Catalytic cleavage of cis- and trans-acting

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zole-enhanced cleavage by a general base mechanism, but another possibility is that .... procedure as described above and under these conditions all the substrate ... mined at 1 mM concentrations of Mg2+, Ca2+ and Mn2+ ions. (Fig. ... The shaded segments in cis(W) denote regions changed in the ribozyme variants.
4482–4492 Nucleic Acids Research, 2001, Vol. 29, No. 21

© 2001 Oxford University Press

Catalytic cleavage of cis- and trans-acting antigenomic delta ribozymes in the presence of various divalent metal ions Jan Wrzesinski, Michal Legiewicz, Barbara Smólska and Jerzy Ciesiolka* Institute of Bioorganic Chemistry, Polish Academy of Sciences, Noskowskiego 12/14, 61-704 Poznan, Poland Received June 12, 2001; Revised and Accepted September 11, 2001

ABSTRACT Catalytic activity of four structural variants of the antigenomic delta ribozyme, two cis- and two transacting, has been compared in the presence of selected divalent metal ions that effectively support catalysis. The ribozymes differ in regions that are not directly involved in formation of the ribozyme active site: the region immediately preceding the catalytic cleavage site, the P4 stem and a stretch of the viral RNA sequence extending the minimal ribozyme sequence at its 3′-terminus. The variants show high cleavage activity in the presence of Mg2+, Ca2+ and Mn2+, lower with Co2+ and Sr2+ and some variants are also active with Cd2+ and Zn2+ ions. In the presence of a particular metal ion the ribozymes cleave, however with different initial rates, according to pseudo-first or higher order kinetics and to different final cleavage extents. On the other hand, relatively small differences are observed in the reactions induced by various metal ions. The cleavage of trans-acting ribozymes induced by Mg2+ is partially inhibited in the presence of Na+, spermidine and some other divalent metal ions. The inert Co(NH3)63+ complex is unable to support catalysis, as reported earlier for the genomic ribozyme. The results are discussed in terms of the influence of structural elements peripheral to the ribozyme active site on its cleavage rate and efficiency as well as the role of metal ions in the cleavage mechanism. Some implications concerning further studies and possible applications of delta ribozymes are also considered. INTRODUCTION Hepatitis delta virus RNA (HDV RNA) is a single-stranded circular RNA which replicates via the double rolling circle mechanism. In the genomic RNA strand as well as in its antigenomic counterpart generated during virus replication there are two sequences with ribozyme activities. These ribozymes, the genomic and antigenomic types, are required for selfcleavage of polymeric RNA transcripts into monomeric units (for selected reviews see 1–4). The spatial structure of the

genomic ribozyme determined by X-ray analysis (5,6) and recent propositions on the mechanism of cleavage by delta ribozymes according to a general acid–base catalysis (7–9) has greatly increased our understanding of their functioning. However, one of the most important issues, concerning the role of divalent metal ions in folding or the mechanism of catalysis of delta ribozymes, remains unsolved. Most evidence has consistently supported a requirement for divalent metal ions in HDV catalysis under physiological pH conditions. The delta ribozymes definitely require divalent ions, while other ribozymes, such as the hammerhead, hairpin and VS ribozymes, are also active in very high concentrations of monovalent ions (10). The presence of a ‘general’ metal ionbinding site in the genomic ribozyme has been suggested based on the results of metal ion-induced cleavage experiments. Specific cleavages are induced in the J4/2 region with Pb2+ (11) and also with Mg2+, Ca2+ and Mn2+ (12). In the trans-acting antigenomic ribozyme, a specific Mg2+-induced cleavage occurs at the bottom of the P2 stem (13). Moreover, although the 3′,5′-phosphodiester linkage at ‘the functional cleavage site’ is cleaved slightly faster in Ca2+ than in Mg2+, the 2′,5′-linkage is cleaved in Mg2+ (or Mn2+) but not Ca2+ (14). This dramatic difference is strongly suggestive of a crucial metal ion interaction at the active site. Recently, it has been shown that imidazole buffer rescues the activity of a mutant ribozyme with a C76→U substitution (7). These data are consistent with imidazole-enhanced cleavage by a general base mechanism, but another possibility is that imidazole could coordinate a catalytic metal ion, presumably replacing a ligand lost with mutation of C76 (7). In another mechanistic proposition (8), C75 acts as the general acid with the pKa of the ring nitrogen N3 shifted upwards to ∼7 as a consequence of interaction with a phosphate residue and the presence of a tight metal ion-binding site in its vicinity. An ionized metal ion hydrate acts as the general base in the proposed mechanism. All the above observations suggest the presence of essential divalent metal ion-binding sites in the delta ribozymes, although the crystal structure of the 3′ cleavage product of a genomic ribozyme (5,6) does not reveal a metal ion in the catalytic pocket. Catalytic activity of delta ribozymes in the presence of various divalent metal ions has been compared for the genomic variant (15). For the antigenomic ribozyme, several authors describe experiments for testing the activity of different variants with divalent ions (14,16–18). In most cases, however, these data cannot be directly compared since it is not possible to separate the two effects on catalysis: the kind of catalytic divalent

*To whom correspondence should be addressed. Tel: +48 61 8528503; Fax: +48 61 8520532; Email: [email protected]

Nucleic Acids Research, 2001, Vol. 29, No. 21 4483

metal ion and the structure of a particular ribozyme variant. Precise kinetic data are needed in order to compare the activity of the same ribozyme in the presence of a broader spectrum of divalent metal ions. In order to gain more information on the role of divalent metal ions in catalysis we compared the activity of closely related variants of the antigenomic ribozyme in the presence of various divalent ions. The variants differed in regions that were not directly involved in formation of the ribozyme catalytic core. Thus the role of these peripheral elements in modulating ribozyme activity could also be assessed. MATERIALS AND METHODS Materials The materials used in this study were from the following sources. [γ-32P]ATP (5000 Ci/mmol) was from Amersham and all the chemicals were from Serva or Fluka. Polynucleotide kinase, T7 RNA polymerase, RNase inhibitor and T4 DNA ligase were purchased from MBI Fermentas. AmpliTaq polymerase was from Perkin Elmer. Chemically synthesized oligoribonucleotides S1 (5′-GGGCGGGUCGG-3′), S2 (5′-CUUCGGGUCGG-3′) and S3 (5′-CUUUCCUCUUCGGGUCGGCA-3′) were purchased from Xeragon AG. DNA template constructs All oligodeoxyribonucleotides were synthesized at the 0.2 µmol scale, deprotected after synthesis and purified by electrophoresis on denaturing 8% (w/v) polyacrylamide gels. DNA bands were excised, eluted with 0.3 M sodium acetate, pH 5.2, 1 mM EDTA and precipitated with ethanol. The DNA was recovered by centrifugation and dissolved in TE buffer. The DNA templates for in vitro transcription of the cis(W) ribozyme and the R oligomer, the ribozyme component of trans(S1) and trans(S2), were prepared as follows (12,19). For cis(W), two DNA oligomers were synthesized, A1 (5′CTCCCTTAGCCATCCGAGTGGACGTGCGTCCTCCTTCGGATGCCCAGGTCGGACCGCGAGGAGGTGGAGATGCCATGCCGACCC-3′) and B1 (5′-TAATACGACTCACTATAGGGTCCTTCTTTCCTCTTCGGGTCGGCATGGCA-3′) (letters in italic mark the T7 RNA polymerase promoter; complementary sequences are underlined). For the R oligomer, two other oligomers were synthesized, A2 (5′-GAAAAGTGGCTCTCCCTTAGCCATCCGAGTGCTCGGATGCCCAGGTCGGACCGCGAGGAGGTGGAGATGCCC-3′) and B2 (5′TAATACGACTCACTATAGGGCATCTCCACC-3′). Equimolar amounts of both oligomers (A1, B1 or A2, B2) were annealed and double-stranded DNA templates were generated by PCR. The reaction mixtures contained 1 µM both DNA oligomers, 10 mM Tris–HCl, pH 8.3, 2 mM MgCl2, 50 mM KCl, 200 µM each dNTP and 25 U/ml AmpliTaq polymerase. The reactions were performed on a Biometra UNO II thermocycler for five cycles of 30 s at 94°C, 30 s at 46°C and 2 min at 72°C. The mixtures were extracted with phenol/chloroform (1:1) and the reaction products precipitated with ethanol, dissolved in TE buffer and used in transcription reactions. RNA preparation The in vitro transcription reactions contained 0.4 µM DNA template, 40 mM Tris–HCl, pH 8.0, 10 mM MgCl2, 2 mM

spermidine, 5 mM DTT, 1 mM each NTP, 750 U/ml RNase inhibitor and 2000 U/ml T7 RNA polymerase (MBI Fermentas). To obtain the R oligomer with a monophosphate group at its 5′-end for subsequent preparation of cis(L), 5 mM 5′-GMP was used in the transcription reaction. Following incubation of the mixtures at 37°C for 4 h, the RNA transcripts were purified on 8% denaturing polyacrylamide gels, localized by UV shadowing, eluted with 0.3 M sodium acetate, pH 5.5, 1 mM EDTA, precipitated with ethanol and dissolved in sterile water containing 0.1 mM EDTA. The cis(W) ribozyme was internally labeled with 32P during transcription by including [γ-32P]ATP in the reaction mixture. To minimize ribozyme self-cleavage the mixture was incubated at 4°C for 48 h, the RNA purified and, following autoradiography, recovered as described above. 5′-32P-labeled cis(L) was prepared by ligation of 5′-32P-labeled oligomer S3 and oligomer R with T4 DNA ligase in the presence of a splint oligodeoxynucleotide spanning 10 nt on each side of the ligation junction (14,20). Equimolar amounts of the three oligomers were heated at 95°C for 2 min, cooled to 25°C for 10 min in 10 mM Tris–HCl, pH 7.5, 1 mM EDTA, 50 mM NaCl buffer. Subsequently, the ligation reaction (total volume 15 µl) was carried out in 50 mM Tris–HCl, pH 7.5, 16.7 mM NaCl, 11 mM MgCl2, 10 mM DTT, 0.3 mM EDTA, 1 mM ATP buffer with 1000 U/ml T4 DNA ligase for 4 h at room temperature. The ligated RNA was purified on a denaturing 8% polyacrylamide gel. The oligoribonucleotide substrates S1, S2 and S3 were labeled at their 5′-ends using [γ-32P]ATP and T4 polynucleotide kinase under standard conditions. Cleavage reaction Prior to self-cleavage the cis-acting ribozymes, internally cis(W) (100 000 c.p.m., 0.5–1 pmol RNA in 50 µl reaction volume) or 5′-32P-labeled cis(L) (∼50 000 c.p.m., 0.1 pmol RNA in 50 µl reaction volume), were subjected to a denaturation–renaturation procedure in the standard reaction buffer, 50 mM Tris–HCl, pH 7.5, 0.1 mM EDTA by incubating at 100°C for 2 min, 0°C for 10 min and finally at 37°C for 10 min. The reactions of cis(L) were performed in the presence of 100 mM NaCl which was added during the denaturation– renaturation procedure, after incubation of the samples at 0°C (17). The trans-acting ribozymes, trans(S1) or trans(S2), were prepared by mixing the 5′-32P-labeled substrates S1 or S2 (∼50 000 c.p.m., 0.1 pmol RNA in 50 µl reaction volume) with the R oligomer in the standard reaction buffer to obtain the final RNA concentrations of