Patent Publication Number: US-2004058330-A1

Title: Methods of use for thermostable RNA ligases

Description:
BACKGROUND OF THE INVENTION  
       [0001] RNA ligase is abundant in T4-infected cells and has been purified in high yields. Bacteriophage T4 RNA ligase catalyzes the ATP-dependent ligation of a 5′-phosphoryl-terminated nucleic acid donor (i.e. RNA or DNA) to a 3′-hydroxyl-terminated nucleic acid acceptor. The reaction can be either intramolecular or intermolecular, i.e., the enzyme catalyzes the formation of circular DNA/RNA, linear DNA/RNA dimers, and RNA-DNA or DNA-RNA block co-polymers. The use of a 5′-phosphate, 3′-hydroxyl terminated acceptor and a 5′-phosphate, 3′-phosphate terminated donor limits the reaction to a unique product. Thus, RNA ligase can be an important tool in the synthesis of DNA of defined sequence (McCoy and Gumport,  Biochemistry  19:635-642 (1980), Sugion, A. et al.,  J. Biol. Chem.  252:1732-1738 (1977)).  
       [0002] The practical use of T4 RNA ligase has been demonstrated in many ways. Various ligation-anchored PCR amplification methods have been developed, where an anchor of defined sequence is directly ligated to single strand DNA (following primer extension, e.g. first strand cDNA). The PCR resultant product is amplified by using primers specific for both the DNA of interest and the anchor (Apte, A. N., and P. D. Siebert,  BioTechniques.  15:890-893 (1993); Troutt, A. B., et al.,  Proc. Natl. Acad. Sci. USA.  89: 9823-9825 (1992); Zhang, X. H., and V. L. Chiang,  Nucleic Acids Res.  24:990-991(1996)). Furthermore, T4 RNA ligase has been used in fluorescence-, isotope- or biotin-labelling of the 5′-end of single stranded DNA/RNA molecules (Kinoshita Y., et al.,  Nucleic Acid Res.  25: 3747-3748 (1997)), synthesis of circular hammer head ribozymes (Wang, L., and D. E. Ruffner.  Nucleic Acids Res  26: 2502-2504 (1998)), synthesis of dinucleoside polyphosphates (Atencia, E. A., et al  Eur. J. Biochem.  261: 802-811 (1999)), and for the production of composite primers (Kaluz, S., et al.,  BioTechniques,  19: 182-186 (1995)).  
       [0003] The use of thermostable enzymes has revolutionized the field of recombinant DNA technology. Thermostable enzymes, foremost DNA polymerases used in amplification of DNA, are of great importance in the research industry today. In addition, thermophilic enzymes are also used in commercial settings (e.g., proteases and lipases used in washing powder, hydrolidic enzymes used in bleaching). Identification of new thermophilic enzymes will facilitate continued DNA research as well as assist in improving commercial enzyme-based products.  
       SUMMARY OF THE INVENTION  
       [0004] This invention pertains to thermostable RNA ligases derived from bacteriophage that infect thermophilic bacteria and their use in various applications including nucleotide labeling, oligonucleotide synthesis, gene synthesis, gene amplification and amplification of mRNA synthesis of cDNA. In certain embodiments, the invention relates to RNA ligase isolates from bacteriophage that infect the thermophilic bacteria,  Rhodothermus marinus  and  Thermus scotoductus.  These thermophilic RNA ligases can replace T4 RNA ligase in methodologies that utilize T4 RNA ligase.  
       [0005] The invention relates to methods of ligating nucleotides or nucleotide analogs or nucleic acids containing nucleotides or nucleotide analogs, comprising contacting nucleotides or nucleic acids with a thermostable RNA ligase, wherein the ligase catalyzes a reaction of ligation of the nucleotides, nucleotide analogs or nucleic acids. In certain embodiments, the thermostable RNA ligase can be derived from a thermostable bacteriophage; the nucleic acids can be RNA or DNA; the RNA or DNA can be single stranded; and the nucleotide analogs contain modified bases, modified sugars and/or modified phosphate groups. The RNA ligase is selected from the group comprising: a RNA ligase obtained from a bacteriophage infecting a thermophilic bacteria; a polypeptide comprising the amino acid sequence of SEQ ID NO: 2 or SEQ ID NO: 4; a polypeptide encoded by a nucleic acid comprising the sequence of SEQ ID NO: 1 or SEQ ID NO: 3; a polypeptide having at least 30% sequence identity with the amino acid sequence of SEQ ID NO: 2 or SEQ ID NO: 4; or a fragment or derivative thereof.  
       [0006] In another embodiment, a method of forming a phosphodiester bond between a 3′ hydroxyl nucleic acid acceptor and a 5′ phosphate nucleic acid donor is described, comprising: contacting a 3′ hydroxyl nucleic acid acceptor; and a 5′ phosphate nucleic acid donor with a thermostable RNA ligase and forming a phophodiester bond between the nucleic acids.  
       [0007] In yet another embodiment, a method of synthesizing an oligonucleotide polymer by repeating cycles of combining a primer oligonucleotide and a blocked oligonucleotide is described, comprising: a) combining the primer oligonucleotide and an oligonucleotide blocked at the 3′ or 5′ end in the presence of a thermostable RNA ligase, thereby forming an extended primer with a blocked 3′ or 5′ end; b) enzymatically removing the blocked phosphate group at the 3′ or 5′ end or enzymatically adding a phosphate group to the 5′ end of the extended primer; and c) repeating a) and b) using the extended primer from b) as the primer for a) wherein an oligonucleotide polymer is formed. In certain embodiments, the formed oligonucleotide polymer comprises a gene or a part of a gene coding for a polypeptide.  
       [0008] The invention further pertains to a method of forming a phosphodiester bond between a 3′ hydroxyl nucleic acid acceptor and a 5′ phosphate nucleic acid donor, comprising: contacting a 3′ hydroxyl nucleic acid acceptor and a 5′ phosphate nucleic acid donor with a thermostable RNA ligase, wherein a phophodiester bond is formed between the nucleic acids.  
       [0009] The invention additionally pertains to a method for synthesizing a recombinant gene product, comprising: providing an array of immobilized oligonucleotides comprising predetermined areas on a surface of a solid support, each area having immobilized thereon copies of an oligonucleotide; hybridizing to said immobilized oligonucelotides first single stranded terminal regions of first nucleic acid strands to be ligated; and ligating with a thermostable RNA ligase, the hybridized first end of a first nucleic acid and a second nucleic acid.  
       [0010] The invention also pertains to a method of detecting nucleic acids, comprising: contacting a first probe, a second probe, a target nucleic acid sample and a thermostable RNA ligase, wherein the first probe and the second probe hybridize to the target nucleic acid sample such that the 5′ end of the first probe and the 3′ end of the second probe are adjacent and can be ligated, wherein at least the 5′ terminal nucleotide of the first probe and the 3′ terminal nucleotide of the second probe are deoxyribonucleotides; and incubating the first probe, second probe, target sample and RNA ligase under conditions that promote hybridization of the probes to the target sequence and that promote ligation of the probes are ligated if the target sequence is present in the target sample.  
       [0011] The invention further relates to method of amplifying nucleic acids, comprising: a) contacting a nucleic acid containing sample, wherein the sample comprises a pool of mRNAs having a poly-A tail, with i) an oligonucleotide with a 5′ end and a 3′ end comprising an oligo-dT sequence at the 3′ end, a promoter sequence recognized by a RNA polymerase at the 5′ end and a transcription initiation region located between the oligo-dT sequence and the promoter sequence wherein the oligonucleotide is blocked at the 3′ end to prohibit extension, ii) an enzyme having reverse transcription activity which forms a double stranded promoter-primer sequence, iii) at least one enzyme having RNase H activity, iv) an enzyme having RNA polymerase activity, and v) sufficient amounts of dNTPs and rNTPs; b) maintaining the resulting reaction mixture under appropriate conditions for a sufficient amount of time for enzymatic activity, such that antisense RNA is formed in the absence of cDNA intermediates; c) contacting the multiple copies of RNA with i) a thermostable RNA ligase, ii) a double stranded DNA complex comprising a double stranded DNA promoter sequence, wherein each strand contains a 5′ end and a 3′ end, the promoter sequence recognizable by a RNA polymerase wherein one strand of said complex has a stretch of RNA attached to the 5′ end thereof, iii) an enzyme having RNA polymerase activity, and iv) sufficient amounts of dNTPs and rNTPs; and d) maintaining the resulting reaction mixture under appropriate conditions for a sufficient amount of time for the enzymatic processes to occur.  
       [0012] In another embodiment, the invention pertains to a method for selectively isolating total cell mRNA, comprising: contacting a cell lysate comprising total cell mRNA and non-isolated ribosome with a thermostable RNA ligase under conditions wherein the ligase adds a 3′ label to the total cell mRNA to form modified total cell mRNA; and isolating the modified total cell mRNA.  
       [0013] In yet another embodiment, the invention relates to a method for synthesizing a repeat region of an oligonucleotide having a defined sequence, the repeat region including a repeated nucleotide that appears more than once in succession, comprising: a) ligating an oligonucleotide primer to a 3′-phosphate-blocked repeated nucleotide to form a 3′-phosphate-blocked primer; b) removing the 3′-phosphate blocking group from the 3′-phosphate-blocked primer using a 3′-phosphatase enzyme, thereby making a deblocked primer without removing the 3′-phosphate blocking group from unreacted 3′-phosphate-blocked repeated nucleotide; and c) repeating steps (a) and (b) using unreacted 3′-phosphate-blocked repeated nucleotide from step (b) as the 3′-phosphate-blocked repeated nucleotide of step (a) and the deblocked primer product of step (b) as the oligonucleotide primer of step (a) without prior separation of the unreacted 3′-phosphate-blocked repeated nucleotide from the deblocked primer product, whereby the cycles are repeated to form an oligonucleotide having a defined sequence.  
       [0014] Additionally, the invention relates to a method for insertion of a single-stranded RNA sequence into a cloning vector, comprising: ligating in the presence of a thermostable RNA ligase, both termini of the single-stranded RNA sequence with a linear double-stranded cloning vector having single-stranded termini complementary to both termini of the single-stranded RNA sequence to form an annealed product, in which the complementary termini of the single-stranded RNA sequence is formed by attaching oligonucleotide linkers to the RNA sequence with a thermostable RNA ligase.  
       [0015] Also described herein is a method for forming a library of DNA sequences, comprising: a) forming a library of target RNA fragments by contacting multiple copies of non-denatured target RNA sequences with a library of random oligonucleotides in the presence of a hydrolytic agent under conditions where a subgroup of the library of random oligonucleotides hybridize to the target RNA, whereupon the hydrolytic agent hydrolyzes the target RNA at a site near the 5′ end of each hybridized random oligonucleotide, and wherein the 3′ ends of each fragment contains the entire sequence to which a random oligonucleotide in the subgroup hybridized; and b) forming a library of templates for primer extension from the library of target RNA fragments; and forming a library of DNA sequences that are complementary to the target RNA fragments from the library of templates for primer extension by attaching a nucleic acid primer complement sequence to the 3′ end of each target RNA fragment with a thermostable RNA ligase.  
       [0016] The invention further pertains to a method for amplifying a 5′ end region of target mRNA, comprising: a) dephoshorylating mRNA molecules with a free phosphate group at the 5′-end; b) removing the 5′-cap on full-length mRNAs; c) ligating linkers to the 5′-end of decapped mRNA molecules using a thermostable RNA ligase; d) synthesize cDNA using reverse transcriptase; and e) amplifying the cDNA by PCR.  
       [0017] In another embodiment, the invention relates to a method of amplifying mRNA, comprising: a) synthesizing cDNA, wherein the first strand of cDNA is synthesized by reverse transcription of target mRNA using a 5′ end-phosphorylated RT-primer that is specific for the target RNA wherein a hybrid DNA-RNA is generated; b) degrading the hybrid DNA-RNA by treatment with RNase H to remove RNA; c) circularizing the single-stranded cDNA or form concatemers with a thermostable RNA ligase; and d) amplifying DNA by PCR using specific primers that are complementary to known sequences.  
       [0018] Also described herein is a method of amplifying nucleic acids with a single primer, comprising: a) hybridizing a mixture of nucleic acids with a degenerate or non-degenerate primer targeted to a single region in a target sequence; b) synthesizing single-stranded DNA complementary to a region of said target sequence, said synthesis being primed by said degenerate or non-degenerate primer and catalyzed by a DNA polymerase or a reverse transcriptase, thereby performing linear amplification of said target sequence by repeated thermal cycling; c) providing a second primer site to said single-stranded DNA by ligating an oligonucleotide to its 3′ end, wherein the ligation is catalyzed by a thermostable RNA; d) amplifying the single-stranded DNA using a primer pair wherein a first primer comprises at least a part of the degenerate or non-degenerate primer sequence, or wherein a first primer is targeted to a region downstream to the degenerate or non-degenerate primer sequence, and the second primer is complementary to the 3′ primer site of step (c). In certain embodiments, the single stranded DNA synthesized in step b) is purified.  
       [0019] Also described is a method for sequencing oligonucleotides, comprising contacting a target oligonucleotide with a thermostable RNA ligase under conditions wherein the ligase adds an auxiliary oligonucleotide to the 3′ end of the target oligonucleotide; and sequencing the oligonucleotide.  
       [0020] In particular embodiments, methods utilizing the thermophilic RNA ligases are performed at temperatures of about 50° C. to about 75° C.  
       [0021] Although the T4 RNA ligase can be utilized for many useful applications, it is only functional up to about 40° C. The thermostable RNA ligase enzymes as described herein have advantageous properties, such as different substrate specificity, in particular, increased activity to ssDNA as compared to T4 ligase and prevention of undesirable secondary structures due to the enzyme&#39;s ability to function at higher temperatures.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0022] The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.  
     [0023]FIG. 1 is a schematic representation of the single primer method where an adaptor sequence is ligated to the 3′ end of the single stranded copy-DNA to provide a second primer site for the second amplification step.  
     [0024]FIG. 2 is a gel depicting screening of amylases with random gene retrieval by single primer method where different RNA ligases were used. Lane 1:1 Kb ladder (New England Biolabs), lane 2: T4 RNA ligase and PCR with primer Am508, lane 3: T4 RNA ligase and PCR with primer oli11, lane 4: T4 RNA ligase and PCR with primers Am508 and oli11, lane 5: RM378 RNA ligase and PCR with primer Am508, lane 6: RM378 RNA ligase and PCR with primer oli11, line 7: RM378 RNA ligase and PCR with primers Am508 and oli11 and line 8: 1 Kb ladder (New England Biolabs).  
     [0025]FIG. 3 is a gel which depicts synthesis of IGFA by T4 RNA ligase and RM378 RNA ligase. Lane 1: 100 bp ladder (New England Biolabs), lane 2: T4 RNA ligase and PCR with primer IGFA-r and IGFA-f giving the whole gene, lane 3: T4 RNA ligase and PCR with primer IGFA-r and IGFA-2f giving the oligoA (partial gene), lane 4: RM378 RNA ligase and PCR with primer IGFA-r and IGFA-f giving the whole gene, lane 5: RM378 RNA ligase and PCR with primer IGFA-r and IGFA-2f giving the oligoA (partial gene) and lane 6: 100 bp ladder (New England Biolabs).  
     [0026]FIG. 4A is a nucleic acid sequence of RM 378 RNA ligase (SEQ ID NO: 1) and the amino acid sequence of RM 378 RNA ligase (SEQ ID NO: 2) and FIG. 4B is the nucleic acid sequence of TS2126 RNA ligase (SEQ ID NO: 3) and the amino acid sequence of TS2126 RNA ligase (SEQ ID NO: 4)  
     [0027]FIG. 5 is a plot which depicts the relative activity of RM378 RNA ligase as a function of pH. MOPS buffer is shown in diamonds and TRIS HCl buffer is shown in squares.  
     [0028]FIG. 6 is a plot of temperature profiles for the activity of RM378 RNA ligase and T4 RNA ligase.  
     [0029]FIG. 7 is a plot which depicts the relative activity of RM378 RNA ligase (squares) and T4RNA ligase (diamonds) after incubation at various temperatures.  
     [0030]FIG. 8 is a plot which depicts the percentage of activity of RM378 RNA ligase at 60° C. over time in MOPS buffer without template.  
     [0031]FIG. 9 is a plot depicting a time curve showing RNA ligase activity on rA20 template (10 μM) using 0.2 T4 RNA ligase enzyme and 0.2 μg and 0.4 μg RM378 RNA ligase enzyme. The reactions were done at 37° C. and 64° C. for T4 and RM378 RNA ligase, respectively.  
     [0032]FIG. 10 is a plot which depicts the percent ligation of total DNA substrate 5′[ 32 P]dA 20  versus time for T4 RNA ligase and RM378 RNA ligase.  
     [0033]FIG. 11 is a bar graph which depicts ligation of 90mer ssDNA substrate using RM378 RNA ligase and T4 RNA ligase. As seen the RM 378 RNA ligase ligation is very efficient for long DNA oligos (50-80%) even without the PEG6000 for RM378 RNA ligase but low for T4 RNA ligase (3% without PEG6000 and 14% with PEG6000).  
     [0034]FIG. 12 is a bar graph which depicts inter-molecular ligation of DNA donors (dA10-dideoxy or —NH 3   + ) to dephosphorylated ssDNA (dA10) oligo or dephosphorylated 17 n.t. RNA (agcgtttttttcgctaa oligo; SEQ ID NO: 5). RM378 RNA ligase shows 20 and 25% ligation when ligating  32 PdA10dd to dA10 without PEG, with PEG, respectively. Ligation dA10-NH 3   +  to DNA and RNA oligos shows 27% and 28% ligation, respectively. T4 RNA ligase shows little activity ligating dA10-NH3 to DNA acceptor but high activity to RNA acceptor (6% and 60% ligation, respectively).  
     [0035]FIG. 13 is a gel depicting the results from control PCR in RLM-RACE using RM378 RNA ligase, and using primers GeneRacer 5′ nested primer and B1 control primer. Lane 1: PCR product; Lane 2: Lambda HindIII marker. A PCR product of about 800-900 bp in size is seen, the expected product is 858 bp.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0036] A description of preferred embodiments of the invention follows.  
     [0037] This invention pertains to methods of using thermostable RNA ligases, in particular ligases that are derived from bacteriophages which infect thermophilic bacteria. In certain embodiments, RNA ligases derived from bacteriophages that infect the bacteria,  Rhodothermus marinus  (RM378 RNA ligase) and  Thermus scotoductus  (TS2126 RNA ligase) are utilized in methods of manipulating nucleic acids. The RM378 RNA ligase is described in U.S. patent application Ser. No. 09/585,858, incorporated herein by reference in its entirety. The TS2126 RNA ligase is described in Attorney Docket No. 2739.2008-000 entitled “Thermostable RNA Ligase from Thermus Phage”, also incorporated by reference in its entirety.  
     [0038] These enzymes are functionally analogous to the widely used T4 RNA ligase yet provide advantageous properties such as, different substrate specificity, in particular, increased activity to DNA and prevention of undesirable secondary structures due to the enzymes ability to function at higher temperatures. Interestingly, these ligases have about 30% sequence identity to each other and to the T4 RNA ligase.  
     [0039] The invention is directed to methods and processes using thermostable RNA ligases, in particular RM378 RNA ligase and TS2126 RNA ligase and other substantially similar enzymes for ligation of nucleic acids, such as ribonucleic acids, deoxyribonucleic acids and nucleic acid analogs. The present invention is directed to specific processes including synthesis of oligonucleotides, gene synthesis, gene shuffling, amplification of RNA including amplification of full-length mRNA through cDNA synthesis, labeling of nucleic acids, sequencing, analysis of single-nucleotide polymorphism, gene amplification, mutation analysis and processing of detector molecules and others. In particular embodiments, the methods and processes include catalysis of a chemical reaction by thermostable RNA ligases at high temperatures such as above 40° C., for example, at temperatures of about 50° C. to about 75° C., even more preferably at temperatures of about 55° C. to about 70° C.  
     [0040] In certain embodiments, the invention pertains to processes using a thermostable RNA ligase such as RM378 RNA ligase and TS2126 RNA ligase to catalyze the ATP-dependent formation of a phosphodiester bond between a 3′-hydroxyl nucleic acid and a 5′-phosphate nucleic acid donor. These processes includes ligation of two oligonucleotides as well as the circularization of a single oligonucleotide.  
     [0041] RNA ligase activity was originally identified as activity induced through infection of  E. coli  by T-even bacteriophages (Silber, R., et al.  Proc. Natl. Acad. USA,  69:3009-3013 (1972)). The RNA ligase from bacteriophage T4 is the product of gene 63 (Snopek, T. J., et al.,  Proc. Natl. Acad. Sci. USA,  74:3355-3359 (1977)) and is the best characterized RNA ligase of very few known homologous RNA ligases.  
     [0042] The properties of RNA ligase from bacteriophage T4 have been extensively studied including its ability to catalyze reactions with various substrates (for review see Gumport and Uhlenbeck, in “Gene Amplification and Analysis”, Vol. II: Analysis of Nucleic Acid Structure by Enzymatic Methods, Chirikjian and Papas, eds. Elsevier North Holland, Inc. (1980)). In general, the T4 RNA ligase catalyzes the ATP-dependent formation of a phosphodiester bond between a 3′-hydroxyl nucleic acid acceptor and a 5′-phosphate nucleic acid donor. T4 RNA ligase can use single-stranded nucleic acids as substrates and does not require a complementary template strand to align donor phosphates with acceptor hydroxyls.  
     [0043] 5′-phosphorylated oligonucleotides are appropriate donors for the ATP-dependent T4 RNA ligase reaction but the minimal donor is a nucleoside 3′,5′-biphosphate (pNp). Suitable minimal acceptor molecules for the T4 RNA ligase reaction are trinucleoside biphosphates.  
     [0044] T4 RNA ligase is adenylated in the presence of ATP thereby forming a covalent bond between AMP and a lysyl residue. The adenyl group can then be transferred from the enzyme to the 5′-phosphate of a nucleic acid. T4 RNA ligase can accept ATP analogs and adenylate nucleic acid substrates with the nucleotide analog. Additionally, T4 RNA ligase is able to catalyze a class of reactions that do not require ATP. The enzyme is able to accept a wide variety of ADP derivatives as substrates and join the extra moiety of the ADP derivative to a nucleic acid acceptor with the elimination of AMP. Examples of ADP derivative of this type include ADP-riboflavin and ADP-hexylamine-biotin (see further Gumport and Uhlenbeck, in “Gene Amplification and Analysis,” Vol. II: Analysis of Nucleic Acid Structure by Enzymatic Methods, Chirikjian and Papas, eds. Elsevier North Holland, Inc (1980)).  
     [0045] T4 RNA ligase has a greater affinity for RNA than DNA. Although RNA and DNA are equally reactive as donors, DNA is a less efficient acceptor than RNA. The efficiency of the RNA ligase reaction is also affected by the nucleotide composition of the acceptor and oligo(A) seems to function as the most efficient acceptor. RNA molecules are good acceptors for the T4 RNA ligase.  
     [0046] The 5′-phosphate of yeast tRNA Phe  is a very poor donor for T4 RNA ligase, indicating that secondary or tertiary structures in the RNA donor molecule is inhibiting the ligase reaction. In contrast, DNA restriction fragments are good donors and little difference is observed between DNA restriction fragments with 5′-staggered ends and blunt ends. On the other hand, the presence of a secondary structure of an RNA acceptor molecule has little effect on the reaction. The 5′-cap (m 7  G 5′ ppp- 5′ ), which is normally formed through addition of methylated guanosine to the 5′ end of eukaryotic mRNA, is neither an acceptor nor a donor for the T4 RNA ligase reaction (Gumport and Uhlenbeck in “Gene Amplification and Analysis,” Vol. II: Analysis of Nucleic Acid Structure by Enzymatic Methods, Chirikjian and Papas, eds. Elsevier North Holland, Inc. (1980)).  
     [0047] T4 RNA ligase is a versatile enzyme with new properties continuing to be discovered. T4 RNA ligase has recently been shown to be able to catalyze the reaction between a 3′-phosphate donor and 5′-hydroxyl acceptor in addition to previously characterized reaction of 5′-phosphate donor and 3′-hydroxyl acceptor (U.S. Pat. No. 6,329,177). T4 RNA ligase has also been shown to have template-mediated DNA ligase activity. Reportedly, the T4 RNA ligase can ligate ends of DNA strands hybridized to RNA, even more efficiently than T4 DNA ligase (U.S. Pat. No. 6,368,801).  
     [0048] Enzymes having RNA ligase activity, but which are apparently not related to the T4 RNA ligase and other homologous proteins in the small family of viral RNA ligases, have been identified. These enzymes have relatively strict substrate specificity whereas the activity of T4 RNA ligase is the most general RNA joining activity known.  
     [0049] The RNA ligases of T-even bacteriophages apparently belong to a very small family of homologous enzymes. However, it is likely that this is a subfamily of much larger superfamily of ligases including DNA ligases and mRNA capping enzymes (Shuman, S. and B. Schwer,  Mol. Microbiol.,  17:405-410 (1995); Timson, D. J., et al.,  Mut. Res.,  460:301-318 (2000)). Until recently, the only clearly identifiable relatives of T4 RNA ligase, found through sequence comparisons (ex. with BLAST software), were from bacteriophage RB69 and  Autographa californica  nuclearpolyhedrosis virus. As disclosed in a previous patent applications (U.S. patent application Ser. No. 09/585,858; PCT Application No. PCT/IB00/00893; European Application No. 00938977.6), the discovery of a bacteriophage from the thermophilic bacterial host  Rhodothermus marinus  and the subsequent genome sequencing identified a new RNA ligase homologous to T4 RNA ligase according to the amino acid sequence of the predicted gene product of a particular open reading frame. Accordingly, these ligases seem to form a family of viral RNA ligase currently only comprising ligases from bacteriophage T4 (and by analogy other T-even bacteriophages), bacteriophage RB69,  Autographa california  nuclearpolyhedrosis virus and the ligases of the instant invention from bacteriophages RM378 and TS2126. The ligases of the instant invention are the only known members of the family from a thermophilic source.  
     [0050] As used herein, “Nucleobase” refers to a nitrogen-containing heterocyclic moiety, e.g., a purine, a 7-deazapurine, or a pyrimidine. Typical nucleobases are adenine, guanine, cytosine, uracil, thymine, 7-deazaadenine, 7-deazaguanine, and the like.  
     [0051] “Nucleoside” as used herein refers to a compound consisting of a nucleobase linked to the C-1′ carbon of a ribose sugar.  
     [0052] “Nucleotide” as used herein refers to a phosphate ester of a nucleoside, as a monomer unit or within a nucleic acid. Nucleotides are sometimes denoted as “NTP,” or “dNTP” and “ddNTP” to particularly point out the structural features of the ribose sugar.  
     [0053] “Nucleotide 5′-triphosphate” refers to a nucleotide with a triphosphate ester group at the 5′ position. The triphosphate ester group can include sulfur substitutions for the various oxygens, e.g., alpha-thio-nucleotide 5′-triphosphates.  
     [0054] As used herein, the term “nucleic acid” encompasses the terms “oligonucleotide” and “polynucleotide” and means single-stranded and double-stranded polymers of nucleotide monomers, including 2′-deoxyribonucleotides (DNA) and ribonucleotides (RNA). The nucleic acid can be composed entirely of deoxyribonucleotides, entirely of ribonucleotides, or chimeric mixtures thereof, linked by internucleotide phosphodiester bond linkages, and associated counter-ions, e.g., H + , NH 4   + , trialkylammonium, Mg 2+ , Na +  and the like. Nucleic acids typically range in size from a few monomeric units, e.g., 5-40 when they are commonly referred to as oligonucleotides, to several thousands of monomeric units. Unless denoted otherwise, whenever an oligonucleotide sequence is represented, it will be understood that the nucleotides are in 5′ to 3′ order from left to right and that “A” denotes deoxyadenosine, “C” denotes deoxycytosine, “G” denotes deoxyguanosine, and “T” denotes deoxythymidine, unless otherwise noted.  
     [0055] “Nucleotide analog” as used herein can be a modified deoxyribonucleoside; a modified ribonucleoside; a base-modified, sugar-modified, a phosphate-modified phosphate group, a phosphorothioate group, a phosphonate group, a methyl-phosphonate group, a phosphoramidate group, a formylacetyl group, a phosphorodithioate group, a boranephosphate group, or a phosphotriester group.  
     [0056] The term “primer” normally refers herein to an oligonucleotide used, for example in amplification of nucleic acids such as PCR. The primer can be comprised of unmodified and/or modified nucleotides, for example modified by a biotin group attached to the nucleotide at the 5′ end of the primer.  
     [0057] “Label” as used herein refers to any moiety covalently attached to a nucleotide that is detectable or imparts a desired functionality or property in the ligation extension product.  
     [0058] “Ligation” is the enzymatic joining by formation of a phosphodiester bond between nucleic acids.  
     [0059] “Peptide nucleic acid” (PNA) refers to synthetic oligomers containing any backbone of acyclic, achiral, and neutral polyamide linkages to which nucleobases are attached.  
     [0060] “Thermostable” is defined herein as having the ability to withstand temperatures above 70° C. for a length of time in minutes without becoming irreversibly denatured and maintaining the ability to catalyze a chemical reaction, such as the formation of a phosphodiester bond, at preferred temperatures above 50° C., such as between 50° C. and 100° C., at preferred temperatures of about 50° C. to about 75° C. and at even more preferred temperatures of about 55° C. to about 70° C.  
     [0061] “Thermophilic bacteria”, also referred to as “thermophiles”, are defined as bacteria having optimum growth temperature above 50° C. “Thermophilic bacteriophages” or “thermostable bacteriophages” are defined as bacteriophages having thermophilic bacteria as hosts.  
     [0062] Methods of producing replicate copies of the same polynucleotide, such as PCR or gene cloning, are collectively referred to herein as “amplification” or “replication.” For example, single or double stranded DNA can be replicated to form another DNA with the same sequence. RNA can be replicated, for example, by RNA directed RNA polymerase, or by reverse transcribing the RNA and then performing a PCR. In the latter case, the amplified copy of the RNA is a DNA with the correlating or homologous sequence.  
     [0063] The polymerase chain reaction (“PCR”) is a reaction in which replicate copies are made of a target polynucleotide using one or more primers, and a catalyst of polymerization, such as a DNA polymerase, and particularly a thermally stable polymerase enzyme. Generally, PCR involves repeatedly performing a “cycle” of three steps: “melting,” in which the temperature is adjusted such that the DNA dissociates to single strands, “annealing,” in which the temperature is adjusted such that oligonucleotide primers are permitted match their complementary base sequence using base pair recognition to form a duplex at one end of the span of polynucleotide to be amplified; and “extension” or “synthesis,” which can occur at the same temperature as annealing, or in which the temperature is adjusted to a slightly higher and more optimum temperature, such that oligonucleotides that have formed a duplex are elongated with a DNA polymerase. The cycle is then repeated until the desired amount of amplified polynucleotide is obtained. Methods for PCR amplification can be found in U.S. Pat. Nos. 4,683,195 and 4,683,202.  
     [0064] When referring to a particular protein such as a RNA ligase, the term “isolated” refers to the preparation of the protein which is substantially free of contaminants.  
     [0065] “Reverse transcription” or “reverse transcribing” refers to the process by which RNA is converted into cDNA through the action of a nucleic acid polymerase such as reverse transcriptase. Methods for reverse transcription are well known in the art and described for example in Ausubel, F. M., et al.,  Short Protocols in Molecular Biology,  John Wiley and Sons (1995); and Innis, M. A. et al.,  PCR Protocols , Academic Press (1990).  
     [0066] The methods disclosed herein involving the molecular manipulation of nucleic acids are known to those skilled in the art. See generally Ausubel, F. M. et al., “Short Protocols in Molecular Biology,” John Wiley and Sons (1995); and Sambrook, J., et al., “Molecular Cloning, A Laboratory Manual,” 2nd ed., Cold Spring Harbor Laboratory Press (1989). The T4 RNA ligase has been utilized for a great variety of applications involving the manipulation of nucleic acids.  
     [0067] After the discovery and early characterization of T4 RNA ligase, it was realized that the enzyme could be used for synthesis of oligonucleotides including oligonucleotides with a defined sequence, even complete genes of DNA or their RNA equivalents (Gumport and Uhlenbeck, in “Gene Amplification and Analysis,” Vol. II: Analysis of Nucleic Acid Structure by Enzymatic Methods, Chirikjian and Papas, eds. Elsevier North Holland, Inc.; McCoy and Gumport,  Biochemistry,  19:635-642 (1980); Sugino, et al.,  J. Biol. Chem.,  252:1732-1738 (1980)).  
     [0068] T4 RNA ligase has been used for the synthesis of circular hammer head ribozymes (Wang, L., and D. E. Ruffner,  Nucleic Acids Res.,  26:2502-2504 (1998)), synthesis of dinucleoside polyphosphates (Atencia, E. A., et al.,  Eur. J. Biochem.,  261:802-811 (1999)), and for the production of composite primers (Kaluz, S., et al,  BioTechniques,  19:182-186 (1995)).  
     [0069] The particular limitations of the T4 RNA ligase can hinder the practical use of applications being designed and prevent development of new applications. An enzyme with similar activity but substantially different properties such as a high working temperature and more equal activity on DNA and RNA substrates can make certain applications more practical and increase possibilities for the development of new methods and applications.  
     [0070] A thermostable RNA ligase can be utilized for improvements of various methods previously utilizing T4 RNA ligase such as methods described above. For example, amplification of mRNA and synthesis of cDNA often involve the use of a complex mixture of RNA containing RNA molecules with various stable secondary structures, which inhibits the action of T4 RNA ligase. The negative influence of secondary structure has been shown using well-defined substrates, both RNA and DNA. An additional heating step prior to ligation is often added to processes to reduce the undesirable secondary structures. The limited efficiency of T4 RNA ligase using natural RNA substrates has also been demonstrated (Gumport and Uhlenbeck, in “Gene Amplification and Analysis,” Vol. II: Analysis of Nucleic Acid Structure by Enzymatic Methods, Chirikjian and Papas, eds. Elsevier North Holland, Inc. (1980); Bruce and Uhlenbeck,  Nucleic Acids Res.,  5:3665-3677 (1978); McCoy and Gumport,  Biochemistry,  19:635-642 (1980)). Some protocols for mRNA amplification, recommended for example by manufactures of commercial kits for rapid amplification of cDNA ends (RACE), include a preheating step and subsequent cooling prior to ligation in an effort to decrease secondary structures of the substrate RNA. The possibility to carry out ligation reactions at higher temperatures using a thermostable RNA ligase will limit the formation of secondary structures and thus improve the amplification of the RNA by increasing the proportion of RNA molecules available for ligation by the enzyme. Similarly, other applications involving ligation of nucleic acids may benefit from decreased formation of secondary structures at higher temperatures.  
     [0071] T4 RNA ligase originates from bacteriophage T4 that has a mesophilic host ( E. coli ) and is not thermostable. The present invention demonstrates the thermostability of RNA ligases analogous to the T4 RNA ligase. The discovery of thermostable homologues of T4 RNA ligase, originating from the thermophilic bacteriophage RM378 having thermophilic a bacterial host  Rhodothermus marinus , represents the first known thermostable RNA ligase comparable to T4 RNA ligase. Further evidence for various other properties of this enzyme in comparison to T4 RNA ligase are described therein demonstrating important similarities and differences in properties of these homologous enzymes. A second thermostable RNA ligase enzyme has also been characterized from a another bacteriophage, TS2126 which infects the bacterial host  Thermus scotoductus . It should be noted that  Thermus scotoductus  is thermophilic like  Rhodothermus marinus  but the two bacterial species are not closely related. The isolated enzyme, originating from bacteriophage TS2126, was predicted to be related to T4 RNA ligase based on a database sequence similarity search but the sequence similarity is low. TS2126 has nucleic acid ligase activity similar to T4 RNA ligase and RM378 RNA ligase. It is also shown that the TS2126 RNA ligase is active at high temperatures.  
     [0072] RM378 RNA ligase, TS2126 RNA ligase and T4 RNA ligase belong to a family of RNA ligases with very few known members, all of viral origin. The discovery of thermostable RNA ligases and the characterization of its activities is a significant contribution to the knowledge and further utilization of a small group of enzymes with versatile and useful activities for the manipulation of nucleic acids. The various uses of T4 RNA ligase already apparent by the many applications currently utilizing this enzyme indicates the utility of other enzymes having similar activities. The present invention provides characterization of RNA ligases with activities generally comparable with that of RNA ligase from bacteriophage T4 but with distinctly different properties making the use of the RM378 RNA ligase for various applications advantageous.  
     [0073] “RM378 RNA ligase” refers to a polypeptide having the amino acid sequence of SEQ ID NO: 2. RM378 RNA ligase originates from bacteriophage RM378 having a thermophilic bacterial host  Rhodothermus marinus.    
     [0074] “TS2126 RNA ligase” refers a polypeptide having the amino acid sequence of SEQ ID NO: 4. TS2126 RNA ligase originates from bacteriophage TS2126 having a thermophilic host  Thermus scotoductus.    
     [0075] The term “RNA ligase” is used herein according to the conventional name of the homologous enzyme from bacteriophage T4 usually referred to as “RNA ligase” (Gumport and Uhlenbeck, in “Gene Amplification and Analysis,” Vol. II: Analysis of Nucleic Acid Structure by Enzymatic Methods, Chirikjian and Papas, eds. Elsevier North Holland, Inc. (1980)). Alternatively, the enzyme can be described by terms such as “nucleic acid ligase” or “oligonucleotide ligase”.  
     [0076] The terms “RM378 RNA ligase, TS2126 RNA ligase or any substantially similar enzyme” and “thermostable RNA ligases of the present invention or any substantially similar enzyme” refers to any polypeptide belonging to a group consisting of:  
     [0077] a) a RNA ligase obtained from a bacteriophage infecting a thermophilic bacteria; i.e., bacteriophage having a thermophilic bacterial host;  
     [0078] b) a polypeptide comprising the amino acid sequence of SEQ ID NO: 2 or SEQ ID NO: 4;  
     [0079] c) a polypeptide encoded by a nucleic acid comprising the sequence of SEQ ID NO: 1 or SEQ ID NO: 3;  
     [0080] d) a polypeptide having at least 30% sequence identity with the amino acid sequence of SEQ ID NO: 2 or SEQ ID NO: 4; or  
     [0081] e) a fragment or derivative of (a), (b), (c), or (d).  
     [0082] The thermostable RNA ligases of the present invention or any substantially similar enzyme can be partially or substantially purified (e.g., purified to homogeneity), and/or are substantially free of other polypeptides. Accordingly, the amino acid sequence of the polypeptide can be that of the naturally-occurring polypeptide or can comprise alterations therein. Polypeptides comprising alterations are referred to herein as “derivatives” of the native polypeptide. Such alterations include conservative or non-conservative amino acid substitutions, additions and deletions of one or more amino acids; however, such alterations should preserve at least one activity of the polypeptide, i.e., the altered or mutant polypeptide should be an active derivative of the naturally-occurring polypeptide. For example, the mutation(s) can preferably preserve the three-dimensional configuration of the binding site of the native polypeptide, or can preferably preserve the activity of the polypeptide (e.g., any mutations preferably preserve the ability of the enzyme to catalyze the ligation of nucleic acids). The presence or absence of activity or activities of the polypeptide can be determined by various standard functional assays including, but not limited to, assays for binding activity or enzymatic activity.  
     [0083] Additionally, the RNA ligases are directed to active fragments of the thermostable RNA ligases of the present invention or any substantially similar enzyme, as well as fragments of the active derivatives described above. An “active fragment,” as referred to herein, is a portion of polypeptide (or a portion of an active derivative) that retains the polypeptide&#39;s activity, as described above.  
     [0084] Appropriate amino acid alterations can be made on the basis of several criteria, including hydrophobicity, basic or acidic character, charge, polarity, size, the presence or absence of a functional group (e.g., —SH or a glycosylation site), and aromatic character. Assignment of various amino acids to similar groups based on the properties above will be readily apparent to the skilled artisan; further appropriate amino acid changes can also be found in Bowie, et al.,  Science,  247:1306-1310 (1990). For example, conservative amino acid replacements can be those that take place within a family of amino acids that are related in their side chains. Genetically encoded amino acids are generally divided into four families: (1) acidic=aspartate, glutamate; (2) basic=lysine, arginine, histidine; (3) nonpolar=alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine, tryptophan; and (4) uncharged polar=glycine, asparagine, glutamine, cystine, serine, threonine, tyrosine. Phenylalanine, tryptophan and tyrosine are sometimes classified jointly as aromatic amino acids. For example, it is reasonable to expect that an isolated replacement of a leucine with an isoleucine or valine, an aspartate with a glutamate, a threonine with a serine or a similar conservative replacement of an amino acid with a structurally related amino acid will not have a major effect on activity or functionality.  
     [0085] The thermostable RNA ligases of the present invention or any substantially similar enzyme can also be fusion polypeptides comprising all or a portion (e.g., an active fragment) of a native polypeptide fused to an additional component, with optional linker sequences. Additional components, such as radioisotopes and antigenic tags, can be selected to assist in the isolation or purification of the polypeptide or to extend the half life of the polypeptide; for example, a hexahistidine tag would permit ready purification by nickel chromatography. The fusion protein can contain, e.g., a glutathione-S-transferase (GST), thioredoxin (TRX) or maltose binding protein (MBP) component to facilitate purification; kits for expression and purification of such fusion proteins are commercially available.  
     [0086] Also included in the thermostable RNA Ligases are polypeptides which are at least about 30% identical (i.e., polypeptides which have substantial sequence identity) to the RM378 RNA ligase or the TS2126 RNA ligase. However, polypeptides exhibiting lower levels of identity are also useful, particular if they exhibit higher identity over one or more particular domains of the polypeptide. For example, polypeptides sharing high degrees of identity over domains necessary for particular activity, such as RNA ligase activity, are included herein. Thus, polypeptides which are at least about 10%, preferably at least about 20%, more preferably at least about 30%, more preferably at least about 40%, even more preferably at least about 50%, yet more preferably at least about 70%, still more preferably at least about 80%, and even more preferably at least about 90% identity, yet more preferably at least about 95% identity, are encompassed by the invention.  
     [0087] The thermostable RNA ligases of the present invention or any substantially similar enzyme can be isolated from naturally-occurring sources (e.g., isolated from host cells infected with bacteriophage RM378, bacteriophage TS2126 or other bacteriophage infecting a thermophilic bacterium). Alternatively, the polypeptides can be chemically synthesized or recombinantly produced. For example, PCR primers can be designed to amplify the ORFs from the start codon to stop codon, using DNA of RM378 or TS2126 or related bacteriophages or respective recombinant clones as a template. The primers can contain suitable restriction sites for an efficient cloning into a suitable expression vector. The PCR product can be digested with the appropriate restriction enzyme and ligated between the corresponding restriction sites in the vector (the same restriction sites, or restriction sites producing the same cohesive ends or blunt end restriction sites).  
     [0088] The thermostable RNA ligases used in the present invention or any substantially similar enzyme can be isolated or purified (e.g., to homogeneity) from cell culture (e.g., from culture of host cells infected with bacteriophage RM378 or bacteriophage TS2126) by a variety of processes. These include, but are not limited to, anion or cation exchange chromatography, ethanol precipitation, affinity chromatography and high performance liquid chromatography (HPLC). The protein can be isolated by conventional means of protein biochemistry and purification to obtain a substantially pure product, i.e., 80, 95 or 99% free of cell component contaminants, as described in Jacoby,  Methods in Enzymology,  Volume 104, Academic Press, New York (1984); Scopes,  Protein Purification, Principles and Practice,  2nd Edition, Springer-Verlag, New York (1987); and Deutscher (ed.),  Guide to Protein Purification, Methods in Enzymology,  Vol. 182 (1990).  
     [0089] The invention pertains to processes using a thermostable RNA ligases such as RM378 RNA ligase, TS2126 ligase or a substantially similar enzyme, to catalyze the ATP-dependent formation of a phosphodiester bond between a 3′-hydroxyl nucleic acid acceptor and a 5′-phosphate nucleic acid donor. This includes ligation of two oligonucleotides as well as the circularization of a single oligonucleotide. The invention is thus directed to processes involving ligation of nucleic acids at relatively high temperatures. For example, the Examples demonstrate the ability of RM378 RNA ligase to catalyze the formation of bond between the 3′-hydroxyl end of a nucleic acid and a 5′-phosphate nucleic acid through circularization of a single nucleic acid substrate. Furthermore, it is shown in that the RM378 RNA ligase of the present invention is able to ligate two separate oligonucleotides thus demonstrating its ability to ligate two oligonucleotides as well as circularize a single oligonucleotide. It is also demonstrated that the RM378 RNA ligase of the present invention is able to catalyze reaction of this type using both RNA and DNA substrates.  
     [0090] The present invention demonstrates that, like the T4 RNA ligase, the RM378 RNA ligase of the invention does not require complementary template strands to align donor phosphates with acceptor hydroxyls. The enzyme is able to use single-stranded nucleic acids as substrates, an activity that is fundamental to various utilities of the enzyme. Importantly, the invention shows that the RM378 RNA ligase is able to catalyze these reactions at elevated temperatures, such as about 40° C. to about 80° C. The invention thus shows for the first time the ability of any enzyme to optimally carry out catalysis of these types of reactions at high temperatures.  
     [0091] The invention also pertains to processes using thermostable RNA ligases such as RM378 RNA ligase or TS2126 RNA ligase to catalyze the ATP-independent formation of a phosphodiester bond between a 3′-hydroxyl nucleic acid acceptor and a 5′-phosphate nucleic acid donor. This can be accomplished by using an ADP-derivative (Ado-5′ pp-X) as donor in enzymatically catalyzed joining of the extra moiety (p-X) to a nucleic acid substrate acceptor with the elimination of AMP.  
     [0092] Further, the invention is directed to processes using the thermostable RNA ligases of the present invention to catalyze the ligation of nucleic acids wherein the nucleic acid can be made of ribonucleotides (RNA), deoxyribonucleotides (DNA) or nucleotide analogs. The nucleic acids or nucleic acid analogs can be oligonucleotides or polynucleotides or single nucleotides such as a nucleoside 3′,5′-biphosphate (pNp). The nucleic acids can be synthetic or natural. The natural nucleic acids can be obtained from biological samples and can be crude samples, or they can be substantially purified. The nucleic acids can further be made through previous amplification of natural nucleic acids. The nucleotide analogs can be analogs such as dideoxyribonucleotides or any base-modified, sugar-modified or phosphate group modified nucleotide. The nucleic acids can be made of natural nucleic acids or synthetic nucleic acids and can contain chimera of natural nucleic acids such as RNA-DNA chimeras. The substrates for the ligase can also contain chimeras of nucleic acids and non-nucleic acids such as chimeras of PNA and DNA, i.e. PNA-DNA (U.S. Pat. No. 6,297,016).  
     [0093] Example 6 shows that the RM378 RNA ligase is able to use a nucleotide analog as a substrate. In the example, the RM378 RNA ligase uses 3′NH 2 -3′dATP instead of ATP for adenylation of a nucleic acid.  
     [0094] Preferred embodiments of the invention include processes involving ligation of nucleic acids at temperatures at about 40° C. to about 75° C. The elevated temperatures can limit formation of secondary structures which limit the use of the conventional T4 RNA ligase which has been shown to less effective on substrates containing secondary structure such as the yeast tRNAPhe (Gumport and Uhlenbeck, in “Gene Amplification and Analysis,” Vol. II: Analysis of Nucleic Acid Structure by Enzymatic Methods (1980); Chirikjian and Papas, eds. Elsevier North Holland, Inc; Bruce &amp; Uhlenbeck,  Nucleic Acids Res.,  5:3665-3677 (1978); McCoy and Gumport,  Biochemistry,  19:635-642 (1980)). Protocols for amplification of mRNA include a heating step prior to ligation with T4 RNA ligase to limit secondary structures of substrate nucleic acid. Furthermore, the utility of a thermostable RNA ligase has been suggested for the synthesis os oligonucleotides (Hyman, U.S. Pat. No. 5,514,569). As shown in Example 3, the RM378 RNA ligase has a temperature profile clearly distinct from T4 RNA ligase. The RM378 RNA ligase is active at higher temperatures than the T4 RNA ligase, such as about 40° C. to about 80° C. The RM378 enzyme has an optimum temperature between 60° C. and 65° C., at a temperature where the T4 enzyme is practically inactive. The enzyme of the present invention is active at higher temperatures than the T4 RNA ligase, such as at about 40° C. to about 80° C., such as temperatures of about 50° C. to about 75° C., and even more preferable at temperatures of about 55° C. to about 65° C. Thus, the thermostable RNA ligases, such as RM378 RNA ligase and TS2126 RNA ligase, can be used for ligation of nucleic acids at temperatures outside the working temperatures of the T4 RNA ligase. Elevated temperatures will limit the formation of secondary structures among substrate nucleic acids and any process involving ligation of nucleic acids containing substantial tertiary or secondary structures can preferably be carried out at higher temperatures, such as above 40° C. For example, natural RNA substrates such as ribosomal RNA and mRNA can contain substantial secondary structure which can limit ligation such as by limiting access to 5′-phosphate groups in the ligation reaction.  
     [0095] The present invention is directed to processes involving ligation of DNA molecules. The T4 RNA ligase has been shown to be much less active on DNA substrates than RNA substrates and reactions involving ligation of DNA catalyzed by T4 RNA ligase have required large amount of enzyme and long reaction times (Gumport and Uhlenbeck, “Gene Amplification and Analysis,” Vol. II: Analysis of Nucleic Acid Structure by Enzymatic Methods, Chirikjian and Papas, eds. Elsevier North Holland, Inc (1980); Gumport, et al.,  Nucleic Acids Symp. Ser.,  7:167-171 (1980)). As shown in Example 5, the invention clearly demonstrates that the RM378 RNA ligase is far more active on DNA substrates than the T4 RNA ligase but under different conditions, optimized for the T4 enzyme, the activity of the two enzymes is more comparable. However, it is shown that the RM378 RNA ligase is able to catalyze these types of reactions at elevated temperatures and determination of optimal conditions for the RM378 enzyme, such as was done for the T4 RNA ligase, can lead to a substantial increase activity. In comparison to T4 RNA ligase, the RM378 RNA Ligase has much more similar activity on DNA and RNA under the conditions detailed in the Examples. From these results, the RM378 prefers DNA over RNA as a substrate which is in contrast to the T4 RNA ligase.  
     [0096] The invention is directed to processes involving ligation of nucleic acids including single stranded DNA and double stranded DNA such as restriction fragments including DNA restriction fragments with 5′-staggered ends or blunt ends.  
     [0097] Oligonucleotide Synthesis  
     [0098] The invention is also directed to processes utilizing thermostable RNA ligases as described herein or any substantially similar enzyme to catalyze a reaction between a 3′-phosphate donor and 5′-hydroxyl acceptor. The invention is further directed to processes involving template-directed ligation of nucleic acids such as template-directed ligation of DNA.  
     [0099] The invention is directed to processes using thermostable or substantially similar enzyme to synthesize oligonucleotides. The synthetic oligonucleotides have widespread use in various fields and can be used for example in applications in molecular biology, including genetic engineering and PCR, in therapeutics, for example for antisense oligonucleotides, for diagnostics and to make catalysts such as ribozymes. The synthetic oligonucleotides can be used as primers for amplification of genetic material. PCR technology, for example, routinely employs oligonucleotides as primers for amplification of genetic material and synthetic genes are made for various purposes including optimization of codon usage for efficient expression. Useful synthetic oligonucleotides include polymers containing natural ribonucleotides and deoxynucleotides as well as polymers containing modified nucleotides such as base-modified, sugar-modified and phosphate-group modified nucleotides.  
     [0100] The invention further includes processes for making oligonucleotides or polynucleotides constituting genes or parts thereof, made of DNA oligonucleotides or their RNA analogs. The synthetic genes can be for inclusion in vectors for expression of a particular gene product. The synthetic genes can comprise codons not used in naturally occurring genes for example for the purpose of optimization of codon usage for efficient expression in a particular host. In the synthesis of oligonucleotides catalyzed by a thermostable RNA ligase, the efficiency of the reactions can be enhanced by blocking the 3′-terminus of donor molecules or de-phosphorylating the 5′-terminus of acceptor molecules; thus driving the reaction to yield products containing a defined order of the oligonucleotide sequences. The use of a thermostable ligase can be preferable to the use of other ligases such as the T4 RNA ligase due to its distinct properties such as its ability to work at high temperatures. This can increase the efficiency of the reaction as well as limit the amount of enzyme and the time needed for synthesis.  
     [0101] The synthesis of oligonucleotides can be performed by repeated cycles of combining a primer oligonucleotide and a blocked oligonucleotide using a thermostable RNA ligase, such as such as RM378 RNA ligase or TS2126 RNA ligase.  
     [0102] In a series of patents (U.S. Pat. Nos. 5,516,664; 5,629,177; 5,514,569 and 5,602,000), Hyman describes the synthesis of oligonucleotides by repeated cycles of combining a primer oligonucleotide and a blocked oligonucleotide using RNA ligase. The method involves few steps: i) combining the primer and a nucleotide having a 3′-end blocked by a phosphate group in the presence of RNA ligase thereby forming an extended primer with a blocked 3′-end; ii) enzymatically removing the blocking phosphate group at the 3′-end of the extended primer using a phosphatase; and iii) repeating the previous steps using the primer-nucleotide from previous cycle (ii) as the primer in the first step (i) in the next cycle. Using Hyman&#39;s method, the primer in each step functions as the acceptor with a free 3′-OH group and the activated adenylated nucleotide to be added as the donor with a 5′-phosphate group. This way, the enzymatic procedure proceeds in the 5′ to 3′ direction. However, Havlina describes the surprising discovery of the capability of RNA ligase to link a 3′-phosphate donor and a 5′-hydroxyl acceptor (U.S. Pat. No. 6,329,177). This allows for the synthesis of oligonucleotides in the 3′ to 5′ direction using RNA ligase, in opposite direction compared to the above procedure described by Hyman. In Havlina&#39;s method, RNA ligase can be used to ligate an oligonucleotide primer to a carrier molecule with a protecting group at the 5′-position or lacking a protecting group at the 3′-position.  
     [0103] The above procedure has been carried out previously using T4 RNA ligase, which is not thermostable, and a phosphatase that was also not thermostable. As pointed out in U.S. Pat. No. 5,516,664, the procedure could benefit from the use of a thermostable RNA ligase and thermal cycling, i.e., performing the ligase reaction at high temperature and then turn off the activity of the ligase by lowering the temperature for subsequent incubation with the phosphatase. Elevating the temperature again in the next cycle would then activate the RNA ligase again.  
     [0104] Using the method outlined above, the synthesis of oligonucleotides proceeds in the 5′ to 3′ direction with the primer in each step functions as the acceptor with a free 3′-OH group and the activated adenylated nucleotide (adenylated 3′,5′-bisphosphate) to be added as the donor with a 5′-phosphate group. The invention also pertains to processes involving synthesis of oligonucleotides in the 3′ to 5′ direction catalyzed by the RNA ligase by linking a 3′-phosphate donor and a 5′-hydroxyl acceptor. The RNA ligase can be used to ligate an oligonucleotide primer to a carrier molecule with a protecting group at the 5′-position or lacking a protecting group at the 3′-position. This method for preparing a nucleotide polymer in the 3′ to 5′ direction can essentially involve the following steps: i) providing a nucleic acid primer and a nucleic acid carrier molecule; ii) ligating the 3′ end of the carrier molecule to the 5′ position of the primer with RNA ligase; thereby obtaining a ligation product; iii) de-protecting the ligation product by removing a protecting group of the carrier molecule; iv) transferring a natural or modified phosphate group to the 5′ position of the ligation product; and v) optionally repeating steps (i) to (iv).  
     [0105] The methods of the invention are also directed the synthesis of oligonucleotides in the 3′- to 5′-direction catalyzed by the RNA ligase by linking a 5′-phosphate donor of a first oligonucleotide and a 3′-hydroxyl acceptor of a second oligonueleotide. The circularization of the individual oligonucleotides can be blocked such as by attaching a chemical group such as biotin to the 3′-end of the oligonucleotide or by having a hydroxyl group at both the 5′ end and the 3′ end. The method involves: i) combining the first oligonucleotide with a 5′-phosphate group and the second oligonucleotide having a 3′-hydroxyl and 5′-hydroxyl group in the presence of the RNA ligase thereby forming an extended oligonucleotide formed through ligation of the 5′-phosphate of the first oligonucleotide and the 3′-hydroxyl group of the second oligonucleotide; ii) adding a phosphate group to the 5′-end of the extended oligonucleotide such as by using polynucleotide kinase; iii) optionally blocking the 5′ end of the unreacted first oligonucleotide; and iv) repeating steps i) to iii) using the extended oligonucleotide from the previous cycle ii) as the first oligonucleotide i) in the next cycle.  
     [0106] The methods of the invention are also directed the synthesis of oligonucleotides in the 5′- to 3′-direction catalyzed by the RNA ligase by linking a 3′-hydroxyl donor of a first oligonucleotide and a 5′-phosphate acceptor of a second oligonucleotide. The circularization of the individual oligonucleotides can be blocked such as by attaching a chemical group such as biotin to the 5′-end of the oligonucleotide or by having a hydroxyl group at both the 5′ end and the 3′ end or by having a phosphate group at both the 5′ end and the 3′ end. The method involves: i) combining the first oligonucleotide with a 3′-hydroxyl group and the second oligonucleotide having a 3′-phosphate and 5′-phosphate group in the presence of the RNA ligase thereby forming an extended oligonucleotide formed through ligation of the 3′-hydroxyl of the first oligonucleotide and the 5′-phosphate group of the second oligonucleotide; ii) removing the phosphate group at the 3′-end of the extended oligonucleotide such as by using a phosphatase; iii) optionally blocking the 3′ end of the unreacted first oligonucleotide; and iv) repeating steps i) to iii) using the extended oligonucleotide from the previous cycle ii) as the first oligonucleotide i) in the next cycle.  
     [0107] The oligonucleotides used for synthesis catalyzed by the ligase can be performed using methods such as chemical synthesis. The RNA ligases described herein can be used to ligate single-stranded oligonucleotides of variable length, such as about 50 to about 100 bases. The blocking of the 5′ end of oligonucleotides can be done for example through chemical modification of the 5′-phosphate group. The extended oligonucleotide can comprise a synthetic gene coding for a gene product comprising the amino acid sequence of a naturally occurring protein or a modification thereof. The oligonucleotide synthesis can further be carried out with a plurality of first oligonucleotides of variable sequence and/or a plurality of second oligonucleotides of variable sequence to produce a plurality of extended oligonucleotides of variable sequence produced by different combinations of various first oligonucleotides with various second oligonucleotides. The procedure can be repeated for additional cycles. The method can thus be used as a method for gene shuffling such as by having the plurality of oligonucleotides comprising the sequence of corresponding fragments of genes of naturally occurring sequences from different sources. Example 9 below describes the synthesis of a gene coding for a protein with the amino acid sequence of a naturally occurring protein. The example demonstrates the synthesis of gene using the RM378 RNA ligase. In the example, the experiment was designed to produce a synthetic gene suitable for cloning and expression of a protein equivalent to the active human insulin-like growth factor. The example demonstrates the synthesis of genes optimized for the production of specific protein having several potential advantages over other methods for producing the same protein. The protein is of human origin and can be difficult to obtain directly. The active protein is only a part of a precursor protein which in vivo requires processing to produce the mature protein chain. Production of the active protein through expression cloning would require genetic engineering, such of a subcloning of a corresponding gene fragment from cDNA and the insertion of a initiator methionine codon to produce an expression vector expressing only the active protein. The composition of the sequence of the human gene, such as codon usage and GC content can not be optimal for expression in a heterological host such as  E. coli.    
     [0108] RNA Amplification  
     [0109] The invention also pertains to methods using the RNA ligase for amplification of RNA, such as methods for amplification of mRNA including synthesis of the corresponding cDNA. The methods can benefit from the use of the thermostable RNA ligase such as a thermostable RNA ligase in favor of the conventional RNA ligase from T4. For example, amplification of mRNA can preferably be carried out at high temperatures such as about 60° C. to limit formation of secondary structures in the nucleic acid substrates that can inhibit the ligase reaction.  
     [0110] Amplification methods can be used for identifying the 5′ and 3′ untranslated regions of genes, studying heterogeneous transcriptional start sites, characterizing promoter regions, obtaining the complete cDNA sequence of a gene and amplifying the full-length cDNA for downstream cloning and expression.  
     [0111] Several methods have been disclosed involving amplification of RNA, especially mRNA through synthesis of the corresponding cDNA. Kempe, et al., U.S. Pat. No. 4,661,450, describes a method for molecular cloning of RNA. In this method the use of T4 RNA ligase is fundamental for the process wherein the RNA ligase is used to attach oligonucleotide linkers to the single-stranded molecule to be cloned. The attached oligonucleotides can be composed of RNA, DNA or mixture of each and facilitate the insertion of the RNA species into a cloning vector. Multiple DNA copies can then be obtained after transformation of the cloning vector into a suitable host. One disadvantage of this particular method is the requirement of having a ribonucleotide at the 3′-terminus of the linker which is attached to the 5′-terminus of the single-stranded RNA molecule to be cloned. This requirement is based on the properties of conventional RNA ligase from bacteriophage T4 which does not effectively use deoxynucleotide with the 3′-hydroxyl group of the acceptor. The use of T4 RNA ligase is thus limited by its substrate specificity.  
     [0112] More recently, methods for amplification of mRNA have mostly been based on synthesis of cDNA with the use of reverse transcriptase and amplification using PCR. Various variations and improvements on the general method of synthesizing cDNA have appeared including methods to obtain cDNA of full-length RNA such as RACE (Rapid amplification of cDNA ends, Maruyama, et al.,  Nucleic Acids Res.,  23:3796-7 (1995)). The methods described often involve the use of RNA ligase for ligation of nucleic acids such as for ligation of oligonucleotide to the 5′-ends of the mRNA or circularization of single-stranded cDNA. One problem associated with traditional RACE methods is the amplification of truncated cDNA (Schaefer, B. C., Anal. Biochem., 227:255-273 (1995)). Ligation-mediated amplification of RNA uses RNA ligase to increase reliability of the process by preserving the termini of the RNA molecules (Volloch, et al.,  Nucleic Acids Res.,  22:2507-2511 (1994)). The presence of the cap structure on the 5′-end of full-length mRNA can be used to selectively produce cDNAs with complete length. First, a phosphatase is used to dephoshorylase mRNA molecules with a free phosphate group at the 5′-end, i.e., degraded and incomplete RNA molecules. After enzymatic removal of the cap on full-length mRNAs, linkers can be added to decapped mRNA molecules which now have a free 5′-phosphate group and can function as substrates for RNA ligase in contrast to the molecules lacking a 5′-phosphate group. A specific oligonucleotide can thus be ligated to the 5′-end of the full-length RNA molecules and cDNA can be produced using reverse transcriptase with for example, a primer containing a poly(T) region complementary to the poly(A) region of eukaryotic mRNA. The cDNA can then be amplified using PCR with primers complementary to the previously ligated oligonucleotide and a gene specific primer or a primer complementary to the poly(A) region (Maruyama &amp; Sugano,  Gene,  138:171-174 (1994)).  
     [0113] U.S. Pat. No. 5,597,713 describes a method of producing cDNAs with complete length by ligation of DNA or DNA-RNA chimeric oligonucleotide to the 5′-end of intact mRNAs after decapping. PCT Patent No. WO 01/04286 describes optimization of a method for constructing full-length cDNA libraries, by minimizing mRNA degradation and increase fullness ratio, through optimization of reaction conditions including the RNA ligase reaction. U.S. Pat. No. 6,242,189 discloses a method for selective isolation of bacterial mRNA after enzymatic modification of the mRNA such as by using RNA ligase. Merenkova, et al. (U.S. Pat. No. 6,022,715) describe a method for specific coupling of the 5′-cap of the mRNA, using chemical modifications, with subsequent isolation of mRNA and preparation of complete cDNA and Zohlnhöfer and Klein (PCT Patent No. WO 01/71027) describe a method for amplification of mRNA involving ligation of poly(C) and poly(G) flanks to cDNA.  
     [0114] Recently identified applications of T4 RNA ligase are based on a target-mediated ligation of DNA by RNA ligase (U.S. Pat. No. 6,368,801). Accordingly, T4 RNA ligase can, more efficiently than T4 DNA ligase, ligate DNA ends hybridized to RNA. This property of T4 RNA ligase can be used for the detection and/or amplification of nucleic acids. Thus, known techniques based on ligation of DNA can be improved using T4 RNA ligase. These methods include ligase chain reaction (LCR), ligation-mediated PCR (LD-PCR), reverse transcription PCR combined with ligation, PCR/ligation detection reaction (PCR/LDR), oligonucleotide ligation assay (OLA), ligation-during-amplification (LDA), iterative gap ligation (IGL) and ligation of padlock probes, open circle probes and other circularizable probes.  
     [0115] A method for amplification of mRNA but not encompassing cDNA synthesis has been described (U.S. Pat. No. 6,338,954). This method uses RNA polymerase for amplification from an attached promoter sequence. RNA ligase is used to attach double-stranded DNA with a promoter sequence to RNA molecules.  
     [0116] In one embodiment, thermostable RNA ligase is used for molecular cloning of RNA wherein the RNA ligase is used to attach oligonucleotide linkers to single-stranded RNA molecule to be cloned. The attached oligonucleotides can be composed of RNA, DNA or mixture of each and facilitate the insertion of the RNA species into a cloning vector. Multiple DNA copies can then be obtained after transformation of the cloning vector into a suitable host.  
     [0117] In preferred embodiments, amplification of mRNA can be based on synthesis of cDNA with the use of reverse transcriptase and amplification using PCR. These embodiments include methods to obtain cDNA of full-length RNA such as methods for rapid amplification of cDNA ends (RACE, Maruyama, et al., Nucleic Acids Res., 23:3796-7 (1995)). These embodiments involve the use of RNA ligase for ligation of nucleic acids such as for ligation of oligonucleotide to the 5′-ends of the mRNA or circularization of single-stranded cDNA. The thermostable RNA ligase can be used for ligation-mediated amplification of RNA by preserving the termini of the RNA molecules. The presence of the cap structure on the 5′-end of full-length mRNA can be used to selectively produce cDNAs with complete length. As an example, the process essentially comprises the following: i) a phosphatase, such as alkaline phosphatase, is used to dephoshorylase mRNA molecules with a free phosphate group at the 5′-end, i.e., degraded and incomplete RNA molecules which lack a 5′-cap; ii) the 5′-cap on full-length mRNAs is removed such as by enzymatic removal such as by using the enzyme tobacco acid pyrophosphatase (TAP); iii) the thermostable RNA ligase, such as RM378 RNA ligase or TS2126 RNA ligase or any substantially similar enzyme is used to add linkers to the 5′-end of decapped mRNA molecules; iv) cDNA is synthesized using reverse transcriptase such as by using a primer containing a poly(T) region complementary to a poly(A) region of the mRNA; and v) the cDNA is amplified such as by using PCR such as with primers complementary to the previously ligated oligonucleotide and a gene specific primer or a primer complementary to a poly (A) region.  
     [0118] The ligation step is preferably carried out at high temperatures, such as about 40° C. to about 80° C., more preferably at temperatures of about 50° C. to about 75° C., and even more preferably at about 55° C. to about 70° C. The linkers added to the 5′-end of RNA molecules can comprise oligonucleotides composed of RNA, DNA, DNA-RNA chimeric oligonucleotides or nucleotide analogs.  
     [0119] Variations on this technique for the rapid amplification of cDNA end (RACE) are also contemplated. For example, a 5′ end region of target mRNA can be amplified by: 1) synthesizing the cDNA, by synthesizing the first strand of cDNA by reverse transcription of target mRNA using a 5′ end-phosphorylated RT-primer that is specific for the target RNA; 2) degrade the hybrid DNA-RNA by treatment with RNase H to remove RNA; 3) circularize the single-stranded cDNA to form concatemers with a thermostable RNA ligase, such as RM378 RNA ligase or TS2126 RNA ligase or any substantially similar enzyme; and 4) amplify DNA by PCR using specifc primers that are complementary to known sequences.  
     [0120] In another embodiment, mRNA is treated with a phosphataseto eliminate the 5′ phosphates from truncted mRNA and non-mRNA, the dephosphorylated RNA is treated the pyrophosphatase to remove the 5′ cap structure from intact, full-length mRNA, which leaves a 5′ phosphate required for ligation to a RNA oligonucleotide. An oligonucleotide is ligated using a thermostable RNA ligase, such as RM378 RNA ligase or TS2126 RNA ligase or any substantially similar enzyme, to the 5′ end of the full-length decapped mRNA. The RNA oligonucleotide provides a priming site for 3′ primers. The ligated mRNA is reverse-transcribed. mRNA is degraded by RNAse H. cDNA generated is then amplified by PCR.  
     [0121] Gene Amplification using a Single Gene Specific Primer  
     [0122] Generally, PCR amplification procedure is based on the application of two specific primers. Therefore, in PCR screening, two conserved target sites with favorable length of interval sequence are required. Although, the method can be adapted in a high throughput manner, most of these single gene PCR methods have only been used on DNA samples from single species harboring limited number of genes.  
     [0123] One approach for single primer PCR (linear PCR) can be performed for example, by using one gene specific primer in each PCR and then ligating an adaptor sequence to the 3′ end of the single stranded copy-DNA to provide a second primer site for the second amplification step. The designed gene specific primers are affinity labeled at the 5′ end (such as preferably labeled with biotin), which allows the separation of the first single stranded DNA product from the complex DNA. After a number copies of the single stranded DNA have been produced by linear amplification, a second reverse priming site can be made available by ligating a single stranded oligonucleotide of known sequence to the 3 end of the single stranded DNA by means of a ligase such as T4 RNA ligase. The modified templates are then re-amplified by using the gene specific primer (unlabelled) and a reverse primer complementing the adapter sequence primer to make double-stranded DNA that can then be amplified by PCR for further cloning and/or sequencing (FIG. 1).  
     [0124] The invention also pertains to processes for using a thermostable RNA ligase, such as RM378 RNA ligase or TS2126 RNA ligase in single primer PCR (linear PCR and Inverse PCR). The RNA ligase may be applied in a PCR method where only one specific primer is used. The second downstream reverse primer (unspecific) binds to an anchor sequence, which is added to the polymerase extension product from the specific primer with RNA ligase. The double stranded DNA generated in this way is then cloned and sequenced. This single specific primer PCR method can be used for screening of family wide gene fragments and further for retrieval of complete genes from various DNA sources such as from pure or mixed cultures and environmental samples or DNA libraries.  
     [0125] The screening method consists of DNA or the mixture of the nucleic acids extracted from the said sample and then the DNA is hybridized with a degenerate primer targeted to a single region in said target sequence. The gene specific primer is degenerate for a highly conserved amino acid sequence region, which is identified by analyzing multiple alignments of proteins from the protein family that is targeted. The degenerate gene specific primer can be designed by a number of methods, including the CODEHOP method [Consensus-Degenerate Hybrid Oligonucleotide Primer] (Rose et al., 1998). The target region of the protein family being targeted should preferably contain at least 3-4 conserved amino acid. Following hybridization, the single stranded copy-DNA-molecules (extension products) are produced with a polymerase or reverse transcriptase by repeated thermal cycling (a linear amplification). The single stranded DNA is then separated from unused primers and template DNA with column purification. A further purification step of the single stranded DNA can be included to reduce background caused by unspecific hybridization of primers and degraded DNA. This is done with specific primers, affinity labeled at the 5 end (preferably labeled with biotin). Thus, the single stranded DNA product can be immobilized to a solid support with the binding to the corresponding ligand molecule (preferably streptavidin, e.g. streptavidin coded beads or wells) and non-immobilized DNA can be washed away. Then a second primer site is provided to the 3′ end of the single stranded copy-DNA with the ligation of a defined 5′-phosphorylated oligonucleotide linker (anchor-molecule) to the 3′ end of the previously produced single stranded DNA molecules with RNA ligase. Following the ligation the single stranded copy-DNA&#39;s can be amplified with a primer pair, which comprises part of the 5′ degenerate primer sequence and a primer complementary to the 3′ anchor molecule. The PCR product can then be cloned and sequenced or directly sequenced.  
     [0126] Additionally, the method for accessing natural diversity and recovering unknown genes and gene fragments from complex DNA isolated from mixed populations of natural microorganisms is an alternative to other conventional methods based on the construction and screening of DNA libraries (Woo, et al.,  Nucleic Acid Res.,  22:4922-4931 (1994); Dalboge H.,  FEMS Microbiol Rev.,  21:29-42 (1997); Rondon et al.,  PNAS USA,  96:6452-6455 (1999); U.S. Pat. No. 5,958,672; Henne, et al.,  Appl. Environ. Microbiol.,  66:3133-316 (2000)) or PCR amplification of environmental samples with two specific degenerate primers (forward and reverse) (U.S. Pat. No. 5,849,491). In fact, the method described above has several advantages over these methods. First of all, it is simple, less time consuming and more targeted to discover new diversity of homologous gene than the library construction methods. Secondly, this method can be used for all gene families, as long as at least one common conserved region is found, containing at least 3 well conserved amino acids and that one specific gene family specific degenerate primer can be designed. This is in contrast to library expression screening where different screening assay, which can be very complicated in procedure, must be employed for different enzyme activities. The requirement of only one conserved region in a protein family instead of two such sites, as are required for the application of the conventional two primer methods, obviously allows for the retrieval of much greater diversity of more distantly related genes with the present invention.  
     [0127] The single primer method has additional advantages over PCR screening methods where two specific primers (forward and reverse) are used. The first step requires the hybridization of only one specific primer before the polymerization starts, and is therefore kinetically more favorable than when hybridization of two primers is required. Consequently greater diversity is obtainable with the single primer method. With the use of only one gene family specific primer in each reaction as in the invented method, fewer reaction are needed (no matrix of forward and reverse primers). This, not only saves time and expensive reagents but also the source DNA. Another significant advantage of the single specific primer method compared to the two primer method is that longer sequences can be obtained in the first reaction since the fragment length is not dependent on the interval between two conserved sites. Few methods based on the application of only one gene specific primer have been described (Jones and Winistorfer,  Nucleic Acids Res.  20:595-600 (1992); Jones and Winistorfer,  Biotechniques,  15:894-904 (1993); Megonigal et al.,  PNAS USA,  97: 9597-9602 (2000); Riley, et al.,  Nucleic Acids Res.,  18:2887-2890 (1990); Rubie, et al.,  Biotechniques,  27:414-418 (1999); Morris et al.  Appl. Environ. Microbiol.,  1998; Rosenthal and Jones,  Nucleic Acids Res.,  18:3095-3096 (1990); Kilstrup and Kristiansen,  Nucleic Acid Res.,  28 E55 (2000); Stokes, et al.,  Appl. Environ Microbiol.,  67:5240-5246 (2001), and Laging, et al.,  Nucleic Acid Res.,  29:E8 (2001)). However these methods have only been described for the purpose of isolation of unknown sequences in a single genome DNA or genome library DNA. Furthermore, in the method described above, one polymerase reaction takes place as the first step, wherein single stranded polynucleotides are produced. Since no restriction or ligation of the source DNA takes place the demands for high quality DNA are not as stringent as for the library-based methods.  
     [0128] The single specific primer—PCR method can be applied for the isolation of sequences flanking a known sequence. Thus the method can be used for the retrieval of complete genes by gene walking from the fragment isolated with the single specific primer PCR screening. In this case two forward specific non-degenerate primers are used in two PCR reactions. In the first reaction, an outer primer labeled with immobilization-ligand (e.g., biotin labeled) is used to generate single stranded DNA molecules, which are purified as described above. In the second reaction the single stranded DNA produced in the first round is used as a template in the second round. A second nested forward primer is then used against the reverse anchor primer in a PCR reaction to improve the specificity of the amplification products. The products can then be cloned and sequenced. Example 8 describes in detail an experiment of PCR with single specific primer using T4 RNA ligase as well as RM378 RNA ligase. The invention is directed to methods involving single primer PCR wherein the ligation step using RNA ligase is preferably carried out at high temperatures, such as at temperatures about 40° C. to about 80° C., more preferably at temperatures of about 50° C. to about 75° C., even more preferable at temperatures of about 55° C. to about 70° C.  
     [0129] Nucleotide Labeling  
     [0130] The invention further pertains to methods for labeling of nucleic acids using a thermostable RNA ligase, such as RM378 RNA ligase or TS2126 RNA ligase or any substantially similar enzyme. RNA ligase can be used for the labeling of oligonucleotide probes, primers or template molecules or polynucleotide probes or template molecules with nucleotide or oligonucleotide labeled with a chemical group. T4 RNA ligase has been used in fluorescence-, isotope or biotin-labeling of the 5′-end of DNA/RNA molecules (Kinoshita, et al.,  Nucl. Acid Res.,  26:2502-2504 (1997)).  
     [0131] Labeling of the nucleic acid (probe or primers) with the RNA ligase can be carried out prior to or following hybridization (cf. PCT WO97/27317). The chemical group can immobilize the hybrid probe/template molecule on a solid surface (see for example U.S. Pat. No. 5,595,908) or it can serve as a ligand which binds to a molecule (antibody) coupled with an enzymatically active group, thus allowing measuring of enzymatic activity and thereby achieving quantitative measure of the specific nucleotide acid in said sample.  
     [0132] Labeled DNA or RNA molecules can be used in various methods of quantitatively detecting nucleic acids and for detection of polynucleotide hybridization. The hybridization of DNA or RNA template molecules with the labeled nucleic acid probes can be carried out in a solution (see for example U.S. Pat. No. 6,136,531) or on a solid surface. If the hybridization takes place on a solid surface, either the nucleic acid probes or the template DNA can be immobilized prior the hybridization. Further, the different probes can be immobilized and organized in an array. The hybrid template/probe molecules can be detected in solution or immobilized on a solid surface.  
     [0133] Diagnostics Assays  
     [0134] In other embodiments, the methods described herein pertain to the use thermostable RNA ligase is used in detection assays for nucleic acids such as in diagnostics assays. This includes detection in various samples such as the detection of DNA contamination in biopharmaceuticals or detection of rare nucleic acids in clinical samples. Template nucleic acid molecules to be detected can be hybridized with binary nucleotide probes complementary to adjacent portions of the target sequence. Following hybridization the probes can be ligated with the RNA ligase in a template dependent manner. The template nucleic acid can be DNA or RNA and the primer molecules can be DNA or RNA. The ligation product can be detected with PCR amplification using appropriate primers, nucleotides and polymerases. The ligation chain reaction (LCR) can also be used for the detection of the ligation product. One of the primers or both can be labeled with radioactive, fluorescent, or electrochemiluminescent molecule, or ligand, which can bind to a molecule (antibody) coupled with an enzymatically active group, thus allowing quantitative measure of the specific nucleotide acid in said sample. Another embodiment of this method is to use a probe, which is complementary on 5′ and 3′ ends to the target nucleic acid. The ends hybridize to adjacent portions of the target DNA and can be ligated with RNA ligase in a template dependent manner thus circularizing the probe. Following ligation, one complementary primer can be added to the circular template and subsequently primer extension can be performed. Also, two primers can be added to the circular template, one reverse complementary and another forward primer, to amplify the circular template. Either the primer or the dNTPs in the PCR reaction can be labeled with radioactive, fluorescent, or electrochemiluminescent molecule, or ligand, which can bind to a molecule (antibody) coupled with an enzymatically active group, thus allowing detection and quantitative measure of the amplification product.  
     [0135] Sequencing  
     [0136] The invention is further directed to methods wherein a thermostable RNA ligase is for a process of sequencing short oligonucleotides. The method can essentially comprise the following steps: an auxiliary oligonucleotide is ligated to the 3′-end of a target oligonucleotide with the RM378 RNA ligase. A labeled primer complementary to the auxiliary oligonucleotide is hybridized to the ligation product. The auxiliary oligonucleotide can be sequenced with the Sanger dideoxy method ( PNAS USA,  74:5463-5467 (1977)).  
     [0137] SNP-Analysis and Mutation Detection  
     [0138] The invention is further directed to processes utilizing a thermostable RNA ligase such as RM 378 RNA ligase or TS2126 RNA ligase for analysis of single nucleotide polymorphisms and detection of mutations. The enzyme can be used in ligase-polymerase mediated genetic bit analysis of single nucleotide polymorphisms. Essentially, two oligonucleotide primers are hybridized to adjacent portions of a target molecule, separated by one nucleotide. One of the primers can be immobilized to a solid support such that the hybridization products will be immobilized. Following immobilization, polymerase extension with corresponding nucleoside triphosphate species, complementary to the nucleotide of said pre selected site, is performed to fill the space between the primers. RNA ligase is then used in the ligation of the extended primer with the downstream primer. Either one of the primers or the dNTP can be labeled for the detection of the extension-ligation product. In another embodiment of the invention, the RNA ligase can be used for detection of mutations, i.e., in direct sequence identification of mutations by cleavage and ligation associated mutation-specific sequencing. The DNA molecule, containing mutations (single base substitutions, insertions, deletions) is immobilized to a solid support. Oligo which do not contain the alteration, are hybridized to the immobilized DNA molecule. Thus, heteroduplex is formed at the mismatch site. In the next step, the hybrids are treated with enzymes such as resolvases, mismatch repair proteins, nucleotide excision repair proteins or combinations thereof so that one or both DNA strands are cleaved within, or in the vicinity of the mismatch region. Example of a resolvase is endonuclease VII from bacteriophage T4. Examples of mismatch proteins are MutY from  E. coli  and the MutS, MutL, and MutH system in  E. coli . Examples of nucleotide excision repair proteins are UvrA, B, C and D. The hybrids formed between the wild-type DNA and the altered DNA (with mutations) are then dissociated by denaturation, and the wild-type DNA and any cleavage product of the target DNA are removed by washing. Then the immobilized remaining target DNA is ligated with the RNA ligase to an oligonucleotide linker of predetermined sequence. This linker serves as a binding site for a sequencing primer. The sequence of the DNA immediately adjacent to the ligated oligonucleotide is then determined by sequence analysis such as by using the Sanger dideoxy method.  
     [0139] RNA-Detector Molecules  
     [0140] The invention further pertains to processes wherein a thermostable RNA ligase is used in the processing of detector molecules, such as in a SELEX process and for Q-beta technology (Ellington and Szostak,  Nature,  346:818-22 (1990)). The detector molecules can be used for the detection of any analytes with RNA affinity such as proteins, nucleotides or amino acids, vitamins, antibiotics, carbohydrates, to which they form a complex through non-nucleic acid base pairing interactions. They can be used in the diagnosis of cancer, infectious and inherited diseases. Each detector molecule consists of three functional parts, each serving a special purpose: one is ligand with high affinity to the target analyte, one is ligated to the corresponding part in the second molecule, one is a template that can be amplified by Q-beta replicase after the ligation of two RNA molecules by RNA ligase. To select specific detector molecules against a defined analyte a library of RNA molecules consisting of the three functional parts are added to a sample with pure analyte. RNA-molecules containing functional part with high affinity to the analyte, bind and form a RNA-target molecule complex. RNA-ligase is then used to ligate two RNA molecules in the complex. Consequently a template for the Q-beta replicase is formed, which enables it to replicate the detector molecule. The molecule can be amplified further by reverse transcriptase and polymerase and cloned and sequenced to analyze the composition of the detector molecule. The RNA molecules contain recognition sites for ribozymes. Following ligation, the sample is treated with ribozymes, which digests all unbound ligated RNA molecules. The specific RNA detector molecules can be produced by transcription of complement DNA sequences in plasmid downstream from promotor such as the T7 promotor. In detection assays they are added to the sample. Then RNA ligase is added for the ligation of the two RNA molecules to form template for the Q-beta replicase. Then the molecule is amplified with Q-beta-replicase, and further with reverse transcriptase and polymerase.  
     [0141] All references cited herein are incorporated herein by reference in their entirety. The following Examples are offered for the purpose of illustrating the present invention and are not to be construed to limit the scope of this invention.  
     EXEMPLIFICATIONS  
     Example 1  
     Expression and Purification of RM378 RNA Ligase  
     [0142] A clone containing the gene for RM378 RNA ligase (pBTac1 expression vector in Topo cells) was cultivated at 37° C. and induced with IPTG at OD 600 =0.4. The cells were harvested and disrupted by sonication. The crude cell extract was centrifuged at 10,000 r.p.m. for 1 hour in JA 25.50 rotor. The supernatant was collected and ammonium sulfate added to 30%. The solution was stirred at 4° C. for 40 minutes and centrifuged 10,000 g for 1 hour in JA 25.50 rotor. The protein pellet was dissolved in 20 mM Tris pH 8 and centrifuged in JA 25.50 at 20.000 rpm for 1 hour. The supernatant was run through Hiprep 26/10 desalting column (Pharmacia) in 20 mM Tris pH 8. The protein was then run on ResQ column (Pharmacia) and eluted with KCl. The majority of the ligase eluted in a single peak, wich was collected. The ligase was concentrated with ammonium sulfate precipitation and dialysed against 20 mM Tris pH 8. The protein was stored in 10 mM Tris pH 8, 50 mM KCl, 1 mM DTT, 0.1 mM EDTA and 50% glycerol.  
     Example 2  
     pH Optimum of RM378 RNA Ligase  
     [0143] A pH activity profile was determined using two different buffers: Tris HCl and MOPS, 500 mM stocks. Both buffers were made over the pH range 4-11. MOPS stock buffers were calibrated at room temperature but Tris at 55° C. due to the drastic effect of temperature on its pH. The reaction mixtures (10 μl) were prepared as follows:  
                                                          MOPS or Tris HCl   50   mM at pH 4-11           ATP   1   mM           MgCl   10   mM           BSA   25   μg/mL           DTT   10   mM             32 P 5′ labeled rA20 oligo   10   μM           RM378 RNA ligase   0.45   μM concentration                      
 
     [0144] Each mixture was incubated at 55° C. for 1 hour, and the reaction terminated by heating at 95° C. for 5 minutes. 30 μl SAP cocktail which includes 5U Shrimp alkaline phosphatase (SAP) in 1×SAP buffer (20 mM Tris-HCl (pH 8.0), 100 mM MgCl 2 ) (USB Corp. Cleveland, Ohio) was then added and incubation continued for 3 hours at 37° C. After the incubation period, 10 μl where spotted on DE81 filters (Whatman plc. Kent, UK), washed twice in 500 mM Phosphate buffers (pH 7) and dried. The filters were transferred to liquid scintillation counter vials, 5 ml OptiGold cocktail added and the filters counted for radioactivity in a liquid scintillation counter (Packard-Tricarb). The results are indicated in FIG. 5. The optimum pH is 6.5 and 7 for Tris and MOPS, respectively. Due to temperature tolerance, MOPS was used in subsequent experiments.  
     [0145]FIG. 5 depicts the relative percentage activity of RM378 RNA ligase as a function of pH using MOPS Buffer and Tris HCl buffer.  
     Example 3  
     Temperature Optimum of RM378 RNA Ligase and T4 RNA Ligase  
     [0146] Reactions were incubated at different temperatures as follows: 4° C., 20° C., 30° C., 37° C., 50° C., 55° C., 60° C., 65° C., 70° C., 80° C. and 90° C. for RM378 RNA ligase and 4° C., 20° C., 37° C., 45° C., 55° C. and 65° C. for T4 RNA ligase (New England Biolabs, Bedford, Mass.). The reaction mixtures (10 μl) were as follows:  
                                                          For RM 378 RNA ligase:                   MOPS   50   mM (pH 7.0)           ATP   1   mM           MgCl   10   mM           BSA   25   μg/ml           DTT   10   mM             32 P 5′ labeled rA20 oligo   10   μM           RM378 RNA ligase   0.45   μM (0.2 μg) μM concentration           For T4 RNA ligase:           MOPS   50   mM (pH 7.8)           ATP   1   mM           MgCl   10   mM           BSA   25   μg/ml           DTT   10   mM             32 P 5′ labeled rA20 oligo   10   μM           RM378 RNA ligase   0.45   μM (0.2 μg) μM concentration                      
 
     [0147] Each mixture was incubated at given temperature for 1 hour, and the reaction was terminated by heating at 95° C. for 5 minutes. 30 μl SAP cocktail which includes 5U Shrimp alkaline phosphatase (SAP) in 1×SAP buffer (20 mM Tris-HCl (pH 8.0), 100 mM MgCl 2 ) (USB Corp. Cleveland, Ohio) and was then added and incubation continued for 3 hours at 37° C. Then, 10 μl were spotted on DE81 filters, washed twice in 500 mM Phosphate buffers (pH 7) and dried. The filters were transferred to liquid scintillation counter vials, 5 ml OptiGold cocktail added and the filters counted for radioactivity in a liquid scintillation counter (Packard-Tricarb). The results are indicated in FIG. 6. The RM378 RNA ligase (squares) has temperature optimum at 60° C. but is active from 40-70° C., whereas the T4 RNA ligase (diamonds) has temperature optimum at 37-45° C. but loses all activity at 55° C. The T4 RNA ligase is not thermostable and has a different temperature profile compared to the RM378 RNA ligase. The T4 RNA ligase has no activity at the optimum temperature for the RM378 enzyme. By optimizing the buffer, by lowering the DTT to 1 mM and MgCl 2  to 5 mM (data not shown) and 1 hour incubation times, we were able to get temperature optimum up to 64° C. but the enzyme is not stable at that temperature and looses activity when the incubation time is longer than 1 hour (data not shown).  
     Example 4  
     Thermostability of RM378 RNA Ligase  
     [0148] Thermostability of the RM378 RNA ligase was determined by investigating RM378 RNA ligase ability to withstand denaturation after incubation at different temperatures. The reaction mixtures (as is described in Example 3) without the template were incubated for 1 hour at 60-90° C. The rA20 template was then added and incubated at 60° C. for 30 minutes. Each reaction was then terminated by heating at 95° C. for 5 min, washed and counted for radioactivity as described previously. To study the stability of T4 RNA ligase, the same experiment was done, but the reactions were incubated at 37° C., 45° C. and 55° C. for 1 hour and then the template was added and incubated for 60 minutes at 37° C. The reaction was terminated by heating at 95° C. for 5 minutes, washed and counted for radioactivity. The results are shown in FIG. 7.  
     [0149] RM378 RNA ligase maintains stability at 60° C. but starts to lose activity at 70° C. Only 40% activity remains after 1 hour at 70° C. All activity was lost at 80° C. The starting point (zero point) on the graph corresponds to the activity of the enzyme measured directly after the enzyme was stored in a −20° C. freezer. As is shown in FIG. 7, the T4 RNA ligase loses 80% of its activity after being incubated at 37° C. for 1 hour in the reaction buffer. Note that the enzymes are without template which may affect their stability. However, under the given conditions, the RM378 enzyme becomes more active whereas the T4 enzyme rapidly loses activity at the same temperature. The MOPS buffer was also subjected to these conditions and the enzyme and template added afterwards. There was less than a 10% decrease in activity after the buffer had been heated at 90° C. for 1 hour (data not shown).  
     [0150] Thermostability at 90° C. over time was also monitored but at 5 minutes at 90° C., the RM378 RNA ligase enzyme lost all activity (data not shown). Stability at 60° C. over longer time was also monitored. As shown in FIG. 8, the enzyme is stable at 60° C. for at least few hours and shows clear signs of thermoactivation. Note that the enzyme is incubated without template in these experiments. Data from dA20 oligo ligation over time indicates that the enzyme is stable in the presence of DNA template for at least 8 hours.  
     Example 5  
     Ligation of Oligonucleotides using RM378 RNA Ligase and T4 RNA Ligase  
     [0151] The ability of RM378 RNA ligase and T4 RNA ligase to catalyze ligation of defined oligoribonucleotide (5′ phosphorylated rA20 mer) and deoxyribonucleotide (5′ phosphorylated dA 20 mer)) substrates was tested.  
     [0152] Activity Measurements on Ribonucleotide (RNA) Substrate  
     [0153] The ligation of a rA20 oligoribonucleotide (RNA) or a 17 n.t. RNA oligoribonucleotide (5′P-agcgtttttttcgctaa (SEQ ID NO: 5) was measured using RM378 RNA ligase and T4 RNA ligase. This corresponds to an end-to-end ligation and consequently circularization of the substrate. The reaction was followed by taking samples at multiple time periods and terminate by heating at 95° C. for 5 minutes.  
     [0154] The reactions (10 μl) were as follows:  
                                                          MOPS   50   mM (pH 7.0)           ATP   1   Mm           MgCl 2     5   mM           BSA   25   μg/ml           DTT   1   mM             32 P 5′ labeled RNA oligo   10   μM           RM378 RNA ligase   0.45   μM concentration                      
 
     [0155] The T4 RNA ligase (New England Biolabs, Bedford, Mass.) reactions were done according to the manufacturer&#39;s instructions, with supplied buffer (50 mM Tris-HCl pH 7.8, 10 mM MgCl 2 , 1 mM ATP, and 10 mM DTT) at 37° C. with the same amount of  32 P 5′ labeled rA20 oligo and protein concentration (measures with Bradford assay as 2 mg/ml, after running 3 μl of the T4 RNA ligase on 10% PAGE gel to observe one major band about 50 kDa). After the heating, 30 μl SAP cocktail (5 U) was added and the mixture incubated at 37° C. for 3 hours. After the incubation, 10 μl were spotted on DE81 filters, washed twice in 500 mM Phosphate buffers (pH 7) and dried. The filters were transferred to Liquid scintillation counter vials, 5 ml OptiGold cocktail added and the filters counted for radioactivity as described previously.  
     [0156] The results shown in FIG. 9 using ribonucleotide (RNA) substrates demonstrate that the RM378 RNA and T4 RNA ligase have similar activity under the given assay, ligating about 30-40% of the total RNA in 1 hour. T4 RNA ligase looks more active, but the RM378 can increase its ligation when protein concentration is increased. The less specific activity could be a result of the thermoactivity of the RM 378 RNA ligase enzyme.  
     [0157] Activity Measurements on Deoxyribonucleotide (DNA) Substrates  
     [0158] Samples were removed from a reaction mixture at multiple time periods and terminated by heating at 95° C. for 5 minutes.  
     [0159] The reaction mixture (10 μL) was prepared as follows:  
                                                          MOPS   50   mM (pH 7.0)           ATP   1   mM           MgCl   10   mM           BSA   25   μg/ml           DTT   10   mM             32 P 5′ labeled dA20 oligo   10   μM RM378           RNA ligase   1.0   μM concentration           H20 to 10 μl volume.                      
 
     [0160] RM378 RNA ligase was assayed at 37 and 60° C. The T4 reactions were done according to the manufacturer&#39;s instructions, with supplied buffer at 20° C. and 37° C. with the same amount of  32 P 5′ labeled dA20 oligo and protein concentration. The processing of the samples were done as described in the rA20 activity assay. As shown in FIG. 10, the results with deoxyribonucleotide (DNA) substrates show that the RM378 RNA ligase is much more active on DNA than the T4 RNA ligase. Note that some chemicals have been reported to increase the T4 RNA ligase activity on DNA, this includes 10-25% PEG6000 and hexamine cobalt chloride.  
     [0161] To demonstrate that RM378 RNA ligase could ligate larger ssDNA molecules a ligation of a (5′-cggcgaattctttatgggtccggaaaccctgtgcggtgctgaactggttgatgctctgcaattcgtttgcggtgatcgtggtttctac ttcaa-3′), (SEQ ID NO: 6) was done under the following reaction condition:  
                                              RM378 RNA ligase   2.0   μM   (1 μg)       5 × MOPS buffer   2   μl   (50 mM MOPS pH 7, 5                   mM MgCl 2 , 1 mM DTT,                   1 mM ATP and 25 μg/ml                   BSA)         32 P 5′ labeled ssDNA oligo   10   μM       PEG6000   0-30%   w/v       H 2 O to 10 μl                  
 
     [0162] The reaction was incubated for 2.5 hours at 60° C., and subjected to phosphatase resistance assay as described above. The same was done for T4 (same amount of enzyme) RNA ligase at 22° C. with 0 and 25% PEG 6000. As seen in FIG. 1 the RM378 RNA ligase have good activity on the ssDNA, and PEG6000 helps the ligation as known for T4 RNA ligase but the activity is still over 50% of total ssDNA without PEG6000. The T4 DNA ligation activity is low and even only about 15% with PEG6000. Note that T4 RNA ligase have been reported to have higher activity when hexamine cobalt cloride is added to DNA ligation mixture.  
     [0163] To demonstrate that the RM378 RNA ligase could ligate two DNA molecules together a  32 P-5′-dA10dd (dideoxy dA, lacking C3-OH group) or  32 P-5′-dA10-Amino modifier (C3-NH 3   + ) oligo (that can not self ligate) was ligated to dA10 with and without PEG6000 as described:  
                                              RM378 RNA ligase   3.3   μM   (1.45 μg)       5 × MOPS buffer   2   μl   (50 mM MOPS pH 7, 5                   mM MgCl2, 1 mM DTT,                   1 mM ATP and 25                   μg/ml BSA)         32 P 5′ labeled ssDNA oligo   10   μM   (donor)       5′ defosforylated oligo   10   μM   (acceptor, RNA or DNA)       PEG6000   0 or 25%   w/v       H 2 O to 10 μl                  
 
     [0164] The reaction was incubated for 1 hour at 64° C., and subjected to phosphatease resistance assay as described above. The same reaction was prepared for T4 (same protein amount) RNA ligase at 22° C. with 25% PEG 6000. As seen in FIG. 12 the RM378 RNA ligase has good activity on the ssDNA, and PEG6000 helps the ligation as known for T4 RNA ligase. The T4 DNA ligation activity is low when ligating to a DNA acceptor but is ore active when ligating to a RNA acceptor. The RM378 RNA ligase does not show discrimination of this kind, resulting in very similar ligation to RNA or DNA acceptor. Note that T4 RNA ligase has been reported to have higher activity when hexamine cobalt cloride is added to DNA ligation mixture.  
     Example 6  
     DNA Adenylation using Modified Nucleotides  
     [0165] RM 378 RNA ligase was used with modified nucleotides to determine its use in modification of nucleic acids such as labeling. ATP was substituted with 3NH 2 -3′ dATP. The reaction was done in the MOPS buffer using 2.0 μM RM378 RNA ligase and 10 μM rA20 or dC14-ddC template in 10 ml reaction volumes, incubated for 8 hours at 60° C., and then put on ice. The samples were desalted using ZipTip (Millipore, Bedford, Mass.) and resuspended in 50% acidonitrile. The samples were spotted onto an Anchor Chip™ (Bruker Daltonics) with a 400 μm anchor. The matrix employed was 7 mg/mL of 3-hydroxypicolinic acid (3-HPA) and 0.7 mg/mL ammonium citrate (dibasic). Mass spectra were recorded and analyzed on a Bruker Reflex m (Bruker Daltonics) that was operated in the linear mode.  
     [0166] RM378 RNA ligase can use modified nucleotides instead of ATP for adenylation of a nucleic acid (Mass spectra results not shown). DNA is less efficient as a substrate, compared to RNA, but adenylation of DNA is still clearly detectable as (note that this C15 substrate has a dideoxynucleotide on its 3′ end and possible interference of this on the adenylation reaction is unknown).  
     [0167] The mass spectra after enzyme-catalyzed adenylation of the 5′P-dC14ddC (DNA) substrate using 3′NH 2 -3′-dATP was also done (results not shown).  
     Example 7  
     DNA Ligation  
     [0168] DNA ligation was done for MALDI-TOF as described in Example 5, using 2.0 μM RM378 RNA ligase and 25 mM 5′P-dC5 and 5′P-dC5 oligodeoxyribonucleotides as substrates. The reaction was carried out at 60° C. for 8 hours. After incubation, the mixture was desalted and analyzed as described in Example 6.  
     [0169] The results demonstrate that the RM378 RNA ligase can be used for single-stranded DNA ligation (data not shown). The mass spectra indicated that the oligos are ligated in three ways: i) 5′P-dC5-dC15; ii) 5′P-dC5-dC5-dC15; and iii) 5′P-dC15-dC15. It is also demonstrated that the ligase can do multiple ligations, i.e., the product of one ligation reaction can be used as substrate for a subsequent reaction. The ligation reaction can also result in circularization of one of the substrates (rather 5′P-dC15 than 5′P-dC5). Although not seen in the mass spectra, this substrate can compete with the linear ligations.  
     Example 8  
     Single Primer Gene Retrieval by using T4 and RM378 RNA Ligases  
     [0170] The purpose of this study was to analyze the RM378 RNA ligase and compare it to the T4 RNA ligase for random gene retrieval by the single primer method as described in the schematic of FIG. 1. This method is based on the principle of using a RNA ligase to ligated a short oligonucleotide to long single stranded PCR products, obtained by single primer PCR.  
     [0171] Environmental Sample Collection and DNA Extraction  
     [0172] An environmental sample (microbial biomass and water sample) was collected from a basin of an alkaline hot spring (pH 8.5) at 80° C. In order to extract the microorganisms from the sediment and biomass, the biomass and the spring water were vigorously mixed together before the DNA isolation. Genomic DNA from the hot spring biomass was extracted as described by Marteinsson, et al.,  Appl. Environ. Microbiol.,  827-833 (2001b).  
     [0173] Construction of a Degenerated Primer  
     [0174] A random degenerated primer (degeneracy of 32) was constructed. The primer was degenerate at the 3′ core region of length 11 bp but it was non-degenerate at the 5′ region (consensus clamp region) of 29 bp. The primer was Am508 (5′-GATATTTAATATGTTTAGCTGCATCAATTckraanccrtc-3′; (SEQ ID NO: 7). Letters in lower case correspond to the core region, and upper case letters to the consensus clamp region.  
     [0175] Linear PCR with a Single Degenerate Random Primer  
     [0176] The DNA from the environmental sample was used as a template for the primer Am508. The primer was biotin labeled at the 5′ end (MWG Biotech, Ebersberg, Germany). The PCR was carried out in 50 μl reaction mixture containing 1-100 ng of genomic DNA (dilutions used), 0.2 μM Am508, 200 μM of each dNTP in 1×DyNAzyme DNA polymerase buffer and 2.0 U DyNAzyme DNA polymerase (Finnzymes) with a MJ Research thermal cycler PTC-0225. The reaction mixture was first denatured at 95° C. for 5 min, followed by 40 cycles of denaturing at 95° C. (50 seconds), annealing at two different temperatures (44° C. and 50° C.) for 50 seconds and extension at 72° C. (2 minutes). Samples were loaded on 1% a TAE agarose gel to identify high priming. The samples with no PCR products from the different annealing temperatures were selected for re-amplification. PCR purification and immobilization of single stranded PCR products. To remove excess of biotin labeled primers, nucleotides and polymerase, the PCR samples were passed through QIAquick PCR purification spin columns (QIAGEN, Germany) by following the manufacturers instructions. Before the purification, samples from the different annealing temperatures were pooled. The samples were eluted with 30 μl of H 2 O and then the biotin labeled PCR products were immobilized by using 150 μg of streptavidin-coated magnetic beads (Dynal, Oslo, Norway) according to the instructions of the manufacturer. The captured biotin labeled PCR products were resuspended in 5 μl of dH 2 O. The immobilized single stranded DNA was then subjected to different ligation reactions as described below. Ligation of an adaptor (oli10) to the single stranded biotin labeled PCR products was done using T4 RNA ligase and RM378 ligase.  
     [0177] In the presence of 20 U of T4 RNA ligase (New England BioLabs, Beverly, Mass., USA), 1×T4 RNA ligation buffer (50 mM Tris-HCl, pH 7.8, 10 mM MgCl 2 , 10 mM DTT and 1 mM ATP), 10% PEG8000, 50 nM of the adaptor 5′-phosphorylated oligodeoxyribonucleotide oli10 (5′-AAGGGTGCCAACCTCTTCAAGGG-3′) (SEQ ID NO:8) was added to the captured DNA in a final volume of 20 μl. The mixture was incubated at 22° C. for 20 hours. Before the ligation reaction, the immobilized DNA was heated for 1 minute at 90° C.  
     [0178] In the presence of 0.5 μg of RM378 RNA ligase (in 50% glycerol), 1×MOPS ligation buffer (50 mM MOPS, pH 7.0, 10 mM MgCl 2 , 10 mM DTT, 0.5 μg BSA and 1 mM ATP), 10% PEG8000, 50 nM of the adaptor 5′-phosphorylated oligodeoxyribonucleotide oli10 (5′-AAGGGTGCCAACCTCTTCAAGGG-3′) (SEQ ID NO: 8) was added to the captured DNA in a final volume of 20 μL. The mixture was incubated at 60° C. for 20 hours. Before the ligation reaction, the immobilized DNA was heated for 1 minute at 90° C.  
     [0179] Re-Amplification PCR from the Ligation Reaction  
     [0180] The exponential re-amplification PCR was carried out in 50 μl reaction mixture containing 2 μl ligation mixture, 1.0 μM unlabelled primer Am508, 1.0 μM oli11 (5′-CTTGAAGAGGTTGGCACCCT-3′) (SEQ ID NO: 9) which is complementary to oli10, 200 μM of each dNTP in 1×DyNAzyme DNA polymerase buffer and 2.0 U DyNAzyme DNA polymerase (Finnzymes, Espoo, Finland) with a MJ Research thermal cycler PTC-0225. The reaction mixture was first carried out by denaturing at 95° C. for 5 minutes, followed by 30 cycles of denaturing at 95° C. (0:50 minutes), annealing at 55° C. for 50 seconds and extension at 72° C. (2 minutes). This was then followed with a final extension for 7 minutes at 72° C. to obtain A overhangs. Control PCRs were also performed with only primer Am508 or only oli11 under the conditions given above.  
     [0181] Analyzing, Purification and Cloning of the PCR Product  
     [0182] Seven microliters of the PCR re-amplification products were taken for 1% TAE agarose gel electrophoresis to confirm the identity of the PCR products and the patterns compared between the control PCRs (primer Am508 or primer oli11) and the main PCRs (primer pair oli11 and Am508). Before cloning, twenty microliters of the PCR products were loaded on thick 1% TAE agarose electrophoresis gels. The same PCR pattern was obtained from the T4 and RM378 RNA ligase ligations (FIG. 2). No amplification was obtained in the oli11 control PCR. Visible amplification DNA products of 0.5-3.0 kb (mostly smears) were observed on agarose gels in the control PCR where only the primer Am508 was used, giving a thick band at approximately 1500 bp. The main PCR (primer pair oli11 and Am508) gave amplification products of 0.2-3.0 kb whereas four bands were visible. Their sizes were approximately 1.5 kb, 0.5 kb and two below 0.5 kb. Compared to the control PCR of the primer Am508, three extra bands are visible, supporting that the oli10 ligation was successful. Bands and smears from the main PCR (primer pair oli11 and Am508) were purified by using spin columns, GFX PCR DNA and Gel Band Purification kit according to the manufacturer (Amersham Biosciences, Hørsholm, Denmark). The samples were eluted with 20 μl of H 2 O. Then the purified PCR products (4 μl) were cloned by the TA cloning method (Zhou and Gomes-Sanchez,  Curr. Issues Mol Bio.  2:1-7(2000)). Clones were grown overnight and their inserts were amplified with M13 reverse and M13 forward primers. The PCR amplification was carried out in 15 μl, containing 0.8 μl overnight culture, 0.5 μM M13 reverse primer, 0.5 μM M13 forward primer, 200 μM of each dNTP in 1×DyNAzyme DNA polymerase buffer and 0.75 U DyNAzyme DNA polymerase (Finnzymes, Espoo, Finland) with a MJ Research thermal cycler PTC-0225. The reaction mixture was first carried out by denaturing at 95° C. for 2 min, followed by 30 cycles of denaturing at 95° C. (0:50 minutes), annealing at 50° C. for 50 seconds and extension at 72° C. (2 minutes). Prior to sequencing, the PCR products were purified with PCR Product Pre-Sequencing Kit according to the instructions of the producer (USB). Inserts in a total of 79 clones were sequencd from samples ligated with T4 RNA ligase and a total of 85 clones were sequenced from samples ligated with RM378 RNA ligases. The gene inserts were sequenced with M13 reverse and M13 forward primers on ABI 3700 DNA sequencers by using a BigDye terminator cycle sequencing ready reaction kit according to the instructions of the manufacturer (PE Applied Biosystems, Foster City, Calif.). All sequences were analyzed in Sequencer 4.0 for Windows (Gene Codes Cooperation, Ann Arbor, Mich.) and XBLAST searched (Altschul, et al., 1990; Altschul, et al., 1997). For both of the RNA ligases the single primer method with primer Am508 retrieved different proteins, in various lengths. Their lengths were from 150-850 bp. The sequences showed the highest sequence identity to variable proteins, e.g. dehydrogenases, amylases, endoglucanases, carboxylases, a kinase and an oxidase.  
     [0183] The main purpose of the study was to analyze the ligation efficiency of the RM378 RNA ligase, that is its efficiency to ligate a short oligonucleotide to longer single stranded PCR products, variable in length. Of the 85 sequences obtained by the RM378 ligation, 66 sequences had ligated the short oli10 oligonucleotide to its end, corresponding to 78% of the sequences. This is according to the FIG. 2 were three new bands are obtained in the PCR by the main Am508/oli11 PCR versus the control Am508 PCR. The results for the T4 RNA ligase showed that 47 out of the 79 sequences (60%) had the oli10 oligonucleotide ligated to its end.  
     Example 9  
     Gene Synthesis using RM378 RNA Ligase  
     [0184] The purpose of the study was to determine if the RM378 RNA ligase could be used for gene synthesis where single stranded oligos of 60-150 bases in length are ligated one after another. Using this approach for the gene synthesis, thermostable enzyme can be important due to the undesirable formation of secondary structures at lower temperatures, which increases with sequence length. Therefore, gene synthesis of this kind is useful. Furthermore, the method can be used to optimize codon usage because a host&#39;s preferred codons for amino acids seem to dramatically improve gene expression. Additionally, the method described herein can be used to modify transcription promoters or translation factor sites or add or remove protein functional domains. The human insulin-like growth factor (IGFA) gene is used as a model for the gene synthesis.  
     [0185] IGFA Oligonucleotides Synthesis  
     [0186] Human insulin-like growth factor (IGFA) gene (ECBI accession number P01343) was synthesized. The codon usage of the IGFA gene was changed to  E. coli  codon usage and then the gene was split up and three oligonucleotides were designed. The 5′ and 3′ end oligonucleotides where designed with 13-14 bases long linkers. The oligonucleotides were I1 (5-cggcgaattctttatgggtccggaaaccctgtgcggtgctgaactggttgatgctctgcaattcgtttgcggtgatcgtggtttctac ttcaa-3′), (SEQ ID NO: 6) I2 (5′-caaaccgaccggttacggttcttcttctcgtcgtgctccgcaaaccggtatcgttgatgaa-3′) (SEQ ID NO: 10) and I3 (5′-tgctgcttccgttgcgatctgcgtcgtctggaaatgtactgcgctccgctgaaaccggctaaatctgcttaaggatcccggcg-3′) (SEQ ID NO: 11). The I3 oligonucleotide was biotin labeled at the 3′ end and phosphorylated at the 5′ end. The oligonucleotides were manufactured by Transgenomics, Cruachem Limited (Glasgow, Scotland).  
     [0187] Immobilization of Biotin Labeled Oligonucleotide I3 with Dynabeads M-280 Streptavidin  
     [0188] The biotin labeled oligonucleotide I3 (10 pmoles) was immobilized by using 150 μg of streptavidin-coated magnetic beads (Dynal, Oslo, Norway) according to the instructions of the manufacturer. The captured biotin labeled olionucleotide I3 was resuspended in 5 μl of dH 2 O. Before the immobilization procedure, the oligonucelotide was heated to 90° C. for 1 minute. The immobilized oligonucleotide I3 was then subjected to different ligation reactions as described below.  
     [0189] Ligation between Oligonucleotides I3 and I2 using T4-RNA Ligase or RM378 Ligase  
     [0190] Prior to ligation, the immobilized oligonucleotides I3 and I2 were heated at 90° C. for 1 minute. In the presence of 20 U of T4 RNA ligase (New England BioLabs, Beverly, Mass.), 1×T4 RNA ligation buffer (50 mM Tris-HCl, pH 7.8, 10 mM MgCl 2 , 10 mM DTT and 1 mM ATP), 10% PEG8000, 50 pmole of oligonucleotide I2 was added to the captured oligonucleotide I3 (10 pmole) in a final volume of 20 μl. The mixture was incubated at 22° C. for 20 hours. The new ligation product was called oligoA-T4.  
     [0191] In the presence of 0.5 μg of RM378 RNA ligase (in 50% glycerol) and 1×MOPS ligation buffer (50 mM MOPS, pH 7.0, 10 mM MgCl 2 , 10 mM DTT, 0.5 μg BSA and 1 mM ATP), 50 pmole of oligonucleotide I2 was added to the captured oligonucleotide I3 (10 pmol) in a final volume of 20 μl. The mixture was incubated at 60° C. for 20 hours. The new ligation product was called oligoA-RM.  
     [0192] After the ligation and the formation of oligoAT4 and oligoA-RM, the rest of oligonucleotide I2 that was not ligated to oligonucleotide I3 was removed by washing the solution twice with 100 μl of dH 2 O. After the washing, the samples were resuspended in 17 μl of dH 2 O.  
     [0193] Phosphorylation of the 5′End of oligoA (Former 5′End of oligonucleotide I2) with Polynucleotide Kinase  
     [0194] In order to subject oligoA-T4 and oligo-RM to further ligation, their 5′ends were phosphorylated in the presence of 10 U of 1 polynucleotide kinase (New England BioLabs, Beverly, Mass.) and 1×PNK buffer (70 mM Tris-HCl, pH 7.6, 10 mM MgCl 2  and 5 mM dithiothreitol) in a final volume of 20 μl. The mixture was incubated at 37° C. for 30 minutes. Inactivation of the enzyme was done be heating at 65° C. for 20 minutes.  
     [0195] After the phosphorylation the solutions were washed twice with 100 μl of dH 2 O as described above. After the washing, the samples were resuspended in 5 μl of dH 2 O.  
     [0196] Ligation between Oligonucleotide I1 and oligoA-T4 using T4-RNA Ligase and Ligation between Oligonucleotide I1 and oligoA-RM using RM378 Ligase  
     [0197] Prior to the next ligation step, the ligation products oligoA-T4 and oligoA-RM were heated at 90° C. for 1 minute as well as oligonucleotide I1 (50 pmol) in order to minimize secondary structures. The ligation reactions were as described above whereas the ligation between oligonucleotide I1 and oligoA-T4 was done with T4-RNA ligase giving the product oligoB-T4 and ligation between oligonucleotide I1 and oligoA-RM by using RM378 ligase, giving the product oligoB-RM. After the ligation, the solutions were washed twice with 100 μl of dH 2 O as described above. After the washing, the samples were resuspended in 20 μl of dH 2 O.  
     [0198] PCR of the Synthesized IGFA Gene (oligoB) by Two Primers  
     [0199] Different PCR amplifications from the ligation solutions allowed to detect if the ligations were successful. This analysis was done by three different PCRs. First to check if the whole gene was formed by using primers complementary for oligonucleotides I1 and I3 (primers IGFA-r and IGFA-f which should give a band of approximately 240 bp). The other two PCRs were to check the ligation reactions. One of the control ligation PCR was to check if the first ligation was successful by using primers complimentary for I3 and I2 (formation of oligoA: primers IGFA-r and IGFA-2f giving a band of approximately 150 bp). The other control ligation PCR was to check if the second ligation was successful by using primers complimentary for I1 and I2 (formation of oligoB: primers IGFA-2r and IGFA-f giving a band of approximately 150 bp). The PCRs were carried out in 50 μl reaction mixture containing 2 μl ligation mixture, 1.0 μM reverse primer IGFA-r (5′-CCGGGATCCTTAAGCAGATT-3′) (SEQ ID NO: 12): complementary to I3) or IGFA-2r (5′-TCATCAACGATACCGGTTTGC-3′) (SEQ ID NO: 13): complementary to I2), 1.0 μM primer IGFA-f (5′-GGCGAATTCTTTATGGGTCCGGAAAC-3′) (SEQ ID NO: 14: complementary to I1) or 1.0 μM primer IGFA-2f (5′-ACCGACCGGTTACGGTTCTTC-3′) (SEQ ID NO: 15): complementary to I2), 200 μM of each dNTP in 1×DyNAzyme DNA polymerase buffer and 2.0 U DyNAzyme DNA polymerase (Finnzymes, Espoo, Finland) with a MJ Research thermal cycler PTC-0225. The reaction mixture was first carried out by denaturing at 95° C. for 5 minutes, followed by 30 cycles of denaturing at 95° C. (0:50 min), annealing at 55° C. for 50 seconds and extension at 72° C. (2 minutes). This was then followed with a final extension for 7 minutes at 72° C. to obtain A overhangs.  
     [0200] Analyzing, Purification and Cloning of the PCR Products  
     [0201] Seven microliters of the PCR reamplification products were taken for 1% TAE agarose gel electrophoresis to confirm that the two ligation reactions had occurred. Visible amplification DNA products of approximately 200 b were observed on agarose gels for both of the ligation reactions and the whole gene PCR (FIG. 3). The bands were cloned to confirm the ligation reactions and the gene synthesis. Before cloning, twenty microliters of the PCR products were loaded on thick 1% TAE agarose electrophoresis gels. The bands were purified by using spin columns, GFX PCR DNA and Gel Band Purification kit according to the manufacturer (Amersham Biosciences, Hørsholm, Denmark). The samples were eluted with 20 μl of H 2 O. The purified PCR products (4 μl) were then cloned by the TA cloning method (Zhou and Gomez-Sanchez,  Curr. Issues Mol. Biol.  2:1-7 (2000)).  
     [0202] Clones were grown overnight and their inserts were amplified with M13 reverse and M13 forward primers. The PCR amplification was carried out in 15 μl, containing 0.8 μl overnight culture, 0.5 μM M13 reverse primer, 0.5 μM M13 forward primer, 200 μM of each dNTP in 1×DyNAzyme DNA polymerase buffer and 0.75 U DyNAzyme DNA polymerase (Finnzymes, Espoo, Finland) with a MJ Research thermal cycler PTC-0225. The reaction mixture was first carried out by denaturing at 95° C. for 2 minutes, followed by 30 cycles of denaturing at 95° C. (0:50 min), annealing at 50° C. for 50 seconds and extension at 72° C. (2 minutes). Prior to sequencing, the PCR products were purified with PCR Product Pre-Sequencing Kit according to the instructions of the producer (USB). The gene inserts were sequenced with M13 reverse primer on ABI 3700 DNA sequencers by using a BigDye terminator cycle sequencing ready reaction kit according to the instructions of the manufacturer (PE Applied Biosystems, Foster City, Calif.). All sequences were analyzed in Sequencer 4.0 for Windows (Gene Codes Cooperation, Ann Arbor, Mich.) and XBLAST searched (Altschul et al.,  J. Mol. Biol.,  215:403-410 (1990); Altschul et al.,  Biotechniques,  15:894-904 (1997)).  
     [0203] The sequencing result for the first ligation reaction (ligation between I3 and I2) were according to the PCR results, that is, the RM378 ligation was successful and a correct oligoA sequence was obtained. Wrong ligation products were also obtained (see below). The sequencing results for the second ligation reaction (ligation between I2 and I1) were according to the PCR results, that is, the ligation was successful. However, not a correct sequence was obtained for this ligation with the RM378 RNA ligase, that is oligonucleotide I1 was right but it was ligated to a truncated sequence of I2. This may be explained by the fact that in the synthesis of long oligonucleotides, different forms of truncated oligonucletides is a common artifact. For the T4 RNA ligase, right I1-I2 and I2-I3 products were formed as well as trunctated ones. The result that PCR product of I1-I2 was seen (although truncated for the RM378 RNA ligase), demonstrates that product I1-I2-I3 was actually formed. This is due to the fact that if I1-I2 was only formed but not I1-I2-I3, the I1-I2 or the I2 oligonucelotide product would have been washed away in one of the many washing steps if not ligated to I3, and therefore not observed. This means that only products ligated to I3 stay in the solution, as that is the only biotin labeled oligonucleotide.  
     [0204] The sequencing of the whole gene PCRs gave only I1-I3 products for both T4 and RM378 RNA ligases. However, as observed for the I1-I2 ligation, not a correct sequence was obtained for this ligation in all cases. About 40% of the I1-I3 products had a wrong sequence that is oligonucleotide I1 was normally right but ligated to a variable truncated sequences of I3. The cloning results indicate that I1-I3 products dominate the IGFA-r/IGFA-f PCR and we cannot see the I1-I2-I3 product, although, the I1-I2 ligations show that the I1-I2-I3 product was actually formed as mentioned above.  
     [0205] Although, the I1-I2-I3 product was not obtained in sequencing, this experiment shows that with the PCRs and sequencing results, the RM378 RNA ligase (as well as T4) can be used in sequential single stranded DNA ligations to ligate long oligonucleotides together, for the purpose of gene synthesis. In order to retrieve the whole gene, different PCR methods can be used, like the known gene splicing overlap (SOE) method (Lefebvre et al., 1995,  Biotechniques  19:186-8).  
     [0206] IGFA gene with a  E. coli  codon usage and linkers (italic and bold) at the 3′ and 5′ ends.  
                          (SEQ ID NO: 16)                           atgggtccggaaaccctgtgcggtgctgaactggttg                   atgctctgcaattcgtttgcggtgatcgtggtttctacttcaacaaaccg               accggttacggttcttcttctcgtcgtgctccgcaaaccggtatcgttga               tgaatgctgcttccgttcttgcgatctgcgtcgtctggaaatgtactgcg               ctccgctgaaaccggctaaatctgct .          
 
     [0207] RM ligations  
                          Sequence for I1 and I2 ligation           (truncated form of I2)       ATGGGTCCGGAAACCCTGTGCGGTGCTGAACTGGTTGATGCTCTGCAATT       CGTTTGCGGTGATCGTGGTTTCTACTTCAAGTTCTTCTTCTCGTCGTGCT       CCGCAAACCGGTATCGTTGATGA               Sequence for I2 and I3 ligation (right ligation)       CCGACCGGTTACGGTTCTTCTTCTCGTCGTGCTCCGCAAACCGGTATCGT       TGATGAATGCTGCTTCCGTTCTTGCGATCTGCGTCGTCTGGAAATGTACT       GCGCTCCGCTGAAACCGGCTAAATCTGCTTAA               Sequence for I1 and I3 ligation (right ligation)       ATGGGTCCGGAAACCCTGTGCGGTGCTGAACTGGTTGATGCTCTGCGATT       CGTTTGCGGTGATCGTGGTTTCTACTTCAATGCTGCTTCCGTTCTTGCGA       TCTGCGTCGTCTGGAAATGTACTGCGCTCCGCTGAAACCGGCTAAATCTG       CTTAA          
 
     Example 10  
     RLM-RACE (RNA Ligase Mediated Rapid Amplification of cDNA Ends)  
     [0208] Generally, this method can be used for example to obtain 5′ ends of mRNA molecules if only a part of the sequence is known. In this example, a RACE experiment was done using some components from the GeneRacer core kit (Invitrogen Inc.) plus additional components. The RNA sample used contained the control RNA provided with the GeneRacer kit.  
     [0209] The RNA was dephosphorylated with calf intestial phosphatase (CIP) which dephosphorylates all RNA except capped mRNA. The reaction conditions were as follows:  
                                          Total RNA   5   μg (Total RNA from HeLa cell line)       10 × CIP buffer   1   μl       RnaseOUT (40 U/ml)   1   μl       CIP (10 U/ml)   1   μl       DEPC treated water to   10   μl                  
 
     [0210] The solution was mixed and incubated for 1 hour at 50° C., then centrifuged and put on ice. The RNA was purified using the RNeasy kit (QIAgen Inc.) according to the manufacturer&#39;s instructions and resuspended in 30 μl DEPC (to remove RNAse contamination) treated water.  
     [0211] Decapping was done on the full length mRNA with Tobacco Acid Pyrophosphatase (TAP). The reaction conditions were as follows:  
                                                          CIP treated RNA   7   μl           10x TAP buffer   1   μl           TAP(0.5 U/ml)   1   μl           RNAseOUT (40 U/ml)   1   μl           Total   10   μl                      
 
     [0212] The solution was mixed and incubated at 37° C. for 1 hour. The RNA was then purified using the RNeasy kit (QIAgen Inc.) and resuspended in 30 μl DEPC treated water.  
     [0213] The GeneRacer RNA Oligo was ligated onto the decapped mRNA with RNA ligase using using T4 RNA ligase (5U per reaction) and RM378 (5 U per reaction), respectively. During this step, the RNA solution (7 μl) was mixed with pre-aliquoted, lyophilized GeneRacer RNA oligonucelotide (0.25 mg) and a second oligonucleotide 5′-CGACUGGAGCACGAGGACACUGACAUGGACUGAAGGAGUAGAAA-3′) (SEQ ID NO: 17).  
     [0214] The RNA solution for the T4 RNA ligase ligation was heated to 65° C. for 5 minutes and then spun down and put on ice, in order to minimize secondary structure. This was not done for the RNA solution for the RM378 RNA ligase reaction. The reaction solution was as follows:  
                                              Decapped RNA   6   μl           10x RNA ligase buffer   1   μl   (MOPS buffer for the RM378 RNA                   ligase, and the supplied buffer with                   the GeneRacer kit for T4 RNA                   ligase)       ATP (10 mM)   1   μl       RNAseOUT   1   μl   (RNAse inhibitor from CHIMERx                   (50 U) was used at 60° C. since it                   is stable below 70° C.)       RNA ligase   1   μl       Total   10   μl                  
 
     [0215] The reaction was incubated at 37° C. for 1 hour for the T4 RNA ligase and 1 hour at 60° C. for the RM 378 RNA ligase. The RNA was then purified using the RNeasy kit (QIAgen Inc.) and resuspended in 30 ml DEPC treated water.  
     [0216] cDNA synthesis was performed in few steps. First, an annealing step with the following conditions:  
                                                          Ligated RNA   18.4   μl           dT20 oligo (50 μM)   1.6   μl           Total   20   μl                      
 
     [0217] The solution was incubated at 70° C. for 10 minutes and cooled on ice.  
     [0218] Second, a first strand synthesis in a solution made of:  
                                                      5× First strand synthesis buffer     6 μl           PowerScript RT (Clontech)   1.5 μl           dNTP mix (10 m each)     3 μl           DTT (100 mM)     3 μl           RNAase OUT (40 U/ml)   1.5 μl           RNA and dT20 mixture    15 μl                      
 
     [0219] The solution was incubated 42° C. for 70 minutes and the reaction terminated by heating at 70° C. for 15 minutes. The solution was then centrifuged and put on ice. 1.5 ml RNAseH solution (2 U/ml, Stratagene Inc.) was then added, the solution mixed and incubated for 20 minutes at 37° C. and put on ice.  
     [0220] Third, a polymerase chain reaction (PCR) was done (Using AmpliTaq Gold™ DNA polymerase (Applied Biosystems), see manufacturer instructions for details) with 0.1-1.0 μl of the previous solution for 30 ml PCR reaction. The primers used were GeneRacer 5′ Primer (SEQ: 5′-CGACTGGAGCACGAGGACACTGA-3′) (SEQ ID NO: 18) or GeneRacer 5′ nested primer (5′-GGACACTGACATGGACTGAAGGAGTA-3′) (SEQ ID NO: 19) and GeneRacer 5′ control primer B1 (Beta actin gene specific primer) (5′-GACCTGGCCGTCAGGCAGCTCG-3′) (SEQ ID NO: 20). The reaction solution was made of:  
                                                      cDNA     1 μl           10× Gold buffer     3 μl           MgC12 solution     3 μl           AmpliTaq (5 U/ml)    0.3 μl           dNTPs (2 mM)     3 μl           Water   19.7 μl                      
 
     [0221] The PCR was done according to the following program:  
                                                       Temperature   Time   Cycles                                                        94° C.   12 min   1           94° C.   30 sec   4           72    2 min           94° C.   30 sec   4           70° C.    2 min           94° C.   30 sec   30           Gradient 55-70° C.   30 sec.           72° C.    2 min                              4° C.   for length of storage                      
 
     [0222] After the reaction, 5 ml of the PCR product were run on a 0.8% agarose gel.  
     [0223] Results:  
     [0224] A PCR product with a size similar to the expected size was generated (FIG. 13). These results demonstrate that the RM378 RNA ligase can be used in a RLM-RACE procedure.  
     [0225] All references cited herein are incorporated by reference in their entirety. While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.