciferase in the presence or absence of dNTPs (lμM final concentration when added), and 10 units of yeast NDPK (Sigma #N0379, Lot #127F81802) . The reactions were assembled with the exception of luciferase and incubated for 15 minutes at room temperature. Luciferase was added and light output (light units) of the reactions was measured immediately using a Turner™ TD-20e Luminometer. The light output values measured are provided in the data table below. These data confirm that NDPK is capable of transferring the phosphate from nucleoside triphosphates to ADP to form ATP, which can be detected using luciferase. Example 6 : Cloning and Expression of a Gene Encoding a NDPK Enzyme from Thermophilic Bacteria Pyrococcus furiosus- 11 The genomic cloning and expression of a DNA segment from the thermophilic bacteria Pyrococcus furiosis [Pfu ; Pfu-Vcl (DSM 3638)] into pZero2 (Invitrogen) is described in Example 1. That nucleic acid, Pf4 (SEQ ID NO:13), isolated from clone CV11, and transferred into expression vector JHEX25 was found to encode an enzyme (SEQ ID NO: 14) that exhibits NDPK activity as demonstrated before. In the work described here, a second cloning of a Pfu DNA segment encoding NDPK activity was performed by PCR amplification of CV11. This second cloning provided an additional 15 nucleotides existing upstream of the 5 ' end of the Pf4 sequence in native Pfu cloned onto the 5 ' end of the Pf4 sequence. These added nucleotides encode an additional five amino acids (MGVLW) of Pfu that are located immediately upstream of the methionine that exists at amino acid residue position one in SEQ ID NO: 14. The original cDNA library of Pfu was constructed in vector pZER02 (Invitrogen) as described previously. PCR amplification primers, 12403 (SEQ ID N0:23) and 10763 (SEQ ID NO:24), were designed to anneal to a Pfu sequence a defined distance upstream from the Pf4 sequence in the CV11 clone. The 12403 primer was constructed to contain a Nco I restriction enzyme site that spans the nucleotides encoding the methionine located five amino acid residues upstream from the methionine at amino acid residue one of SEQ ID NO: 14. Primer 10763 was the same primer used to isolate the Pf4 sequence. This latter primer anneals to Pfu downstream of the 3 ' end of the sequence encoding Pfu NDPK and contains a Xba I restriction enzyme recognition site in order to facilitate cloning of the amplified nucleic acid into expression vector JHEX25. The PCR cloning was performed by first combining the following: Not I -digested CV11 100 ng 2 μL Primer 12403 50 pmol 1 μL Primer 10763 50 pmol 1 μL dNTP mix 10 mM each 1 μL Pfu buffer (Promega Corp.) 10 X 5 μL Pfu DNA polymerase (Promega Corrpp.. )) 1 μL nanopure water 40 μL The relevant Pfu sequence was amplified using the following cycling profile: 96°C x 1 minute, (94°C x 15 seconds, 58°C x 30 seconds, 72°C x 1 minute) x 10 cycles, 72°C x 1 minute. The resulting amplified DNA was then separated on a 1% agarose gel. The single band of about 500 base pairs was obtained as expected and was eluted from the gel using a Wizard ® PCR purification kit according to manufacturer's instructions (Promega Corp., A7170) . The eluted DNA was digested with Nco I and Xba I and then ligated overnight (about 18 hours) at 16°C into expression vector JHEX25 digested with the same enzymes. The ligation mixture was then transformed into JM109 E. coli cells and plated on LB agar plates containing tetracycline. Clones were selected and analyzed by digestion with Nco I and Xba I. This double digest removes a 500 base pair fragment from a correct clone. Two of the clones were sequenced to confirm the presence of the correct and complete nucleotide sequence of Pfu encoding the NDPK enzyme (Pf8, SEQ ID NO: 25) containing two methionines at amino acid residue positions 1 and 5 at the amino-terminus of the enzyme (SEQ ID NO: 3) . The Pfu NDPK enzyme containing one methionine at the 5 ' end is referred to here as PfuNDPK-1, the Pfu NDPK enzyme containing two methionines at the 5' end (amino acids 1 and 5) is referred to here as PfuNDPK-2. Studies were then performed to demonstrate that PfuNDPK-2 is an active enzyme. A 50 mL culture of the Pf8/JHEX25 plasmid in E. coli JM109 was grown in Terrific Broth (Promega, AA363) + tetracycline (10 μg/ml) overnight (about 18 hours) at 37°C. The culture (20 mL) was used to inoculate one liter of Terrific Broth + tetracycline that was further grown for 5 hours at 37°C. The temperature was reduced to 25°C and NDPK expression was induced by addition of IPTG to a final concentration of 1 mM. The culture was grown for an additional 20 hours at 25°C to an ODgQO °f approximately 7.0. The cells were centrifuged and the supernatant discarded. The cell pellet was stored frozen at -80°C. About 5 grams of the cell paste was resuspended in 50 mL of 20 mM Tris-acetate pH 7.4 , 1 mM EDTA, 2 μg/mL aprotinin and 0.1 mg/mL lysozyme. The cell suspension was stirred for 10 minutes at room temperature until homogeneous. The solution was sonicated in two 25 mL aliquots by the following parameters: 35% amplitude, microtip, two minutes total of five seconds on followed by five seconds off. The suspension was centrifuged at 10,000 x g at 4°C for 20 minutes. The supernatant was transferred to a clean tube. The supernatant was heated to 80°C for 20 minutes in a 250 mL flask in an 80°C water bath. The denatured proteins were pelleted by centrifugation at 20,000 x g at 4°C for 20 minutes. The supernatant was transferred to a clean tube. The solution was run on an ATP-sepharose (Sigma, A9264) column and fractions collected as described in Example 2. Fractions 8 through 28 contained the majority of the protein as determined by Coomassie Plus staining and these fractions were pooled, yielding about 35 mL. Five microliters of the pooled fractions were run on a 4- 20% acrylamide gel and stained with Coomassie blue. Both forms of NDPK, PfuNDPK-1 and PfuNDPK-2, were expressed at approximately equal amounts. A doublet of the expected size was observed on the gel, each band of approximately equal intensity. The 35 mL pooled fraction solution was combined with 26.8 grams of solid ammonium sulfate and stirred at room temperature for 15 minutes. The resulting composition was then centrifuged at 14,000 x g at 4°C for 20 minutes to pellet precipitated protein. The pellet was resuspended in 5 mL of TE buffer (10 mM Tris, 1 mM EDTA, pH 8.0). The 5 mL NDPK solution was then added to a 50 mL G-75 Superfine (Amersham Pharmacia Biotech, # 17-0051-01) column previously equilibrated with TE buffer at room temperature. Fractions of eluant were monitored for protein elution using Coomassie Plus reagent. Protein eluted in the fraction around 12 ml of eluant. Two 5 ml fractions of eluant containing the most protein were pooled and demonstrated to have low ATP background by combining 1 μL of eluant with 100 μL of L/L reagent (Promega) and measuring light output on a luminometer. The pooled Pfu NDPK solution (10 mL) was dialysed into storage buffer (10 mM Tris-HCl, pH 7.5 , 1 mM EDTA, 1 mM DTT, 10% glycerol) at 4°C for two hours, and the final yield of protein was determined using Coomassie Plus reagent according to manufacturer's instructions (Pierce) 12403 5' GAGGGAAAACCATGGGGGTGCTTTG 3' (SEQ ID NO: 23) 10763 5' AAGGGCAAAAATTCTAGAGTTCAGCAT 3' (SEQ ID NO:24) Pf8 : ATGGGGGTGC TTTGGATGAA CGAAGTTGAA AGAACATTGG TAATCATAAA 50 GCCCGACGCA GTAGTTAGGG GTCTAATAGG TGAAATTATA AGCAGGTTTG 100 AGAAGAAAGG CCTCAAGATT GTTGGAATGA AGATGATCTG GATAGACAGG 150 GAGTTGGCTG AGAAGCACTA TGAGGAGCAC AAAGGAAAGC CCTTCTTTGA 200 GGCTCTCATA GATTACATAA CGAAAGCTCC AGTAGTTGTT ATGGTGGTTG 250 AGGGAAGGTA TGCAGTAGAA GTAGTTAGAA AGATGGCTGG AGCTACTGAT 300 CCAAAGGACG CAGCACCTGG GACAATTAGG GGAGATTATG GACTTGACAT 350 AGGAGATGCA ATCTACAACG TGATTCATGC CAGTGATTCA AAGGAAAGTG 400 CGGAGAGGGA AATAAGCCTG TACTTTAAAC CTGAAGAAAT TTATGAATAC 450 TGCAAAGCTG CAGATTGGTT TTACAGGGAA AAGAAGCAGG CTAAATGCTG 500 A 501 (SEQ ID NO:25) PfuNKl : MNEVERTLVI IKPDAWRGL IGEIISRFEK KGLKIVGMKM I IDRELAEK 50 HYEEHKGKPF FEALIDYITK APVW WEG RYAVEWRKM AGATDPKDAA 100 PGTIRGDYGL DIGDAIYNVI HASDSKESAE REISLYFKPE EIYEYCKAAD 150 WFYREKKQAK C 161 (SEQ ID NO: 14) PfUNK2 : MGVLWMNEVE RTLVIIKPDA WRGLIGEII SRFEKKGLKI VGMKMIWIDR 50 ELAEKHYEEH KGKPFFEALI DYITKAPVW MWEGRYAVE WRKMAGATD 100 PKDAAPGTIR GDYGLDIGDA lYNVIHASDS KESAEREISL YFKPEEIYEY 150 CKAADWFYRE KKQAKC 166 (SEQ ID NO : 3 ) Example 7: PfuNDPK-1 and PfuNDPK-2 Activity Assay This Example uses a purified protein solution containing approximately equal amounts of PfuNDPK-1 and PfuNDPK-2 enzyme described in Example 6, and the purified protein solution containing PfuNDPK-1 without PfuNDPK-2 enzyme described in Example 3, to demonstrate that both PfuNDPK-1 and PfuNDPK-2 exhibit NDPK activity. Because the purified protein solution of Example 6 contains PfuNDPK-1 and PfuNDPK-2 forms of the enzyme present at about a 1:1 ratio, it was necessary to compare NDPK activity in equal amounts of this solution and the solution containing purified PfuNDPK-1 prepared in Example 3. If PfuNDPK-2 is not active, then the NDPK activity in a solution containing PfuNDPK-1 and PfuNDPK-2 would be less than the NDPK activity in the PfuNDPK-1 solution of Example 3. Solutions containing either fifty picograms of PfuNDPK-1 + PfuNDPK-2 or fifty picograms of PfuNDPK-1 alone were prepared by serial dilution of the appropriate stock solutions into Nanopure water (Promega AA399) , and the solutions were kept on ice. One milliliter of master mix was prepared by combining 889 μL of Nanopure water, 100 μL of 10X DNA polymerase buffer (Promega, M195A) , 1 μL of 10 mM ADP (Sigma, A5285) , and 10 μL of 10 mM dCTP (Promega, U128B) . The following three reaction mixtures were then assembled: 1. 20 μL Master Mix 2. 18.1 μL Master Mix, 1.9 μL PfuNDPK-1 solution (50 pg) 3. 18.8 μL Master Mix, 1.2 μL PfuNDPK-1 + PfuNDPK-2 solution (50 pg) These reactions were incubated at 70°C for 10 minutes. Then 5 μL of each reaction were added to 495 μL TE buffer to stop the reaction (1:100 dilution) . The NDPK activity is stopped because of the presence of EDTA that chelates the magnesium present in the DNA polymerase buffer and required by that enzyme. Then, 10 μL of each 1:100 dilution were separately added to 100 μL of L/L reagent (Promega, F120B) and the light output measured in a Turner™ TD20/20 luminometer. The following results were obtained. Reaction Relative Number Enzyme Light Units 1 no enzyme 15.48 2 PfuNDPK-1 369.2 3 PfuNDPK-1 + PfuNDPK-2 815.6 Because equivalent amounts of protein were used in Reactions 2 and 3, it can be determined that the PfuNDPK-2 form of the enzyme contributes activity in Reaction 3. Because the activity value of Reaction 3 is about two- fold higher than that of Reaction 2, it appears that the PfuNDPK-2 form of the enzyme has a higher specific activity than does the PfuNDPK-1 form of the enzyme. Inasmuch as there are about equal concentrations of both enzymes present in the solution of Reaction 3 and the total amount of enzyme in Reactions 2 and 3 was about the same, one can calculate that the specific activity of PfuNDPK-2 is about three times that of PfuNDPK-1 under the conditions used for this study. From the foregoing, it will be observed that numerous modifications and variations can be effected without departing from the true spirit and scope of the present invention. It is to be understood that no limitation with respect to the specific examples presented is intended or should be inferred. The disclosure is intended to cover by the appended claims modifications as fall within the scope of the claims .

Claims: What is claimed is: 1. An isolated and purified nucleoside diphosphate kinase (NDPK) enzyme that comprises an amino acid residue sequence at least 80 percent identical with the sequence of SEQ ID NO : 3 or 14, and that exhibits higher NDPK activity at a temperature of about 50°C to about 90°C relative to the NDPK activity at 37°C. 2. The isolated and purified NDPK enzyme according to claim 1 that comprises an amino acid residue sequence at least 90 percent identical to the sequence of SEQ ID NO : 3 or 14. 3. The isolated and purified NDPK enzyme according to claim 2 that comprises an amino acid residue sequence at least 95 percent identical to the sequence of SEQ ID NO : 3 or 14. 4. The isolated and purified NDPK enzyme according to claim 3 that comprises the amino acid residue sequence of SEQ ID NO : 3 or 14. 5. The isolated and purified NDPK enzyme according to claim 4 that is encoded by the nucleotide sequence of SEQ ID NO: 6, 13, 26, 27 or 28. 6. A method for producing an NDPK polypeptide that exhibits higher NDPK activity at a temperature of about 50°C to about 90°C relative to the NDPK activity at 37°C comprising the steps of: (a) providing a host cell culture wherein the host cell harbors an expression vector comprising the nucleotide sequence of SEQ ID NO: 6, 13, 26, 27 or 28 or variant thereof that has at least 80 percent identity to the NDPK sequence of SEQ ID NOs : 6 , 13, 26, 27 or 28 and that hybridizes with said NDPK gene under moderate stringency conditions comprising hybridization at a temperature of about 50°C to about 65°C in 0.2 to 0.3 M NaCl, followed by washing at about 50°C to about 55°C in 0.2X SSC, 0.1% SDS. (b) expressing an NDPK polypeptide from the expression vector; and (c) isolating and purifying the NDPK polypeptide from the host cell culture. 7. The method for producing an NDPK polypeptide according to claim 6 wherein said nucleotide sequence has at least 90 percent identity to the NDPK sequence of SEQ ID NOs : 6 , 13, 26, 27 or 28. 8. The method for producing an NDPK polypeptide according to claim 7 wherein said nucleotide sequence comprises the NDPK sequence of SEQ ID NOs:6, 13, 26, 27 or 28. 9. An isolated and purified polynucleotide comprising the nucleotide sequence of SEQ ID NO : 6 , 13, 26, 27 or 28 or variant thereof that has at least 80 percent identity to the NDPK gene of SEQ ID NOs : 6 , 13, 26, 27 or 28 and that hybridizes with said NDPK gene under moderate stringency conditions comprising hybridization at a temperature of about 50°C to about 65°C in 0.2 to 0.3 M NaCl, followed by washing at about 50°C to about 55°C in 0.2X SSC, 0.1% SDS, said nucleotide sequence encoding an enzyme that uses NTPs or dNTPs to convert ADP to ATP with a higher NDPK activity at a temperature of about 50°C to about 90°C than the NDPK activity at 37°C. 10. The isolated and purified polynucleotide according to claim 9 wherein said nucleotide sequence has at least 90 percent identity to the NDPK sequence of SEQ ID NOs : 6 , 13, 26, 27 or 28. 11. The isolated and purified polynucleotide according to claim 10 wherein said nucleotide sequence comprises the NDPK sequence of SEQ ID NOs:6, 13, 26, 27 or 28. 12. An isolated and purified NDPK enzyme encoded by a DNA molecule comprising a BseR l/Dra I restriction fragment of about 240 base pairs and a Dra l/Pst I restriction fragment of about 35 base pairs . 13. The isolated and purified NDPK enzyme according to claim 12 wherein said DNA molecule comprises a restriction site map comprising BseR I, Dra I, and Pst I as shown in Table 1. 14. The isolated and purified NDPK enzyme according to claim 12 wherein said DNA molecule further comprises a PpuM 1 /BseR I restriction fragment of about 180 base pairs. 15. The isolated and purified NDPK enzyme according to claim 14 wherein said DNA molecule comprises a restriction map as shown in Table 1. 16. The isolated and purified NDPK enzyme according to claim 12 wherein said restriction fragments hybridize with the NDPK gene of SEQ ID Nos: 6, 13, 26, 27 or 28 under moderate stringency conditions comprising hybridization at a temperature of about 50°C to about 65°C in 0.2 to 0.3 M NaCl, followed by washing at about 50°C to about 55°C in 0.2X SSC, 0.1% SDS. 17. An isolated and purified analog nucleic acid sequence that encodes an amino acid residue sequence that is at least 80 percent identical to the sequence of an NDPK of SEQ ID NO: 3 or 14, said nucleic acid sequence upon suitable transfection and expression in a host, providing an enzyme that (1) uses NTPs or dNTPs to convert ADP to ATP and (2) exhibits higher NDPK activity at a temperature of about 50°C to about 90°C than at 37°C. 18. The isolated and purified analog nucleic acid sequence according to claim 17 that encodes an amino acid residue sequence that is at least 90 percent identical to the sequence of an NDPK of SEQ ID NO: 3 or 14. 19. The isolated and purified analog nucleic acid sequence according to claim 18 that encodes an amino acid residue sequence that is at least 95 percent identical to the sequence of an NDPK of SEQ ID NOs: 3 or 14. 20. The isolated and purified analog nucleic acid sequence according to claim 18 that encodes an amino acid residue sequence of SEQ ID NOs: 3 or 14. 21. A composition comprising an aqueous solution containing an isolated and purified polynucleotide comprising a sequence that is at least 80 percent identical to the nucleotide sequence of SEQ ID NO:6, 13, 26, 27 or 28. 22. The composition according to claim 21 wherein said polynucleotide comprises a sequence that is at least 90 percent identical to the nucleotide sequence of SEQ ID NO: 6, 13, 26, 27 or 28. 23. The composition according to claim 22 wherein said polynucleotide comprises the nucleotide sequence of SEQ ID NO: 6, 13, 26, 27 or 28. 24. A composition for determining the presence or absence of a predetermined nucleic acid target sequence in a nucleic acid sample comprising an aqueous solution that contains: (A) a purified and isolated enzyme whose activity is to release one or more nucleotides from the 3' terminus of a hybridized nucleic acid probe; (B) at least one nucleic acid probe, said nucleic acid probe being complementary to said predetermined nucleic acid target sequence; and (C) a purified and isolated nucleoside diphosphate kinase (NDPK) enzyme that comprises an amino acid residue sequence at least 80 percent identical with the sequence of SEQ ID NO : 3 or 14, and that exhibits higher NDPK activity at a temperature of about 50°C to about 90°C relative to NDPK activity at 37°C. 25. The composition according to claim 24 wherein said NDPK enzyme comprises an amino acid residue sequence at least 90 percent identical with the sequence of SEQ ID NO : 3 or 14. 26. The composition according to claim 25 wherein said NDPK enzyme comprises the amino acid residue sequence of SEQ ID NO : 3 or 14. 27. A composition for determining the presence or absence of at least one predetermined nucleic acid target sequence in a nucleic acid sample comprising an aqueous solution that contains : (A) a purified and isolated enzyme whose activity in the presence of pyrophosphate is to release identifier nucleotide as a nucleoside triphosphate from the 3 ' end of a nucleic acid probe hybridized to said nucleic acid target sequence; (B) adenosine 5' diphosphate; (C) pyrophosphate; (D) a purified and isolated nucleoside diphosphate kinase (NDPK) enzyme that comprises an amino acid residue sequence at least 80 percent identical with the sequence of SEQ ID NO: 3 or 14, and that exhibits higher NDPK activity at a temperature of about 50 to about 90 degrees C relative to NDPK activity at 37 degrees C; and (E) at least one nucleic acid probe, said nucleic acid probe being complementary to said predetermined nucleic acid target sequence. 28. The composition according to claim 27, wherein said purified and isolated enzyme whose activity in the presence of pyrophosphate is to release identifier nucleotides is selected from the group consisting of the Tne triple mutant DNA polymerase, Bst DNA polymerase, Ath DNA polymerase, Tag DNA polymerase and Tvu DNA polymerase . 29. The composition according to claim 28, wherein said purified and isolated NDPK enzyme comprises an amino acid residue sequence at least 90 percent identical with the sequence of SEQ ID NO : 3 or 14. 30. The composition according to claim 29, wherein said purified and isolated NDPK enzyme comprises the amino acid residue sequence of SEQ ID NO: 3 or 14. 31. A recombinant DNA molecule comprising a vector operatively linked to an exogenous DNA segment that comprises at least 486 base pairs that define a gene for the Pyrococcus furiosus NDPK enzyme or a DNA variant that has at least 80 percent identity to the NDPK gene of SEQ ID NOs : 6 , 13, 26, 27 or 28 and hybridizes with said gene under moderate stringency conditions comprising hybridization at a temperature of about 50°C to about 65°C in 0.2 to 0.3 M NaCl, followed by washing at about 50°C to about 55°C in 0.2X SSC, 0.1% SDS, said nucleotide segment encoding an enzyme that uses NTPs or dNTPs to convert ADP to ATP with a higher activity at a temperature of about 50°C to about 90°C than the NDPK activity at 37°C, and a promoter for driving the expression of said enzyme in host organism cells. 32. The recombinant DNA molecule according to claim 31 wherein said exogenous DNA segment comprises a nucleotide sequence that has at least 90 percent identity to the NDPK gene of SEQ ID NOs:6, 13, 26, 27 or 28. 33. The recombinant DNA molecule according to claim 32 wherein said exogenous DNA segment comprises the nucleotide sequence of SEQ ID NOs : 6 , 13, 26, 27 or 28. 34. The recombinant DNA molecule according to claim 31 wherein the promoter is inducible by an exogenously supplied agent. 35. The recombinant DNA molecule according to claim 34 wherein said promoter is induced by exogenously supplied IPTG. 36. The recombinant DNA molecule according to claim 31 present in a host organism. 37. A recombinant DNA molecule that comprises a vector operatively linked to a promoter for driving the expression of the enzyme in host organism cells and a DNA segment that is an analog nucleic acid sequence that encodes an amino acid residue sequence that is at least 80 percent identical to the sequence of a Pyrococcus furiosus NDPK of SEQ ID NOs : 3 or 14, wherein said recombinant DNA molecule, upon suitable transfection and expression in a host, provides an enzyme that (1) uses NTPs or dNTPs to convert ADP to ATP and (2) exhibits higher NDPK activity at a temperature of about 50°C to about 90°C than at 37°C. 38. The recombinant DNA molecule according to claim 37 wherein said amino acid residue sequence is at least 90 percent identical to the sequence of a Pyrococcus furiosus NDPK of SEQ ID NOs : 3 or 14. 39. The recombinant DNA molecule according to claim 38 wherein said amino acid residue sequence is the sequence of a Pyrococcus furiosus NDPK of SEQ ID NOs: 3 or 14. 40. The recombinant DNA molecule according to claim 37 present in a host organism. 41. A host organism containing a polynucleotide that comprises a sequence at least 80 percent identical to the sequence of SEQ ID NO: 3, 16, 26, 27 or 28. 42. The host organism according to claim 41 wherein the polynucleotide comprises a sequence at least 90 percent identical to the sequence of SEQ ID N0:3, 16, 26, 27 or 28. 43. The host organism according to claim 42 wherein the polynucleotide comprises the sequence of SEQ ID NO:3, 16, 26, 27 or 28.

Inventors: ANDREWS CHRISTINE ANN, HARTNETT JAMES R, ANDREWS, CHRISTINE, ANN, HARTNETT, JAMES, R.
Applicants: PROMEGA CORP, PROMEGA CORPORATION
Application year: 2000
Main classification: C12N9
IPC codes: C12N1, C12N5, C12N9, C12N15, C12Q1, C12R1, G01N33