Mutations in a nonconserved sequence of the Tetrahymena ribozyme increase activity and specificity

Using droplet-based microfluidics to improve the catalytic properties of RNA under multiple-turnover conditions

In vitro evolution methodologies are powerful approaches to identify RNA with new functionalities. While Systematic Evolution of Ligands by Exponential enrichment (SELEX) is an efficient approach to generate new RNA aptamers, it is less suited for the isolation of efficient ribozymes as it does not select directly for the catalysis. In vitro compartmentalization (IVC) in aqueous droplets in emulsions allows catalytic RNAs to be selected under multiple-turnover conditions but suffers severe limitations that can be overcome using the droplet-based microfluidics workflow described in this paper. Using microfluidics, millions of genes in a library can be individually compartmentalized in highly monodisperse aqueous droplets and serial operations performed on them. This allows the different steps of the evolution process (gene amplification, transcription, and phenotypic assay) to be uncoupled, making the method highly flexible, applicable to the selection and evolution of a variety of RNAs, and easily adaptable for evolution of DNA or proteins. To demonstrate the method, we performed cycles of random mutagenesis and selection to evolve the X-motif, a ribozyme which, like many ribozymes selected using SELEX, has limited multiple-turnover activity. This led to the selection of variants, likely to be the optimal ribozymes that can be generated using point mutagenesis alone, with a turnover number under multiple-turnover conditions, k(ss) cat, ∼ 28-fold higher than the original X-motif, primarily due to an increase in the rate of product release, the rate-limiting step in the multiple-turnover reaction.

How a chemist looks at RNA

RNA, just another starting material? Nobel Laureate Tom Cech shows that with an education steeped in kinetics, thermodynamics, and molecular structure, and armed with the ability to synthesize molecules, the chemist is ideally suited to investigate RNA.

Cooperative tertiary interaction network guides RNA folding

Noncoding RNAs form unique 3D structures, which perform many regulatory functions. To understand how RNAs fold uniquely despite a small number of tertiary interaction motifs, we mutated the major tertiary interactions in a group I ribozyme by single-base substitutions. The resulting perturbations to the folding energy landscape were measured using SAXS, ribozyme activity, hydroxyl radical footprinting, and native PAGE. Double- and triple-mutant cycles show that most tertiary interactions have a small effect on the stability of the native state. Instead, the formation of core and peripheral structural motifs is cooperatively linked in near-native folding intermediates, and this cooperativity depends on the native helix orientation. The emergence of a cooperative interaction network at an early stage of folding suppresses nonnative structures and guides the search for the native state. We suggest that cooperativity in noncoding RNAs arose from natural selection of architectures conducive to forming a unique, stable fold.

A role for a single-stranded junction in RNA binding and specificity by the Tetrahymena group I ribozyme

We have investigated the role of a single-stranded RNA junction, J1/2, that connects the substrate-containing P1 duplex to the remainder of the Tetrahymena group I ribozyme. Single-turnover kinetics, fluorescence anisotropy, and single-molecule fluorescence resonance energy transfer studies of a series of J1/2 mutants were used to probe the sequence dependence of the catalytic activity, the P1 dynamics, and the thermodynamics of docking of the P1 duplex into the ribozyme's catalytic core. We found that A29, the center A of three adenosine residues in J1/2, contributes 2 orders of magnitude to the overall ribozyme activity, and double-mutant cycles suggested that J1/2 stabilizes the docked state of P1 over the undocked state via a tertiary interaction involving A29 and the first base pair in helix P2 of the ribozyme, A31·U56. Comparative sequence analysis of this group I intron subclass suggests that the A29 interaction sets one end of a molecular ruler whose other end specifies the 5'-splice site and that this molecular ruler is conserved among a subclass of group I introns related to the Tetrahymena intron. Our results reveal substantial functional effects from a seemingly simple single-stranded RNA junction and suggest that junction sequences may evolve rapidly to provide important interactions in functional RNAs.

Site-specific isotope labeling of long RNA for structural and mechanistic studies

A site-specific isotope labeling technique of long RNA molecules was established. This technique is comprised of two simple enzymatic reactions, namely a guanosine transfer reaction of group I self-splicing introns and a ligation with T4 DNA ligase. The trans-acting group I self-splicing intron with its external cofactor, ‘isotopically labeled guanosine 5′-monophosphate’ (5′-GMP), steadily gave a 5′-residue-labeled RNA fragment. This key reaction, in combination with a ligation of 5′-remainder non-labeled sequence, allowed us to prepare a site-specifically labeled RNA molecule in a high yield, and its production was confirmed with ¹⁵N NMR spectroscopy. Such a site-specifically labeled RNA molecule can be used to detect a molecular interaction and to probe chemical features of catalytically/structurally important residues with NMR spectroscopy and possibly Raman spectroscopy and mass spectrometry.

Motions of the substrate recognition duplex in a group I intron assessed by site-directed spin labeling

The Tetrahymena group I intron recognizes its oligonucleotide substrate in a two-step process. First, a substrate recognition duplex, called the P1 duplex, is formed. The P1 duplex then docks into the prefolded ribozyme core by forming tertiary contacts. P1 docking controls both the rate and the fidelity of substrate cleavage and has been extensively studied as a model for the formation of RNA tertiary structure. However, previous work has been limited to studying millisecond or slower motions. Here we investigated nanosecond P1 motions in the context of the ribozyme using site-directed spin labeling (SDSL) and electron paramagnetic resonance (EPR) spectroscopy. A nitroxide spin label (R5a) was covalently attached to a specific site of the substrate oligonucleotide, the labeled substrate was bound to a prefolded ribozyme to form the P1 duplex, and X-band EPR spectroscopy was used to monitor nitroxide motions in the 0.1-50 ns regime. Using substrates that favor the docked or the undocked states, it was established that R5a was capable of reporting P1 duplex motions. Using R5a-labeled substrates it was found that the J1/2 junction connecting P1 to the ribozyme core controls nanosecond P1 mobility in the undocked state. This may account for previous observations that J1/2 mutations weaken substrate binding and give rise to cryptic cleavage. This study establishes the use of SDSL to probe nanosecond dynamic behaviors of individual helices within large RNA and RNA/protein complexes. This approach may help in understanding the relationship between RNA structure, dynamics, and function.

Challenges in enzyme mechanism and energetics

Since the discovery of enzymes as biological catalysts, study of their enormous catalytic power and exquisite specificity has been central to biochemistry. Nevertheless, there is no universally accepted comprehensive description. Rather, numerous proposals have been presented over the past half century. The difficulty in developing a comprehensive description for the catalytic power of enzymes derives from the highly cooperative nature of their energetics, which renders impossible a simple division of mechanistic features and an absolute partitioning of catalytic contributions into independent and energetically additive components. Site-directed mutagenesis has emerged as an enormously powerful approach to probe enzymatic catalysis, illuminating many basic features of enzyme function and behavior. The emphasis of site-directed mutagenesis on the role of individual residues has also, inadvertently, limited experimental and conceptual attention to the fundamentally cooperative nature of enzyme function and energetics. The first part of this review highlights the structural and functional interconnectivity central to enzymatic catalysis. In the second part we ask: What are the features of enzymes that distinguish them from simple chemical catalysts? The answers are presented in conceptual models that, while simplified, help illustrate the vast amount known about how enzymes achieve catalysis. In the last section, we highlight the molecular and energetic questions that remain for future investigation and describe experimental approaches that will be necessary to answer these questions. The promise of advancing and integrating cutting edge conceptual, experimental, and computational tools brings mechanistic enzymology to a new era, one poised for novel fundamental insights into biological catalysis.

Imaging Tetrahymena ribozyme splicing activity in single live mammalian cells

Tetrahymena ribozymes hold promise for repairing genetic disorders but are largely limited by their modest splicing efficiency and low production of final therapeutic proteins. Ribozyme evolution in intact living mammalian cells would greatly facilitate the discovery of new ribozyme variants with high in vivo activity, but no such strategies have been reported. Here we present a study using a new reporter enzyme, beta-lactamase, to report splicing activity in single living cells and perform high-throughput screening with flow cytometry. The reporter ribozyme constructs consist of the self-splicing Tetrahymena thermophila group I intron ribozyme that is inserted into the ORF of the mRNA of beta-lactamase. The splicing activity in single living cells can be readily detected quantitatively and visualized. Individual cells have shown considerable heterogeneity in ribozyme activity. Screening of Tetrahymena ribozymes with insertions in the middle of the L1 loop led to identification of better variants with at least 4-fold more final in vivo activity than the native sequence. Our work has provided a new reporter system that allows high-throughput screening with flow cytometry of single living mammalian cells for a direct and facile in vivo selection of desired ribozyme variants.

A modular, bifunctional RNA that integrates itself into a target RNA

Nature often combines independent functional domains to achieve complex function, but this approach has not been extensively explored with artificial enzymes. Here, a group I ribozyme, which can act as an endoribonuclease, was partnered with the R3C ribozyme, which catalyzes the ligation of RNA molecules. The conjoined ribozymes have the potential to perform successive RNA cleavage and joining reactions, resulting in their mutual integration into a target RNA substrate. When simply joined together, however, the ribozymes were unable to achieve this outcome because of inefficient transfer of the substrate between the two catalytic subunits. In vitro evolution was used to optimize the behavior of the conjoined ribozymes, resulting in bifunctional molecules with substantially improved integration activity. The ligase subunit of these molecules was unchanged, whereas the group I subunit acquired several mutations, mostly in peripheral regions. The generation and study of this bifunctional assembly helps shed light on the evolution of modular enzymes and the obstacles that must be overcome in bringing together independent functional domains. These molecules also may be useful as tools for the insertional mutagenesis of target mRNAs.

Exploration of the transition state for tertiary structure formation between an RNA helix and a large structured RNA

Docking of the P1 duplex into the pre-folded core of the Tetrahymena group I ribozyme exemplifies the formation of tertiary interactions in the context of a complex, structured RNA. We have applied Phi-analysis to P1 docking, which compares the effects of modifications on the rate constant for docking (k(dock)) with the effects on the docking equilibrium (K(dock)). To accomplish this we used a single molecule fluorescence resonance energy transfer assay that allows direct determination of the rate constants for formation of thermodynamically favorable, as well as unfavorable, states. Modification of the eight groups of the P1 duplex that make tertiary interactions with the core and changes in solution conditions decrease K(dock) up to 500-fold, whereas k(dock) changes by </=2-fold. The absence of effects on k(dock), both from atomic modifications and global perturbations, strongly suggests that the transition state for docking is early and does not closely resemble the docked state. These results, the slow rate of docking of 3s(-1), and the observation that a modification that is expected to increase the degrees of freedom between the P1 duplex and the ribozyme core accelerates docking, suggest a model in which a kinetic trap(s) slows docking substantially. Nonetheless, urea does not increase k(dock), suggesting that there is little change in the exposed surface area between the trapped, undocked state and the transition state. The findings highlight that urea and temperature dependencies can be inadequate to diagnose the presence of kinetic traps in a folding process. The results described here, combined with previous work, provide an in-depth view of an RNA tertiary structure formation event and suggest that large, highly structured RNAs may have local regions that are misordered.

Guiding ribozyme cleavage through motif recognition: the mechanism of cleavage site selection by a group ii intron ribozyme

The mechanism by which group II introns cleave the correct phosphodiester linkage was investigated by studying the reaction of mutant substrates with a ribozyme derived from intron ai5gamma. While fidelity was found to be quite high in most cases, a single mutation on the substrate (+1C) resulted in a dramatic loss of fidelity. When this mutation was combined with a second mutation that induces a bulge in the exon binding site 1/intron binding site 1 (EBS1/IBS1) duplex, the base-pairing register of the EBS1/IBS1 duplex was shifted and the cleavage site moved to a downstream position on the substrate. Conversely, when mismatches were incorporated at the EBS1/IBS1 terminus, the duplex was effectively truncated and cleavage occurred at an upstream site. Taken together, these data demonstrate that the cleavage site of a group II intron ribozyme can be tuned at will by manipulating the thermodynamic stability and structure of the EBS1/IBS1 pairing. The results are consistent with a model in which the cleavage site is not designated through recognition of specific nucleotides (such as the 5'-terminal residue of EBS1). Instead, the ribozyme detects a structure at the junction between single and double-stranded residues on the bound substrate. This finding explains the puzzling lack of phylogenetic conservation in ribozyme and substrate sequences near group II intron target sites.

Reading DNA differently

Product inhibition has provided the limiting barrier to efficient template-directed ligation and polymerization reactions. Here we review the attempts to circumvent this limitation and outline a translation strategy that does overcome the barrier and allows the information encoded in DNA to be read and amplified into backbone-modified oligonucleotides.

Isolation and characterization of the C-terminal nuclease domain from the RecB protein of Escherichia coli

The RecB subunit of the Escherichia coli RecBCD enzyme has been shown in previous work to have two domains: an N-terminal 100 kDa domain with ATP-dependent helicase activity, and a C-terminal 30 kDa domain. The 30 kDa domain had nuclease activity when linked to a heterologous DNA binding protein, but by itself it appeared unable to bind DNA and lacked detectable nuclease activity. We have expressed and isolated this 30 kDa domain, called RecB(N), and show that it does have nuclease activity detectable at high protein concentration in the presence of polyethylene glycol, added as a molecular crowding agent. The activity is undetectable in a mutant RecB(N)protein in which an aspartate residue has been changed to alanine. Structural analysis of the wild-type and mutant RecB(N)proteins by second derivative absorbance and circular dichroism spectroscopy indicates that both are folded proteins with very similar secondary and tertiary structures. The results show that the Asp-->Ala mutation has not caused a significant structural change in the isolated domain and they support the conclusion that the C-terminal domain of RecB has the sole nuclease active site of RecBCD.

Specialized conservation of surface loops of myosin: evidence that loops are involved in determining functional characteristics

The molecular motor myosin has been the focus of considerable structure-function analysis. Of key interest are the portions of the protein that control the rate of ATP hydrolysis, the affinity for actin, and the velocity at which myosin moves actin. Two regions that have been implicated in determining these parameters are the "loop" regions at the junctions of the 25 kDa and 50 kDa domains and the 50 kDa and 20 kDa domains of the protein. However, the sequences of these regions are poorly conserved between different myosin families, suggesting that they are not constrained evolutionarily, and thus are relatively unimportant for myosin function. In order to address this apparent incongruity, we have performed an analysis of relative rates of observed evolutionary change. We found that the sequences of these loop regions appear to be actually more constrained than the sequences of the rest of the myosin molecule, when myosins are compared that are known to be kinetically or developmentally similar. This suggests that these loop regions could play an important role in myosin function and supports the idea that they are involved in modulating the specific kinetic characteristics that functionally differentiate one myosin isoform from another. Apparently "unconserved" loops may generally play a role in determining kinetic properties of enzymes, and similar analyses of relative rates of evolution may prove useful for the study of structure-function relationships in other protein families.

A hydrogen-bonding triad stabilizes the chemical transition state of a group I ribozyme

BACKGROUND

The group I intron is an RNA enzyme capable of efficiently catalyzing phosphoryl-transfer reactions. Functional groups that stabilize the chemical transition state of the cleavage reaction have been identified, but they are all located within either the 5'-exon (P1) helix or the guanosine cofactor, which are the substrates of the reaction. Functional groups within the ribozyme active site are also expected to assist in transition-state stabilization, and their role must be explored to understand the chemical basis of group I intron catalysis.

RESULTS

Using nucleotide analog interference mapping and site-specific functional group substitution experiments, we demonstrate that the 2'-OH at A207, a highly conserved nucleotide in the ribozyme active site, specifically stabilizes the chemical transition state by approximately 2 kcal mol-1. The A207 2'-OH only makes its contribution when the U(-1) 2'-OH immediately adjacent to the scissile phosphate is present, suggesting that the 2'-OHs of A207 and U(-1) interact during the chemical step.

CONCLUSIONS

These data support a model in which the 3'-oxyanion leaving group of the transesterification reaction is stabilized by a hydrogen-bonding triad consisting of the 2'-OH groups of U(-1) and A207 and the exocyclic amine of G22. Because all three nucleotides occur within highly conserved non-canonical base pairings, this stabilization mechanism is likely to occur throughout group I introns. Although this mechanism utilizes functional groups distinctive of RNA enzymes, it is analogous to the transition states of some protein enzymes that perform similar phosphoryl-transfer reactions.

Towards a new concept of gene inactivation: specific RNA cleavage by endogenous ribonuclease P

In the first part of this chapter, general concepts for gene inactivation, antisense techniques and catalytic RNAs (ribozymes) are presented. The requirements for modified oligonucleotides are discussed with their effects on the stability of base-paired hybrids and on resistance against nuclease attack. This also includes the problems in the choice of an optimal target sequence within the inactivated RNA and the options of cellular delivery systems. The second part describes the recently introduced antisense concept based on the ubiquitous cellular enzyme ribonuclease P. This system is unique, since the substrate recognition requires the proper tertiary structure of the cleaved RNA. General properties and possible advantages of this approach are discussed.

Effects of helical structures formed by the binding arms of DNAzymes and their substrates on catalytic activity

As a part of our efforts to clarify structure-function relationships in reactions catalyzed by deoxyribozymes (DNAzymes), which were recently selected in vitro , we synthesized various chimeras and analyzed the kinetics of the corresponding cleavage reactions. We focused on the binding arms and generated helices composed of binding arms and substrates that consisted of RNA and RNA, of RNA and DNA or of DNA and DNA. As expected for the rate limiting chemical cleavage step in reactions catalyzed by DNAzymes, a linear relationship between log( k cat) and pH was observed. In all cases examined, introduction of DNA into the binding helix enhanced the rate of chemical cleavage. Comparison of CD spectra of DNAzyme. substrate complexes suggested that higher levels of B-form-like helix were associated with higher rates of cleavage of the substrate within the complex. To our surprise, the enhancement of catalytic activity that followed introduction of DNA into the binding helix (enhancement by the presence of more B-form-like helix) was very similar to that observed in the case of the hammerhead ribozymes that we had investigated previously. These data, together with other observations, strongly suggest that the reaction mechanism of metal-ion-dependent DNAzymes is almost identical to that of hammerhead ribozymes.

Ribozyme-mediated trans-splicing of a trinucleotide repeat

Trinucleotide repeat expansions (TREs) are a recently described class of mutations characterized by a change in the size of the genomic fragment due to amplification of the repeated unit. A number of diseases have been attributed to TRE, including Huntington disease and myotonic dystrophy (DM), but attempts at genetic therapy have yet to prove successful. A potential therapeutic approach would be to repair the expanded repeat using the trans-splicing ability of group I intron ribozymes. We have used DM as a model to test this hypothesis. A group I intron ribozyme (DMPK-RZ1) was designed to modify the TRE at the 3' end of the human myotonic dystrophy protein kinase (DMPK) transcripts. DMPK-RZ1 was shown to ligate a small DMPK mRNA fragment, contained within the ribozyme, to a simple DMPK-target RNA in vitro. It also modified a larger target transcript, leading to replacement of twelve repeats with five repeats, both in vitro and in mammalian cells. Finally, this ribozyme successfully replaced the 3' end of endogenous DMPK mRNA in fibroblasts with a different 3' region. Ribozyme-mediated RNA repair may thus form a novel therapeutic strategy for diseases associated with repeat expansions.

Mechanistic aspects of enzymatic catalysis: lessons from comparison of RNA and protein enzymes

A classic approach in biology, both organismal and cellular, is to compare morphologies in order to glean structural and functional commonalities. The comparative approach has also proven valuable on a molecular level. For example, phylogenetic comparisons of RNA sequences have led to determination of conserved secondary and even tertiary structures, and comparisons of protein structures have led to classifications of families of protein folds. Here we take this approach in a mechanistic direction, comparing protein and RNA enzymes. The aim of comparing RNA and protein enzymes is to learn about fundamental physical and chemical principles of biological catalysis. The more recently discovered RNA enzymes, or ribozymes, provide a distinct perspective on long-standing questions of biological catalysis. The differences described in this review have taught us about the aspects of RNA and proteins that are distinct, whereas the common features have helped us to understand the aspects that are fundamental to biological catalysis. This has allowed the framework that was put forth by Jencks for protein catalysts over 20 years ago (1) to be extended to RNA enzymes, generalized, and strengthened.

New loop-loop tertiary interactions in self-splicing introns of subgroup IC and ID: a complete 3D model of the Tetrahymena thermophila ribozyme

BACKGROUND

Group I introns self-splice via two consecutive trans-esterification reactions in the presence of guanosine cofactor and magnesium ions. Comparative sequence analysis has established that a catalytic core of about 120 nucleotides is conserved in all known group I introns. This core is generally not sufficient for activity, however, and most self-splicing group I introns require non-conserved peripheral elements to stabilize the complete three-dimensional (3D) structure. The physico-chemical properties of group I introns make them excellent systems for unraveling the structural basis of the RNA-RNA interactions responsible for promoting the self-assembly of complex RNAs.

RESULTS

We present phylogenetic and experimental evidence for the existence of three additional tertiary base pairings between hairpin loops within peripheral components of subgroup IC1 and ID introns. Each of these new long range interactions, called P13, P14 and P16, involves a terminal loop located in domain 2. Although domains 2 of IC and ID introns share very strong sequence similarity, their terminal loops interact with domains 5 and 9 (subgroup IC1) and domain 6 (subgroup ID). Based on these tertiary contacts, comparative sequence analysis, and published experimental results such as Fe(II)-EDTA protection patterns, we propose 3D models for two entire group I introns, the subgroup IC1 intron in the large ribosomal precursor RNA of Tetrahymena thermophila and the SdCob.1 subgroup ID intron found in the cytochrome b gene of Saccharomyces douglasii.

CONCLUSIONS

Three-dimensional models of group I introns belonging to four different subgroups are now available. They all emphasize the modular and hierarchical organization of the architecture of group I introns and the widespread use of base-pairings between terminal hairpin loops for stabilizing the folded and active structures of large and complex RNA molecules.

Tagging ribozyme reaction sites to follow trans-splicing in mammalian cells

In mammalian cells, genetic instructions are usually revised by RNA splicing before they are translated to proteins. Here we demonstrate that a trans-splicing group I ribozyme can be employed to intentionally modify the sequence of targeted transcripts in tissue culture cells. By analyzing the ribozyme reaction products, we demonstrate that targeted trans-splicing can proceed in murine fibroblasts with high fidelity, providing direct evidence that ribozymes function as anticipated in a therapeutically relevant setting. Trans-splicing is not very specific however, and the ribozyme reacted with and tagged a variety of cellular transcripts with its 3' exon sequence. RNA tagging provides a unique approach to study RNA catalysis in mammalian cells. Such analysis should facilitate the logical development of safe, therapeutic ribozymes that can repair mutant RNAs associated with a variety of inherited diseases.

Two group I introns with a C.G basepair at the 5' splice-site instead of the very highly conserved U.G basepair: is selection post-translational?

In virtually all of the 200 group I introns sequenced thus far, the specificity of 5' splice-site cleavage is determined by a basepair between a uracil base at the end of the 5' exon and a guanine in an intron guide sequence which pairs with the nucleotides flanking the splice-site. It has been reported that two introns in the cytochrome oxidase subunit I gene of Aspergillus nidulans and Podospora anserina are exceptions to this rule and have a C.G basepair in this position. We have confirmed the initial reports and shown for one of them that RNA editing does not convert the C to a U. Both introns autocatalytically cleave the 5' splice-site. Mutation of the C to U in one intron reduces the requirement for Mg2+ and leads to an increase in the rate of cleavage. As the C base encodes a highly conserved amino acid, we propose that it is selected post-translationally at the level of protein function, despite its inferior splicing activity.

Protein facilitation of group I intron splicing by assembly of the catalytic core and the 5' splice site domain

The yeast mitochondrial group I intron b15 undergoes self-splicing at high Mg2+ concentrations, but requires the splicing factor CBP2 for reaction under physiological conditions. Chemical accessibility and UV cross-linking experiments now reveal that self-processing is slow because functional elements are not properly positioned in an active tertiary structure. Folding energy provided by CBP2 drives assembly of two RNA domains that comprise the catalytic core and meditates association of an approximately 100 nt 5' domain that contains the 5' splice site. Thus, the protein assembles RNA secondary structure elements into a specific three-dimensional array while the RNA provides the catalytic center. The division of labor between RNA and protein illustrated by this simple system reveals principles applicable to complex ribonucleoprotein assemblies such as the spliceosome and ribosome.

Colocalizing ribozymes with substrate RNAs to increase their efficacy as gene inhibitors

The ability to target ribozymes to specifically cleave viral RNAs in vitro has led to much speculation about their potential therapeutic value as antiviral agents in vivo. To transfer a ribozyme's potential as an antiviral agent from test tubes to cells and organisms successfully, the characteristics that distinguish these settings must be considered. In vitro, ribozymes and substrate RNAs freely diffuse in solution in test tubes, and trans-cleavage reactions are dependent on a diffusive step. In eukaryotic cells, by contrast, many RNAs do not appear to diffuse freely. Instead, they appear to be highly compartmentalized and actively sorted to specific cellular locations. Such RNA trafficking may result in localization of substrate RNAs in a different compartment than ribozymes, which would effectively reduce substrate RNA availability to ribozymes and therefore limit the effectiveness of ribozymes as gene inhibitors.

A kinetic mechanism for cleavage of precursor tRNA(Asp) catalyzed by the RNA component of Bacillus subtilis ribonuclease P

A kinetic mechanism is presented for the cleavage of Bacillus subtilis precursor tRNA(Asp) catalyzed by the RNA component of B. subtilis ribonuclease P (RNase P) under optimal conditions (50 mM Tris Cl (pH 8.0), 100 mM MgCl2, and 800 mM NH4Cl, 37 degrees C). This kinetic mechanism was derived from measuring pre-steady-state, steady-state, single-turnover, and binding kinetics using a combination of quench-flow, gel filtration, and gel shift techniques. A minimal kinetic description involves the following: (1) binding of pre-tRNA(Asp) to RNase P RNA rapidly (6.3 x 10(6) M-1 s-1), but slower than the diffusion-controlled limit; (2) cleavage of the phosphodiester bond with a rate constant of 6 s-1; (3) dissociation of products in a kinetically preferred pathway, with the 5' RNA fragment dissociating first (> or = 0.2 s-1) followed by rate-limiting tRNA dissociation (0.02 s-1); and (4) formation of a second conformer of RNase P RNA during the catalytic cycle that is less stable and binds pre-tRNA(Asp) significantly more slowly (7 x 10(4) M-1 s-1). This scheme involves the isolation of individual steps in the reaction sequence, is consistent with steady-state data, and pinpoints the rate-determining step under a variety of conditions. This kinetic mechanism will facilitate a more accurate definition of the role of metals, pH, and the protein component in each step of the reaction and provide an essential background for understanding the influence of structural changes on the catalytic activity.

A tyrosyl-tRNA synthetase can function similarly to an RNA structure in the Tetrahymena ribozyme

Group I introns are highly structured RNAs which catalyse their own splicing by guanosine-initiated transesterification reactions. Their catalytic core is generally stabilized by RNA-RNA interactions within the core and with peripheral RNA structures. Additionally, some group I introns require proteins for efficient splicing in vivo. The Neurospora CYT-18 protein, the mitochondrial tyrosyl-transfer RNA synthetase (mt TyrRS), promotes splicing of the Neurospora mitochondrial large ribosomal RNA (LSU) and other group I introns by stabilizing the catalytically active structure of the intron core. We report here that CYT-18 functions similarly to a peripheral RNA structure, P5abc, that stabilizes the catalytic core of the Tetrahymena LSU intron. The CYT-18 protein and P5abc RNA bind to overlapping sites in the intron core, inducing similar conformational changes correlated with splicing activity. Our results show that a protein can play the role of an RNA structure in a catalytic RNA, a substitution postulated for the evolution of nuclear pre-messenger RNA introns from self-splicing introns.

A tertiary interaction in the Tetrahymena intron contributes to selection of the 5' splice site

The utilization of cryptic splice sites has been observed in a number of RNA splicing reactions. In the self-splicing group I intron of Tetrahymena thermophila, point mutations of either A57 or A95 promote cleavage at two sites other than the normal 5' splice site, suggesting that these nucleotides are involved in a common tertiary interaction. These results are unusual since A57 and A95 are neither at nor near the 5' splice site in the sequence or secondary structure. Cleavage at the alternative sites appears to occur by intron cyclization, a reaction with well-established structural and mechanistic similarities to the first step of RNA self-splicing. Alternative docking of P1 (the helix containing the 5' splice site paired to the internal guide sequence of the intron) into the catalytic core accounts for cleavage at the cryptic reaction sites. We propose that the A57/A95 interaction, along with an element implicated previously (J1/2), provide structural connectivity from the reaction site in P1 to the catalytic core of the Tetrahymena intron. It seems likely that RNA splicing in general will require such tertiary interactions to position RNA helices.

Translocation of an RNA duplex on a ribozyme

RNA cleavage by the Tetrahymena ribozyme requires recognition of the reaction-site helix by the catalytic apparatus. This binding can occur in several registers, each of which results in reaction at a different nucleotide in the helix. We now identify commensurate sets of 2'-hydroxyl interactions on both strands of the reaction-site helix that account for its translocation into alternative binding registers. These results indicate that the ribozyme has a relatively rigid substrate-binding pocket into which the helix can bind in different alignments. A similar mechanism of reaction site recognition is proposed to occur during intron circularization and ribozyme polymerase activity. Translocation of the reaction site duplex provides an example of structural heterogeneity in packing of helices during the tertiary folding of RNA.

The efficiency and versatility of catalytic RNA: implications for an RNA world

Because RNA is chemically and structurally dissimilar from protein, the finding of catalytic activity in RNA was initially surprising. Quantitative measurements of reaction rates show that RNA can be as efficient a catalyst as protein. On the other hand, the potential versatility of RNA to catalyze diverse types of reactions has only begun to be explored. Understanding the efficiency and versatility of RNA as a catalyst helps us evaluate origin-of-life scenarios involving self-replicating RNA, and may explain why RNA catalysis remains important in contemporary cells.

Tertiary interactions with the internal guide sequence mediate docking of the P1 helix into the catalytic core of the Tetrahymena ribozyme

The L-21 ScaI ribozyme catalyzes sequence-specific cleavage of an oligonucleotide substrate. Cleavage is preceded by base pairing of the substrate to the internal guide sequence (IGS) at the 5' end of the ribozyme to form a short RNA duplex (P1). Tertiary interactions between P1 and the catalytic core dock P1 into the active site of the ribozyme. These include interactions between the catalytic core and 2'-hydroxyls of the substrate at nucleotide positions -3u and perhaps -2c. In this study, 2'-hydroxyls of the IGS strand that contribute to P1 recognition by the ribozyme are identified. IGS 2'-hydroxyls (nucleotide positions 22-27) were individually modified to either 2'-deoxy or 2'-methoxynucleotides within full-length semisynthetic L-21 ScaI ribozymes generated using T4 DNA ligase. Thermodynamic and kinetic characterization of the resulting IGS variant ribozymes justify the following conclusions: (i) 2'-Hydroxyls at nucleotide positions G22 and G25 play a critical energetic role in docking P1 into the catalytic core, contributing 2.6 and 2.1 kcal.mol-1, respectively. (ii) The loss of binding energy is manifest primarily as an increase in the rate of dissociation. Because turnover for the wild-type ribozyme is limited by product dissociation, G22 and G25 deoxy variants display up to a 20-fold increase in the multiple-turnover rate at saturating substrate. (iii) IGS tertiary interactions are energetically coupled with the tertiary interactions made to the substrate, consistent with P1 becoming undocked from its binding site in J8/7 upon substitution of either the G22 or G25 2'-hydroxyl. (iv) The G22 deoxy variant loses energetic coupling between guanosine and substrate binding, suggesting that in this variant the P1 helix is also undocked from its binding site in J4/5, the proposed site of guanosine and substrate interaction. Therefore, in combinations with previous studies four P1 2'-hydroxyls are implicated as important for docking. The contributions of the 2'-hydroxyl tertiary interactions are not equivalent and follow the hierarchical order G22 > G25 >> -3u > -2c. Because the G22 2'-hydroxyl appears to mediate P1 docking into both J8/7 and J4/5, it may serve as the molecular linchpin for the recognition of P1 by the catalytic core.

Guanosine binding to the Tetrahymena ribozyme: thermodynamic coupling with oligonucleotide binding

The L-21 Sca I ribozyme derived from the group I intron of Tetrahymena thermophila pre-rRNA catalyzes an endonuclease reaction analogous to the first step of self-splicing. Guanosine (G) is bound by the ribozyme, and its 3'-hydroxyl group acts as the nucleophile. Here, we provide evidence that Km for G in several single-turnover reactions is equal to the equilibrium dissociation constant for G. This evidence includes the observation that removal of the 2'-hydroxyl group at the cleavage site of the oligoribonucleotide substrate [from CCCUCUA to CCCUC(dU)A] decreases the rate of cleavage approximately 1000-fold but has no effect on either the Km for G (0.17 mM) or for guanosine 5'-monophosphate (pG) (0.09 mM). In the course of this study, it was observed that Km for G or pG was lower by a factor of 5 for reactions with the ribozyme-CCCUC(dU)A complex compared with the free ribozyme, indicating a modest amount of thermodynamic coupled binding of the two substrates. The decrease in the rate of oligonucleotide dissociation upon addition of saturating pG provides independent support for this coupling. Coupling is lost with a substrate that cannot make the normal tertiary interactions with the ribozyme, providing evidence that coupled binding requires docking of the substrate into the catalytic core. Surprisingly, the binding of product CCCUCU and G is slightly anticooperative, indicating that the cleaved pA is important for coupling with substrate. Coupled binding suggests a splicing model in which the intron binds G tightly to promote the first step of reaction, after which its binding is an order of magnitude weaker, thereby facilitating the second step.

Nuclease-resistant chimeric ribozymes containing deoxyribonucleotides and phosphorothioate linkages

Hammerhead ribozymes are considered to be potential therapeutic agents for HIV virus because of their site-specific RNA cleavage activities. In order to elucidate structure--function relationship and also to hopefully endow ribozymes with resistance to ribonucleases, we firstly synthesized chimeric DNA/RNA ribozymes in which deoxyribonucleotides were substituted for ribonucleotides at noncatalytic residues (stems I, II, and III). Kinetic analysis revealed that (i) DNA in the hybridizing arms (stems I and III) enhanced the chemical cleavage step. (ii) stem II and its loop do not affect its enzymatic activity. Secondly, we introduced deoxyribonucleotides with phosphorothioate linkages to the same regions (stems I, II, and III) in order to test whether such thio-linkages further improve their resistance to nucleases. Kinetic measurements revealed that this chimeric thio-DNA/RNA ribozyme had seven-fold higher cleavage activity (kcat = 27 min-1) than that of the all-RNA ribozyme. In terms of stability in serum, DNA-armed ribozymes gained about 10-fold higher stability in human serum but no increase in stability was recognized in bovine serum, probably because the latter serum mainly contained endoribonucleases that attacked unmodified catalytic-loop regions of these ribozymes. Thirdly, in order to protect them from endoribonucleases, three additional modifications were made at positions U7, U4 and C3 within the internal catalytic-loop region, that succeeded in gaining more than a hundred times greater resistance to nucleases in both serums. More importantly, these catalytic-loop modified ribozymes had the comparable cleavage activity (kcat) to the wild-type ribozyme. Since these chimeric thio-DNA/RNA ribozymes are more resistant to attack by both exonucleases and endoribonucleases than the wild-type all-RNA ribozymes in vivo and since their cleavage activities are not sacrificed, they appear to be better candidates than the wild type for antiviral therapeutic agents.

Footprinting the sites of interaction of antibiotics with catalytic group I intron RNA

Aminoglycoside inhibitors of translation have been shown previously to inhibit in vitro self-splicing by group I introns. Chemical probing of the phage T4-derived sunY intron shows that neomycin, streptomycin, and related antibiotics protected the N-7 position of G96, a universally conserved guanine in the binding site for the guanosine cofactor in the splicing reaction. The antibiotics also disrupted structural contacts that have been proposed to bring the 5' cleavage site of the intron into proximity to the catalytic core. In contrast, the strictly competitive inhibitors deoxyguanosine and arginine protected only the N-7 position of G96. Parallels between these results and previously observed protection of 16S ribosomal RNA by aminoglycosides raise the possibility that group I intron splicing and transfer RNA selection by ribosomes involve similar RNA structural motifs.

An anti-lymphocytic choriomeningitis virus ribozyme expressed in tissue culture cells diminishes viral RNA levels and leads to a reduction in infectious virus yield

Ribozymes, RNA molecules which cleave RNA in a sequence-specific manner, are a promising tool in the development of specific antiviral therapies. The viruses most susceptible to ribozymes may be those in which all aspects of the viral life cycle depend on RNA, with no DNA intermediate. Consequently, we have chosen as a model one such virus, the arenavirus lymphocytic choriomeningitis virus (LCMV), and have previously reported the design of specific anti-LCMV ribozymes (Z. Xing and J. L. Whitton, J. Virol. 66:1361-1369, 1992). Here we describe the establishment of several cell lines, each stably expressing an antiarenaviral ribozyme of different specificity. Expression of a single ribozyme leads to a reduction in LCMV RNA levels, and stimulation of ribozyme transcription amplifies the effect. Target site selection may be an important determinant of antiviral effectiveness, since the extent of the antiviral effect, measured by assay of viral RNA, varies with the specificity of the antiviral ribozyme expressed. Furthermore, infectious virus production is reduced approximately 100-fold. This effect is LCMV specific, as yield of a related arenavirus is not similarly curtailed. We are currently investigating the mechanism underlying the ribozyme-mediated antiviral effect.

Mutations at the guanosine-binding site of the Tetrahymena ribozyme also affect site-specific hydrolysis

Self-splicing group I introns use guanosine as a nucleophile to cleave the 5' splice site. The guanosine-binding site has been localized to the G264-C311 base pair of the Tetrahymena intron on the basis of analysis of mutations that change the specificity of the nucleophile from G (guanosine) to 2AP (2-aminopurine ribonucleoside) (F. Michel et al. (1989) Nature 342, 391-395). We studied the effect of these mutations (G-U, A-C and A-U replacing G264-C311) in the L-21 ScaI version of the Tetrahymena ribozyme. In this enzymatic system (kcat/Km)G monitors the cleavage step. This kinetic parameter decreased by at least 5 x 10(3) when the G264-C311 base pair was mutated to an A-U pair, while (kcat/Km)2AP increased at least 40-fold. This amounted to an overall switch in specificity of at least 2 x 10(5). The nucleophile specificity (G > 2AP for the G-C and G-U pairs, 2AP > G for the A-U and A-C pairs) was consistent with the proposed hydrogen bond between the nucleotide at position 264 and N1 of the nucleophile. Unexpectedly, the A-U and A-C mutants showed a decrease of an order of magnitude in the rate of ribozyme-catalyzed hydrolysis of RNA, in which H2O or OH- replaces G as the nucleophile, whereas the G-U mutant showed a decrease of only 2-fold. The low hydrolysis rates were not restored by raising the Mg2+ concentration or lowering the temperature. In addition, the mutant ribozymes exhibited a pattern of cleavage by Fe(II)-EDTA indistinguishable from that of the wild type, and the [Mg2+]1/2 for folding of the A-U mutant ribozyme was the same as that of the wild type. Therefore the guanosine-binding site mutations do not appear to have a major effect on RNA folding or stability. Because changing G264 affects the hydrolysis reaction without perturbing the global folding of the RNA, we conclude that the catalytic role of this conserved nucleotide is not limited to guanosine binding.