NUCLEIC ACID CATALYSTS WITH ENDONUCLEASE ACTIVITY

Abstract: Catalytic nucleic acids, i.e. ribozymes, with new motifs giving an altered endoclease specificity, methods of screening, synthesis and expression, and uses thereof in modulating gene expression.

Description: DESCRIPTION NUCLEIC ACID CATALYSTS WITH ENDONUCLEASE ACTIVITY Background of the Invention This invention relates to nucleic acid molecules with catalytic activity and derivatives thereof. The following is a brief description of enzymatic nucleic acid molecules. This summary is not meant to be complete but is provided only for understanding of the invention that follows. This summary is not an admission that all of the work described below is prior art to the claimed invention. Enzymatic nucleic acid molecules (ribozymes) are nucleic acid molecules capable of catalyzing one or more of a variety of reactions, including the ability to repeatedly cleave other separate nucleic acid molecules in a nucleotide base sequence-specific manner. Such enzymatic nucleic acid molecules can be used, for example, to target virtually any RNA transcript (Zaug et al, 324, Nature 429 1986 ; Cech, 260 JAMA 3030, 1988; and Jefferies et al, 17 Nucleic Acids Research 1371, 1989). Because of their sequence-specificity, tr< s-cleaving enzymatic nucleic acid molecules show promise as therapeutic agents for human disease (Usman & McSwiggen, 1995 Ann. Rep. Med. Chem. 30, 285-294; Christoffersen and Marr, 1995 J. Med. Chem. 38, 2023-2037). Enzymatic nucleic acid molecules can be designed to cleave specific RNA targets within the background of cellular RNA. Such a cleavage event renders the mRNA non-functional and abrogates protein expression from that RNA. In this manner, synthesis of a protein associated with a disease state can be selectively inhibited. There are seven basic varieties of naturally-occurring enzymatic RNAs. Each can catalyze the hydrolysis of RNA phosphodiester bonds in trans (and thus can cleave other RNA molecules) under physiological conditions. In general, enzymatic nucleic acids act by first binding to a target RNA. Such binding occurs through the target binding portion of a enzymatic nucleic acid which is held in close proximity to an enzymatic portion of the molecule that acts to cleave the target RNA. Thus, the enzymatic nucleic acid first recognizes and then binds a target RNA through complementary base-pairing, and once bound to the correct site, acts enzymatically to cut the target RNA. Strategic cleavage of such a target RNA will destroy its ability to direct synthesis of an encoded protein. After an enzymatic nucleic acid has bound and cleaved its RNA target, it is released from that RNA to search for another target and can repeatedly bind and cleave new targets. In addition, several in vitro selection (evolution) strategies (Orgel, 1979, Proc. R Soc. London, B 205, 435) have been used to evolve new nucleic acid catalysts capable of catalyzing a variety of reactions, such as cleavage and ligation of phosphodiester linkages and amide linkages, (Joyce, 1989, Gene, 82, 83-87; Beaudry et al, 1992, Science 257, 635-641; Joyce, 1992, Scientific American 267, 90-97; Breaker et al, 1994, TIBTECH 12, 268; Bartel et Λ/., 1993, Science 261:1411-1418; Szostak, 1993, TIBS 17, 89-93; Kumar et al, 1995, FASEB J., 9, 1183; Breaker, 1996, Curr. Op. Biotech., 7, 442). The enzymatic nature of a ribozyme is advantageous over other technologies, since the effective concentration of ribozyme necessary to effect a therapeutic treatment is generally lower than that of an antisense oligonucleotide. This advantage reflects the ability of the ribozyme to act enzymatically. Thus, a single ribozyme (enzymatic nucleic acid) molecule is able to cleave many molecules of target RNA. In addition, the ribozyme is a highly specific inhibitor, with the specificity of inhibition depending not only on the base-pairing mechanism of binding, but also on the mechanism by which the molecule inhibits the expression of the RNA to which it binds. That is, the inhibition is caused by cleavage of the RNA target and so specificity is defined as the ratio of the rate of cleavage of the targeted RNA over the rate of cleavage of non-targeted RNA. This cleavage mechanism is dependent upon factors additional to those involved in base- pairing. Thus, it is thought that the specificity of action of a ribozyme is greater than that of antisense oligonucleotide binding the same RNA site. The development of ribozymes that are optimal for catalytic activity would contribute significantly to any strategy that employs RNA-cleaving ribozymes for the purpose of regulating gene expression. The hammerhead ribozyme functions with a catalytic rate (λc at of ~1 πώr 1 in the presence of saturating (10 mM) concentrations of Mg 2+ cofactor. However, the rate for this ribozyme in Mg 2+ concentrations that are closer to those found inside cells (0.5 - 2 mM) can be 10- to 100-fold slower. In contrast, the RNase P holoenzyme can catalyze pre-tRNA cleavage with a A a t of ~30 πώr 1 under optimal assay conditions. An artificial 'RNA ligase' ribozyme has been shown to catalyze the corresponding self-modification reaction with a rate of -100 min" . In addition, it is known that certain modified hammerhead ribozymes that have substrate binding arms made of DNA catalyze RNA cleavage with multiple turn-over rates that approach 100 min" 1 . Finally, replacement of a specific residue within the catalytic core of the hammerhead with certain nucleotide analogues gives modified ribozymes that show as much as a 10-fold improvement in catalytic rate. These findings demonstrate that ribozymes can promote chemical transformations with catalytic rates that are significantly greater than those displayed in vitro by most natural self-cleaving ribozymes. It is then possible that the structures of certain self-cleaving ribozymes may not be optimized to give maximal catalytic activity, or that entirely new RNA motifs could be made that display significantly faster rates for RNA phosphoester cleavage. An extensive array of site-directed mutagenesis studies have been conducted with the hammerhead ribozyme to probe relationships between nucleotide sequence and catalytic activity. These systematic studies have made clear that most nucleotides in the conserved core of the hammerhead ribozyme cannot be mutated without significant loss, of catalytic activity. In contrast, a combinatorial strategy that simultaneously screens a large pool of mutagenized ribozymes for RNAs that retain catalytic activity could be used more efficiently to define immutable sequences and to identify new ribozyme variants. Although similar in vitro selection experiments have been conducted with the hammerhead ribozyme (Nakamaye & Eckstein, 1994, Biochemistry 33, 1271; Long & Uhlenbeck, 1994, Proc. Natl. Acad. Set, 91, 6977; Ishizaka et al, 1995, BBRC 214, 403; Vaish et al, 1997, Biochemistry 36, 6495), none of these efforts have successfully screened full-sized hammerhead ribozymes for all possible combinations of sequence variants that encompass the entire catalytic core. The hammerhead ribozyme is one of the smallest ribozymes known and has thus attracted much attention for the study of structure-function relationships in catalytic RNAs as well as for its potential for the sequence-specific inhibition of gene expression (Usman et al, supra). The hammerhead cleaves RNA sequence-specifically adjacent to the general triplet sequence NUX, where N is any nucleotide and X can be A, U or C. Cleavage behind a guanosine such as in GUG is very slow (4.3 x 10 '5 min "1 ) compared to the triplet substrate GUC (1 min' 1 ) (Baidya et al, 1997, Biochemistry 36, 1108). Although the X-ray structure of this ribozyme has been solved and a mechanism proposed (Pley et al, 1994 Nature, 372, 68; Scott et al, 1996, Science 274, 2065), the question of what determines its specificity for the NUX sequence is still largely unresolved. One way of obtaining an insight into this problem might be to compare sequences of hammerhead ribozymes with different triplet cleaving specificities. In previous publications it was demonstrated, by in vitro selection, that the native hammerhead sequence to cleave typical cleavage triplet, NUX, can not be altered (Nakamaye & Eckstein, 1994, Biochemistry 33, 1271; Long & Uhlenbeck, 1994, Proc. Natl. Acad. Set, 91, 6977; Ishizaka et al, 1995, BBRC 214, 403; Vaish et al, 1997, Biochemistry 36, 6495). Tang et al, 1997, RNA 3, 914, reported novel ribozyme sequences with endonuclease activity, where the authors used an in vitro selection approach to isolate these ribozymes. The references cited above are distinct from the presently claimed invention since they do not disclose and/or contemplate the catalytic nucleic acid molecules of the instant invention. Summary of the Invention This invention relates to novel nucleic acid molecules with catalytic activity, which are particularly useful for cleavage of RNA or DNA. The nucleic acid catalysts of the instant invention are distinct from other nucleic acid catalysts known in the art. The nucleic acid catalysts of the instant invention do not share sequence homology with other known ribozymes. Specifically, nucleic acid catalysts of the instant invention are capable of catalyzing an intermolecular or intramolecular endonuclease reaction. In a preferred embodiment, the invention features a nucleic acid molecule with catalytic activity having one of the formulae I-IH: Formula I ^^ (N) n — C — C — C X— 3' L ^ (N) m — J Y— 5' Formula III ^^ (N) n -- G ~ C — G X— 3' L "^ (N) m — K Y— 5' In each of the above formula, N represents independently a nucleotide or a non- nucleotide linker, which may be same or different; X and Y are independently oligonucleotides of length sufficient to stably interact (e.g., by forming hydrogen bonds with complementary nucleotides in the target) with a target nucleic acid molecule (the target can be an RNA, DNA or RNA/DNA mixed polymers); m and n are integers independently greater than or equal to 1 and preferably less than about 100, wherein if. (N) m and (N) n are nucleotides, (N)m and (N)n are optionally able to interact by hydrogen bond interaction; J is an oligonucleotide of length 7 nucleotides; K is an oligonucleotide of length 6 nucleotides; L is a linker which may be present or absent (i.e., the molecule is assembled from two separate molecules), but when present, is a nucleotide and/or a non-nucleotide linker, which may be a single-stranded and/or double- stranded region; and represents a chemical linkage (e.g. a phosphate ester linkage, amide linkage or others known in the art). C and G represent cytidine and guanosine nucleotides, respectively. The nucleotides in the each of the formula I-III are unmodified or modified at the sugar, base, and/or phosphate as known in the art. In a preferred embodiment, the invention features nucleic acid molecules of Formula I, where the sequence of oligonucleotide J is selected from the group comprising 5'-GGCAUCC-3', 5'-AGCAUU-3\ 5'-GCAUCA-3\ 5'-AGCAUC-3', and. 5'- AGCGUC-3'. In another preferred embodiment, the invention features nucleic acid molecules of Formula II, where the sequence of oligonucleotide J is 5'-GGAAUC-3\ In a further preferred embodiment, the invention features nucleic acid molecules of Formula III, where the sequence of oligonucleotide K is selected from the group comprising 5'-UGAUG-3\ 5'-UGAUC-3\ and 5'-UGGGC-3\ In yet another embodiment, the nucleotide linker (L) is a nucleic acid aptamer, such as an ATP aptamer, HIV Rev aptamer (RRE), HIV Tat aptamer (TAR) and others (for a review see Gold et al, 1995, Anna. Rev. Biochem., 64, 763; and Szostak & Ellington, 1993, in The RNA World, ed. Gesteland and Atkins, pp 511, CSH Laboratory Press). A "nucleic acid aptamer" as used herein is meant to indicate nucleic acid sequence capable of interacting with a ligand. The ligand can be any natural or a synthetic molecule, including but not limited to a resin, metabolites, nucleosides, nucleotides, drugs, toxins, transition state analogs, peptides, lipids, proteins, amino acids, nucleic acid molecules, hormones, carbohydrates, receptors, cells, viruses, bacteria and others. In yet another embodiment, the non-nucleotide linker (L) is as defined herein. The term "non-nucleotide" as used herein include either abasic nucleotide, polyether, polyamine, polyamide, peptide, carbohydrate, lipid, or polyhydrocarbon compounds! Specific examples include those described by Seela and Kaiser, Nucleic Acids Res. 1990, 75:6353 and Nucleic Acids Res. 1987, 75:3113; Cload and Schepartz, J. Am. Chem. Soc. 1991, 773:6324; Richardson and Schepartz, J. Am. Chem. Soc. 1991, 775:5109; Ma et al., Nucleic Acids Res. 1993, 27:2585 and Biochemistry 1993, 32:1751; Durand et al., Nucleic Acids Res. 1990, 75:6353; McCurdy et al., Nucleosides & Nucleotides 1991, 70:287; Jschke et al., Tetrahedron Lett. 1993, 3..301; Ono et al., Biochemistry 1991, 30:9914; Arnold et al, International Publication No. WO 89/02439; Usman et al, International Publication No. WO 95/06731; Dudycz et al, International Publication No. WO 95/11910 and Ferentz and Verdine, J. Am. Chem. Soc. 1991, 773:4000, all hereby incorporated by reference herein. Thus, in a preferred embodiment, the invention features an enzymatic nucleic acid molecule having one or more non-nucleotide moieties, and having enzymatic activity to cleave an RNA or DNA molecule. By the term "non- nucleotide" is meant any group or compound which can be incorporated into a nucleic acid chain in the place of one or more nucleotide units, including either sugar and/or phosphate substitutions, and allows the remaining bases to exhibit their enzymatic activity. The group or compound is abasic in that it does not contain a commonly recognized nucleotide base, such as adenosine, guanine, cytosine, uracil or thymine. The terms "abasic" or "abasic nucleotide" as used herein encompass sugar moieties lacking a base or having other chemical groups in place of base at the 1' position. In preferred embodiments, the enzymatic nucleic acid includes one or more stretches of RNA, which provide the enzymatic activity of the molecule, linked to the non-nucleotide moiety. The necessary RNA components are known in the art (see for e.g., Usman et al, supra). By RNA is meant a molecule comprising at least one ribonucleotide residue. Thus, in one preferred embodiment, the invention features ribozymes that inhibit gene expression and or cell proliferation. These chemically or enzymatically synthesized nucleic acid molecules contain substrate binding domains that bind to accessible regions of specific target nucleic acid molecules. The nucleic acid molecules also contain domains that catalyze the cleavage of target. Upon binding, the enzymatic nucleic acid molecules cleave the target molecules, preventing for example, translation and protein accumulation. In the absence of the expression of the target gene, cell proliferation, for example, is inhibited. In another aspect of the invention, enzymatic nucleic acid molecules that cleave target molecules are expressed from transcription units inserted into DNA or RNA vectors. The recombinant vectors are preferably DNA plasmids or viral vectors. Ribozyme expressing viral vectors could be constructed based on, but not limited to, adeno-associated virus, retrovirus, adenovirus, or alphavirus. Preferably, the recombinant vectors capable of expressing the ribozymes are delivered as described above, and persist in target cells. Alternatively, viral vectors may be used that provide for transient expression of ribozymes. Such vectors might be repeatedly administered as necessary. Once expressed, the ribozymes cleave the target mRNA. Delivery of ribozyme expressing vectors could be systemic, such as by intravenous or intramuscular administration, by administration to target cells ex-planted from the patient followed by reintroduction into the patient, or by any other means that would allow for introduction into the desired target cell (for a review see Couture and Stinchcomb, 1996, TIG., 12, 510). In a preferred embodiment, an expression vector comprising nucleic acid sequence encoding at least one of the nucleic acid catalyst of the instant invention is disclosed. The nucleic acid sequence encoding the nucleic acid catalyst of the instant invention is operable linked in a manner which allows expression of that nucleic acid molecule. In one embodiment, the expression vector comprises: a transcription initiation region (e.g., eukaryotic pol I, II or III initiation region); b) a transcription termination region (e.g., eukaryotic pol I, II or III termination region); c) a gene encoding at least one of the nucleic acid catalyst of the instant invention; and wherein said gene is operably linked to said initiation region and said termination region, in a manner which allows expression and/or delivery of said nucleic acid molecule. The vector may optionally include an open reading frame (ORF) for a protein operably linked on the 5' side or the 3 '-side of the gene encoding the nucleic acid catalyst of the invention; and/or an intron (intervening sequences). By "patient" is meant an organism which is a donor or recipient of explanted cells or the cells themselves. "Patient" also refers to an organism to which enzymatic nucleic acid molecules can be administered. Preferably, a patient is a mammal or mammalian cells. More preferably, a patient is a human or human cells. By "vectors" is meant any nucleic acid- and/or viral-based technique used to deliver a desired nucleic acid. Another means of accumulating high concentrations of a ribozyme(s) within cells is to incorporate the ribozyme-encoding sequences into a DNA or RNA expression vector. Transcription of the ribozyme sequences are driven from a promoter for eukaryotic RNA polymerase I (pol I), RNA polymerase II (pol II), or RNA polymerase III (pol III). Transcripts from pol II or pol III promoters will be expressed at high levels in all cells; the levels of a given pol II promoter in a given cell type will depend on the nature of the gene regulatory sequences (enhancers, silencers, etc.) present nearby. Prokaryotic RNA polymerase promoters are also used, providing that the prokaryotic RNA polymerase enzyme is expressed in the appropriate cells (Elroy-Stein and Moss, 1990 Proc. Natl. Acad Sci. USA, 87, 6743-7; Gao and Huang 1993 Nucleic Acids Res., 21, 2867-72; Lieber et al., 1993 Methods Enzymol, 217, 47-66; Zhou et al., 1990 Mo/. Cell. Biol, 10, 4529-37). Several investigators have demonstrated that ribozymes expressed from such promoters can function in mammalian cells (e.g. Kashani-Sabet et al., 1992 Antisense Res. Dev., 2, 3-15; Ojwang et al., 1992 Proc. Natl Acad Sci. U S A, 89, 10802-6; Chen et α/., 1992 Nucleic Acids Res., 20, 4581-9; Yu et al., 1993 Proc. Natl. Acad. Sci. U S A, 90, 6340-4; L'Huillier et al, 1992 EMBO J. 11, 4411-8; Lisziewicz et al, 1993 Proc. Natl. Acad. Sci. U. S. A., 90, 8000-4;Thompson et al, 1995 Nucleic Acids Res. 23, 2259; Sullenger & Cech, 1993, Science, 262, 1566). The above ribozyme transcription units can be incorporated into a variety of vectors for introduction into mammalian cells, including but not restricted to, plasmid DNA vectors, viral DNA vectors (such as adenovirus or adeno-associated virus vectors), or viral RNA vectors (such as retroviral or alphavirus vectors) (for a review see Couture and Stinchcomb, 1996, supra). In a preferred embodiment, the invention features a method of synthesis of enzymatic nucleic acid molecules of instant invention which follows the procedure for normal chemical synthesis of RNA as described in Usman et al, 1987 J. Am. Chem. Soc, 109, 7845; Scaringe et al, 1990 Nucleic Acids Res., 18, 5433; and Wincott et al, 1995 Nucleic Acids Res. 23, 2677-2684 and makes use of common nucleic acid protecting and coupling groups, such as dimethoxytrityl at the 5'-end, and phosphoramidites at the 3'-end. Small scale synthesis were conducted on a 394 Applied Biosystems, Inc. synthesizer using a modified 2.5 μmol scale protocol with a 5 min coupling step for alkylsilyl protected nucleotides and 2.5 min coupling step for 2'-O- methylated nucleotides. Table I outlines the amounts, and the contact times, of the reagents used in the synthesis cycle. A 6.5-fold excess (163 μL of 0.1 M = 16.3 μmol) of phosphoramidite and a 24-fold excess of S-ethyl tetrazole (238 μL of 0.25 M = 59.5 μmol) relative to polymer-bound 5'-hydroxyl is used in each coupling cycle. Average coupling yields on the 394 Applied Biosystems, Inc. synthesizer, determined by colorimetric quantitation of the trityl fractions, is 97.5-99%. Other oligonucleotide synthesis reagents for the 394 Applied Biosystems, Inc. synthesizer : detritylation solution was 2% TCA in methylene chloride (ABI); capping was performed with 16% N-methyl imidazole in THF (ABI) and 10% acetic anhydride/10% 2,6-lutidine in THF (ABI); oxidation solution was 16.9 mM I2, 49 mM pyridine, 9% water in THF (Millipore). B & J Synthesis Grade acetonitrile is used directly from the reagent bottle. S-Ethyl tetrazole solution (0.25 M in acetonitrile) is made up from the solid obtained from American International Chemical, Inc. In a preferred embodiment, deprotection of the chemically synthesized nucleic acid catalysts of the invention is performed as follows. The polymer-bound oligoribonucleotide, trityl-off, is transferred from the synthesis column to a 4mL glass screw top vial and suspended in a solution of methylamine (MA) at 65 °C for 10 rm'n After cooling to -20 °C, the supernatant is removed from the polymer support. The support is washed three times with 1.0 mL of EtOH:MeCN:H20/3:1:1, vortexed and the supernatant is then added to the first supernatant. The combined supematants, containing the oligoribonucleotide, are dried to a white powder. The base-deprotected oligoribonucleotide is resuspended in anhydrous TEA » HF/NMP solution (250 μL of a solution of 1.5mL N-methylpyrrolidinone, 750 μL TEA and 1.0 mL TEA*3HF to provide a 1.4M HF concentration) and heated to 65°C for 1.5 h. The resulting, fully deprotected, oligomer is quenched with 50 mM TEAB (9 mL) prior to anion exchange desalting. For anion exchange desalting of the deprotected oligomer, the TEAB solution is loaded on to a Qiagen 500® anion exchange cartridge (Qiagen Inc.) that is prewashed with 50 mM TEAB (10 mL). After washing the loaded cartridge with 50 mM TEAB (10 mL), the RΝA is eluted with 2 M TEAB (10 mL) and dried down to a white powder. The average stepwise coupling yields are generally >98% (Wincott et al, 1995 Nucleic Acids Res. 23, 2677-2684). Ribozymes of the instant invention are also synthesized from DΝA templates using bacteriophage T7 RΝA polymerase (Milligan and Uhlenbeck, 1989, Methods Enzymol. 180, 51). Ribozymes are purified by gel electrophoresis using general methods or are purified by high pressure liquid chromatography (HPLC; See Wincott et al, supra) the totality of which is hereby incorporated herein by reference) and are resuspended in water. In another preferred embodiment, catalytic activity of the molecules described in the instant invention can be optimized as described by Draper et al., supra. The details will not be repeated here, but include altering the length of the ribozyme binding arms, or chemically synthesizing ribozymes with modifications (base, sugar and/or phosphate) that prevent their degradation by serum ribonucleases and or enhance their enzymatic activity (see e.g., Eckstein et al, International Publication No. WO 92/07065; Perrault et al, 1990 Nature 344, 565; Pieken et al., 1991 Science 253, 314; Usman and Cedergren, 1992 Trends in Biochem. Sci. 17, 334; Usman et al, International Publication No. WO 93/15187; and Rossi etal., International Publication No. WO 91/03162; Sproat, US Patent No. 5,334,711; and Burgin et al, supra; all of these describe various chemical modifications that can be made to the base, phosphate and/or sugar moieties of enzymatic RNA molecules). Modifications which enhance their efficacy in cells, and removal of bases from stem loop structures to shorten RNA synthesis times and reduce chemical requirements are desired. (All these publications are hereby incorporated by reference herein.). By "enhanced enzymatic activity" is meant to include activity measured in cells and/or in vivo where the activity is a reflection of both catalytic activity and ribozyme stability. In this invention, the product of these properties is increased or not significantly (less that 10 fold) decreased in vivo compared to an all RNA ribozyme. In yet another preferred embodiment, nucleic acid catalysts having chemical modifications which maintain or enhance enzymatic activity is provided. Such nucleic acid is also generally more resistant to nucleases than unmodified nucleic acid. Thus, in a cell and/or in vivo the activity may not be significantly lowered. As exemplified herein such ribozymes are useful in a cell and/or in vivo even if activity over all is reduced 10 fold (Burgin et al, 1996, Biochemistry, 35, 14090). Such ribozymes herein are said to "maintain" the enzymatic activity on all RNA ribozyme. Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims. Description of the Preferred Embodiments The drawings will first briefly be described. Drawings: Figure 1 A) is a diagrammatic representation of the hammerhead ribozyme domain known in the art. Stem II can be ≥ 2 base-pair long. Each N is independently any base or non-nucleotide as used herein; B) is a diagrammatic representation of a self-cleaving hammerhead ribozyme; C) is a diagrammatic representation of a random pool of self- cleaving RNA. N indicates the region with random nucleotides. Figure 2 is a schematic representation of a non-limiting in vitro selection strategy used to evolve nucleic acid catalysts. Figure 3 shows sequences of individual RNAs that represent new classes of self- cleaving ribozymes. Also shown are rates of catalysis (k in .-j S (min "1 )) for each of the new self-cleaving RNA sequences. The underlined sequences indicate the regions capable of interacting with each other.. Figure 4 summarizes structure mapping studies on clone 40 RNA. Figure 5 shows sequence and possible secondary structure of a trans-cleaving ribozyme of the invention. Arrow identifies the site of cleavage. Figure 6 shows non-limiting examples of tr_n_-cleaving ribozyme-substrate complex of the present invention. Figure 7 shows non-limiting examples of chemically modified trα/w-cleaving ribozyme-substrate complex of the present invention. Nucleic Acid Catalysts The invention provides nucleic acid catalysts and methods for producing a class of enzymatic nucleic acid cleaving agents which exhibit a high degree of specificity for the nucleic acid sequence of a desired target. The enzymatic nucleic acid molecule is preferably targeted to a highly conserved sequence region of a target such that specific diagnosis and/or treatment of a disease or condition in a variety of biological system can be provided with a single enzymatic nucleic acid. Such enzymatic nucleic acid molecules can be delivered exogenously to specific cells as required. In the preferred hammerhead motif the small size (less than 60 nucleotides, preferably between 30-40 nucleotides in length) of the molecule allows the cost of treatment to be reduced. By the phrase "nucleic acid catalyst" is meant a nucleic acid molecule capable of catalyzing (altering the velocity and/or rate of) a variety of reactions including the ability to repeatedly cleave other separate nucleic acid molecules (endonuclease activity) in a nucleotide base sequence-specific manner. Such a molecule with endonuclease activity may have complementarity in a substrate binding region (e.g. X and Y in formulae I-III) to a specified gene target, and also has an enzymatic activity that specifically cleaves RNA or DNA in that target. That is, the nucleic acid molecule with endonuclease activity is able to intramolecularly or intermolecularly cleave RNA or DNA and thereby inactivate a target RNA or DNA molecule. This complementarity functions to allow sufficient hybridization of the enzymatic RNA molecule to the target RNA or DNA to allow the cleavage to occur. 100% complementarity is preferred, but complementarity as low as 50-75% may also be useful in this invention. The nucleic acids may be modified at the base, sugar, and/or phosphate groups. The term enzymatic nucleic acid is used interchangeably with phrases such as ribozymes, catalytic RNA, enzymatic RNA, catalytic DNA, nucleozyme, DNAzyme, RNA enzyme, endoribonuclease, endonuclease, minizyme, leadzyme, oligozyme or DNA enzyme. All of these terminologies describe nucleic acid molecules with enzymatic activity. By "nucleic acid molecule" as used herein is meant a molecule comprising nucleotides. The nucleic acid can be composed of modified or unmodified nucleotides or non-nucleotides or various mixtures and combinations thereof. By "complementarity" is meant a nucleic acid that can form hydrogen bond(s) with other RNA sequence by either traditional Watson-Crick or other non-traditional types (for example, Hoogsteen type) of base-paired interactions. By "oligonucleotide" as used herein, is meant a molecule comprising two or more nucleotides. The specific enzymatic nucleic acid molecules described in the instant application are not limiting in the invention and those skilled in the art will recognize that all that is important in an enzymatic nucleic acid molecule of this invention is that it has a specific substrate binding site (e.g., X and Y of Formulae I-III above) which is complementary to one or more of the target nucleic acid regions, and that it have nucleotide sequences within or surrounding that substrate binding site which impart a nucleic acid cleaving activity to the molecule. By "enzymatic portion" is meant that part of the ribozyme essential for cleavage of an RNA substrate. By "substrate binding arm" or "substrate binding domain" is meant that portion of a ribozyme which is complementary to (i.e., able to base-pair with) a portion of its substrate. Generally, such complementarity is 100%, but can be less if desired. For example, as few as 10 nucleotides out of a total of 14 may be base-paired. Such arms are shown generally in Figure 1A and as X and Y in Formulae I-III. That is, these arms contain sequences within a ribozyme which are intended to bring ribozyme and target RNA together through complementary base-pairing interactions. The ribozyme of the invention may have binding arms that are contiguous or non-contiguous and may be of varying lengths. The length of the binding arm(s) are preferably greater than or equal to four nucleotides; specifically 12-100 nucleotides; more specifically 14-24 nucleotides long. The two binding arms are chosen, such that the length of the binding arms are symmetrical (i.e., each of the binding arms is of the same length; e.g., five and five nucleotides, six and six nucleotides or seven and seven nucleotides long) or asymmetrical (i.e., the binding arms are of different length; e.g., six and three nucleotides; three and six nucleotides long; four and five nucleotides long; four and six nucleotides long; four and seven nucleotides long; and the like). Therapeutic ribozymes must remain stable within cells until translation of the target RNA has been inhibited long enough to reduce the levels of the undesirable protein. This period of time varies between hours to days depending upon the disease state. Clearly, ribozymes must be resistant to nucleases in order to function as effective intracellular therapeutic agents. Improvements in the chemical synthesis of RNA (Wincott et al, 1995 Nucleic Acids Res. 23, 2677; incorporated by reference herein) have, expanded the ability to modify ribozymes to enhance their nuclease stability. The term "nucleotide" is used as recognized in the art to include natural bases, and modified bases well known in the art. Such bases are generally located at the 1' position of a sugar moiety. Nucleotide generally comprise a base, sugar and a phosphate group. The nucleotides can be unmodified or modified at the sugar, phosphate and/or base moeity, (see for example, Usman and McSwiggen, supra; Eckstein et al, International PCT Publication No. WO 92/07065; Usman et al, International PCT Publication No. WO 93/15187; all hereby incorporated by reference herein). There are several examples of modified nucleic acid bases known in the art and has recently been summarized by Limbach et al, 1994, Nucleic Acids Res. 22, 2183. Some of the non-limiting examples of base modifications that can be introduced into enzymatic nucleic acids without significantly effecting their catalytic activity include, inosine, purine, pyridin-4-one, pyridin-2-one, phenyl, pseudouracil, 2, 4, 6-trimethoxy benzene, 3-methyluracil, dihydrouridine, naphthyl, aminophenyl, 5-alkylcytidines (e.g., 5-methylcytidine), 5-alkyluridines (e.g., ribothymidine), 5-halouridine (e.g., 5-bromouridine) or 6-azapyrimidines or 6-alkylpyrimidines (e.g. 6-methyluridine) and others (Burgin et al, 1996, Biochemistry, 35, 14090). By "modified bases" in this aspect is meant nucleotide bases other than adenine, guanine, cytosine and uracil at l 1 position or their equivalents; such bases may be used within the catalytic core of the enzyme and/or in the substrate- binding regions. In particular, the invention features modified ribozymes having a base substitution selected from pyridin-4-one, pyridin-2-one, phenyl, pseudouracil, 2, 4, 6-trimethoxy benzene, 3-methyluracil, dihydrouracil, naphthyl, 6-methyl-uracil and aminophenyl. There are several examples in the art describing sugar and phosphate modifications that can be introduced into enzymatic nucleic acid molecules without significantly effecting catalysis and witi significant enhancement in their nuclease stability and efficacy. Ribozymes are modified to enhance stability and/or enhance catalytic activity by modification with nuclease resistant groups, for example, 2'-amino, 2'-C-allyl, 2'- flouro, 2'-O-methyl, 2'-H, nucleotide base modifications (for a review see Usman and Cedergren, 1992 TIBS 17, 34; Usman et al, 1994 Nucleic Acids Symp. Ser. 31, 163; Burgin et al, 1996 Biochemistry 35, 14090). Sugar modification of enzymatic nucleic acid molecules have been extensively described in the art (see Eckstein et al, International Publication PCT No. WΟ 92/07065; Perrault et al. Nature 1990, 344, 565-568; Pieken et al. Science 1991, 253, 314-317; Usman and Cedergren, Trends in Biochem. Sci. 1992, 17, 334-339; Usman et al. International Publication PCT No. WO 93/15187; Sproat, US Patent No. 5,334,711 and Beigelman et al, 1995 J Biol Chem. 270, 25702). Such publications describe general methods and strategies to determine the location of incorporation of sugar, base and/or phosphate modifications and the like into ribozymes without inhibiting catalysis, and are incorporated by reference herein. In view of such teachings, similar modifications can be used as described herein to modify the nucleic acid catalysts of the instant invention. As the term is used in this application, non-nucleotide-containing enzymatic nucleic acid means a nucleic acid molecule that contains at least one non-nucleotide component which replaces a portion of a ribozyme, e.g., but not limited to, a double- stranded stem, a single-stranded "catalytic core" sequence, a single-stranded loop or a single-stranded recognition sequence. These molecules are able to cleave (preferably, repeatedly cleave) separate RNA or DNA molecules in a nucleotide base sequence specific manner. Such molecules can also act to cleave intramolecularly if that is desired. Such enzymatic molecules can be targeted to virtually any RNA transcript. Administration of Ribozymes Sullivan et al, PCT WO 94/02595, describes the general methods for delivery of enzymatic RNA molecules. Ribozymes may be administered to cells by a variety of methods known to those familiar to the art, including, but not restricted to, encapsulation in Uposomes, by iontophoresis, or by incorporation into other vehicles, such as hydrogels, cyclodextrins, biodegradable nanocapsules, and bioadhesive microspheres. For some indications, ribozymes may be directly delivered ex vivo to cells or tissues with or without the aforementioned vehicles. Alternatively, the RNA vehicle combination is locally delivered by direct injection or by use of a catheter, infusion pump or stent. Other routes of delivery include, but are not limited to, intravascular, intramuscular, subcutaneous or joint injection, aerosol inhalation, oral (tablet or pill form), topical, systemic, ocular, intraperitoneal and/or intrathecal delivery. More detailed descriptions of ribozyme delivery and administration are provided in Sullivan et al, supra and Draper et al, PCT WO93/23569 which have been incorporated by reference herein. The molecules of the instant invention can be used as pharmaceutical agents. Pharmaceutical agents prevent, inhibit the occurrence, or treat (alleviate a symptom to some extent, preferably all of the symptoms) of a disease state in a patient. The negatively charged polynucleotides of the invention can be administered (e.g., RNA, DNA or protein) and introduced into a patient by any standard means, with or without stabilizers, buffers, and the like, to form a pharmaceutical composition. When it is desired to use a liposome delivery mechanism, standard protocols for formation of liposomes can be followed. The compositions of the present invention may also be formulated and used as tablets, capsules or elixirs for oral administration; suppositories for rectal administration; sterile solutions; suspensions for injectable administration; and the like. The present invention also includes pharmaceutically acceptable formulations of the compounds described. These formulations include salts of the above compounds, e.g., acid addition salts, for example, salts of hydrochloric, hydrobromic, acetic acid, and benzene sulfonic acid. A pharmacological composition or formulation refers to a composition or formulation in a form suitable for administration, e.g., systemic administration, into a cell or patient, preferably a human. Suitable forms, in part, depend upon the use or the route of entry, for example oral, transdermal, or by injection. Such forms should not prevent the composition or formulation to reach a target cell (i.e., a cell to which the negatively charged polymer is desired to be delivered to). For example, pharmacological compositions injected into the blood stream should be soluble. Other factors are known in the art, and include considerations such as toxicity and forms which prevent the composition or formulation from exerting its effect. By "systemic administration" is meant in vivo systemic absorption or accumulation of drugs in the blood stream followed by distribution throughout the entire body. Administration routes which lead to systemic absorption mclude, without limitations: intravenous, subcutaneous, intraperitoneal, inhalation, oral, intrapulmonary and intramuscular. Each of these administration routes expose the desired negatively charged polymers, e.g., nucleic acids, to an accessible diseased tissue. The rate of entry of a drug into the circulation has been shown to be a function of molecular weight or size. The use of a liposome or other drug carrier comprising the compounds of the instant invention can potentially localize the drug, for example, in certain tissue types, such as the tissues of the reticular endothelial system (RES). A liposome formulation which can facilitate the association of drug with the surface of cells, such as, lymphocytes and macrophages is also useful. This approach may provide enhanced delivery of the drug to target cells by taking advantage of the specificity of macrophage and lymphocyte immune recognition of abnormal cells, such as the cancer cells. The invention also features the use of the a composition comprising surface- modified liposomes containing poly (ethylene glycol) lipids (PEG-modified, or long- circulating liposomes or stealth liposomes). These formulations offer an method for increasing the accumulation of drugs in target tissues. This class of drug carriers resists opsonization and elimination by the mononuciear phagocytic system (MPS or RES), thereby enabling longer blood circulation times and enhanced tissue exposure for the encapsulated drug (Lasic et al. Chem. Rev. 1995, 95, 2601-2627; Ishiwataet al, Chem. Pharm. Bull. 1995, 43, 1005-1011). Such liposomes have been shown to accumulate selectively in tumors, presumably by extravasation and capture in the neovascularized target tissues (Lasic et al, Science 1995, 267, 1275-1276; Oku et al.,1995, Biochim. Biophys. Acta, 1238, 86-90). The long-circulating liposomes enhance the pharmacokinetics and pharmacodynamics of DNA and RNA, particularly compared to conventional cationic liposomes which are known to accumulate in tissues of the MPS (Liu et al, J. Biol. Chem. 1995, 42, 24864-24870; Choi et al, International PCT Publication No. WO 96/10391; Ansell et al, International PCT Publication No. WO 96/10390; Holland et al, International PCT Publication No. WO 96/10392; all of these are incorporated by reference herein). Long-circulating liposomes are also likely to protect drugs from nuclease degradation to a greater extent compared to cationic liposomes, based on their ability to avoid accumulation in metabolically aggressive MPS tissues such as the liver and spleen. All of these references are incorporated by reference herein. The present invention also includes compositions prepared for storage or administration which include a pharmaceutically effective amount of the desired compounds in a pharmaceutically acceptable carrier or diluent. Acceptable carriers or diluents for therapeutic use are well known in the pharmaceutical art, and are described, for example, in Remington's Pharmaceutical Sciences, Mack Publishing Co. (A.R. Gennaro edit. 1985) hereby incorporated by reference herein. For example, preservatives, stabilizers, dyes and flavoring agents may be provided. Id. at 1449. These include sodium benzoate, sorbic acid and esters of p-hydroxybenzoic acid. In addition, antioxidants and suspending agents may be used. A pharmaceutically effective dose is that dose required to prevent, inhibit the occurrence, or treat (alleviate a symptom to some extent, preferably all of the symptoms) of a disease state. The pharmaceutically effective dose depends on the type of disease, the composition used, the route of administration, the type of mammal being treated, the physical characteristics of the specific mammal under consideration, concurrent medication, and other factors which those skilled in the medical arts will recognize. Generally, an amount between 0.1 mg/kg and 100 mgkg body weight/day of active ingredients is administered dependent upon potency of the negatively charged polymer. In a one aspect, the invention provides enzymatic nucleic acid molecules that can be delivered exogenously to specific cells as required. The enzymatic nucleic acid molecules are added directly, or can be complexed with cationic lipids, packaged within liposomes, or otherwise delivered to smooth muscle cells. The RNA or RNA complexes can be locally administered to relevant tissues through the use of a catheter, infusion pump or stent, with or without their incorporation in biopolymers. Using the methods described herein, other enzymatic nucleic acid molecules that cleave target nucleic acid may be derived and used as described above. Specific examples of nucleic acid catalysts of the instant invention are provided below in the Tables and figures. Alternatively, the enzymatic nucleic acid molecules of the instant invention can be expressed within cells from eukaryotic promoters (e.g., Izant and Weintraub, 1985 Science 229, 345; McGarry and Lindquist, 1986 Proc. Natl Acad. Sci. USA 83, 399; Scanlon et α/., 1991, Proc. Natl. Acad. Sci. USA, 88, 10591-5; Kashani-Sabet et al, 1992 Antisense Res. Dev., 2, 3-15; Dropulic et al, 1992 J. Virol, 66, 1432-41; Weerasinghe et al, 1991 J. Virol, 65, 5531-4; Ojwang et al, 1992 Proc. Natl. Acad. Sci. USA 89, 10802-6; Chen et al, 1992 Nucleic Acids Res., 20, 4581-9; Sarver et al, 1990 Science 247, 1222-1225; Thompson et a , 1995 Nucleic Acids Res. 23, 2259; Good et al, 1997, Gene Therapy, 4, 45; all of the references are hereby incorporated in their totality by reference herein). Those skilled in the art realize that any nucleic acid can be expressed in eukaryotic cells from the appropriate DNA/RNA vector. The activity of such nucleic acids can be augmented by tiieir release from the primary transcript by a ribozyme (Draper et al, PCT WO 93/23569, and Sullivan et al, PCT WO 94/02595; Ohkawa et α/., 1992 Nucleic Acids Symp. Ser., 27, 15-6; Tairaet α/., 1991, Nucleic Acids Res., 19, 5125-30; Ventura et al, 1993 Nucleic Acids Res., 21, 3249-55; Chowrira et al, 1994 J. Biol. Chem. 269, 25856; all of the references are hereby incorporated in their totality by reference herein). Transcription of the ribozyme sequences are driven from a promoter for eukaryotic RNA polymerase I (pol I), RNA polymerase II (pol II), or RNA polymerase III (pol III). Transcripts from pol II or pol III promoters will be expressed at high levels in all cells; the levels of a given pol II promoter in a given cell type will depend on the nature of the gene regulatory sequences (enhancers, silencers, etc.) present nearby. Prokaryotic RNA polymerase promoters are also used, providing that the prokaryotic RNA polymerase enzyme is expressed in the appropriate cells (Elroy-Stein and Moss, 1990 Proc. Natl Acad. Sci. USA, 87, 6743-7; Gao and Huang 1993 Nucleic Acids Res., 21, 2867-72; Lieber et al., 1993 Methods Enzymol, 217, 47-66; Zhou et al., 1990 ?/. Cell. Biol, 10, 4529-37). Several investigators have demonstrated that ribozymes expressed from such promoters can function in mammalian cells (e.g. Kashani-Sabet et al., 1992 Antisense Res. Dev., 2, 3-15; Ojwang et al., 1992 Proc. Natl. Acad Sci. U S A, 89, 10802-6; Chen et al, 1992 Nucleic Acids Res., 20, 4581-9; Yu et al, 1993 Proc. Natl. Acad Sci. U S A, 90, 6340-4; LΗuillier et ' al, 1992 EMBO J. 11, 4411-8; Lisziewicz et al, 1993 Proc. Natl Acad Sci. U. S. A., 90, 8000-4; Thompson et al, 1995 Nucleic Acids Res. 23, 2259; Sullenger & Cech, 1993, Science, 262, 1566). More specifically, transcription units such as the ones derived from genes encoding U6 small nuclear (snRNA), transfer RNA (tRNA) and adenovirus VA RNA are useful in generating high concentrations of desired RNA molecules such as ribozymes in cells (Thompson et al, supra; Couture and Stinchcomb, 1996, supra; Noonberg et al, 1994, Nucleic Acid Res., 22, 2830; Noonberg et al, US Patent No. 5,624,803; Good et al, 1997, Gene Ther. 4, 45; Beigelman et al, International PCT Publication No. WO 96/18736; all of these publications are incorporated by reference herein. Examples of transcription units suitable for expression of ribozymes of the instant invention are shown in Figure 15. The above ribozyme transcription units can be incorporated into a variety of vectors for introduction into mammalian cells, including but not restricted to, plasmid DNA vectors, viral DNA vectors (such as adenovirus or adeno-associated virus vectors), or viral RNA vectors (such as retroviral or alphavirus vectors) (for a review see Couture and Stinchcomb, 1996, supra). In yet another aspect the invention features an expression vector comprising nucleic acid sequence encoding at least one of the catalytic nucleic acid molecule of the invention, in a manner which allows expression of that nucleic acid molecule. The expression vector comprises in one embodiment; a) a transcription initiation region; b) a . transcription termination region; c) a gene encoding at least one said nucleic acid molecule; and wherein said gene is operably linked to said initiation region and said termination region, in a manner which allows expression and or delivery of said nucleic acid molecule. In another preferred embodiment the expression vector comprises: a) a transcription initiation region; b) a transcription termination region; c) an open reading frame; d) a gene encoding at least one said nucleic acid molecule, wherein said gene is operably linked to the 3'-end of said open reading frame; and wherein said gene is operably linked to said initiation region, said open reading frame and said termination region, in a manner which allows expression and/or delivery of said nucleic acid molecule. In yet another embodiment the expression vector comprises: a) a transcription initiation region; b) a transcription termination region; c) an intron; d) a gene encoding at least one said nucleic acid molecule; and wherein said gene is operably linked to said initiation region, said intron and said termination region, in a manner which allows expression and/or delivery of said nucleic acid molecule. In another embodiment, d e expression vector comprises: a) a transcription initiation region; b) a transcription termination region; c) an intron; d) an open reading frame; e) a gene encoding at least one said nucleic acid molecule, wherein said gene is operably linked to the 3'-end of said open reading frame; and wherein said gene is operably linked to said initiation region, said intron, said open reading frame and said termination region, in a manner which allows expression and/or delivery of said nucleic acid molecule. By "consists essentially of is meant that the active ribozyme contains an enzymatic center or core equivalent to those in the examples, and binding arms able to bind target nucleic acid molecules such that cleavage at the target site occurs. Other sequences may be present which do not interfere with such cleavage. Examples The following are non-limiting examples showing the selection, isolation, synthesis and activity of enzymatic nucleic acids of the instant invention. An extensive array of site-directed mutagenesis studies have been conducted with the hammerhead to probe relationships between nucleotide sequence and catalytic activity. These systematic studies have made clear that most nucleotides in the conserved core of the hammerhead ribozyme (Forster & Symons, 1987, Cell, 49, 211) cannot be mutated without significant loss of catalytic activity. In contrast, a combinatorial strategy that simultaneously screens a large pool of mutagenized ribozymes for RNAs that retain catalytic activity could be used more efficiently to define immutable sequences and to identify new ribozyme variants (Breaker, 1997, supra). For example, Joseph and Burke (1993; J. Biol. Chem., 268, 24515) have used an in vitro selection approach to rapidly screen for sequence variants of the 'hairpin' self- cleaving RNA that show improved catalytic activity. This approach was successful in identifying two mutations in the hairpin ribozyme that together give a 10-fold improvement in catalytic rate. Although similar in vitro selection experiments have been conducted with the hammerhead ribozyme (Nakamaye & Eckstein, 1994, supra; Long & Uhlenbeck, 1994, supra; Ishizaka et al, 1995, supra), none of these efforts have successfully screened full-sized hammerhead ribozymes for all possible combinations of sequence variants that encompass the entire catalytic core. Applicant employed in vitro selection strategy to comprehensively test whether the natural consensus sequence for the core of the hammerhead ribozyme produces maximal catalytic rates, or whether sequence variants of this ribozyme could catalyze endonuclease reaction similar to or better than the hammerhead ribozyme. The procedure reported herein to select for intramolecular AUG cleaving sequences is based on Applicant's previously reported selection of alternative GUC cleaving hammerhead sequences (Vaish et al, supra; incorporated by reference herein). Example 1 : In vitro selection of self-cleaving RNAs from a random pool It was of interest to see what changes have to be imposed on the native hammerhead sequence for it to cleave after AUG, which usually resists cleavage, and thus arrive at a ribozyme with a new specificity. To achieve this, an in vitro selection was undertaken, where the number of randomized positions in the starting pool corresponded to the number of nucleotides typically found in the core and stem-loop II region of a hammerhead ribozyme. This would allow all possible sequence permutations to be explored in the search for a hammerhead which is able to cleave behind AUG (Robertson & Joyce, 1990; Ellington and Szostak, 1990; Tuerk & Gold, 1990; Wright & Joyce, 1997; Carmi et al, 1997; for reviews see Breaker, 1997; Abelson, 1996; Ellington, 1997; Chapman & Szostak, 1994). An initial pool of dsDNAs containing 22 randomized positions flanked by twσ constant regions were synthesized. These were transcribed, with transcription being initiated with GTPγS, to produce a pool of potential ribozymes. Self-cleavage of active ribozymes occurs during transcription and the cleavage products can then be separated from intact transcripts by means of a mercury gel (Igloi, Biochemistry 27, 3842). The recovered cleavage products were then reverse transcribed and amplified by PCR to give a dsDNA pool of selected ribozymes. This selection procedure was repeated for 13 cycles before the dsDNA pool was cloned and sequenced. Oligodeoxyribonucleotides and oligoribonucleotides were chemically synthesized and purified as previously described (Vaish et al., supra). Primers used in the selection: Primer 1: 5'- TGGTGCAAGCTTAATACGACTCACTATAGGGAGACTGTCTAGATC f TGA GGATGCTA-3'; Primer 2: 5'- TCTCGGATCCTGCAGATCATNNNNNNNNNNNNNN^ AGCATCCTCAT-3*. These two primers were used for the initial Sequenase reaction, and Primer 2 also functioned as restoration primer for the reintroduction of the lost sequences during cleavage. Reverse transcription (RT)-Primer, 5'-TCTCGGATCCTGCAGATCAT-3'; and polymerase chain reaction (PCR)-Primer, 5'- TGGTGCAAGCTTAATACGACTCA-3', with Hindlll (5'-end) and BamHI and Pstl (3'-end) sites in boldface; T7 promoter is underlined; ribozyme cleavage triplet in italics; N, randomized nucleotides. RNA selection was performed as described (Vaish et al, supra, incorporated by reference herein) with modifications to the initial two cycles of selection. Transcriptions from Pool 0 and Pool 1 were performed on a larger scale (10 x 500 μl and 1 x 250 μl, respectively) for 12 h and PCR amplification (Step 5) was omitted in each case. Successive transcriptions were performed at lOOμl volume. Transcription reactions were for decreasing periods of time from 12 h (1 st cycle) to 1 min (13 th cycle), where reactions for short periods were quenched by addition of EDTA (75 mM final cone). All transcriptions were performed with lμM DNA. DNA template in the PCR mixture was at least 1 x 10 "15 M. The minimum concentration of RNA template was 1 x 10 "8 M for reverse transcription. The concentrations were determined assuming a molar extinction coefficient of 6600. A selection pressure was exerted by progressively reducing the time of transcription from 12 h for the first six cycles, to 6 h for the seventh, 1 h for the eighth, 30 min for the ninth and tenth, 5 min for the eleventh, and 1 min for the twelve and thirteenth cycle. White colonies were selected randomly each from pools 7 and 10 and those showing self-cleavage upon transcription were sequenced. There was no homology among the sequences. Cloning and sequencing was performed as described (Vaish et al, supra) except that the dsDNA was digested with Hindlll and BamHI. Clones from pools seven and ten were tested for transcript self-cleavage from linearized plasmids. For clones from Pool 13 plasmid DNA was amplified by PCR, using the RT- and PCR-primers, and then transcribed. Rates of intramolecular cleavage of transcripts were determined as described but with 10 u of enzyme per μl. Active clones were sequenced. A transcript of only 50 nucleotides, without the extra sequences for cloning beyond stem III, showed a kin-cis f 0-32 min *1 . By "randomized region" is meant a region of completely random sequence and/or partially random sequence. By completely random sequence is meant a sequence wherein theoretically there is equal representation of A, T, G and C nucleotides or modified derivatives tiiereof, at each position in the sequence. By partially random sequence is meant a sequence wherein there is an unequal representation of A, T, G and C nucleotides or modified derivatives thereof, at each position in the sequence. A partially random sequence can therefore have one or more positions of complete randomness and one or more positions with defined nucleotides. Characterization of New self-cleaving ribozyme sequences The intramolecular cleavage rate of the most active sequence of pool 7 was 0.03 min "1 and that of pool 10 was 0.06 min '1 (Figure 3). Seventy clones from pool thirteen were picked and amplified by PCR. Of these 20 gave full length DNA and were transcribed and of these 14 showed intramolecular cleavage. These 14 active ribozymes could be divided into two groups: one group contained transcripts with 22 nucleotides in the randomized region and the second group contained a deletion in this region with only 21 nucleotides being present. Several sequences were represented twice such as those of clones 29, 36 and 40. One of die most active sequences, from clone 13/40, with kj n - cts °f 0.52 min '1 was chosen for further study. In comparison, under these conditions the intramolecular cleavage of the native hammerhead with a GUC triplet was m ^ s of 0.56 min "1 , indicating that the cleavage activity of the selected ribozyme, Rzl3/40, with an AUG triplet is comparable. In order to acquire some information on the secondary structure of Rzl3/40 a limited nuclease digest of the 3'-end-labeled, full-length ribozyme was performed. Rz 13/40 was prepared by transcription in such a way that cleavage was minimized and full-length ribozyme. A full length transcript for the intramolecularly cleaving ribozyme was generated by transcription at 12°C for 8 h (Frank et al, 1996, RNA, 2, 1179; incorporated by reference herein) and 3 '-end labeled with 32 pCp. Limited nuclease digestions with RNase A, RNase Tl and nuclease SI were performed as Hodson et al, 1994, Nucleic Acids Res., 22, 1620; Loria etα/., 1996, RNA, 2,551; both are incorporated by reference herein). Alkaline hydrolysis of labeled RNA was performed in 50 mM NaHCO 3 at 100°C for 7.5 minutes. Limited digestion with RNase A and Tl and nuclease SI, which indicates single- stranded regions, gave a digestion pattern consistent with Rzl3/40 adopting a hammerhead-type structure, which comprised three base-paired helices surrounding a single-stranded core (Fig. 4). Since the structure of Rzl3/40 resembles the hammerhead structure, the same numbering system has been adopted, witii positions 3 and 9 in the core being considered vacant. The data of nuclease digestion corresponded fairly well with the MFOLD structure except for a few discrepancies. MFOLD shows base pairing in the core region between U 7 and G 14 ; G 8 and C 13 ; G m and U LZ5 ; and U 4 and G 17 although there is cleavage at these positions by the nucleases. Ribozyme Engineering Sequence, chemical and structural variants of ribozymes of the present invention can be engineered using the techniques shown above and known in the art to cleave a separate target RNA or DNA in trans. For example, the size of ribozymes can, be reduced or increased using the techniques known in the art (Zaug et al, 1986, Nature, 324, 429; Ruffner et al, 1990, Biochem., 29, 10695; Beaudry et al, 1990, Biochem., 29, 6534; McCall et al, 1992, Proc. Natl. Acad. Sci., USA.. 89, 5710; Long et al. 1994; Supra; Hendry et al. 1994, BBA 1219, 405; Benseler et al, 1993, JACS, 115, 8483; Thompson et al, 1996, Nucl Acids Res., 24, 4401;Michels et al, 1995, Biochem., 34, 2965; Been et al, 1992, Biochem., 31, 11843; Guo et al, 1995, EMBO. J, 14, 368; Pan et al, 1994, Biochem., 33, 9561; Cech, 1992, Curr. Op. Struc. Bio., 2, 605; Sugiyama et al, 1996, FEBSLett., 392, 215; Beigelman et al, 1994, Bioorg. Med. Chem., 4, 1715; all are incorporated in its totality by reference herein). For example the stem-loop II domain of the ribozymes may not be essential for catalytic activity and " hence could be systematically reduced in size using a variety of methods known in the art, to the extent that the overall catalytic activity of the ribozyme is not significantly decreased. Further rounds of in vitro selection strategies described herein and variations thereof can be readily used by a person skilled in the art to evolve additional nucleic acid catalysts and such new catalysts are within the scope of the instant invention. Example 2: Trans-cleaving Ribozymes The self-cleaving ribozymes can be divided into separate ribozyme and substrate domains to create a functional bimolecular complex. This ribozyme presumably interacts with the substrate domain by forming base-paired regions that are analogous to helices I and II of the hammerhead ribozyme (Figure 1). Likewise, the substrate specificity of ribozymes can presumably be altered by changing the sequences of the substrate-binding arms to complement the sequence of the desired substrate molecule, as was achieved with the ribozyme from clone 40 self-cleaving RNA (Figure 5). To further characterize the selected ribozyme for intermolecular cleavage, Rzl 3/40 was divided into a catalytic portion and the corresponding substrate strand (Fig 5). The initial ribozyme construct was designed with 5 bp each in stems I and III for annealing to die substrate. Stems of this lengtii were chosen because they had been reported to give reliable kinetic data with die hammerhead. Cleavage kinetics of intermolecularly cleaving ribozymes was performed witii chemically synthesized ribozyme and substrate in 50 mM Tris-HCl (pH 8.0), and 10 mM MgCl 2 with 50 to 500 nM ribozyme and 25 nM substrate for single turnover, and 50 to 500 nM substrate with 5 to 25 nM ribozyme for multiple turnover kinetics as described (Hertel et al, Biochemistry 33, 3374, 1994). The intermolecular version of Rzl3/40 of the present invention gave a very high K M - (1.1 mM) and low k cat (0.1 min *1 ) under single turnover conditions and was inactive under multiple turnover conditions. As there was no doubt about the intramolecular cleavage of Rzl3/40, the lack of catalytic activity under multiple turnover conditions must be associated with the design of the intermolecular version. After analysis of the ribozyme-substrate complex by native gel it was concluded that formation of the complex was inefficient under the conditions used. A second ribozyme construct was synthesized, where stem III was extended to 10 base-pairs. Native gel analysis confirmed the formation of the ribozyme-substrate complex and the ribozyme cleaved its corresponding substrate efficiently under both single and multiple turnover conditions (Table U). The discrepancy between kcat and kcat' (0.06 and 0.91 min "1 respectively) is presumably the result of product inhibition, which is a common effect observed with the native hammerhead. The time course of cleavage was followed for about 12 half-lives under single turnover conditions. Reactions were first order up to 60% cleavage witiiin die first minute and reached a total of 80%. First order end points with the conventional hammerhead are commonly 70 to 75% (19). Steady state cleavage rates were linear for several turnovers. Although the hammerhead and class I ribozymes cleave at different intemucleotide linkages (Figure 1), both ribozymes appear to proceed by a similar chemical mechanism. The hammerhead ribozyme is known to produce a 2 ',3 '-cyclic phosphate at the terminus of the 5'- cleavage product, thereby leaving a 5'-hydroxyl terminus on the 3 '-cleavage fragment. The site of substrate cleavage was confirmed as being behind G of the AUG cleavage triplet by comparison of the ribozyme cleavage product, obtained under single turnover conditions for 30 min, with the products of a limited alkaline hydrolysis of the 5 '-end-labeled substrate and with a partial RNase Tl digest. Reactions were performed in 10 μl containing RNA, 50 mM Tris (pH 8) and CNPase (Sigma Chemicals) (0.62 u to 0.012 u/μl) and incubation at 45°C for 1.5 h. Product analysis was on a 20% PAGE where the cyclic phosphate-terminated product runs slightly slower than the ring- opened derivative. In addition, the cleavage products were also shown to be die 2', 3'- cyclic phosphate and a 5'-hydroxyl by hydrolysis with CNPase and labeling with T4 PNK respectively. Thus Rzl3/40 cleaves in a similar manner to the hammerhead. In order to investigate die properties and sequence requirements of the selected ribozyme Rzl3/40, mutations were made on die substrate and ribozyme strands. In the native hammerhead, stem II has been shortened to as few as two base-pair without significant loss in activity. However, the removal of a single base-pair (10.3/11.3) in Rzl3/40 resulted in a 1000-fold loss in activity for the cleavage behind an AUG triplet indicating the stem-loop structure has a wider role in catalysis. The effect of mutations around the cleavage site have also been investigated under single and multiple turnover conditions (Table II). Surprisingly, the AUA triplet was also cleaved quite efficiently even though it was not selected for, again yielding a 2', 3'- cyclic phosphate. Triplets terminating in a uridine or cytidine were either cleaved extremely slowly or not at all. Thus this ribozyme is purine nucleoside specific. Whether this indicates a necessity for base pairing between U 4 and d e nucleotide to be cleaved, as possible with AUG and AUA, is uncertain at present. As the native hammerhead strictly requires a uridine in the middle of d e triplet, it was of interest to test whether this was applicable to Rz 13/ 40. Interestingly the triplet AAG was cleaved witii a k caf of 0.57 min "1 under single turnover conditions, multiple turnover conditions showed a k^ of 0.09 min "1 . It was also tested whetiier the nucleotide next to die cleavage site was of importance. Cleavage of the triplet AUG with a following U or G was found to be only fractionally slower in single turnover than cleavage of AUG-A (Table H). Inspection of the ribozyme sequences reveals some interesting features. The sequence 5'-GCGCG at positions 11.2 to 14 is present in all ribozymes from pool 10 onwards. This would indicate that this sequence may be an indispensable part for activity just as is the case for the GAA sequence in the conventional hammerhead. Of the two classes of ribozyme isolated in Pool 13, the ribozymes with a deletion in the randomized region are die more active intramolecular cleavers. These ribozymes appear to have an overall secondary structure which is similar to that of die native hammerhead, but mere are significant differences. Obvious differences include die 'vacancies' at positions 3 and 9 in the core as well as die altered sequence to positions 12 to 14 already mentioned. More subtle differences are associated with stem-loop II. In the hammerhead a variety of structures are tolerated: d e stem can be reduced to two base-pair, and d e loop can be of virtually any size and even contain non-nucleotidic linkers. In the selected ribozymes similar to Rzl3/40, less variation in die stem-loop structure is observed. In the active ribozymes of this class, stem-loop II consists of two G:C base-pairs adjacent to the core; followed by two A:U base-pairs; and finally terminating in a five nucleotide loop, where variations at only positions L2.4 and L2.5 have been detected. The length of die stem appears to have a large effect on catalytic efficiency, since the removal of a single base-pair (10.3/11.3) abolishes activity. The most active sequences in pool 13 have a fully complementary stem II. One mismatch at 10.2/11.2 as in Rzl3/1 approximately halves the activity. It is interesting to note that in Pool 13, the more active ribozymes contain a deletion in the randomized region, which is 21 nucleotides instead of 22. Of tiiese only Rz 13/31 and 13/29 show reasonable activity. One can draw a secondary structure for these with a stem II witii two mismatches and die extra nucleotide in the core region. The new triplet specificity should be useful for the application of die ribozyme family of die present invention for the inhibition of gene expression in tiiat it expands die targets on a mRNA for cleavage. Although the conventional NUX triplets might be considered sufficient for a wide application, die accessibility of oligodeoxynucleotides to mRNAs is very restricted. Thus die specificity of the new ribozyme for cleavage at purines, apparentiy independent of d e nature of the neighboring nucleotides, should represent a considerable advantage over die native NUX-cleaving ribozyme for this application. Diagnostic uses Enzymatic nucleic acids of this invention may be used as diagnostic tools to examine genetic drift and mutations within diseased cells or to detect the presence of target RNA in a cell. The close relationship between ribozyme activity and the structure of the target RNA allows the detection of mutations in any region of the molecule which alters the base-pairing and tiiree-dimensional structure of the target RNA. By using multiple ribozymes described in this invention, one may map nucleotide changes which are important to RNA structure and function in vitro, as well as in cells and tissues. Cleavage of target RNAs with ribozymes may be used to inhibit gene expression and define die role (essentially) of specified gene products in the progression of disease. In tiiis manner, other genetic targets may be defined as important mediators of the disease. These experiments will lead to better treatment of die disease progression by affording d e possibility of combinational tiierapies (e.g, multiple ribozymes targeted to different genes, ribozymes coupled witii known small molecule inhibitors, or intermittent treatment witii combinations of ribozymes and/or other chemical or biological molecules). Otiier in vitro uses of ribozymes of this invention are well known in die art, and include detection of die presence of mRNAs associated with disease condition. Such RNA is detected by determining the presence of a cleavage product after treatment with a ribozyme using standard methodology. In a specific example, ribozymes which can cleave only wild-type or mutant forms of the target RNA are used for the assay. The first ribozyme is used to identify wild-type RNA present in the sample and die second ribozyme will be used to identify mutant RNA in the sample. As reaction controls, synthetic substrates of both wild- type and mutant RNA will be cleaved by botii ribozymes to demonstrate die relative ribozyme efficiencies in the reactions and d e absence of cleavage of the "non-targeted" RNA species. The cleavage products from the synthetic substrates will also serve to generate size markers for the analysis of wild-type and mutant RNAs in the sample population. Thus each analysis will require two ribozymes, two substrates and one unknown sample which will be combined into six reactions. The presence of cleavage products will be determined using an RNAse protection assay so tiiat full-length and cleavage fragments of each RNA can be analyzed in one lane of a polyacrylamide gel. It is not absolutely required to quantify die results to gain insight into die expression of mutant RNAs and putative risk of die desired phenotypic changes in target cells. The expression of mRNA whose protein product is implicated in die development of the phenotype is adequate to establish risk. If probes of comparable specific activity are used for botii transcripts, then a qualitative comparison of RNA levels will be adequate and will decrease the cost of die initial diagnosis. Higher mutant form to wild-type ratios will be correlated with higher risk whetiier RNA levels are compared qualitatively or quantitatively. Additional Uses Potential usefulness of sequence-specific enzymatic nucleic acid molecules of the instant invention might have many of the same applications for the study of RNA that DNA restriction endonucleases have for the study of DNA (Nathans et al, 1975 Ann. Rev. Biochem. 44:273). For example, the pattern of restriction fragments could be used to establish sequence relationships between two related RNAs, and large RNAs could be specifically cleaved to fragments of a size more useful for study. The ability to engineer sequence specificity of the ribozyme is ideal for cleavage of RNAs of unknown sequence. Other embodiments are within die following claims. Table I Table 2.5 μmol RNA Synthesis Cycle Reagent Equivalents Amount Wait Time* Phosphoramidites 65 163 μL 25 S-Ethyl Tetrazole 23.8 238 μL 25 Acetic Anhydride 100 233 μL 5 sec ΛAMethyl Imidazole 186 233 μL 5 sec TCA 83.2 1.73 mL 21 sec Iodine 8.0 1.18 mL 45 sec Acetonitrile NA 6.67 L NA * Wait time does not include contact time during delivery. Table II. Rate constants for ribozyme 40 cleavage at various triplets with next nucleotide specified. Reaction conditions as in text, la, activity too low for quantitation; na, no activity detectable. MTO, multiple turnover; STO, single turnover MTO kinetics STO kinetics Cleavable Triplet k cat (min * ) K M (nm) k c a t ^min "1 ) AUG-A 0.06 50.71 0.91 AUA-A 0.09 24.70 0.34 AUU-A la la 0.007 AUC-A na na na AAG-A 0.57 AUG-U 0.35 AUG-G 0.66

Claims: Claims 1. A nucleic acid molecule witii an endonuclease activity having the formula I: ^- ( ) n — G — C — G X-- 3' L (N) m — J Y— 5' wherein, each N represents independently a nucleotide or a non-nucleotide linker, which may be same or different; X and Y are independent oligonucleotides of length sufficient to stably interact with a target nucleic acid molecule; m and n are integers greater than or equal to 1 , wherein if (N)m and (N) n are nucleotides, (N)m and (N)n may interact by hydrogen bond interaction; J is an oligonucleotide of length 7 nucleotides; L is a linker which may be present or absent, wherein said linker, when present, is a nucleotide " and/or a non-nucleotide linker, wherein said nucleotide linker comprises a single-stranded and/or double-stranded region; represents a chemical linkage; and C and G represent cytidine and guanosine nucleotides, respectively. 2. A nucleic acid molecule with an endonuclease activity having the formula II: ^ (N) n — C — C — C X— 3' L ^ (N) m — J Y— 5' wherein, each N represents independently a nucleotide or a non-nucleotide linker, which may be same or different; X and Y are independent oligonucleotides of lengtii sufficient to stably interact with a target nucleic acid molecule; m and n are integers greater than or equal to 1, wherein if (N)m and (N) n are nucleotides, (N)m and (N)n may interact by hydrogen bond interaction; J is an oligonucleotide of lengtii 7 nucleotides; L is a linker which may be present or absent, wherein said linker, when present, is a nucleotide and or a non-nucleotide linker, wherein said nucleotide linker comprises a single-stranded and or double-stranded region; represents a chemical linkage; and C represent cytidine nucleotide. 3. A nucleic acid molecule with endonuclease activity having the formula HI: ^- (N) n — G — C — G X— 3' L ^ (N) m — K Y— 5 ' wherein, each N represents independently a nucleotide or a non-nucleotide linker, which may be same or different; X and Y are independent oligonucleotides of length sufficient to stably interact with a target nucleic acid molecule; m and n are integers greater tiian or equal to 1 , wherein if (N)m and (N)n are nucleotides, (N)m and (N)n may interact by hydrogen bond- interaction; K is an oligonucleotide of lengtii 6 nucleotides; L is a linker which may be present or absent, wherein said linker, when present, is a nucleotide and/or a non-nucleotide linker, wherein said nucleotide linker comprises a single-stranded and/or double-stranded region; represents a chemical linkage; and C and G represent cytidine and guanosine nucleotides, respectively. 4. The nucleic acid molecule of claim 1, wherein, the sequence of said J in said nucleic acid molecule is a sequence selected from die group consisting of 5'- GGCAUCC-3', 5'-AGCAUU-3\ 5'-GCAUCA-3', 5'-AGCAUC-3\ and. 5'- AGCGUC-3'. 5. The nucleic acid molecule of claim 2, wherein, the sequence of said J in said nucleic acid molecule is 5'-GGAAUC-3'. 6. The nucleic acid molecule of claim 3, wherem, the sequence of said K in said nucleic acid molecule is a sequence selected from the group consisting of 5'- UGAUG-3', 5'-UGAUC-3\ and 5'-UGGGC-3'. 7. The nucleic acid molecules of any of claims 1-6, wherein said L in said nucleic acid molecule is nucleotide linker. 8. The nucleic acid molecule of claim 7, wherein said nucleotide linker is a nucleic acid aptamer. 9. The nucleic acid molecule of claim 8, wherem said aptamer is an ATP aptamer. 10. The nucleic acid molecule of any of claims 1-6, wherem said L is non-nucleotide linker. 11. The nucleic acid molecule of any of claims 1-6, wherein said (N)m and (N)n each have a nucleotide sequence. 12. The nucleic acid molecule of claim 11, wherein said nucleotide sequence of said (N)m is complementary to the nucleotide sequence of said (N)n. 13. The nucleic acid molecule of any of claims 1-6, wherein said nucleic acid cleaves a separate nucleic acid molecule. 14. The nucleic acid molecule of claim 13, wherein said separate nucleic acid molecule is RNA. 15. The nucleic acid molecule of claim 13, wherein said nucleic acid comprises between 12 and 100 bases complementary to said separate nucleic acid molecule. 16. The nucleic acid molecule of claim 13, wherein said nucleic acid comprises between 14 and 24 bases complementary to said separate nucleic acid molecule. 17. A cell including die nucleic acid molecule of any of claims 1 -6. 18. The cell of claim 17, wherein said cell is a mammalian cell. 19. The cell of claim 18, wherein said cell is a human cell. 20. An expression vector comprising nucleic acid sequence encoding at least one of die nucleic acid molecule of any of claims 1-6, in a manner which allows expression of that nucleic acid molecule. 21. A cell including die expression vector of claim 20 22. The cell of claim 21 , wherein said cell is a mammalian cell. 23. The cell of claim 21 , wherein said cell is a human cell. 24. A pharmaceutical composition comprising die nucleic acid molecule of any of claims 1-5. 25. A meti od for modulating expression of a gene in a plant cell by administering to said cell d e nucleic acid molecule of any of claims 1-6. 26. A mediod for modulating expression of gene in a mammalian cell by administering to said cell die nucleic acid molecule of any of claims 1-6. 27. A method of cleaving a separate nucleic acid comprising, contacting the nucleic acid molecule of any of claims 1-6 with said separate nucleic acid molecule under conditions suitable for the cleavage of said separate nucleic acid molecule. 28. The mediod of claim 27, wherein said cleavage is carried out in the presence of a divalent cation. 29. The method of claim 28, wherein said divalent cation is Mg2 + . 30. The nucleic acid molecule of claims 1-6, wherein said nucleic acid is chemically synthesized. 31. The expression vector of claim 19, wherein said vector comprises: a) a transcription initiation region; b) a transcription termination region; c) a gene encoding at least one said nucleic acid molecule; and wherein said gene is operably linked to said initiation region and said termination region, in a manner which allows expression and/or delivery of said nucleic acid molecule. 32. The expression vector of claim 20, wherein said vector comprises: a) a transcription initiation region; b) a transcription termination region; c) an open reading frame; d) a gene encoding at least one said nucleic acid molecule, wherein said gene is operably linked to d e 3'-end of said open reading frame; and wherein said gene is operably linked to said initiation region, said open reading frame and said termination region, in a manner which allows expression and/or delivery of said nucleic acid molecule. 33. The expression vector of claim 20, wherein said vector comprises: a) a transcription initiation region; b) a transcription termination region; c) an intron; d) a gene encoding at least one said nucleic acid molecule; and wherein said gene is operably linked to said initiation region, said intron and said termination region, in a manner which allows expression and/or delivery of said nucleic acid molecule. 34. The expression vector of claim 20, wherein said vector comprises: a) a transcription initiation region; b) a transcription termination region; c) an intron; d) an open reading frame; e) a gene encoding at least one said nucleic acid molecule, wherein said gene is operably linked to the 3'-end of said open reading frame; and wherein said gene is operably linked to said initiation region, said intron, said open reading frame and said termination region, in a manner which allows expression and/or delivery of said nucleic acid molecule.

Inventors: ECKSTEIN FRITZ, HEATON PAUL A, VAISH NARENDRA K, ECKSTEIN, FRITZ, HEATON, PAUL, A., VAISH, NARENDRA, K.
Applicants: ECKSTEIN FRITZ, HEATON PAUL A, MAX PLANCK GESELLSCHAFT, VAISH NARENDRA K, ECKSTEIN, FRITZ, HEATON, PAUL, A., MAX-PLANCK-GESELLSCHAFT ZUR FORDERUNG DER WISSENSC, VAISH, NARENDRA, K.
Application year: 1998
Main classification: C12N15
IPC codes: A61K31, A61K38, C12N5, C12N9, C12N15, C12Q1