TEMPLATE-DEPENDENT LIGATION WITH PNA-DNA CHIMERIC PROBES

Abstract: The invention provides methods, kits, and compositions for ligation of PNA-DNA chimeric probes and oligonucleotides when they are hybridized adjacently to template nucleic acids using ligases and ligation reagents. Structural requirements of the chimeras for ligation include 5 to 15 contiguous PNA monomer units, 2 or more contiguous nucleotides, and a 3' hydroxyl or 5' hydroxyl terminus. The chimera and/or oligonucleotide may be labelled with fluorescent dyes or other labels. The methods include, for example, oligonucleotide-ligation assays (OLA) and single nucleotide polymorphism detection.

Description: TEMPLATE-DEPENDENT LIGATION WITH PNA-DNA CHIMERIC PROBES I. FIELD OF THE INVENTION The invention relates generally to the fields of enzymology and nucleic acid analogs. Specifically, this invention is directed to template-dependent ligation of PNA-DNA chimeras and oligonucleotides with ligase enzymes. II. REFERENCES Andrus, A. "Chemical methods for 5' non-isotopic labelling of PCR probes and primers" (1995) in PCR 2: A Practical Approach, Oxford University Press, Oxford, pp. 39-54. Barany, F. "Genetic disease detection and DNA amplification using cloned thermostable ligase", Proc. Natl. Acad. Sci. USA 88:189-93 (1991). Barany, F., Hood, L., Kaiser, R., Nickerson, D., Zebala, J. " New thermostable DNA ligase - obtd. using DNA from Thermus aquaticus strain HB8, used for ligation of oligonucleotides", WO 91/17239, Intl. Publ. Date Nov. 14, 1991. Beaucage, S. and Iyer, R. "Advances in the synthesis of oligonucleotides by the phosphoramidite approach", Tetrahedron 48:2223-2311 (1992). Benson, S., Menchen, S., Theisen, P., Uphadhya, K., and Hauser, J. "Aromatic-substituted xanthene dyes", WO 99/16832, Intl. Publ. Date Apr. 8, 1999. Bergot, B., Chakerian, V., Connell, C, Eadie, J., Fung, S., Hershey, N., Lee, L., Menchen, S. and Woo, S. "Spectrally resolvable rhodamine dyes for nucleic acid sequence determination", U.S. Patent 5,366,860, issued Nov. 22, 1994. Blackburn, G. and Gait, M. Eds. "DNA and RNA structure" in Nucleic Acids in Chemistry and Biology, 2 nd Edition, (1996) Oxford University Press. Breipohl, G., Will, D.W., Peyman, A. & Uhlmann, E. "Novel synthetic routes to PNA monomers and PNA-DNA linker molecules", Tetrahedron 53:14671-86 (1997). Bronstein, I. and Voyta, J., "Methods of using chemiluminescent 1,2-dioxetanes" U.S. Patent 4,931 ,223, issued Jun. 5, 1990. Buchardt, O., Egholm, M., Nielsen, P., and Berg, R. "Peptide Nucleic Acids", WO 92/20702, Intl. Pub. Date 26 November 1992. Caruthers, M. and Beaucage, S. "Phosphoramidite compounds and processes", U.S. Patent No. 4,415,732, issued Nov. 15, 1983. Caruthers, M. and Matteucci, M. "Process for preparing polynucleotides", US Patent No. 4,458,066, issued July 3, 1984. Clegg, R., "Fluorescence resonance energy transfer and nucleic acids", Meth. Enzymol., 211 :353-388 (1992). Cook, P. "PNA-DNA-PNA Chimeric macromolecules"; US Patent No. 5,700,922, issued Dec. 23, 1997. Delahunty, C, Ankener, W., Deng, Q., Eng, J. and Nickerson, D. "Testing the feasibility of DNA typing for human identification by PCR and an oligonucleotide ligation assay", Am. J. Hum. Genetics 58:1239-46 (1996). Egholm, M., Christensen, L., Dueholm, K., Buchardt, O., Coull, J., and Nielsen, P. "Efficient pH-independent sequence-specific DNA binding by pseudoisocytosine- containing bis-PNA", Nucleic Acids Res. 23:217-22 (1995). Egholm, M., Buchardt, O., Christensen, L., Behrens, C, Freier, S., Driver, D., Berg, R. and Kim, S. "PNA hybridizes to complementary oligonucleotides obeying the Watson- Crick hydrogen bonding rules", Nature 365:566-68 (1993). Finn, P.J., Gibson, N.J., Fallon, R., Hamilton, A. & Brown, T. "Synthesis and properties of DNA-PNA Chimeric oligomers", Nucleic Acids Research 24:3357-63 (1996). Flanagan, W., Wagner, R., Grant, D., Lin, K. and Matteucci, M. "Cellular penetration and antisense activity by a phenoxazine-substituted heptanucleotide", Nature Biotech. 17:48-52 (1999). Fodor, S., Pirrung, M., Read, J., and Stryer, L. "Array of oligonucleotides on a solid substrate", US Patent No. 5,445,934, issued Aug. 29, 1995. Grossman, P., Bloch, W., Brinson,E., Chang, C, Eggerding, F., Fung, S., lovannisci, D., Woo, S. and Winn-Deen, E. "High-density multiplex detection of nucleic acid sequences: oligonucleotide ligation assay and sequence-coded separation", Nucl. Acids Res. 22:4527-34 (1994). Guo, Z., Guilfoyle, R., Thiel, A., Wang, R. and Smith, L. "Direct fluorescence analysis of genetic polymorphisms by hybridization with oligonucleotide arrays on glass supports", Nucl. Acids Res. 22:5456065 (1994). - 7 . Jensen, K., 0rum, H., Nielsen, P., Norden, B. "Kinetics for hybridization of peptide nucleic acids (PNA) with DNA and RNA studied with the BIAcore technique", Biochem 36:5072-77 (1997). Khan, S., Menchen, S., Rosenblum, B. "Substituted propargylethoxyamido nucleosides, oligonucleotides and methods for using same", U.S. Patent 5,770,716, issued Jun. 23, 1998, and "Propargylethoxyamino nucleotides", U.S. Patent 5,821,356, issued Oct. 13, 1998 Koppitz, M., Nielsen, P., Orgel, L. "Formation of Oligonucleotide-PNA-Chimeras by Template-Directed Ligation", J. Am. Chem. Soc, 120:4563-69 (1998). Kornberg, A. in DNA Replication (1980), W.H. Freeman and Co., San Francisco, pp. 261- 76. Kricka, L. in Nonisotopic DNA Probe Techniques (1992), Academic Press, San Diego, pp. 3-28. Kubista, M. and Svanvik, N. "Probe for analysis of nucleic acids", WO 97/45539, Intl. Publ. Date Dec. 4, 1997. Kutyavin, I., Lukhtanov, E., Gamper, H. and Meyer, R. "Covalently linked oligonucleotide minor groove binder conjugates", WO 96/32496, Intl. Publ. Date Oct. 17, 1996. Kyger, E., Krevolin, M. and Powell, M. "Detection of the hereditary hemochromatosis gene mutation by real-time fluorescence polymerase gene reaction and peptide nucleic acid clamping", Anal. Biochem. 260:142-48 (1998). Landegren, U., Kaiser, R., Sanders, J. and Hood, L. "A ligase mediated gene detection technique", Science 241 : 1077-80 (1988). Lee, L., Spurgeon, S., Rosenblum, B. "Energy transfer dyes with enhanced fluorescence", U.S. Patent 5,800,996, issued Sep. 1, 1998. Lutz, M., Will, D., Breipohl, G, Benner, S. and Uhlmann, E. "Synthesis of a monocharged peptide nucleic acid (PNA) analog and its recognition as substrate by DNA polymerase", Nucleosides & Nucleotides 18:393-401 (1999). Lutz, M., Benner, S., Hein, S., Breipohl, G. and Uhlmann, E. "Recognition of uncharged polyamide-linked nucleic acid analogs by DNA polymerases and reverse transcriptatses", J. Am. Chem. Soc. 1 19:3177-78 (1997). Menchen, S., Lee, L., Connell, C., Hershey, N., Chakerian, V., Woo, S. and Fung, S. "4,7- Dichlorofluorescein dyes as molecular probes", U.S. patent 5,188,934, issued Feb. 23, Meyer, R. "Incorporation of modified bases in oligonucleotides" in Protocols for Oligonucleotide Conjugates, Ed. S. Agrawal (1994) Humana Press, Totowa, NJ, pp. 73-92. Mullah, B. and Andrus, A. "Solid support reagents for the direct synthesis of 3'-labeled polynucleotides", U.S. Patent 5,736,626, issued Apr. 7, 1998. Nickerson, D., Kaiser, R., Lappin, S., Stewart, J., Hood, L. and Landegren, U. "Automated DNA diagnostics using an ELISA-based oligonucleotide assay" Proc. Natl. Acad. Sci USA 87:8923-27 (1990) Nielsen, P., Egholm, M., Berg, R. and Buchardt, O. "Sequence-selective recognition of DNA by strand displacement with a thymidine-substituted polyamide", Science 254: 1497-1500 (1991). Rajur, S., Robles, J., Wiederholt, K., Kuimelis, R. and McLaughlin, L. "Hoechst 33258 tethered by a hexa (ethylene glycol) linker to the 5 '-termini of oligodeoxynucleotide 15-mers: duplex stabilization and fluorescence properties", J. Organic Chem. 62:523- 29 (1997). Stanton, T., Schindele, D., Renzoni, G., Pepich, B., Anderson, N., Clagett, J. and Opheim, K. "Preparation and use of monomeric phthalocyanine reagents", WO 8804777, Intl. Publ. Date: June 30, 1988. Stetsenko, D.A., Lubyako, E.N., Potapov, N.K., Azhikina, T.L. & Sverdlov, E. "New Approach to Solid Phase Synthesis of Polyamide Nucleic Acids Analogues (PNA) and PNA-DNA Conjugates", Tetrahedron Lett. 37:3571-74 (1996). Takahashi, M. etal, "Thermophilic DNA ligase", J. Biol. Chem. 259:10041-47 (1984). Theisen, P., McCollum, C. and Andrus, A. "Fluorescent dye phosphoramidite labelling of oligonucleotides", in Nucleic Acid Symposium Series No. 27, Oxford University Press, Oxford, pp. 99-100 (1992). Uhlmann, E. "Peptide nucleic acids (PNA) and PNA-DNA chimeras: from high binding affinity towards biological function", Biol. Chem. 379:1045-52 (1998). Uhlmann, E., Will, D.W., Breipohl, G., Langner, D. & Ryte, A. "Synthesis and properties of PNA DNA chimeras", Angew. Chem., Intl. Ed. Eng. 35:2632-35 (1996). Uhlmann, E., Peyman, A., Breipohl, G. and Will, D. "PNA: Synthetic polyamide nucleic acids with unusual binding properties", Angew. Chem., Intl. Ed. Eng. 37:2796-2823 (1998). Van der Laan, A.C. et al. "Optimization of the binding properties of PNA-(5')-DNA Chimerae", Bioorg. Med. Chem. Lett. 8:663-68 (1998). Van der Laan, A., Brill, R., Kuimelis, R., Kuyl-Yeheskiely, E., van Boom, J., Andrus, A. and Vinayak, R. "A convenient automated solid-phase synthesis of PNA-(5')-DNA- (3')-PNA chimera", Tetrahedron Lett. 38:2249-52 (1997). Vinayak, R., van der Laan, A., Brill, R., Otteson, K., Andrus, A., Kuyl-Yeheskiely, E. and van Boom, J. "Automated chemical synthesis of PNA-DNA chimera on a nucleic synthesizer", Nucleosides & Nucleotides 16:1653-56 (1997). Vinayak, R. "Process and compounds for RNA synthesis", U.S. Patent No. 5,281,701, issued Jan. 25, 1994. Wengel, J. "Oligonucleotide analogs", WO 99/14226, Intl. Publ. Date 25 Mar. 1999. Whiteley, N., Hunkapiller, M. and Glazer, A. "Detection of specific sequences in nucleic acids", U.S. Patent 4,883,750, issued 1989. Will, D.W., Breipohl, G., Langner, D., Knolle, J. & Uhlmann, E. "The Synthesis of Polyamide Nucleic Acids using a Novel Monomethoxytrityl Protecting-Group Strategy", Tetrahedron 51 :12069-12082 (1995). III. BACKGROUND The covalent joining of nucleic acid probes by ligase enzymes is one of the most useful tools available to molecular biologists. When two probes are annealed to a template nucleic acid where the two probes are adjacent and without intervening gaps, a phosphodiester bond can be formed by a ligase enzyme (Whiteley, 1989). The ligation bond is formed between a 5' terminus of one probe and the 3' terminus of the other probe. The events of annealing and ligation each require a high level of fidelity, i.e. complementarity, between the sequences of the ligating probes and the template nucleic acid. Both events are inefficient when base-pairing mismatches occur. Generally, DNA ligase can join two adjacent probes only when they perfectly complement a denatured template nucleic acid, such as a PCR product (Landegren, 1988; Nickerson, 1990). Even a single nucleotide mismatch at, or near, the ligation site of the probes will prevent ligation of the annealed probes. Oligonucleotide ligation assays detect the presence of specific sequences in target DNA sample. For example, allelic discrimination assays rely on probes representing the complementary sequences of the allelic forms to the target. Ligation to a common, second target-complementary probe indicates the presence of the polymoφhic site (Whiteley, 1989; Landegren, 1988). Absence of ligation indicates the lack of the polymoφhic site. Ligation can be detected through detectable labels on the allelic probe and electrophoretic separation of the ligation products (Grossman, 1994). It is desirable to provide optimized probes and methods of annealing and ligation. Such methods would improve assays and tests that benefit from greater precision and accuracy. IV. SUMMARY The invention relates to chimeric molecules of PNA and DNA monomer units and their use in ligation methods to generate ligation products. The invention is based in part on the discovery that a ligase enzyme can ligate a PNA-DNA chimeric probe and a second probe under a broad range of experimental conditions and variables. PNA-DNA chimeras of the invention comprise at least two moieties covalently linked together, preferably: i) a contiguous moiety of 3 to 15 PNA monomer units, and ii) a contiguous moiety of at least two nucleotides. The nucleotide moiety has a ligatable terminus, such that the PNA-DNA chimera can be ligated to a second probe. In a first aspect, the invention provides a method of producing a template-dependent ligation product by ligating a PNA-DNA chimeric probe, annealed to a template nucleic acid in the presence of a ligase and a ligation reagent, to a second probe annealed adjacent to the chimeric probe on the template nucleic acid. The second probe is capable of supporting template-dependent ligation. The second probe is a PNA-DNA chimera or an oligonucleotide. The second probe may be 5 to 100 monomer units or nucleotides (nt) in length. Preferably the second probe is 10 to 30 nt. Together, the chimeric probe and the second probe may be 10 to 100 nt. In one illustrative embodiment of the invention, the PNA-DNA chimera has the formula: where each P is independently a PNA monomer, x is an integer from 3 to 15, L represents a covalent linkage between P and N, each N is independently a nucleotide, y is an integer from 2 to 15, and the terminal N has either a 3' hydroxyl group or 5' hydroxyl group. In a preferred embodiment, the PNA moiety, i.e., P x , of the PNA-DNA chimera is a 2-aminoethylglycine peptide nucleic acid. The DNA moiety, i.e., N y , of the PNA-DNA chimera may be comprised of 2'- deoxynucleotides (DNA), ribonucleotides (RNA), and modified sugars or internucleotide linkages thereof, especially those that confer greater specificity, affinity, rate of hybridization, and chemical stability. The chimera and/or the second probe may be labelled with a non-radioisotopic label such that the ligation product is non-radioisotopically labelled. In embodiments employing a labelled PNA-DNA chimera, the PNA-DNA chimera may be labelled at: (i) a nucleobase, e.g. the 7-deaza or C-8 positions of a purine or a deazapurine nucleobase, or the C-5 position of a pyrimidine nucleobase; (ii) a sugar; (iii) the PNA backbone; or (iv) an amino, a sulfide, a hydroxyl, and/or a carboxyl group. Preferably, the chimera is labelled at the amino terminus of the PNA moiety. In embodiments employing a labelled oligonucleotide, the oligonucleotide is preferably labelled at the opposite terminus from the ligation site, 3' or 5'. Alternatively, the oligonucleotide may be labelled at a nucleobase, but may also be labelled at other positions provided that the label does not interfere adversely with hybridization affinity or specificity, or with ligase efficiency. Labels may be fluorescent dyes, fluorescence quenchers, hybridization-stabilizers, energy-transfer dye pairs, electrophoretic mobility modifiers, chemi luminescent dyes, amino acids, proteins, peptides, enzymes, and affinity ligands. Preferably, the label is detectable upon illumination with light, e.g. laser sources at infrared, visible or ultraviolet excitation wavelengths. The PNA and DNA moieties of the chimeric probe are covalently linked together. The linkage, L, between the PNA and DNA moieties may be a bond, e.g. the carbonyl- nitrogen bond in an amide group where the moieties are linked without intervening atoms, or a multi-atom linker. The linkage may comprise a phosphodiester group or a phosphoramidate group. The template or target nucleic acid can be any nucleic acid or nucleic acid analog capable of mediating template-directed nucleic acid synthesis. Examples of suitable template nucleic acids include, e.g., genomic DNA, DNA digests, DNA fragments, DNA transcripts, plasmids, vectors, viral DNA, PCR products, RNA, and synthetic nucleic acids. The template nucleic acid may also be a metaphase or inteφhase chromosome. Preferably, the chromosome is denatured prior to PNA-DNA chimera hybridization and ligation. Template nucleic acids may be single-stranded or double-stranded and can range from as few as about 20-30 to as many as millions of nucleotides (nt) or base-pairs (bp), depending on the particular application. The template nucleic acid, the PNA-DNA chimera, or the second probe may be immobilized on a solid substrate Ligations may be conducted where one of the probes or template is attached to a solid support or surface V. BRIEF DESCRIPTION OF THE DRAWINGS FIG 1 Structures of PNA and PNA-DNA chimeras with (lA) two 2'- deoxynucleotides, and (IB) three 2'-deoxynucleotιdes B is a nucleobase FIG 2 Structures of linker reagents and linkages (2 A) linker reagents to form amide and phosphodiester linkages, (2B) bis-amide linkage of 2-(2-ammoethoxy) ethoxyacetic acid, and (2C) amide, phosphate linkage of 2-(2-ammoethoxy)ethanol FIG 3 Generalized schematic of ligation (3 A) between a 3 '-hydroxyl PNA-DNA chimera and a 5 '-phosphate oligonucleotide hybndized to a DNA template with DNA ligase to form a PNA-DNA ligation product, and (3B) probe sequences and a 38nt perfect match DNA template for ligation expeπments FIG 4 Scanned images of PAGE analysis of ligation expeπments (4A top) with T4 DNA ligase and (4A bottom) without hgase, and (4B) quantitative estimate of ligation by densitometry, SpotDenso program FIG 5 Scanned images of PAGE analysis of ligation expenments with T4 DNA ligase (5A) SYBR-Green stained gel image, and (5B) schematic of ligation of PNA, PNA-DNA chimera, and DNA to 5'-phosphate oligonucleotides hybndized to a DNA template 38nt FIG 6 Scanned image of PAGE analysis of PNA-DNA chimera ligase reactions detecting wild-type and mutant sequences Specificity of PNA-DNA chimenc probe oligonucleotide ligation relative to oligonucleotide oligonucleotide ligation FIG 7 MALDI-TOF Mass Spectroscopy analysis of ligation reaction products (7A) without ligase and (7B) with ligase FIG 8 Oligonucleotide Ligation Assay (OLA) with PNA-DNA chimenc probes (8A) Determining the nature of a locus with different dye labels, (8B) Multiplex OLA with mismatched base at 5'-phosphate of oligonucleotides of different lengths and/or mobility modifiers, (8C) Multiplex OLA with mismatched base at 3' terminus of PNA-DNA chimeras of different lengths and/or mobility modifiers FIG 9 Oligonucleotide ligation assay with PNA-DNA chimera probes to discriminate mutations in human CFTR loci (9A) human pCFTR621G-T exon 4, (9B) human pCFTR1078delT exon 7, (9C) human pCFTRCό l D exon 1 1 (UPPER CASE - PNA, lower case - DNA), (9D) OLA with PNAι 0m er-DNA 3me r at CFTR locus 621G-T Visualized and recorded under UV illumination (top) and with SYBR Green staining (bottom) VI. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made m detail to the preferred embodiments of the invention While the invention will be descnbed in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to those embodiments On the contrary, the invention is intended to cover alternatives, modifications, and equivalents, which may be included withm the invention as defined by the appended claims VI 1 DEFINITIONS Unless stated otherwise, the following terms and phrases as used herein are intended to have the following meanings "Nucleobase" refers to a nitrogen-containing heterocyclic moiety, e g a punne, a 7- deazapunne, or a pynmidine. Typical nucleobases are adenine, guanine, cytosine, uracil, thymine, 7-deazaadenme, 7-deazaguanιne, and the like "Nucleoside" refers to a compound consisting of a nucleobase linked to the C-l' carbon of a nbose sugar "Nucleotide" 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 nbose sugar "Nucleotide 5'-tnphosphate" refers to a nucleotide with a tnphosphate ester group at the 5' position The tnphosphate ester group may include sulfur substitutions for the vanous oxygens, e g α-thio-nucleotide 5'-tnphosphates As used herein, the term "nucleic acid" encompasses the terms "oligonucleotide" and "polynucleotide" and means smgle-stranded and double-stranded polymers of nucleotide monomers, including 2'-deoxynbonucleotιdes (DNA) and nbonucleotides (RNA) The nucleic acid may be composed entirely of deoxyribonucleotides, entirely of nbonucleotides, or chimenc mixtures thereof, linked by mtemucleotide phosphodi ester bond linkages, and associated countenons, e g , H + , NH + , tnalkylammoniurn, Mg 2+ , Na + and the like Nucleic acids typically range in size from a few monomenc units, e g 5-40 when they are commonly referred to as oligonucleotides, to several thousands of monomenc 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 nght and that "A" denotes deoxyadenosme, "C" denotes deoxycytidine, "G" denotes deoxyguanosme, and "T" denotes thymidine, unless otherwise noted The term "Watson/Cnck base-pamng" refers to the hydrogen-bondmg base pamng commonly observed in double-stranded DNA "Attachment site" refers to a site on a moiety, e g a chimera or nucleotide, to which is covalently attached a linker "Linker" refers to a moiety that links one moiety to another, e g (l) a label to an oligonucleotide or PNA-DNA chimera, or (n) the PNA moiety to a DNA moiety in a PNA- DNA chimera "PNA-DNA Chimera" refers to an oligomer compnsed of (l) a contiguous moiety of PNA monomer units and (n) a contiguous moiety of nucleotide monomer units with an enzymatically-extendable terminus "Alkyl" refers to a saturated or unsaturated, branched, straight-chain, branched, or cyclic hydrocarbon radical denved by the removal of one hydrogen atom from a single carbon atom of a parent alkane, alkene, or alkyne Typical alkyl groups include, but are not limited to, methyl, ethyl, propyl, butyl, and the like In preferred embodiments, the alkyl groups consist of 1-12 saturated and/or unsaturated carbons "Cycloalkyl" refers to a cyclic alkyl radical Nitrogen atoms with cycloalkyl substituents may form azmdmyl, azetidinyl, pyrrohdinyl, pipendmyl, larger nngs, and substituted forms of heterocycles thereof "Alkyldiyl" refers to a saturated or unsaturated, branched, straight chain or cyclic hydrocarbon radical of 1-20 carbon atoms, and having two monovalent radical centers denved by the removal of two hydrogen atoms from the same or two different carbon atoms of a parent alkane, alkene or alkyne Typical alkyldiyl radicals include, but are not limited to, 1,2- ethyldiyl, 1 ,3-propyldιyl, 1 ,4-butyldιyl, and the like "Aryldiyl" refers to an unsaturated cyclic or polycyclic hydrocarbon radical of 6-20 carbon atoms having a conjugated resonance electron system and at least two monovalent radical centers denved by the removal of two hydrogen atoms from two different carbon atoms of a parent aryl compound Typical aryldiyl groups include, but are not limited to, radicals denved from benzene, substituted benzene, naphthalene, anthracene, biphenyl, and the like "Label" refers to any non-radioisotopic moiety covalently attached to a chimera or nucleotide that is detectable or imparts a desired functionality or property m the ligation extension product "Ligation" is the enzymatic joining by formation of a phosphodiester bond between a PNA-DNA chimenc probe and a second probe oligonucleotide when the chimera and the second probe are hybndized (annealed) adjacently and to a template nucleic acid VI.2 PNA-DNA CHIMERA In one aspect, the present invention utilizes chimenc probes which contain PNA moieties and DNA moieties The PNA moieties may be any backbone of acyclic, achiral, and neutral polyamide linkages to which nucleobases are attached A preferred form of the PNA moiety is a backbone of N-(2-amιnoethyl)-glycme, a peptide-hke, amide-hnked unit (Buchardt, 1992, Nielsen, 1991), as shown below in a partial structure with a carboxyl- terminal amide PNA oligomers themselves are not substrates for nucleic acid processing enzymes, such as DNA polymerases (Lutz, 1999, Kyger, 1998, Lutz, 1997) PNA-DNA chimeras are oligomers compnsed of 1) a contiguous moiety of PNA monomer units and 2) a contiguous moiety of nucleotides The two moieties are covalently linked together The nucleotide moiety of the chimera may be 2'-deoxynucleotιdes, nbonucleotides, or a mixture thereof The nucleotide moiety of the chimera has a 3' hydroxyl terminus The preferred length of the PNA moiety is from 3 to 15 PN Λ monomer units, reflecting optimum enzymatic activity, hybndization specificity and affinity, economy of synthesis reagents, and ease of chimera synthesis and punfication The length of the DNA moiety is from 2 to 15 nucleotides The prefened length of the DNA moiety is the shortest sequence which promotes efficient ligation, l e at least two 2'-deoxynucleotιdes (Figure 1A) Prefened nucleobases in one or more PNA monomer units include, but are not limited to, adenine, guanine, cytosine, uracil, thymine, 7-deazaadenme, 7-deazaguanιne, C-5- alkyl pynmidines, 2-thιopynmιdιne, 2,6-dιamιnopunne, C-5-propyne pynmidine, phenoxazme (Flanagan, 1999), 7-deazapunne, isocytidine, pseudo-isocytidine (Egholm, 1995), isoguanosme, 4(3 H)-pyπmιdone, hypoxanthme, and 8-oxopunnes (Meyer, 1994) The increased affinity and specificity (Egholm, 1993, Jensen, 1997) conferred by the PNA moiety in a PNA-DNA chimera allows for shorter probes to be used in hybndization expenments and assays (Uhlmann and Peyman, 1998, Uhlmann, 1998, Cook, 1997, Uhlmann, 1996 In general, shorter probes are more specific than corresponding longer probes, I e the relative structural perturbation is larger in a smaller probe Also, shorter probes are more economical, I e cheaper to synthesize, and require less sequence information to design It is desirable to provide methods by which PNA-DNA chimeras can be ligated to oligonucleotides and other PNA-DNA chimeras to form PNA-contammg ligation products Binding of the PNA moiety in a PNA-DNA chimera to its DNA or RNA complement can occur in either a parallel or anti-parallel onentation The anti-parallel duplex, where the carboxyl terminus of PNA is aligned with the 5' terminus of the complement DNA, and the amino terminus of PNA is aligned with the 3' terminus of the DNA complement, is typically more stable than the parallel duplex, where the carboxyl terminus of PNA is aligned with the 3' terminus of the DNA complement and the amino terminus of PNA is aligned with the 5' terminus of the DNA complement (Koppitz, 1998, Egholm, 1993) The exemplary chimeras shown here are designed such that the PNA moiety anneals in the anti-parallel onentation with the target sequences Whenever a PNA sequence is represented as a senes of letters, it is understood that the ammo terminus is at the left side and the carboxyl terminus is at the nght side Chimera sequences are typically completely complementary to a portion of the target sequence However, chimera sequences may contain mixed-base ("redundant" or "degenerate") sites whereby a chimera sample may be a mixture of sequences with one or more base positions represented by two or more different nucleobases The mixed -base site may be located in the PNA or DNA moieties of the ohgomer Mixed-base chimeras are mixtures of sequences with varying levels of complementanty to a particular target sequence Mixed-base chimeras may be useful for random pnming or where template sequence information is unknown or uncertain Although certain features of the invention are illustrated herein using single-stranded probes and template nucleic acids, it will also be appreciated that any of the probes and template nucleic acids may contain double-stranded regions It is also contemplated that PNA-DNA chimeras may undergo ligation as one or both strands of a duplex ligating with a second duplex, where both strands of each duplex may ligate with overhangs ("sticky ends") For example, the chimenc probe can be provided in double-stranded form with a sticky end such that the overhang strand contains the DNA moiety and at least a portion of the PNA moiety which is complementary to the template nucleic acid, and such that the recessed strand of the chimenc probe, upon hybndization of the chimenc probe to the template, is positioned either immediately adjacent to, or spaced by a gap of one or more nucleotide positions from, a terminal end of the template nucleic acid Also contemplated are templates containing a PNA moiety of one or more PNA monomers which allow ligation of adjacently hybndized probes PNA-DNA chimeras can be synthesized by covalently linking PNA monomers and nucleotides in virtually any combination or sequence, using the respective conventional methods of synthesis of PNA oligomers, DNA oligonucleotides, and RNA oligonucleotides (Vinayak, 1997, Uhlmann, 1996, Van der Laan, 1997) Efficient and automated methods have been developed for synthesizing PNA/DNA chimera at a 2-25 μmole scale on commercially available, automated synthesizers, e g "Expedite™", Model 433A and Model 394 Synthesizers (PE Biosystems), and with commercially available reagents (Uhlmann, 1996, Vinayak, 1997, Van der Laan, 1997) In this approach, the chimeras can be made continuously, in a single column and on a single synthesizer Typically, synthesis of chimeras is initiated by detntylation of the 5'-dιmethoxytntyl (DMT) group of commercially available, high-cross link, non-swellmg polystyrene beads packed in a synthesis column The supports are loaded at 20-30 μmole/g with 5'-DMT deoxynucleosides (A bz , G' bu , C b/ , T) linked through the 3' hydroxyl to the support through a base-labile succinate/hydroxymethylbenzoic acid linker (Vinayak, 1997) 5'-DMT, 3'- cyanoethyl phosphoramidite deoxynucleoside monomers (Beaucage, 1992) are dissolved in dry acetomtnle and delivered concurrently with tetrazole activator and coupled to the support-bound 5'-hydroxyl Coupling is followed by capping with acetic anhydnde of unreacted 5'-hydroxyls, and iodine oxidation to the pentavalent mtemucleotide phosphate tnester The DNA synthesis cycle is repeated until the last deoxynucleoside addition, where a 5' monomethoxytntyl (MMT) ammo nucleoside phosphoramidite is employed to furnish a 5' amino terminus on the support-bound DNA moiety, for coupling to a PNA monomer at the linkage between DNA and PNA in the chimera The MMT group is favored for protection of the backbone ammo in the synthesis of PNA-DNA chimeras because of its acid-lability The MMT group is efficiently and rapidly removed from nitrogen under mild acidic conditions which do not cause depunnation or other damage to the chimera. To initiate synthesis of the PNA moiety, the 5' MMT group is removed with 3% tnchloroacetic acid in dichloromethane and the amino group is coupled with a PNA monomer and a coupling reagent. The backbone amino group of the PNA monomers is preferably protected with MMT and the nucleobase exocychc amines are protected as A bz , G ,bLI , and C bz (Breipohl, 1997, Finn, 1996; Will, 1995). Any conventional peptide coupling reagent may be used, but HBTU and HATU are preferred coupling reagents PNA monomers may be dissolved in 1 :1 DMF: acetomtnle to a concentration of about 0.2M. Pnor to delivery to the synthesis column, the monomer solution is mixed with an equal volume of 0.2M HBTU (O-benzotnazol-l-yl-N,N,N',N'-tetramethyluronιum hexafluorophosphate), also m 1.1 DMF: acetomtnle (Vinayak, 1997) The solution is delivered to the column concurrently with 0.2M dnsopropylefhylamme in 1 :1 DMF acetomtnle The synthesis cycles for the PNA and DNA moieties in a chimera on a Model 394 synthesizer at a 2 μmole scale are summanzed in Table 1 below. Table 1 PNA-DNA chimera synthesis cycles PNA DNA After synthesis is complete, the ammo terminus may be acetylated to minimize migration or cychzation, or reacted as a nucleophile in labelling The crude chimera is cleaved from the support, and all protecting groups are removed with concentrated ammonium hydroxide at 55 °C for 8-16 hours The chimeras are analyzed and punfied by reverse-phase HPLC or polyacrylamide gel electrophoresis (PAGE), analyzed by mass spectroscopy, and quantified by correlating UN absorbance at 260 nm with mass Chimeras with a DΝA moiety compnsmg nbonucleotides can be synthesized with the appropnate RΝA phosphoramidite nucleosides and/or 5' DMT protected nbonucleotides support (Vinayak, 1994) The 2' hydroxyl of RΝA phosphoramidites are typically protected with the tert-butyldimethylsilyl (TBDMS) group and the exocychc amino groups of the nucleobases are protected as A bz , G dmf , C bz After synthesis, TBDMS groups are removed with a fluonde reagent, e g tetrabutylammomum fluonde m tetrahydrofuran Otherwise, the synthesis, punfication, and analysis methods for nbonucleotide-contammg PΝA-DΝA chimeras are virtually the same as for chimeras with only 2'-deoxynucleotιde containing DΝA moieties The linkage between the PΝA and DΝA moieties of the chimenc probes of the invention may be a direct bond, e g an amide bond formed by the amino group at the 5' of a deoxynucleotide and the carboxyl group at the carboxyl terminal of the PΝA moiety without an intervening atom (Figures 1 A- IB) Alternatively, the linkage L may be a phosphodiester or phosphoramidate group The linkage may also comprise one or more units of a non-base painng moiety such as ethyleneoxy, linked to the PΝA and DΝA moieties by amide (Figure 2B) or phosphate (Figure 2C) bonds Ethyleneoxy linkage units between the PΝA and DΝA moieties can be installed by coupling reagents such as protected forms of 2-[2-(2-amιnoethoxy) ethoxy] acetic acid The O-linker, 2-[2-(2-amιnoethoxy] acetic acid, is coupled as the MMT-amino protected amide-forming carboxy c acid, or phosphoramidite synthons (Figure 2A) One or more O linker units can act as a flexible, non-base pairing, linkage between the PΝA and DΝA moieties Figure 2 shows a bis- ethyleneoxy-acetamido linker (2B) and a bis-ethyleneoxy-phosphate linker (2C) Other exemplary linkers include alkydiyl, e g hexyldiyl (Vinayak, 1997), or 1,4-phenyldιyl (Figure 2A) VI 3 OLIGONUCLEOTIDES Generally, the oligonucleotides of the present invention are prepared by the phosphoramidite synthesis method, preferred because of its efficient and rapid coupling and the stability of the starting nucleoside monomers (Caruthers, 1983, Beaucage, 1983, Beaucage, 1992) The phosphoramidite method entails cyclical addition of nucleotide monomer units to an oligonucleotide chain growing on a solid-support, most commonly in the 3' to 5' direction in which the 3' terminus nucleoside is attached to the solid-support at the beginning of synthesis The method is usually practiced using automated, commercially available synthesizers (Caruthers, 1984) Typically, phosphoramidite nucleoside monomer units include where, Ri is a protecting group or substituent, e g cyanoethyl, methyl, lower alkyl, substituted alkyl, phenyl, aryl, and substituted aryl, R and R are amine substituents, e g isopropyl, moφhohno, methyl, ethyl, lower alkyl, cycloalkyl, and aryl, R is an exocychc nitrogen protecting group such as benzoyl, isobutyryl, acetyl, phenoxyacetyl, aryloxyacetyl, dimethylformamidine, dialkylformamidme, and dialkylacetamidme, and R 5 is an acid-labile protecting group such as DMT, MMT, pixyl, tntyl, and tnalkylsilyl Preferred nucleobases in one or more nucleosides include, but are not limited to, adenine, guanine, cytosine, uracil, thymine, 7-deazaadenιne, 7-deazaguanιne, C-5-alkyl pynmidmes, 2-thιopynmιdιne, 2,6-dιamιnopunne, C-5-propyne pynmidme, phenoxazine (Flanagan, 1999), 7-deazapuπne, lsocytidme, pseudo-isocytidme (Egholm, 1995), isoguanosme, 4(3 H)-pyrimidone, hypoxanthme, and 8-oxopunnes (Meyer, 1994) Preferred sugars in one or more of the nucleosides include, but are not limited to, 2'- deoxynbose, nbose, and 2'- or 3'-nbose modifications where the 2'- or 3'-posιtιon may be hydrogen, hydroxy, methoxy, ethoxy, allyloxy, isopropoxy, butoxy, isobutoxy, methoxyethyl, alkoxy, phenoxy, azido, ammo, alkylamino, fluoro, chloro and bromo Other preferred sugars include 4'- -anomenc nucleotides, l '-α-anomenc nucleotides, and 2'-4'-lmked and other "locked", bicyclic sugar modifications (Wengel, 1999) VI 4 LIGASE ENZYMES The ligase enzyme used in the present invention can be any ligase that performs ligation of the PNA-DNA chimenc probe to the second probe, when the chimenc probe and second probe are annealed to adjacent regions in a target template DNA hgases join DNA sequences by forming a phosphodiester bond between a 5'-phosphate and a 3'-hydroxyl on two probes which are adjacent, l e hybndized immediately next to each other (Kornberg, 1980, Whiteley, 1989) Alternatively, the 3'-phosphate end of one probe and the 5'- hydroxyl of the other probe may form a phosphodiester bond In the present invention, one or both probes contain a PNA moiety For example, the DNA ligase from bacteπophage T4 can join both DNA and RNA sequences, and it can use either DNA or RNA templates to align the sequences to be ligated Reactions involving only DNA strands proceed with greater efficiency Two DNA duplexes with base-paired blunt ends can be joined by the phage ligase Certain hgases need cofactors such as NAD or ATP A number of hgases have recently been isolated from thermophile organisms and which have significant activity above 60 °C and survive conditions that denature DNA A preferred thermostable ligase is denved from Thermus aquaticus (Takahashi, 1984) and can also be prepared recombman ly (Barany, WO 91/17239, 1991) The thermostable hgases also exhibit a reduced activity of joining duplex DNA with blunt ends or short complementary overhang ends These properties result in increased specificity of detection and convenience in many analytical assays and applications involving ligation VI 5 LABELS It is desirable to provide methods by which labelled PNA-DNA chimeras and labelled oligonucleotides can be enzymatically ligated as probes to form non- radioisotopically labelled ligation products Fluorescence has largely supplanted radioactivity as the prefened detection method for many ligation expenments and applications, such as the oligonucleotide ligation assay and other in vitro DNA probe-based diagnostic tests Therefore, fluorescent labels are a prefened class of detection labels Labels which enhance hybridization specificity and affinity are also preferred, e.g. minor- groove binders. Affinity ligand labels are also prefened. Biotin is a useful affinity ligand label for chimeric probes and oligonucleotides for capture and isolation of ligation products. In certain experiments, biotin labelling of the template nucleic acid may be useful for capture, isolation, removal, or retrieval puφoses. The PNA-DNA chimeras and the oligonucleotides participating in ligation may bear covalently attached labels. Labeling can be accomplished using any one of a large number of known techniques employing known labels, linkages, linking groups, standard reagents and reaction conditions, and analysis and purification methods. Generally, the linkage linking the dye and oligonucleotide or chimera should not (i) interfere with ligation, (ii) inhibit ligase activity, or (iii) adversely affect the fluorescence properties of the dye, e.g. quenching or bleaching. PNA-DNA chimeras and oligonucleotides can be labelled at sites including a nucleobase, a sugar, the aminoethylglycine backbone, amino, sulfide, hydroxyl, and carboxyl. Nucleobase label sites generally include the 7-deaza or C-8 positions of the purine or deazapurine, and the C-5 position of the pyrimidine. The linkage between the label and the chimera or oligonucleotide (NUC) may be acetylenic-amido or alkenic-amido linkages (Khan, 1998). Typically, a carboxyl group on the label is activated by forming an active ester, e.g. N- hydroxysuccinimide (NHS) ester and reacted with an amino group on the alkynylamino- or alkenylamino-derivatized chimera or nucleotide. The resulting linkage is 3-(carboxy)amino-l- propyn-1-yl having the structures: O II NUC— C≡C— CH 2 -NH— C— Label O II Chimera— C≡ C—CH 2 -NH—C— Label Labels may be attached to oligonucleotides at any suitable terminal or internal attachment sites, including: (i) a terminus, e.g. 5' and/or 3' (Mullah, 1998), (ii) an internucleotide linkage, (iii) a sugar, or (iv) a nucleobase. Labels are most conveniently and efficiently introduced at the 5' terminus with fluorescent dyes (FAM, HEX, TET) and other labels which have been functionalized as phosphoramidite reagents, as part of the automated protocol (Theisen, 1992). A preferred class of labels provide a signal for detection of the labelled oligonucleotide by fluorescence, chemiluminescence, and electrochemical luminescence (Kncka, 1992) Fluorescent dyes useful for labelling oligonucleotides include fluoresceins (Menchen, 1993), rhodammes (Bergot, 1994), energy-transfer dyes (Lee and Spurgeon, 1998), cyanmes (Kubista, 1997), and metal poφhynn complexes (Stanton, 1988) Examples of fluorescem dyes include 6-carboxyfluorescem (6-FAM), 2',4',1 ,4,- tetrachlorofluorescein (TET), 2',4',5',7',1,4-hexachlorofluorescem (HEX), 2',7'-dιmethoxy- 4',5'-dιchloro-6-carboxyrhodamme (JOE), and aromatic-substituted xanthene dyes (Benson, 1997) The 5-carboxyl, and other regio-isomers, may also have useful detection properties Another prefened class of labels include fluorescence quenchers The emission spectra of a quencher overlaps with a proximal intramolecular or mtermolecular fluorescent dye such that the fluorescence of the fluorescent dye is substantially diminished, or quenched, by the phenomenon of fluorescence resonance energy transfer "FRET" (Clegg, 1992) An example of FRET m the present invention is where the PNA-DNA chimenc probe is labelled with a fluorescent dye and the second probe is labelled with a fluorescence quencher Alternatively, the chimera may be labelled with a fluorescent quencher and the second probe is labelled with a fluorescent dye Pnor to hybndization and ligation, the fluorescent dye is substantially unquenched After ligation, the fluorescent dye of the ligation product is substantially quenched by FRET Particularly preferred quenchers include but are not limited to (l) rhodamme dyes selected from the group consisting of tetramethyl-6-carboxyrhodamme (TAMRA), tetrapropano-6-carboxyrhodamιne (ROX), and (n) DABSYL, DABCYL, cyan e dyes including mtrothiazole blue (NTB), anthraquinone, malachite green, nιtrothιazole,and troimidazole compounds and the like Nitro-substituted forms of quenchers are especially preferred Energy-transfer dyes are a preferred class of chimera and oligonucleotide labels An energy-transfer dye label includes a donor dye linked to an acceptor dye (Lee and Spurgeon, 1998) Light, e g from a laser, at a first wavelength is absorbed by a donor dye, e g FAM The donor dye emits excitation energy absorbed by the acceptor dye The acceptor dye fluoresces at a second, longer wavelength The donor dye and acceptor dye moieties of an energy-transfer label may be attached by a linker linking the 4' or 5' positions of the donor dye, e g FAM, and a 5- or 6-carboxyl group of the acceptor dye Other ngid and non-ngid linkers may be useful Metal poφhyrin complexes, e.g. aluminum phthalocyanine tetrasulfonate (Stanton, 1988) and chemiluminescent compounds, e.g 1 ,2-dioxetane chemi luminescent moieties (Bronstein, 1990) are also preferred classes of chimera and oligonucleotide labels. Another prefened class of labels, referred to herein as hybridization-stabilizing moieties, include but are not limited to minor groove binders (Blackburn, 1996, p.337-46), intercalators, polycations, such as poly-lysine and spermine, and cross-linking functional groups. Hybridization-stabilizing moieties may increase the stability of base-pairing, i.e. affinity, or the rate of hybridization, exemplified by high thermal melting temperatures, Tm, of the duplex. Hybridization-stabilizing moieties may also increase the specificity of base- pairing, exemplified by large differences in Tm between perfectly complementary oligonucleotide and target sequences and where the resulting duplex contains one or more mismatches of Watson/Crick base-pairing (Blackburn, 1996, pp. 15-81). Preferred minor groove binders include Hoechst 33258 (Rajur, 1997), CDPIμ3 (Kutyavin, 1996), netropsin, and distamycin. Other useful labels include electrophoretic mobility modifiers, amino acids, peptides, enzymes, and affinity ligands, e.g. biotin and digoxigenin. Linkers between a label and the PNA/DNA chimera can be an amide bond, e.g. where the active ester form of a label is coupled with an amino group of the chimera. Also, linkers can comprise alkyldiyl, aryldiyl, or one or more ethyleneoxy units (Rajur, 1997). VI.6 LIGATION Figure 3A shows a generalized schematic of ligation between a 3'-hydroxyl PNA- DNA chimera probe and a 5 '-phosphate oligonucleotide, as the second probe, hybridized to a DNA template with DNA ligase to form a PNA-DNA ligation product. The first probe has a 3'-hydroxyl terminus and the second probe has a 5'-phosphate terminus. Alternatively, the first probe has a 5'-phosphate terminus and the second probe has a 3'-hydroxyl terminus. A ligation mixture generally includes a DNA template, a PNA-DNA chimeric probe, a second probe which is another PNA-DNA chimeric probe or an oligonucleotide, ligase, and other ligation reagents. DNA probes corresponding to the PNA-DNA chimeras may be used as a control or comparison. Under typical conditions, the probes are used at a final concentration of about 1 μM each. Additionally, the ligation mixture may contain reagents such as IX T4 DNA ligase buffer [50 mM Tris-HCI, pH 7.5, 10 mM MgCl 2 , 10 mM dithiothreitol, 1 mM ATP, 25 μg/ml bovine serum albumin], and 1 ,000 units of T4 DNA ligase plus 10 units of T4 polynucleotide kinase in a volume of 50 μl. The ligation reagent may also contain ligase co-factors, e.g. NAD and ATP, polyethylene glycol, EDTA, KC1, ammonium sulfate, dithiothreitol, BSA, MgCl , Tris-HCI, glycerol, water, NaCl, mercaptoethanol, and other salts or buffers. The mixture may be incubated at 22 to 25° C for 3 hours or more. During the ligation, another addition of 10 to 20 units of T4 polynucleotide kinase may be helpful for the ligation. Typically after incubation, ligase reaction products are heat-inactivated at approximately 80 °C for about 20 minutes and placed on ice or at 4 °C for a short period. For analysis, typically, 5 to 25 pmol of the ligation product is mixed with a final concentration of IX loading buffer (45 mM Tris base, 45 mM boric acid, 0.4 mM EDTA, 3% Ficoll, 0.02% bromophenol blue, 0.02% xylene cyanol) and denatured at 95 °C for 10 to 20 min. The sample is loaded into a 10 to 15% denaturing PAGE gel and run in lχ TBE (89 mM Tris base, 89 mM boric acid, 2 mM EDTA, pH 8.3) at 100 to 160 V, 70 °C for 25 to 60 min. The ligation product is visualized by staining the gel with SYBR-Green (Molecular Probes, Eugene, OR) in a volume of 40 to 120 ml in lχ TBE for 10 to 30 min. The image may be captured in a gel documentation system (e.g. Chemilmager 4000 Imaging System, Alpha Innotech Coφoration, San Leandro, CA). Figure 3B shows template and probe sequences for a ligation assay in a model system. The probe sequences include two oligonucleotide probes, 6nt and 9nt, and five PNA-DNA chimeras containing 0 to 4 DNA 2'-deoxynucleotides. The oligonucleotides and chimeras hybridize to a 38nt DNA template. The template is labelled with biotin at the 3' or 5' end to facilitate isolation and transfer. The oligonucleotide ligation assay (OLA) is a convenient, highly-stringent method that permits distinction among known DNA sequence variants (Landegren, 1988). Multiplex analysis of highly polymoφhic loci is useful for identification of individuals, e.g., for paternity testing and in forensic science, organ transplant donor-receiver matching, genetic disease diagnosis, prognosis, and pre-natal counseling, and other genetic-based testing which depend on the discrimination of single-base differences at a multiplicity of loci (Delahunty, 1996). Products of a multiplex oligonucleotide ligation assay (OLA) may be resolved electrophoretically from one another and from unligated probes under denaturing conditions with fluorescence detection (Grossman, 1994). For example, Figures 8A-8C shows different assays where two PNA-DNA chimeras, a wild-type (WT) sequence chimera and a mutant sequence chimera, bear different fluorescent dyes. Only when the mutant sequence is present m the target sample, will the mutant sequence chimera ligate to the adjacently annealed second probe (ohgo) if the mutant base pair is at the ligation site (Figure 8A) The ligation products may be discnminated by separation based on (1) size using electrophoresis or chromatography and/or (π) detectable labels (Grossman, 1994) With a plurality of fluorescent dyes labelled to chimeras with sequences targetting unique target sequences, multiplexed OLA can be conducted on a single sample in a single vessel Requirements for efficient multiplex OLA include probes that anneal and ligate in a highly specific and rapid manner The chimeras and second probe sequences may be selected such that the mutant base, or single base polymoφhism, may be at the 5'-phosphate of the second probe (Figure 8B) or the 3'- terminus of the chimera (Figure 8C) It is contemplated that OLA expenments of the present invention may be conducted on solid substrates where the template nucleic acid, PNA-DNA chimenc probe, or the second probe may be immobilized on a solid particle or bead, or a solid porous or non- porous surface When immobilized, the template, chimera or second probe is preferably covalently attached to the solid substrate, e g via a terminal monomer unit The solid substrate may be polystyrene, controlled-pore-glass, silica gel, silica, polyacrylamide, magnetic beads, polyacrylate, hydroxyethylmethacrylate, polyamide, polyethylene, polyethyleneoxy, and copolymers and grafts of any of the above solid substrates The configuration or format of the solid substrate may be small particles or beads of approximately 1 to 50 μm in diameter, membranes, frits, slides, plates, micromachmed chips, alkanethiol-gold layers, non-porous surfaces, and polynucleotide-immobihzmg media For example, a PNA-DNA chimenc probe is covalently attached by a linker at the amino terminus to a non-porous, inorganic surface, e g glass (Guo, 1994) A template nucleic acid sample is allowed to hybndized to the chimenc probe under conditions that promote hybndization A second probe with a sequence complementary to the template is added to the hybndized duplex and the second probe hybndizes to the template adjacent to the chimenc probe The chimenc probe and second probe are ligated together with ligase The ligated product may be detected and/or isolated where the chimenc probe, template, or second probe bear a label or affinity ligand and an oligonucleotide ligation assay is thereby performed In a preferred embodiment, an array of chimenc probes are assembled on a solid substrate where a chimenc probe of known sequence occupies a defined area on a two- dimensional surface The number of chimenc probes on any particular surface may be hundreds or even thousands, limited by the spatial requirements for synthesis, attachments, and detection (Fodor, 1995) Alternatively an array of probes may be immobilized on beads or particles contained in wells or vessels A template, or mixture of templates, may be added for hybndization to the immobilized chimenc probes on the surface Where sufficient sequence complementarity exists under the defined hybndization conditions, duplex formation will occur A mixture of second probes may be added separately, or with the template sample, for hybridization In the presence of ligase, ligation will occur only where chimenc probes and second probes are hybndized adjacently Unhybndized probes and template samples may be removed by washing under conditions that maintain hybridization, or under denatunng conditions Where the second probe bears a label, e g fluorescent dye, the ligation product is covalently immobilized on the surface and can be detected, e g laser-induced fluorescence From the knowledge of the immobilized chimenc probe sequences, the presence of certain sequences m the template sample can be deduced from the locatιon(s) of detected fluorescence on the anay surface In a second aspect of the invention, a kit for ligation is provided In one embodiment, for example, the kit, which is useful for practicing the method of the invention, compnses (l) a PNA-DNA chimera having from 3 to 15 contiguous PNA monomer units, from 2 to 15 contiguous nucleotides, and a 3' hydroxyl, (n) a second probe where the second probe is a PNA-DNA chimera or an oligonucleotide and, (in) a ligase enzyme The chimera and/or the oligonucleotide may be labelled with a non-radioisotopic label In another embodiment, the kit additionally includes a template compnsmg a sequence complementary to the chimera or containing one or more mismatches to the chimera In another embodiment, the kit additionally includes a polynucleotide kinase From the foregoing discussion, it can be seen how various features and advantages of the invention are met The present invention provides a method for detecting selected target sequences that is highly sensitive and accurate Selected target sequences can be detected using chimenc PNA/DNA probes containing target-specific sequences shorter than all-DNA probes used in previous oligonucleotide ligation assays The chimenc probes thus require less target sequence information to design and can be less expensive to synthesize 21 In addition, the present invention can be adapted to a wide variety of target sequences and assay formats, and can be readily automated. VI.7 EXAMPLES The invention is further illustrated by the following examples, which are intended to be purely exemplary of the present invention and not to limit its scope in any way. Example 1 Labelling of PNA-DNA chimera TAMRA and NTB labelling: Labelling is performed with 5 mg of NHS ester of TAMRA or NTB dissolved in 100 μl DMF or NMP and 10 μl DIEA. The labelling mixture is added to the support bound PNA-DNA chimera and allowed to react for 2 to 18 hours (typically overnight). The support is washed following the labelling with DMF and subsequently DCM prior to cleavage. CDPI labelling: CDPI 3 is attached to the chimera by three consecutive couplings of Fmoc-CDPI (Lukhtanov, 1995) to the amino terminus of a PNA-DNA chimera to give CDPI 3 -labelled PNA-DNA chimera. The CDPI monomer unit, l,2-dihydro-(3H)-pyrrolo[3,2-e]indole-7- carboxylate, protected with Fmoc (5 mg, 0.012 mmole) is dissolved in 100 μl NMP and activated by 0.95 equivalents ΗATU (0.2M in DMF) and 2 equivalents DIEA (0.4M in DMF). After one hour at room temperature, the activated Fmoc-CDPI solution is added to the support bound chimera and allowed to couple for another hour at room temperature. The resin is washed following the coupling with 20 ml DMF. The Fmoc is removed by treatment of the resin support with 1 :4 piperidine:DMF for 10 minutes at room temperature. This coupling and deprotection cycle is repeated two additional times for a total of 3 manual couplings to give CDPl 3 -labelled PNA-DNA chimera. Example 2 Ligation Figure 4 shows ligation experiments where a ligation mixture contained a template and two probes at a final concentration of 1 μM each, IX T4 DNA ligase buffer [50 mM Tris-ΗCl, pΗ 7.5, 10 mM MgCl 2 , 10 mM dithiothreitol, 1 mM ATP, 25 μg/ml bovine serum albumin], and 1 ,000 units T4 DNA ligase (New England BioLabs, Beverly, MA) in a volume of 50 μl. T4 ligase can also be purchased from Boehringer-Mannheim. Approximately 70 units of NE Biolabs T4 DNA ligase is equal to 1 Weiss unit of Boehringer-Mannheim T4 ligase. The same reactions were also conducted without ligase. After incubation at 22.5 to 25 °C for 3 to 4 hours, the reaction mixture was heated at 80 °C for 20 min. and then stored at 4 °C. Five μl of the reaction mixture was analyzed by ( 15%) polyacrylamide gel electrophoresis (PAGE) under denaturing conditions (7M urea) at 120- 140 V for 20-60 min (Figure 4A). Ligation of 5 '-phosphate oligonucleotides (DNA2, DNA4 or DNA5) to an oligonucleotide DNA3 or PNA-DNA chimeras were conducted on templates (DNAl or DNA6) according to Table 2. Ligation with T4 ligase (top gel image) is evident in lanes 3,4,6,8,10 between PNA-DNA chimeras and 5'-phosphate oligonucleotides by the appearance of new bands. Ligation was effective with chimeras having 6 PNA monomers and from 2 to 4 2'-deoxynucleotide monomers. Control ligation between two oligonucleotide probes (lane 2) shows a ligation product band. Control ligation with only one probe (lanes 1 ,5,7,9) does not show a new band. Experiments without T4 ligase (bottom gel image) show bands for the templates and probes. Table 2. Figures 4A-4B Lane Template PNA-DNA chimera 5 '-phosphate or oligonucleotide oligonucleotide 1 DNAl DNA2 2 DNAl DNA3 DNA2 3 DNAl PNA 6 -DNA 3 DNA2 4 DNAl PNA 6 -DNA 3 DNA2 5 DNAl DNA4 6 DNAl PNA 6 -DNA 2 DNA4 7 DNAl DNA5 8 DNAl PNA 6 -DNA 4 DNA5 9 DNA6 DNA5 M DNA oligo ladder DNAl (SEQ. ID NO. 9) Biotin- cgctcaacacatagcatggtctagaactaagcctggaa DNA6 (SEQ. ID NO. 15) cgctcaacaca agcatggtaaagccgggacctaactgtt DNA3 tagttctag (SEQ. ID NO. 2) PNA 6 -DNA 2 TAGTTC- ta (SEQ. ID NO. 5) PNA 6 -DNA 3 TAGTTC- tag (SEQ. ID NO. 6) PNA 6 -DNA 4 TAGTTC- taga (SEQ. ID NO. 7) PNA 1 1 -DNA4 TAGGTCCCGGC- ttta (SEQ. ID NO. 10) DNA2 5 ' -phos accatgctatgtgttgagcg (SEQ. ID NO. 11) DNA4 5 ' -phos -gaccatgctatgtgttgagcg-biotin (SEQ. ID NO. 12) DNA5 5 ' -phos -ccatgctatgtgttgagcg-biotin (SEQ. ID NO. 8) (UPPER CASE - PNA, lower case - DNA) Figure 4B shows the quantitative estimate of the ligase reactions in Figure 4A by densitometry with the SpotDenso program. The bands enclosed by the boxes are the 5'- phosphate probes remaining after ligation. The negative control experiments in lane 1,5,7, and 9 established the levels of 5 '-phosphate probe remaining when no ligation occurred. The positive control experiment in lane 2 gave the ligation efficiency between two all-DNA probes. From these quantified values, the ligation efficiencies with PNA-DNA chimeric probes can be calculated from the areas (IDV - Integrated Density Value) in the chart (right). Figure 5 shows ligation experiments using T4 DNA ligase with PNA-DNA chimeric probes to which 1 to 4 DNA bases are attached at 3' terminus of the PNA oligomer and a second probe, a 5'-phosphate oligonucleotide of variable lengths (lanes 4-7). The lengths of the chimera and the oligonucleotide probes are chosen to form ligation products of equivalent length (Table 3). In addition, control ligations were conducted where instead of the chimera, no probe (lane 2), an all PNA probe (lane 3), an all-DNA probe, 6nt (lane 8) and an all-DNA probe, 9nt (lane 9) were used. The ligation mixture contained a 38nt DNA template and the two probes at a final concentration of 1 μM each, IX T4 DNA ligase buffer [50 mM Tris-HCI, pH 7.5, 10 mM MgCl 2 , 10 mM dithiothreitol, 1 mM ATP, 25 μg/ml bovine serum albumin], and 1 ,000 units of T4 DNA ligase in a volume of 50 μl. The ligation reaction mixture was incubated at 22.5 to 25°C for 3 to 4 h. After incubation, the reaction mixture was heated at 80 °C for 20 min and then stored at 4 °C. Five μl of the reaction mixture was loaded onto a 15% polyacrylamide gel and electrophoresed at 120-140 V under denatunng conditions (7M urea, 70°C) for 20-60 min., then stained with SYBR- Green Figure 5A shows a scanned image of the stained gel Figure 5B is a schematic of ligation of PNA, PNA-DNA chimera, and DNA to 5'-phosphate oligonucleotides hybndized to a DNA template 38nt (SEQ. ID NO 9). It is evident from new bands below the template bands that PNA-DNA chimeras are ligated where the chimera has 2, 3, or 4 DNA monomers (2'-deoxynucleotιdes), lanes 5-7 respectively No ligation is evident for an all-PNA probe or a chimera containing only 1 DNA monomer, lanes 3 and 4 respectively. Lane 1 is a negative control. Lanes 8 and 9 are positive controls, where 6nt and 9nt oligonucleotides are ligated to 5'-phosphate oligonucleotides. The electrophoretic retardation of PNA in the ligation products of chimeras, lanes 5-7, is evident compared to all-DNA ligation products, lanes 8 and 9. Table 3. Figures 5A-5B Probes Lane DNA Temnlλte 5 '-phosphate DNA PNA-DNA chimera 2 38nt 23nt None 38nt 23nt PNA 6 4 38nt 22nt PNA 6 -DNA, 5 38nt 21nt PNA 6 -DNA 2 6 38nt 20nt PNA 6 -DNA 3 7 38nt 19nt PNA 6 -DNA 4 8 38nt 23nt DNA 6 9 38nt 20nt DNA 9 1 M = DNA oligonucleotide ladder DNA () tagttc (SEQ. ID NO. 1) DNAq tagttctag (SEQ ID NO. 2) PNA* TAGTTC (SEQ. ID NO. 3) PNA (1 -DNAι TAGTTC - 1 (SEQ. ID NO. 4) PNA 6 -DNA 2 TAGTTC- a (SEQ ID NO 5) PNAc-DNAi TAGTTC - tag (SEQ ID NO. 6) PNA 6 -DNA 4 TAGTTC- taga (SEQ ID NO. 7) 5'-phosphate ohgos 19nt 5 ' -phos-ccatgctatgtgttgagcg-biotin (SEQ. ID NO.8) 20nt 5 ' -phos-accatgctatgtgttgagcg-biotin (SEQ. ID NO.11) 21nt 5 ' -phos-gaccatgctatgtgttgagcg-biotin (SEQ. ID NO.12) 22nt 5 ' -Phos-agaccatgctatgtgttgagcg-biotin (SEQ. ID NO.13) 23nt 5 ' -Phos-tagaccatgctatgtgttgagcg-biotin (SEQ. ID NO.14) DNA template 38nt : (SEQ. ID NO. 16) cgctcaacacatagcatggtccagaactaagcctggaa (UPPER CASE - PNA, lower case - DNA) Figure 6 shows the specificity of ligation with PNA-DNA chimeric probes. PNA- DNA chimeras and 5'-phosphate oligonucleotides were ligated on templates, as perfect matches and with mismatches. Ligation mixtures contained a template and two probes at a final concentration of 1 μM each, 1 X T4 DNA ligase buffer [50 mM Tris-HCI, pH 7.5, 10 mM MgCl 2 , 10 mM dithiothreitol, 1 mM ATP, 25 μg/ml bovine serum albumin], and 1,000 units of T4 DNA ligase in a volume of 50 μl. Ligation reactions were incubated at 22.5 to 25°C for 3 to 4 h. After incubation, the reaction mixture was heated at 80 °C for 20 min and then stored at 4 °C. Five μl of the reaction mixture was used for PAGE analysis, electrophoresing at 120- 140 V for 20-60 min. Table 4 Figure 6 Phosphorylated DNA PNA-DNA chimera or Lane DNA Temnlate Matched Mismatched DNA oliponu lentide 1 DNA1 C DNA2 DNA 9 2 DNA1 C DNA2 DNA2A DNA 9 3 DNA1C DNA2 PNA 6 -DNA 3 4 DNA1 C DNA2 DNA2A PNA 6 -DNA 3 5 DNA1 C DNA3 PNA 6 -DNA 4 (mismatched) 6 DNA2C DNA2A DNA 9 (mismatched) 7 DNA2C DNA2A PNA 6 -DNA 3 (mismatched) 8 DNA2C DNA4 PNA 6 -DNA 2 (mismatched) M DNA oligonucleotide ladder DNA Template DNA1C (SEQ. ID NO. 27) Biotin-cgctcaacacatagcatggcctagaactaagcctggaa DNA2C (SEQ. ID NO. 16) Biotin-cgctcaacaca agcatggtccagaactaagcctggaa Phosphorylated DNA DNA2 5 ' - Phos -gccatgctatgtgtt -Biotin (SEQ. ID NO. 17) DNA2A 5 ' -Phos -accatgctatgtgtt -Biotin (SEQ. ID NO. 18) DNA3 5 ' - Phos - ccatgctatgtgttgagcg-Biotin (SEQ. ID NO. 8) DNA4 5 ' - Phos-accatgctatgtgttgagcg-Biotin (SEQ. ID NO. 11) DNA DNA 9 tagttctag (SEQ. ID NO.2) PNA-DNA Chimera PNA 6 -DNA 2 TAGTTC- a (SEQ. ID NO.5) PNA 6 -DNA 3 TAGTTC -tag (SEQ. ID NO.6) PNA 6 -DNA 4 TAGTTC -taga (SEQ. ID NO.7) (UPPER CASE - PNA, lower case - DNA) The experiments in Figure 6 show that PNA-DNA chimeric probes require a high level of sequence complementarity for ligation to occur. When a mismatch occurs either in the chimeric probe or the second probe, ligation is not detectable, within the limits of the system shown. By comparison, all-DNA probes are less specific. When a mismatch occurs in either all-DNA probe, some ligation is still evident. Lane 1 is a positive control ligation where two oligonucleotides are perfectly matched (W) to the template and ligate to form a ligation product migrating at the expected rate of 24nt. The experiment of lane 2 additionally has a mismatched (W+M) probe which ligates, evidenced by a new band between the perfect match product and the template (38nt) The mismatched probe, DNA2A, has a mismatch at the ligation site, the A base at the 5' terminus of the 5'- phosphate probe The expenment of lane 3 hgates perfectly matched PNA 6 -DNA 3 chimera and DNA2 second probe, giving a new band, migrating faster than template The expenment of lane 4 additionally has the mismatched DN A2 A probe Unlike the all-DNA expenment of lane 2, mismatched DN A2 A does not ligate with the chimera, demonstrating greater specificity conferred by the PNA moiety The expenments of lanes 5 and 7 likewise have mismatches at the 3' terminus of the chimera The 3' terminus of the chimera in the expenment of lane 8 has a deletion and a mismatch These experiments, lanes 5,7,8 show no ligation product The expenment of lane 6 has a mismatch in the DNA 9 probe at the penultimate base near the 3' terminus In this expenment, a ligation product is evident, reflecting the lower specificity of ligation of all-DNA probes In summary, the ligation experiments shown Figure 6 illustrate that PNA-DNA chimenc probes when ligated to oligonucleotide probes are better able to discnmmate base- pair mismatches (specificity) than hgations between two all-DNA, oligonucleotide probes, whether the mismatch occurs m the chimera probe or the oligonucleotide probe Example 3 MALDI-TOF analysis of ligation reaction Mass spectra were acquired on a MALDI-TOF MS (Voyager DE, PerSeptive Biosystems, division of PE Coφoration) workstation Desalted samples are mixed 1 1 with matnx solution consisting of 50 mg/ml 3-hydroxy picohmc acid, 50 mM ammonium citrate, and 30% acetomtnle, and are spotted onto a sample plate Time-of-fhght data from 20 to 50 individual laser pulses are recorded and averaged on a transient digitizer, after which the averaged spectra are automatically converted to mass by data processing software Figure 7 of ligation reactions A 3' biotmylated 20nt oligonucleotide (mass 6303) and a PNA-DNA chimera (mass 2539) were hybndized to 5' biotmylated DNA template 38nt (mass 12358) Figure 7A shows MALDI-TOF Mass Spectroscopy analysis of the mixture without ligase The analysis shows only ion peaks of the starting matenals When the ligation mixture contains ligase (Figure 7B), a ligation product is evident with the expected mass of 8823 8 template . (SEQ ID NO 9) Biotm-cgctcaacacatagcatggtctagaactaagcctggaa 5 ' -phos-accatgctatgtgttgagcg-biotin (SEQ ID NO 11) (mass 6303) ^0 Ac -TAGTTC- tag (mass 2539 ) (SEQ. ID NO. 6) ligation product : Ac -TAGTTC- tagaccatgctatgtgttgagcg-biotin (SEQ. ID NO. 19) (mass 8824 ) Ac = acetylated amino terminus (UPPER CASE - PNA, lower case - DNA) Example 4 Multiplex Oligonucleotide Ligation Assay for CFTR loci OLA reactions for CFTR loci were multiplexed in one tube. Two differentially labelled (i.e. FAM- or TET- at 5' site) PNA-DNA chimeras and one 5' phosphorylated DNA oligonucleotide were used for the analysis of each mutation. The sequences of the probes and templates are given in Fig. 8. All multiplex OLA reactions are carried in a 20 μl reaction volume containing 20 mM Tris-HCI, pH 7.6, 25 mM potassium acetate, 10 mM magnesium acetate, 10 mM DTT, 1 mM NAD, 0.1% Triton X-100, 1 to 50 nM each probe, 5 to 10 μl of pooled PCR product, and 2 to 10 units of thermostable ligase such as Thermus aquaticus ligase. Linear amplification of product is achieved by 20 to 30 cycles at 94 °C for 30 sec and 30 to 50 °C for 1 to 3 min, followed by heating at 95 °C for 10 to 20 min in a Model 9700 Thermocycler (PE Biosystems division of PE Coφoration). An aliquot of 2 μl of each multiplex OLA product was mixed with 2.5 μl of deionized formamide, 0.5 μl of dextran blue loading buffer, and 0.5 μl of GENESCAN-500 TAMRA size marker. The mixture was denatured at 95 °C for 3 min and then rapidly chilled on ice prior to loading the gel. OLA products were electrophoresed for 3.5 h at 2,500 V on a Model 373A fluorescence-scanning DNA sequencer (PE Biosystems, division of PE Coφoration) using an 8% acrylamide, 19:1 acrylamide:bisacrylamide, denaturing gel containing 8.3 M urea, 89 mM Tris, 89 mM boric acid, and 2 mM EDTA. The resulting gel data are analyzed for peak color and fragment size using the GENESCAN Fragment Analysis Software and the Genotyper Software (PE Biosystems Division of PE Coφoration). Figure 9 shows the oligonucleotide ligation assay with PNA-DNA chimera probes to discriminate mutations in human CFTR loci. Ligation mixtures contained a template and two probes at a final concentration of 1 μM each, IX T4 DNA ligase buffer [50 mM Tris- HC1, pH 7.5, 10 mM MgCl 2 , 10 mM dithiothreitol, 1 mM ATP, 25 μg/ml bovine serum albumin], and 1 ,000 units of T4 DNA ligase (New England Biolabs) plus 10 units of T4 polynucleotide kinase in a volume of 50 μl. The mixtures were incubated at 22.5 to 25°C for 3 to 4 h. After incubation the reaction mixture was heated at 80 °C for 20 min and then stored at 4°C. Five μl of the reaction mixture was used for PAGE analysis, electrophoresing at 120-140 V for 20-60 min. (9A) human pCFTR621G-T: Exon 4; (9B) human pCFTRl 078delT: Exon 7; (9C) human pCFTRG551D: Exon 11. (UPPER CASE - PNA, lower case - DNA) Figure 9D shows scanned images of OLA experiments with PNA-DNA chimeric probes and an all-DNA control probe (lane 10). Ligation of a 3'-TAMRA-labelled, 5'- phosphate oligonucleotide with PNAιo-DNA 3 chimera at the CFTR locus 621G-T with T4 ligase gave a fluorescent labelled product, visible without staining under UV light (9D, top gel) and by staining with SYBR-Green (9D, bottom gel). Ligation products with PNA- DNA chimeric probes are also evident from experiments in lanes 3 and 9. Table 5 Figure 9D Lane DNA template PNA-DNA chimera 5'-phosphate or oligonucleotide oligonucleotide 1 la 2 l a 2 (FAM) 3 l a PNA 10 -DNA 3 2 4 lb 5 lb 3 (TAMRA) 6 lb PNA, 0 -DNA 3 3 (TAMRA) 7 lc 8 lc 4 10 lc DNA 9 4 M DNA oligo ladder DNA Template DNA la (SEQ. ID NO. 20) gtttgat tataagaaggtaatacttccttgcacag DNA lb (SEQ. ID NO. 21 ) cacagataaaaacaccacaaagaaccctgagaagaagaag DNAlc (SEQ. ID NO. 9) Biot in- cgctcaacacatagcatggtctagaactaagcctggaa PNA-DNA Chimera - 7, 2 - PNA, 0 -DNA 3 (lane 3) Ac -CAAGGAAGTA-tta (SEQ. ID NO. 22) PNA 10 -DNA 3 (lane 6) Ac -CTTCTCAGGG- ttc (SEQ. ID NO. 23) PNA 6 -DNA 4 TAGTTC - taga (SEQ. ID NO. 7) DNA DNA 9 tagttctag (SEQ. ID NO. 2) 5'-phosphate oligonucleotides 2(FAM) 5' -Phos-ccttcttata-FAM-3 ' (SEQ. ID NO.24) 2 5 ' -Phos-ccttcttata-3 ' (SEQ. ID NO.24) 3(TAMRA) 5' -Phos-tttgtggtgtttt-TAMRA-3 ' (SEQ. ID NO.25) 4 5 ' -Phos-ccatgctatgtgtt-Biotin-3 ' (SEQ. ID NO.26) All publications and patent applications are herein incoφorated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incoφorated by reference. Although certain embodiments have been described in detail above, those having ordinary skill in the art will clearly understand that many modifications are possible in the preferred embodiments without departing from the teachings thereof. All such modifications are intended to be encompassed within the following claims.

Claims: CLAIMS We claim: 1. A method of producing a template-dependent ligation product comprising the step of enzymatically ligating a PNA-DNA chimeric probe to a second probe in the presence of a template nucleic acid and a ligase, said chimeric probe having a PNA moiety and DNA moiety, said DNA moiety having at least two nucleotides and a 3' hydroxyl or 5' hydroxyl terminus, wherein the chimeric probe and the second probe are each hybridized to the template nucleic acid and adjacent to each other, and at least a portion of the PNA moiety is hybridized to the template, and wherein the second probe is a PNA-DNA chimera or an oligonucleotide. 2. The method of claim 1 in which the PNA-DNA chimera has the structure: P x - L - N y wherein: each P is independently a PNA monomer; x is an integer from 3 to 15; each N is independently a nucleotide; and y is an integer of 2 or more; L represents a covalent linkage between P and N; with the proviso that the terminal nucleotide N has a 3'-hydroxyl group or a 5 '-hydroxyl group. 3. The method of claim 2 wherein the 3'-terminal nucleotide N of the chimera contains a 3'-phosphate group and the 5' end of the second probe contains a 5'-hydroxyl. 4. The method of claim 2 wherein the 3'-terminal nucleotide N of the chimera contains a 3'-hydroxyl and the 5' end of the second probe contains a 5'-phosphate group. 5. The method of claim 2 wherein the 5'-terminal nucleotide N of the chimera contains a 5'-phosphate group and the 3' end of the second probe contains a 3'-hydroxyl. 6 The method of claim 2 wherein the 5' terminal nucleotide N of the chimera contains a 5'-hydroxyl and the 3' end of the second probe contains a 3'-phosphate group 7 The method of claim 2 wherein P x is a 2-ammoethylglycιne peptide nucleic acid 8 The method of claim 2 in which each nucleotide N is independently a 2'- deoxyπbonucleotide 9 The method of claim 2 in which each nucleotide N is independently a πbonucleotide 10 The method of claim 2 wherein the nucleobases of N y are selected from the group consisting of ademne, guanine, cytosine, uracil, thymine, 7-deazaadenme, 7- deazaguamne, C-5-alkyl pynmidme, 2,6-dιamιnopunne, 2-thιopyrιmιdme, C-5-propyne pynmidme, phenoxazme, isocytidme, pseudo-isocytidine, isoguanosme, hypoxanthme, 8- oxopunne, and 4(3 H)-pynmιdone 11 The method of claim 2 wherein the sugars of N y are each independently selected from the group consisting of 2'-0-alkyl-nbonucleotιdes, 2'-O-methyl- nbonucleotides, 2'-O-allyl-nbonucleotιdes, 2'-allyl nbonucleotides, 2'-halo- nbonucleotides, 2'-0-methoxyethyl-nbonucleotιdes, 4'-α-anomenc nucleotides, l'-α- anomenc nucleotides, 2',4'-hnked nucleotides, and bicyclic nucleotides 12 The method of claim 1 in which the PNA-DNA chimera and/or the oligonucleotide are non-radioisotopically labelled 13 The method of claim 12 wherein the PNA-DNA chimera is labelled at the amino terminus of the PNA moiety 14 The method of claim 12 wherein the oligonucleotide is labelled at a nucleobase 15 The method of claim 14 wherein the nucleobases are labelled at the 7-deaza or C-8 positions of the punne or deazapunne, and the C-5 position of the pynmidme i 16. The method of claim 12 wherein the label is selected from the group consisting of fluorescent dyes, fluorescence quenchers, hybridization-stabilizers, energy- transfer dye sets, electrophoretic mobility modifiers, chemiluminescent dyes, amino acids, proteins, peptides, enzymes, and affinity ligands. 17. The method of claim 16 where the label is a fluorescent dye selected from the group consisting of FAM, TET, HEX, JOE, TAMRA, ROX, aromatic-substituted xanthene dyes, 4,7-dichloro-fluoresceins, 4,7-dichloro-rhodamines, and cyanines. 18. The method of claim 16 where the label is a fluorescence quencher selected from the group consisting of TAMRA, d-TAMRA, ROX, DABCYL, DABSYL, malachite green, NTB, and cyanines. 19. The method of claim 16 where the label is a hybridization-stabilizer that is a minor groove binder. 20. The method of claim 16 where the minor groove binder is selected from the group consisting of Hoechst 33258, CDPI] -3 , MGB1, netropsin, and distamycin. 21. The method of claim 16 where the label is an affinity ligand selected from the group consisting of biotin, 2,4-dinitrophenyl, digoxigenin, cholesterol, polyethyleneoxy, peptides, and fluorescent. 22. The method of claim 12 wherein the oligonucleotide is labelled at a 3' terminus. 23. The method of claim 12 wherein the oligonucleotide is labelled at a 5' terminus. 24. The method of claim 2 wherein L is selected from the group consisting of a covalent bond, phosphate, phosphoramidate, alkyldiyl consisting of 1-20 carbon atoms, aryldiyl consisting of 6-20 carbon atoms, O-linker, and -(CH 2 CH2θ)m- where m is 1 to 6. 25. The method of claim 1 in which the template nucleic acid is a DNA and the ligase is selected from the group consisting of T4 DNA ligase, E. coli DNA ligase, and a thermostable ligase. 26 The method of claim 1 m which the template nucleic acid is an RNA and the ligase is an RNA ligase 27 The method of claim 1 in which the PNA-DNA chimenc probe, the second probe, or the template nucleic acid is immobilized on a solid substrate 28 The method of claim 27 in which the PNA-DNA chimenc probe, the second probe, or the template nucleic acid is covalently attached to the solid substrate, optionally with the aid of a linker 29 The method of claim 28 wherein the solid substrate is selected from the group consisting of polystyrene, controlled-pore-glass, glass, silica gel, silica, polyacrylamide, magnetic beads, polyacrylate, hydroxyethylmethacrylate, polyamide, polyethylene, polyethyleneoxy, and copolymers and grafts of any of the above solid substrates 30 The method of claim 28 wherein the solid substrate is selected from the group consisting of small particles, beads, membranes, frits, slides, plates, micromachined chips, alkanethiol-gold layers, non-porous surfaces, and polynucleotide-immobihzing media 31 A method of template-dependent ligation, compnsmg the steps of a) generating a ligation product by enzymatically hgatmg a PNA-DNA chimenc probe to a second probe in the presence of a template to which the chimenc probe and the second probe are complementary and hybndized adjacently, wherein the second probe is a PNA-DNA chimera or an oligonucleotide, and b) detecting the ligation product 32 The method of claim 31 in which the chimenc probe and/or the second probe is non-radioisotopically labelled 33 A kit for template-dependent ligation compnsmg a PNA-DNA chimenc probe, said probe compnsmg 5 to 15 contiguous PNA monomer units, 2 to 15 contiguous nucleotides, and a 3' hydroxyl terminus, a second probe which is a PNA-DNA chimera or an oligonucleotide, and a hgase enzyme 34 The kit of claim 33 in which the chimera probe and/or the second probe is non-radioisotopically labelled 35 The kit of claim 33 further compnsmg a template nucleic acid comprising a sequence complementary to the chimenc probe or containing one or more mismatches relative to the chimenc probe 36 The kit of claim 33 further comprising a polynucleotide kinase 37 A duplex hybnd comprising a PNA-DNA chimenc probe, a second probe which is a PNA-DNA chimera or an oligonucleotide, and a template nucleic acid with a sequence complementary to the chimenc probe or containing one or more mismatches relative to the chimenc probe, wherein a terminus of the second probe hybndizes adjacent to a terminus of the chimenc probe on the template and said terminii can be ligated together by a ligase enzyme 38 The duplex hybrid of claim 37 m which the chimenc probe and/or the second probe is non-radioisotopically labelled 39 The duplex hybnd of claim 37 further compnsmg a polynucleotide kinase

Inventors: CHEN CAIFU, EGHOLM MICHAEL, CHEN, CAIFU, EGHOLM, MICHAEL
Applicants: PE CORP, PE CORPORATION
Application year: 2000
Main classification: C12Q1
IPC codes: C12N15, C12Q1, G01N21