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Self-cleaving RNAs (ribozymes) can be engineered to cleave target RNAs of choice in a sequence-specific manner (1). Consequently, they could be used to inhibit virus replication or to analyse host gene function in vivo. However, ribozymes that are catalytic in vitro are generally disappointing when analysed in cells unless expressed at high levels relative to their target RNAs (2, 3). Here we provide evidence that this can be overcome by optimizing ribozyme structure using cellular rather than cell-free assays. We show that ribozymes of relatively long flanking complementary regions (FCRs), while poor catalysts in vitro, can produce profound inhibition of HIV replication in cells. By examining a series of ribozymes in which the FCRs vary from 9 to 564 nucleotides, we establish that the optimum length for activity in the cell is > or = 33 nucleotides.

Analysis of the self-cleavage of ribozymes derived from the genomic RNA of Hepatitis delta virus (HDV) has revealed that certain co-transcribed vector sequences significantly affect the activity of the ribozyme. Specifically, the t1/2 of self-cleavage for a 135 nucleotide HDV RNA varied, at 42 degrees C, from 5 min to 88 min, depending on the vector-derived sequences flanking the 5' end of the ribozyme. Further analysis suggested that this phenomenon was most likely due to the interaction of vector-derived sequences with a 16 nucleotide region found at the 3' end of the ribozyme. These findings have implications for studies of ribozymes transcribed from cDNA templates, and may provide information regarding the catalytic structure of the HDV ribozyme.

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Controlling RNA splicing opens up possibilities for the synthetic biologist. The Tetrahymena ribozyme is a model group I self-splicing ribozyme that has been shown to be useful in synthetic circuits. To create additional splicing ribozymes that can function in synthetic circuits, we generated synthetic ribozyme variants by rationally mutating the Tetrahymena ribozyme. We present an alignment visualization for the ribozyme termed as structure information diagram that is similar to a sequence logo but with alignment data mapped on to secondary structure information. Using the alignment data and known biochemical information about the Tetrahymena ribozyme, we designed synthetic ribozymes with different primary sequences without altering the secondary structure. One synthetic ribozyme with 110 nt mutated retained 12% splicing efficiency in vivo. The results indicate that our biochemical understanding of the ribozyme is accurate enough to engineer a family of active splicing ribozymes with similar secondary structure but different primary sequences.

Ribozymes are small, catalytic RNA molecules that can be engineered to down-regulate gene expression by cleaving specific mRNA. Here we report the selection of hairpin ribozymes that inhibit human immunodeficiency virus (HIV) replication from a combinatorial ribozyme library. We identified a total of 17 effective ribozymes, each capable of inhibiting HIV infection of human CD4+ cells. These ribozymes target diverse steps of the viral replication cycle, ranging from entry to transcription. One ribozyme suppressed HIV integration and transcription by inhibiting the expression of the Ku80 subunit of the DNA-activated protein kinase. Another ribozyme specifically inhibited long terminal repeat transactivation, while two additional ones blocked a step that can be bypassed by vesicular stomatitis virus G-protein pseudotyping. The function of Ku80 in HIV replication and its mechanism of action were further confirmed using short interfering RNA. Identification of the gene targets of these and other selected ribozymes may reveal novel therapeutic targets for combating HIV infection.

How life emerged on this planet is one of the most important and fundamental questions of science. Although nearly all details concerning our origins have been lost in the depths of time, there is compelling evidence to suggest that the earliest life might have exploited the catalytic and self-recognition properties of RNA to survive. If an RNA based replicating system could be constructed in the laboratory, it would be much easier to understand the challenges associated with the very earliest steps in evolution and provide important insight into the establishment of the complex metabolic systems that now dominate this planet. Recent progress into the selection and characterization of ribozymes that promote nucleotide synthesis and RNA polymerization are discussed and outstanding problems in the field of RNA-mediated RNA replication are summarized.

Synthetic RNA replication systems could provide insight into how life first emerged. Recently developed ribozymes that promote nucleotide synthesis and polymerization bring us closer to this goal.

The discovery of ribozymes strengthened the RNA world hypothesis, which assumes that these precursors of modern life both stored information and acted as catalysts. For the first time among extensive studies on ribozymes, we have investigated the influence of hydrostatic pressure on the hairpin ribozyme catalytic activity. High pressures are of interest when studying life under extreme conditions and may help to understand the behavior of macromolecules at the origins of life. Kinetic studies of the hairpin ribozyme self-cleavage were performed under high hydrostatic pressure. The activation volume of the reaction (34 ± 5 ml/mol) calculated from these experiments is of the same order of magnitude as those of common protein enzymes, and reflects an important compaction of the RNA molecule during catalysis, associated to a water release. Kinetic studies were also carried out under osmotic pressure and confirmed this interpretation and the involvement of water movements (78 ± 4 water molecules per RNA molecule). Taken together, these results are consistent with structural studies indicating that loops A and B of the ribozyme come into close contact during the formation of the transition state. While validating baro-biochemistry as an efficient tool for investigating dynamics at work during RNA catalysis, these results provide a complementary view of ribozyme catalytic mechanisms.

Non-coding RNAs of complex tertiary structure are involved in numerous aspects of the replication and processing of genetic information in many organisms; however, an understanding of the complex relationship between their structural dynamics and function is only slowly emerging. The Neurospora Varkud Satellite (VS) ribozyme provides a model system to address this relationship. First, it adopts a tertiary structure assembled from common elements, a kissing loop and two three-way junctions. Second, catalytic activity of the ribozyme is essential for replication of VS RNA in vivo and can be readily assayed in vitro. Here we exploit single molecule FRET to show that the VS ribozyme exhibits previously unobserved dynamic and heterogeneous hierarchical folding into an active structure. Readily reversible kissing loop formation combined with slow cleavage of the upstream substrate helix suggests a model whereby the structural dynamics of the VS ribozyme favor cleavage of the substrate downstream of the ribozyme core instead. This preference is expected to facilitate processing of the multimeric RNA replication intermediate into circular VS RNA, which is the predominant form observed in vivo.

The RNA world hypothesis states that the early evolution of life went through a stage in which RNA served both as genome and as catalyst. The central catalyst in an RNA world organism would have been a ribozyme that catalyzed RNA polymerization to facilitate self-replication. An RNA polymerase ribozyme was developed previously in the lab but it is not efficient enough for self-replication. The factor that limits its polymerization efficiency is its weak sequence-independent binding of the primer/template substrate. Here we tested whether RNA polymerization could be improved by a cationic arginine cofactor, to improve the interaction with the substrate. In an RNA world, amino acid-nucleic acid conjugates could have facilitated the emergence of the translation apparatus and the transition to an RNP world. We chose the amino acid arginine for our study because this is the amino acid most adept to interact with RNA. An arginine cofactor was positioned at ten different sites on the ribozyme, using conjugates of arginine with short DNA or RNA oligonucleotides. However, polymerization efficiency was not increased in any of the ten positions. In five of the ten positions the arginine reduced or modulated polymerization efficiency, which gives insight into the substrate-binding site on the ribozyme. These results suggest that the existing polymerase ribozyme is not well suited to using an arginine cofactor.

The glmS ribozyme-riboswitch is the first known example of a naturally occurring catalytic RNA that employs a small molecule as a coenzyme. Binding of glucosamine-6-phosphate (GlcN6P) activates self-cleavage of the bacterial ribozyme, which is part of the mRNA encoding the metabolic enzyme GlcN6P-synthetase. Cleavage leads to negative feedback regulation. GlcN6P binds in the active site of the ribozyme, where its amine could function as a general acid and electrostatic catalyst. The ribozyme is pre-folded but inactive in the absence of GlcN6P, demonstrating it has evolved strict dependence on the exogenous small molecule. The ribozyme showcases the ability of RNA to co-opt non-covalently bound small molecules to expand its chemical repertoire. Analogue studies demonstrate that some molecules other than GlcN6P, such as l-serine (but not d-serine), can function as weak activators. This suggests how coenzyme use by RNA world ribozymes may have led to evolution of proteins. Primordial cofactor-dependent ribozymes may have evolved to bind their cofactors covalently. If amino acids were used as cofactors, this could have driven the evolution of RNA aminoacylation. The ability to make covalently bound peptide coenzymes may have further increased the fitness of such primordial ribozymes, providing a selective pressure for the invention of translation.

The glmS ribozyme is a self-cleaving RNA catalyst that resides in the 5′-untranslated region of glmS mRNA in certain bacteria. The ribozyme is specifically activated by glucosamine-6-phosphate (GlcN6P), the metabolic product of the GlmS protein, and is thus proposed to provide a feedback mechanism of riboswitch regulation. Both phylogenetic and biochemical analyses of the glmS ribozyme have established a highly conserved core sequence and secondary structure required for GlcN6P-dependent self-cleavage. However, the high degree of nucleotide conservation offers few clues regarding the higher-order structural organization of the catalytic core. To further investigate core ribozyme structure, minimal ‘consensus-type’ glmS ribozymes that retain GlcN6P-dependent activity were produced. Mutational analyses of consensus-type glmS ribozymes support a model for core ribozyme folding through a pseudoknot structure formed by the interaction of two highly conserved sequence segments. Moreover, GlcN6P-dependent function is demonstrated for bimolecular constructs in which substrate interaction with the ribozyme is minimally comprised of sequence representing that involved in putative pseudoknot formation. These studies suggest that the glmS ribozyme adopts an intricate multi-strand catalytic core through the formation of a pseudoknot structure, and provide a refined model for further considering GlcN6P interaction and GlcN6P-dependent ribozyme function.

HDV ribozymes catalyze their own scission from the transcript during rolling circle replication of the hepatitis delta virus. In vitro selection of self-cleaving ribozymes from a human genomic library revealed an HDV-like ribozyme in the second intron of the human CPEB3 gene and recent results suggest that this RNA affects episodic memory performance. Bioinformatic searches based on the secondary structure of the HDV/CPEB3 fold yielded numerous functional ribozymes in a wide variety of organisms. Genomic mapping of these RNAs suggested several biological roles, one of which is the 5′ processing of non-LTR retrotransposons. The family of HDV-like ribozymes thus continues to grow in numbers and biological importance.

Naturally occurring hammerhead ribozymes are produced by rolling circle replication followed by self-cleavage. This results in monomer-length catalytic RNAs which have self-complementary sequences that can occupy their trans-binding domains and potentially block their ability to cleave other RNA strands. Here we show, using small self-processed ribozymes, that this self-binding does not necessarily inhibit trans-cleavage and can result in greatly elevated discrimination against mismatches. We utilized a designed 63 nt circular DNA to encode the synthesis of a self-processed ribozyme, MDR63. Rolling circle transcription followed by self-processing produced the desired 63 nt ribozyme, which potentially can bind mdr-1 RNA with 9+9 nt of complementarity or bind itself with 4+5 nt of self-complementarity by folding back its ends to form hairpins. Kinetics of trans-cleavage of short complementary and mismatched RNAs were measured under multiple turnover conditions, in comparison to a standard 40 nt ribozyme (MDR40) that lacks the self-complementary ends. The results show that MDR63 cleaves an mdr-1 RNA target with a kcat/Km almost the same as MDR40, but with discrimination against mismatches up to 20 times greater. Based on folding predictions, a second self-processed ribozyme (UG63) having a single point mutation was synthesized; this displays even higher specificity (up to 100-fold) against mismatches. The results suggest that self-binding ends may be generally useful for increasing sequence specificity of ribozymes.

RNA catalysts (ribozymes) designed to cleave sequences unique to viral RNA's might be developed as therapeutics. For this purpose, they would require high catalytic efficiency and resistance to nucleases. Reported here are two approaches that can be used in combination to improve these properties. First, catalytic efficiency can be improved by oligonucleotides (facilitators) that bind to the substrate contiguously with the 3'-end of the ribozyme. Second, 2'-O-methylation of flanking sequences of the ribozyme increases catalytic activity as well as resistance to nucleases. In combination with a facilitator oligodeoxynucleotide, the cleavage rate was increased 20 fold over that of the unmodified ribozyme.

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Early life presumably required polymerase ribozymes capable of replicating RNA. Known polymerase ribozymes best approximating such replicases use as their catalytic engine an RNA-ligase ribozyme originally selected from random RNA sequences. Here, we report 3.15 Å crystal structures of this ligase trapped in catalytically viable pre-ligation states, with the 3′-hydroxyl nucleophile positioned for in-line attack on the 5′-triphosphate. Guided by metal and solvent-mediated interactions, the 5′-triphosphate hooks into the major groove of the adjoining RNA duplex in an unanticipated conformation. Two phosphates and the nucleophile jointly coordinate an active-site metal ion. Atomic mutagenesis experiments demonstrate that active-site nucleobase and hydroxyl groups also participate directly in catalysis, collectively playing a role that in proteinaceous polymerases is performed by a second metal ion. Thus artificial ribozymes can employ complex catalytic strategies that differ dramatically from those of analogous biological enzymes.

Background

The group I intron, a ribozyme that catalyzes its own splicing reactions in the absence of proteins in vitro, is a potential target for rational engineering and attracted our interest due to its potential utility in gene repair using trans-splicing. However, the ribozyme activity of a group I intron appears to be facilitated by RNA chaperones in vivo; therefore, the efficiency of self-splicing could be dependent on the structure around the insert site or the length of the sequence to be inserted. To better understand how ribozyme activity could be modulated in cultured mammalian cells, a group I intron was inserted into a short hairpin RNA (shRNA), and silencing of a reporter gene by the shRNA was estimated to reflect self-splicing activity in vivo. In addition, we appended a theophylline-binding aptamer to the ribozyme to investigate any potential effects caused by a trans-effector.

Results

shRNA-expression vectors in which the loop region of the shRNA was interrupted by an intron were constructed to target firefly luciferase mRNA. There was no remarkable toxicity of the shRNA-expression vectors in Cos cells, and the decrease in luciferase activity was measured as an index of the ribozyme splicing activity. In contrast, the expression of the shRNA through intron splicing was completely abolished in 293T cells, although the silencing induced by the shRNA-expressing vector alone was no different from that in the Cos cells. The splicing efficiency of the aptamer-appended intron also had implications for the potential of trans-factors to differentially promote self-splicing among cultured mammalian cells.

Conclusions

Silencing by shRNAs interrupted by a group I intron could be used to monitor self-splicing activity in cultured mammalian cells, and the efficiency of self-splicing appears to be affected by cell-type specific factors, demonstrating the potential effectiveness of a trans-effector.

The ribozymes of hepatitis delta virus (HDV) have so far been studied primarily in vitro. Several structural models for HDV ribozymes based on truncated HDV RNA fragments, which are different from the hammerhead or the hairpin/paperclip ribozyme model proposed for plant viroid or virusoid RNAs, have been proposed. Whether these structures actually exist in vivo and whether ribozymes actually function in the HDV replication cycle have not been demonstrated. We have now developed an in vivo ribozyme self-cleavage assay capable of detecting self-cleavage of dimer or trimer HDV RNA in vivo. By site-directed mutagenesis and compensatory mutations to disrupt and restore potential base pairing in the ribozyme domain of the full-length HDV RNA according to the various structural models, a close correlation between the detected in vivo and the predicted in vitro ribozyme activities of various mutant RNAs was demonstrated. These results suggest that the proposed in vitro ribozyme structure likely exists and functions during the HDV replication cycle in vivo. Furthermore, the pseudoknot model most likely represents the structure responsible for the ribozyme activity in vivo. All of the mutants that had lost the ribozyme activity could not replicate, indicating that the ribozyme activities are indeed required for HDV RNA replication. However, some of the compensatory mutants which have restored both the cleavage and ligation activities could not replicate, suggesting that the ribozyme domains are also involved in other unidentified functions or in the formation of an alternative structure that is required for HDV RNA replication. This study thus established that the ribozyme has important biological functions in the HDV life cycle.

A hammerhead ribozyme designed to cleave in trans the R region of HIV-1 RNA was inserted into a eukaryotic expression vector. This ribozyme was studied in vitro using the T3 RNA polymerase promoter located upstream of the eukaryotic promoter. The ribozyme showed no activity against its specific target sequence under any condition tested. To decrease the influence of potential cis inhibitory sequences in such a ribozyme transcript, a specific target sequence was inserted upstream of the ribozyme-coding sequence. This insertion allowed the release by cis cleavage of a short RNA bearing ribozyme activity and able to cleave in trans an external RNA target. The cis cleavage reaction generated two RNA molecules: the shorter RNA species, which included the catalytic domain, showed a trans cleavage reaction. This self-cleavable ribozyme was active in vitro at 37 degrees C against three distinct HIV-1 transcripts sharing the specific target sequence. Ribozyme activity was thus attained by self-cleavage of the ribozyme-containing sequence from the longer vector transcript.

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The hairpin ribozyme is a prominent member of the group of small catalytic RNAs (RNA enzymes or ribozymes) because it does not require metal ions to achieve catalysis. Biochemical and structural data have implicated guanine 8 (G8) and adenine 38 (A38) as catalytic participants in cleavage and ligation catalyzed by the hairpin ribozyme, yet their exact role in catalysis remains disputed. To gain insight into dynamics in the active site of a minimal self-cleaving hairpin ribozyme, we have performed extensive classical, explicit-solvent molecular dynamics (MD) simulations on timescales of 50-150 ns. Starting from the available X-ray crystal structures, we investigated the structural impact of the protonation states of G8 and A38, and the inactivating A−1(2′-methoxy) substitution employed in crystallography. Our simulations reveal that a canonical G8 agrees well with the crystal structures while a deprotonated G8 profoundly distorts the active site. Thus MD simulations do not support a straightforward participation of the deprotonated G8 in catalysis. By comparison, the G8 enol tautomer is structurally well tolerated, causing only local rearrangements in the active site. Furthermore, a protonated A38H+ is more consistent with the crystallography data than a canonical A38. The simulations thus support the notion that A38H+ is the dominant form in the crystals, grown at pH 6. In most simulations, the canonical A38 departs from the scissile phosphate and substantially perturbs the structures of active site and S-turn. Yet, we occasionally also observe formation of a stable A−1(2′-OH)…A38(N1) hydrogen bond, which documents the ability of the ribozyme to form this hydrogen bond, consistent with a potential role of A38 as general base catalyst. The presence of this hydrogen bond is, however, incompatible with the expected in-line attack angle necessary for self-cleavage, requiring a rapid transition of the deprotonated 2′-oxyanion to a position more favorable for in-line attack after proton transfer from A−1(2′-OH) to A38(N1). The simulations revealed a potential force field artifact, occasional but irreversible formation of ‘ladder-like’, underwound A-RNA structure in one of the external helices. Although it does not affect the catalytic center of the hairpin ribozyme, further studies are under way to better assess possible influence of such force field behavior on long RNA simulations.

We report the structural basis for the modulation of an ATP-sensitive ribozyme that was engineered by modular rational design. This allosteric ribozyme is composed of two independently functioning domains, one a receptor for ATP and the other a self-cleaving ribozyme. When fused in the appropriate fashion, the conjoined aptamer-ribozyme construct functions as an allosteric ribozyme that is inhibited in the presence of ATP. The aptamer domain remains conformationally heterogeneous in the absence of ATP, but folds into a distinct structure upon ligand binding. This ATP-induced conformational change causes a reduction in catalytic activity of the adjacent ribozyme domain due to steric interference between the aptamer and ribozyme tertiary structures. This mechanism for structural and functional modulation of nucleic acids is one of several possible mechanisms by which the function of ribozymes could be specifically controlled by small effector molecules.

Background

The hypothesis of an RNA-based origin of life, known as the "RNA world", is strongly affected by the hostile environmental conditions probably present in the early Earth. In particular, strong UV and X-ray radiations could have been a major obstacle to the formation and evolution of the first biomolecules. In 1951, J. D. Bernal first proposed that clay minerals could have served as the sites of accumulation and protection from degradation of the first biopolymers, providing the right physical setting for the evolution of more complex systems. Numerous subsequent experimental studies have reinforced this hypothesis.

Results

The ability of the possibly widespread prebiotic, clay mineral montmorillonite to protect the catalytic RNA molecule ADHR1 (Adenine Dependent Hairpin Ribozyme 1) from UV-induced damages was experimentally checked. In particular, the self-cleavage reaction of the ribozyme was evaluated after UV-irradiation of the molecule in the absence or presence of clay particles. Results obtained showed a three-fold retention of the self-cleavage activity of the montmorillonite-protected molecule, with respect to the same reaction performed by the ribozyme irradiated in the absence of the clay.

Conclusion

These results provide a suggestion with which RNA, or RNA-like molecules, could have overcame the problem of protection from UV irradiation in the RNA world era, and suggest that a clay-rich environment could have favoured not only the formation of first genetic molecules, but also their evolution towards increasingly complex molecular organization.

In order to determine the parameters that govern the activity of a ribozyme in vivo, we made a systematic analysis of chimeric tRNAVal ribozymes by measuring their cleavage activities in vitro as well as the steady-state levels of transcripts, the half-lives of transcribed tRNAVal ribozymes, and their activities in both HeLa and H9 cells. These analyses were conducted by the use of transient expression systems in HeLa cells and stable transformants that express ribozymes. Localization of transcripts appeared to be determined by the higher-order structure of each transcribed tRNAVal ribozyme. Since colocalization of the ribozyme with its target RNA is important for strong activity of the ribozyme in vivo, the best system for tRNA-based expression seems to be one in which the structure of the transcript is different from that of the natural tRNA precursor so that processing of the tRNAVal ribozyme can be avoided. At the same time, the structure of the transcript must be similar enough to allow recognition, probably by an export receptor, so that the transcript can be exported to the cytoplasm to ensure colocalization with its target. In the case of several tRNAVal ribozymes that we constructed, inspection of computer-predicted secondary structures enabled us to control the export of transcripts. We found that only a ribozyme that was transcribed at a high level and that had a sufficiently long half-life, within cells, had significant activity when used to withstand a challenge by human immunodeficiency virus type 1.

Ribozymes are RNA molecules that act as chemical catalysts. In contemporary cells, most known ribozymes carry out phosphoryl transfer reactions. The nucleolytic ribozymes comprise a class of five structurally-distinct species that bring about site-specific cleavage by nucleophilic attack of the 2′-O on the adjacent 3′-P to form a cyclic 2′,3′-phosphate. In general, they will also catalyse the reverse reaction. As a class, all these ribozymes appear to use general acid–base catalysis to accelerate these reactions by about a million-fold. In the Varkud satellite ribozyme, we have shown that the cleavage reaction is catalysed by guanine and adenine nucleobases acting as general base and acid, respectively. The hairpin ribozyme most probably uses a closely similar mechanism. Guanine nucleobases appear to be a common choice of general base, but the general acid is more variable. By contrast, the larger ribozymes such as the self-splicing introns and RNase P act as metalloenzymes.

By linking a guide sequence to the catalytic RNA subunit of RNase P (M1 RNA), we constructed a functional ribozyme (M1GS RNA) that targets the overlapping mRNA region of two human cytomegalovirus (HCMV) capsid proteins, the capsid scaffolding protein (CSP) and assemblin, which are essential for viral capsid formation. The ribozyme efficiently cleaved the target mRNA sequence in vitro. Moreover, a reduction of >85% in the expression of CSP and assemblin and a reduction of 4000-fold in viral growth were observed in the HCMV-infected cells that expressed the functional ribozyme. In contrast, there was no significant reduction in viral gene expression and growth in virus-infected cells that either did not express the ribozyme or produced a ‘disabled’ ribozyme carrying mutations that abolished its catalytic activity. Characterization of the effects of the ribozyme on the HCMV lytic replication cycle further indicates that the expression of the functional ribozyme specifically inhibits the expression of CSP and assemblin, and consequently blocks viral capsid formation and growth. Our results provide the direct evidence that RNase P ribozymes can be used as an effective gene-targeting agent for antiviral applications, including abolishing HCMV growth by blocking the expression of the virus-encoded capsid proteins.

Hepatitis delta virus RNAs possess self-cleavage activities that produce 2′,3′-cyclic phosphate and 5′-hydroxyl termini (i.e. cis-acting delta ribozyme). Trans-acting delta ribozymes have been engineered by removing a junction from the cis version, thereby producing one molecule possessing the substrate sequence and the other the catalytic domain. According to the pseudoknot model, the secondary structure of the delta ribozyme includes a pseudoknot (i.e. P1.1 stem) formed by two base pairs from residues of the L3 loop and J1/4 junction. A collection of 48 P1.1 stem mutants was synthesized in order to provide an original characterization of both the importance and the structure of this pseudoknot in a trans-acting version of the ribozyme. Several structural differences were noted compared to the results reported for cis-acting ribozymes. For example, a combination of two stable Watson–Crick base pairs composing the essential P1.1 stem was demonstrated to be crucial for a significant level of activity, while the cis version required only one base pair. In addition, we present the first physical evidences revealing that the composition of the P1.1 stem affects the substrate specificity for ribozyme cleavage. Depending on the residues forming the J1/4 junction, non-productive ribozyme–substrate complexes can be observed. This phenomenon is proposed to be important for further development of a gene-inactivation system based on delta ribozyme.

Ribozymes are small catalytic RNA molecules that can be engineered to enzymatically cleave RNA transcripts in a sequence-specific fashion and thereby inhibit expression and function of the corresponding gene product. With their simple structures and site-specific cleavage activity, they have been exploited as potential therapeutic agents in a variety of human disorders, including hepatitis C virus (HCV) infection. We have designed a hairpin ribozyme (Rz3′X) targeting the HCV minus-strand replication intermediate at position 40 within the 3′X tail. Surprisingly, Rz3′X was found to induce ganciclovir (GCV)-resistant colonies in a bicistronic cellular reporter system with HCV internal ribosome entry site (IRES)-dependent translation of herpes simplex virus thymidine kinase (TK). Rz3′X-transduced GCV-resistant HeLa reporter cells showed substantially reduced IRES-mediated HCV core protein translation compared with control vector-transduced cells. Since these reporter systems do not contain the HCV 3′X tail sequences, the results indicate that Rz3′X probably exerted an inhibitory effect on HCV IRES activity fortuitously through another gene target. A novel technique of ribozyme cleavage-based target gene identification (cleavage-specific amplification of cDNA ends) (M. Krüger, C. Beger, P. J. Welch, J. R. Barber, and F. Wong-Staal, Nucleic Acids Res. 29:e94, 2001) revealed that human 20S proteasome α-subunit PSMA7 mRNA was a target RNA recognized and cleaved by Rz3′X. We then showed that additional ribozymes directed against PSMA7 RNA inhibited HCV IRES activity in two assay systems: GCV resistance in the HeLa IRES TK reporter cell system and a transient transfection assay performed with a bicistronic Renilla-HCV IRES-firefly luciferase reporter in Huh7 cells. In contrast, ribozymes were inactive against IRES of encephalomyocarditis virus and human rhinovirus. Additionally, proteasome inhibitor MG132 exerted a dose-dependent inhibitory effect on HCV IRES-mediated translation but not on cap-dependent translation. These data suggest a principal role for PSMA7 in regulating HCV IRES activity, a function essential for HCV replication.