Monitoring the structure of Escherichia coli RNase P RNA in the presence of various divalent metal ions

Lead(II)-induced cleavage can be used as a tool to probe conformational changes in RNA. In this report, we have investigated the conformation of M1 RNA, the catalytic subunit of Escherichia coli RNase P, by studying the lead(II)-induced cleavage pattern in the presence of various divalent metal ions. Our data suggest that the overall conformation of M1 RNA is very similar in the presence of Mg(2+), Mn(2+), Ca(2+), Sr(2+) and Ba(2+), while it is changed compared to the Mg(2+)-induced conformation in the presence of other divalent metal ions, Cd(2+) for example. We also observed that correct folding of some M1 RNA domains is promoted by Pb(2+), while folding of other domain(s) requires the additional presence of other divalent metal ions, cobalt(III) hexamine or spermidine. Based on the suppression of Pb(2+) cleavage at increasing concentrations of various divalent metal ions, our findings suggest that different divalent metal ions bind with different affinities to M1 RNA as well as to an RNase P hairpin-loop substrate and yeast tRNA(Phe). We suggest that this approach can be used to obtain information about the relative binding strength for different divalent metal ions to RNA in general, as well as to specific RNA divalent metal ion binding sites. Of those studied in this report, Mn(2+) is generally among the strongest RNA binders.     

Cleavage mediated by the P15 domain of bacterial RNase P RNA

Independently folded domains in RNAs frequently adopt identical tertiary structures regardless of whether they are in isolation or are part of larger RNA molecules. This is exemplified by the P15 domain in the RNA subunit (RPR) of the universally conserved endoribonuclease P, which is involved in the processing of tRNA precursors. One of its domains, encompassing the P15 loop, binds to the 3'-end of tRNA precursors resulting in the formation of the RCCA-RNase P RNA interaction (interacting residues underlined) in the bacterial RPR-substrate complex. The function of this interaction was hypothesized to anchor the substrate, expose the cleavage site and result in re-coordination of Mg(2+) at the cleavage site. Here we show that small model-RNA molecules (~30 nt) carrying the P15-loop mediated cleavage at the canonical RNase P cleavage site with significantly reduced rates compared to cleavage with full-size RPR. These data provide further experimental evidence for our model that the P15 domain contributes to both substrate binding and catalysis. Our data raises intriguing evolutionary possibilities for 'RNA-mediated' cleavage of RNA.     

The precursor tRNA 3'-CCA interaction with Escherichia coli RNase P RNA is essential for catalysis by RNase P in vivo

The L15 region of Escherichia coli RNase P RNA forms two Watson-Crick base pairs with precursor tRNA 3'-CCA termini (G292-C75 and G293-C74). Here, we analyzed the phenotypes associated with disruption of the G292-C75 or G293-C74 pair in vivo. Mutant RNase P RNA alleles (rnpBC292 and rnpBC293) caused severe growth defects in the E. coli rnpB mutant strain DW2 and abolished growth in the newly constructed mutant strain BW, in which chromosomal rnpB expression strictly depended on the presence of arabinose. An isosteric C293-G74 base pair, but not a C292-G75 pair, fully restored catalytic performance in vivo, as shown for processing of precursor 4.5S RNA. This demonstrates that the base identity of G292, but not G293, contributes to the catalytic process in vivo. Activity assays with mutant RNase P holoenzymes assembled in vivo or in vitro revealed that the C292/293 mutations cause a severe functional defect at low Mg2+ concentrations (2 mM), which we infer to be on the level of catalytically important Mg2+ recruitment. At 4.5 mM Mg2+, activity of mutant relative to the wild-type holoenzyme, was decreased only about twofold, but 13- to 24-fold at 2 mM Mg2+. Moreover, our findings make it unlikely that the C292/293 phenotypes include significant contributions from defects in protein binding, substrate affinity, or RNA degradation. However, native PAGE experiments revealed nonidentical RNA folding equilibria for the wild-type versus mutant RNase P RNAs, in a buffer- and preincubation-dependent manner. Thus, we cannot exclude that altered folding of the mutant RNAs may have also contributed to their in vivo defect.     

Multiple binding modes of substrate to the catalytic RNA subunit of RNase P from Escherichia coli.

M1 RNA that contained 4'-thiouridine was photochemically cross-linked to different substrates and to a product of the reaction it governs. The locations of the cross-links in these photochemically induced complexes were identified. The cross-links indicated that different substrates share some contacts but have distinct binding modes to M1 RNA. The binding of some substrates also results in a substrate-dependent conformational change in the enzymatic RNA, as evidenced by the appearance of an M1 RNA intramolecular cross-link. The identification of the cross-links between M1 RNA and product indicate that they are shared with only one of the three cross-linked E-S complexes that were identified, an indication of noncompetitive inhibition by the product. We also examined whether the cross-linked complexes between M1 RNA and substrate(s) or product are altered in the presence of the enzyme's protein cofactor (C5 protein) and in the presence of different concentrations of divalent metal ions. C5 protein enhanced the yield of certain M1 RNA-substrate cross-linked complexes for both wild-type M1 RNA and a deletion mutant of M1 RNA (delta[273-281]), but not for the M1 RNA-product complex. High concentrations of Mg2+ increased the yield of all M1 RNA-substrate complexes but not the M1 RNA-product complex.     

Solution structure and thermodynamics of a divalent metal ion binding site in an RNA pseudoknot.

Identification and characterization of a metal ion binding site in an RNA pseudoknot was accomplished using cobalt (III) hexammine, Co(NH3)63+, as a probe for magnesium (II) hexahydrate, Mg(H2O)62+, in nuclear magnetic resonance (NMR) structural studies. The pseudoknot causes efficient -1 ribosomal frameshifting in mouse mammary tumor virus. Divalent metal ions, such as Mg2+, are critical for RNA structure and function; Mg2+preferentially stabilizes the pseudoknot relative to its constituent hairpins. The use of Co(NH3)63+as a substitute for Mg2+was investigated by ultraviolet absorbance melting curves, NMR titrations of the imino protons, and analysis of NMR spectra in the presence of Mg2+or Co (NH3)63+. The structure of the pseudoknot-Co(NH3)63+complex reveals an ion-binding pocket formed by a short, two-nucleotide loop and the major groove of a stem. Co(NH3)63+stabilizes the sharp loop-to-stem turn and reduces the electrostatic repulsion of the phosphates in three proximal strands. Hydrogen bonds are identified between the Co(NH3)63+protons and non-bridging phosphate oxygen atoms, 2' hydroxyl groups, and nitrogen and oxygen acceptors on the bases. The binding site is significantly different from that previously characterized in the major groove surface of tandem G.U base-pairs, but is similar to those observed in crystal structures of a fragment of the 5 S rRNA and the P5c helix of the Tetrahymena thermophila group I intron. Changes in chemical shifts occurred at the same pseudoknot protons on addition of Mg2+as on addition of Co(NH3)63+, indicating that both ions bind at the same site. Ion binding dissociation constants of approximately 0.6 mM and 5 mM (in 200 mM Na+and a temperature of 15 degrees C) were obtained for Co(NH3)63+and Mg2+, respectively, from the change in chemical shift as a function of metal ion concentration. An extensive array of non-sequence-specific hydrogen bond acceptors coupled with conserved structural elements within the binding pocket suggest a general mode of divalent metal ion stabilization of this type of frameshifter pseudoknot. These results provide new thermodynamic and structural insights into the role divalent metal ions play in stabilizing RNA tertiary structural motifs such as pseudoknots.     

Base pairing between Escherichia coli RNase P RNA and its substrate.

Base pairing between the substrate and the ribozyme has previously been shown to be essential for catalytic activity of most ribozymes, but not for RNase P RNA. By using compensatory mutations we have demonstrated the importance of Watson-Crick complementarity between two well-conserved residues in Escherichia coli RNase P RNA (M1 RNA), G292 and G293, and two residues in the substrate, +74C and +75C (the first and second C residues in CCA). We suggest that these nucleotides base pair (G292/+75C and G293/+74C) in the ribozyme-substrate complex and as a consequence the amino acid acceptor stem of the precursor is partly unfolded. Thus, a function of M1 RNA is to anchor the substrate through this base pairing, thereby exposing the cleavage site such that cleavage is accomplished at the correct position. Our data also suggest possible base pairing between U294 in M1 RNA and the discriminator base at position +73 of the precursor. Our findings are also discussed in terms of evolution.     

RNase E, an RNA processing enzyme from Escherichia coli.

An activity, RNase E, was purified about 100-fold from Escherichia coli cells, it can process p5 rRNA from a 9 S RNA molecule which accumulates in a mutant of E. coli defective in the maturation of 5 S rRNA. The enzyme requires Na+, K+, or NH4+, and Mg2+ or Mn2+. The molecular weight of the enzyme is about 70,000 and its pH optimum is 7.6 to 8.0. Its temperature optimum is around 30 degrees C, and it can be irreversibly inactivated at 50 degrees C. It has a very high degree of specificity but the reaction can be inhibited by nonspecific RNAs. We interpret its mode of action in producing p5 RNA as being accomplished in two steps, 9 S RNA is first processed to 7 S and 4 S, and subsequently 7 S is further processed to p5.     

Importance of the +73/294 interaction in Escherichia coli RNase P RNA substrate complexes for cleavage and metal ion coordination

We have studied an interaction, the "73/294-interaction", between residues 294 in M1 RNA (the catalytic subunit of Escherichia coli RNase P) and +73 in the tRNA precursor substrate. The 73/294-interaction is part of the "RCCA-RNase P RNA interaction", which anchors the 3' R(+73)CCA-motif of the substrate to M1 RNA (interacting residues underlined). Considering that in a large fraction of tRNA precursors residue +73 is base-paired to nucleotide -1 immediately 5' of the cleavage site, formation of the 73/294-interaction results in exposure of the cleavage site. We show that the nature/orientation of the 73/294-interaction is important for cleavage site recognition and cleavage efficiency. Our data further suggest that this interaction is part of a metal ion-binding site and that specific chemical groups are likely to act as ligands in binding of Mg(2+) or other divalent cations important for function. We argue that this Mg(2+) is involved in metal ion cooperativity in M1 RNA-mediated cleavage. Moreover, we suggest that the 73/294-interaction operates in concert with displacement of residue -1 in the substrate to ensure efficient and correct cleavage. The possibility that the residue at -1 binds to a specific binding surface/pocket in M1 RNA is discussed. Our data finally rationalize why the preferred residue at position 294 in M1 RNA is U.     

The P4 metal binding site in RNase P RNA affects active site metal affinity through substrate positioning

Although helix P4 in the catalytic domain of the RNase P ribozyme is known to coordinate magnesium ions important for activity, distinguishing between direct and indirect roles in catalysis has been difficult. Here, we provide evidence for an indirect role in catalysis by showing that while the universally conserved bulge of helix P4 is positioned 5 nt downstream of the cleavage site, changes in its structure can still purturb active site metal binding. Because changes in helix P4 also appear to alter its position relative to the pre-tRNA cleavage site, these data suggest that P4 contributes to catalytic metal ion binding through substrate positioning.     

Studies of the RNA degradosome-organizing domain of the Escherichia coli ribonuclease RNase E

The hydrolytic endoribonuclease RNase E, which is widely distributed in bacteria and plants, plays key roles in mRNA degradation and RNA processing in Escherichia coli. The enzymatic activity of RNase E is contained within the conserved amino-terminal half of the 118 kDa protein, and the carboxy-terminal half organizes the RNA degradosome, a multi-enzyme complex that degrades mRNA co-operatively and processes ribosomal and other RNA. The study described herein demonstrates that the carboxy-terminal domain of RNase E has little structure under native conditions and is unlikely to be extensively folded within the degradosome. However, three isolated segments of 10-40 residues, and a larger fourth segment of 80 residues, are predicted to be regions of increased structural propensity. The larger of these segments appears to be a protein-RNA interaction site while the other segments possibly correspond to sites of self-recognition and interaction with the other degradosome proteins. The carboxy-terminal domain of RNase E may thus act as a flexible tether of the degradosome components. The implications of these and other observations for the organization of the RNA degradosome are discussed.     

Helix P4 is a divalent metal ion binding site in the conserved core of the ribonuclease P ribozyme

The ribonuclease P ribozyme (RNase P RNA), like other large ribozymes, requires magnesium ions for folding and catalytic function; however, specific sites of metal ion coordination in RNase P RNA are not well defined. To identify and characterize individual nucleotide functional groups in the RNase P ribozyme that participate in catalytic function, we employed self-cleaving ribozyme-substrate conjugates that facilitate measurement of the effects of individual functional group modifications. The self-cleavage rates and pH dependence of two different ribozyme-substrate conjugates were determined and found to be similar to the single turnover kinetics of the native ribozyme. Using site-specific phosphorothioate substitutions, we provide evidence for metal ion coordination at the pro-Rp phosphate oxygen of A67, in the highly conserved helix P4, that was previously suggested by modification-interference experiments. In addition, we detect a new metal ion coordination site at the pro-Sp phosphate oxygen of A67. These findings, in combination with the proximity of A67 to the pre-tRNA cleavage site, support the conclusion that an important role of helix P4 in the RNase P ribozyme is to position divalent metal ions that are required for catalysis.     

Product release is a rate-limiting step during cleavage by the catalytic RNA subunit of Escherichia coli RNase P.

The kinetic constants for cleavage of the tRNA(Tyr)Su3 precursor by the M1 RNA of E. coli RNase P were determined in the absence and presence of the C5 protein under single and multiple (steady state) turnover conditions. The rate constant of cleavage in the reaction catalyzed by M1 RNA alone was 5 times higher in single turnover than in multiple turnovers, suggesting that a rate-limiting step is product release. Cleavage by M1 RNA alone and by the holoenzyme under identical buffer conditions demonstrated that C5 facilitated product release. Addition of different product-like molecules under single turnover reaction conditions inhibited cleavage both in the absence and presence of C5. In the presence of C5, the Ki value for matured tRNA was approximately 20 times higher than in its absence, suggesting that C5 also reduces the interaction between the 5'-matured tRNA and the enzyme. In a growing cell the number of tRNA molecules is approximately 1000 times higher than the number of RNase P molecules. A 100-fold excess of matured tRNA over enzyme clearly inhibited cleavage in vitro. We discuss the possibility that RNase P is involved in the regulation of tRNA expression under certain growth conditions.     

The Escherichia coli major exoribonuclease RNase II is a component of the RNA degradosome

Multiprotein complexes that carry out RNA degradation and processing functions are found in cells from all domains of life. In Escherichia coli, the RNA degradosome, a four-protein complex, is required for normal RNA degradation and processing. In addition to the degradosome complex, the cell contains other ribonucleases that also play important roles in RNA processing and/or degradation. Whether the other ribonucleases are associated with the degradosome or function independently is not known. In the present work, IP (immunoprecipitation) studies from cell extracts showed that the major hydrolytic exoribonuclease RNase II is associated with the known degradosome components RNaseE (endoribonuclease E), RhlB (RNA helicase B), PNPase (polynucleotide phosphorylase) and Eno (enolase). Further evidence for the RNase II-degradosome association came from the binding of RNase II to purified RNaseE in far western affinity blot experiments. Formation of the RNase II–degradosome complex required the degradosomal proteins RhlB and PNPase as well as a C-terminal domain of RNaseE that contains binding sites for the other degradosomal proteins. This shows that the RNase II is a component of the RNA degradosome complex, a previously unrecognized association that is likely to play a role in coupling and coordinating the multiple elements of the RNA degradation pathways.     

Distinct modes of mature and precursor tRNA binding to Escherichia coli RNase P RNA revealed by NAIM analyses

We have analyzed by nucleotide analog interference mapping (NAIM) pools of precursor or mature tRNA molecules, carrying a low level of Rp-RMPalphaS (R = A, G, I) or Rp-c7-deaza-RMPalphaS (R = A, G) modifications, to identify functional groups that contribute to the specific interaction with and processing efficiency by Escherichia coli RNase P RNA. The majority of interferences were found in the acceptor stem, T arm, and D arm, including the strongest effects observed at positions G19, G53, A58, and G71. In some cases (interferences at G5, G18, and G71), the affected functional groups are candidates for direct contacts with RNase P RNA. Several modifications disrupt intramolecular tertiary contacts known to stabilize the authentic tRNA fold. Such indirect interference effects were informative as well, because they allowed us to compare the structural constraints required for ptRNA processing versus product binding. Our ptRNA processing and mature tRNA binding NAIM analyses revealed overlapping but nonidentical patterns of interference effects, suggesting that substrate binding and cleavage involves binding modes or conformational states distinct from the binding mode of mature tRNA, the product of the reaction.     

Evidence for a polynuclear metal ion binding site in the catalytic domain of ribonuclease P RNA

Interactions with divalent metal ions are essential for the folding and function of the catalytic RNA component of the tRNA processing enzyme ribonuclease P (RNase P RNA). However, the number and location of specific metal ion interactions in this large, highly structured RNA are poorly understood. Using atomic mutagenesis and quantitative analysis of thiophilic metal ion rescue we provide evidence for metal ion interactions at the pro-R(P) and pro-S(P) non-bridging phosphate oxygens at nucleotide A67 in the universally conserved helix P4. Moreover, second-site modifications within helix P4 and the adjacent single stranded region (J3/4) provide the first evidence for metal ion interactions with nucleotide base functional groups in RNase P RNA and reveal the presence of an additional metal ion important for catalytic function. Together, these data are consistent with a cluster of metal ion interactions in the P1-P4 multi-helix junction that defines the catalytic core of the RNase P ribozyme.     

The NMR structure of 31mer RNA domain of Escherichia coli RNase P RNA using its non-uniformly deuterium labelled counterpart [the 'NMR-window' concept].

The NMR structure of a 31mer RNA constituting a functionally important domain of the catalytic RNase P RNA from Escherichia coli is reported. Severe spectral overlaps of the proton resonances in the natural 31mer RNA (1) were successfully tackled by unique spectral simplifications found in the partially-deuterated 31 mer RNA analogue (2) incorporating deuterated cytidines [C5 (>95 atom % 2H), C2' (>97 atom % 2H), C3' (>97 atom % 2H), C4' (>65 atom % 2H) and C5' (>97 atom % 2H)] [for the 'NMR-window' concept see: Földesi,A. et al. (1992) Tetrahedron, 48, 9033; Foldesi,A. et al. (1993) J. Biochem. Biophys. Methods, 26, 1; Yamakage,S.-I. et al. (1993) Nucleic Acids Res., 21, 5005; Agback,P. et al. (1994) Nucleic Acids Res., 22, 1404; Földesi,A. et al. (1995) Tetrahedron, 51, 10065; Földesi,A. et al. (1996) Nucleic Acids Res., 24, 1187-1194]. 175 resonances have been assigned out of total of 235 non-exchangeable proton resonances in (1) in an unprecedented manner in the absence of 13C and 15N labelling. 41 out of 175 assigned resonances could be accomplished with the help of the deuterated analogue (2). The two stems in 31mer RNA adopt an A-type RNA conformation and the base-stacking continues from stem I into the beginning of the loop I. Long distance cross-strand NOEs showed a structured conformation at the junction between stem I and loop I. The loop I-stem II junction is less ordered and shows structural perturbation at and around the G11 -C22 base pair.