Structural studies of the tRNA domain of tmRNA

Insights into the ribosomal trans-translation rescue system: lessons from recent structural studies

The arrest of protein synthesis caused when ribosomes stall on an mRNA lacking a stop codon is a deadly risk for all cells. In bacteria, this situation is remedied by the trans-translation quality control system. Trans-translation occurs because of the synergistic action of two main partners, transfer-messenger RNA (tmRNA) and small protein B (SmpB). These act in complex to monitor protein synthesis, intervening when necessary to rescue stalled ribosomes. During this process, incomplete nascent peptides are tagged for destruction, problematic mRNAs are degraded and the previously stalled ribosomes are recycled. In this 'Structural Snapshot' article, we describe the mechanism at the molecular level, a view updated after the most recent structural studies using cryo-electron microscopy.

Structures of tmRNA and SmpB as they transit through the ribosome

In bacteria, trans-translation is the main rescue system, freeing ribosomes stalled on defective messenger RNAs. This mechanism is driven by small protein B (SmpB) and transfer-messenger RNA (tmRNA), a hybrid RNA known to have both a tRNA-like and an mRNA-like domain. Here we present four cryo-EM structures of the ribosome during trans-translation at resolutions from 3.0 to 3.4 Å. These include the high-resolution structure of the whole pre-accommodated state, as well as structures of the accommodated state, the translocated state, and a translocation intermediate. Together, they shed light on the movements of the tmRNA-SmpB complex in the ribosome, from its delivery by the elongation factor EF-Tu to its passage through the ribosomal A and P sites after the opening of the B1 bridges. Additionally, we describe the interactions between the tmRNA-SmpB complex and the ribosome. These explain why the process does not interfere with canonical translation.

It's a loop world - single strands in RNA as structural and functional elements

Unpaired regions in RNA molecules - loops - are centrally involved in defining the characteristic three-dimensional (3D) architecture of RNAs and are of high interest in RNA engineering and design. Loops adopt diverse, but specific conformations stabilised by complex tertiary structural interactions that provide structural flexibility to RNA structures that would otherwise not be possible if they only consisted of the rigid A-helical shapes usually formed by canonical base pairing. By participating in sequence-non-local contacts, they furthermore contribute to stabilising the overall fold of RNA molecules. Interactions between RNAs and other nucleic acids, proteins, or small molecules are also generally mediated by RNA loop structures. Therefore, the function of an RNA molecule is generally dependent on its loops. Examples include intermolecular interactions between RNAs as part of the microRNA processing pathways, ribozymatic activity, or riboswitch-ligand interactions. Bioinformatics approaches have been successfully applied to the identification of novel RNA structural motifs including loops, local and global RNA 3D structure prediction, and structural and conformational analysis of RNAs and have contributed to a better understanding of the sequence-structure-function relationships in RNA loops.

Trans-translation exposed: understanding the structures and functions of tmRNA-SmpB

Ribosome stalling is a serious issue for cell survival. In bacteria, the primary rescue system is trans-translation, performed by tmRNA and its protein partner small protein B (SmpB). Since its discovery almost 20 years ago, biochemical, genetic, and structural studies have paved the way to a better understanding of how this sophisticated process takes place at the cellular and molecular levels. Here we describe the molecular details of trans-translation, with special mention of recent cryo-electron microscopy and crystal structures that have helped explain how the huge tmRNA-SmpB complex targets and delivers stalled ribosomes without interfering with canonical translation.

Fluorescence characterization of the transfer RNA-like domain of transfer messenger RNA in complex with small binding protein B

Transfer messenger RNA (tmRNA) and small binding protein B (SmpB) are the main components of the trans-translation rescue machinery that releases stalled ribosomes from defective mRNAs. Little is known about how SmpB binding affects the conformation of the tRNA-like domain (TLD) of tmRNA. It has been previously hypothesized that the absence of a D stem in the TLD provides flexibility in the elbow region of tmRNA, which can be stabilized by its interaction with SmpB. Here, we have used Förster resonance energy transfer to characterize the global structure of the tRNA-like domain of tmRNA in the presence and absence of SmpB and as a function of Mg(2+) concentration. Our results show tight and specific binding of SmpB to tmRNA. Surprisingly, our data show that the global conformation and flexibility of tmRNA do not change upon SmpB binding. However, Mg(2+) ions induce an 11 Å compaction in the tmRNA structure, suggesting that the flexibility in the H2a stem may allow different conformations of tmRNA as the TLD and mRNA-like domain need to be positioned differently while moving through the ribosome.

tmRNA on its way through the ribosome: two steps of resume, and what next?

Trans-translation is a universal quality-control process eubacteria use to degrade incompletely synthesized proteins and rescue ribosome stalled on defective mRNAs. This process is facilitated by a ribonucleoprotein complex composed of transfer-messenger RNA (tmRNA)-a chimera made of a tRNA-like molecule and a short open reading frame (ORF) -and small protein B (SmpB). Determination of the structure of tmRNA and SmpB in complex with the ribosome, at the stage when translation has resumed on tmRNA, has provided an increased understanding of the structure of tmRNA as it transits the ribosome, and unique insights into the complex mechanism of template switching on the ribosome and SmpB-driven selection of the correct reading frame on tmRNA's ORF.

Accommodation of tmRNA-SmpB into stalled ribosomes: a cryo-EM study

In eubacteria, translation of defective messenger RNAs (mRNAs) produces truncated polypeptides that stall on the ribosome. A quality control mechanism referred to as trans-translation is performed by transfer-messenger RNA (tmRNA), a specialized RNA acting as both a tRNA and an mRNA, associated with small protein B (SmpB). So far, a clear view of the structural movements of both the protein and RNA necessary to perform accommodation is still lacking. By using a construct containing the tRNA-like domain as well as the extended helix H2 of tmRNA, we present a cryo-electron microscopy study of the process of accommodation. The structure suggests how tmRNA and SmpB move into the ribosome decoding site after the release of EF-Tu.GDP. While two SmpB molecules are bound per ribosome in a preaccommodated state, our results show that during accommodation the SmpB protein interacting with the small subunit decoding site stays in place while the one interacting with the large subunit moves away. Relative to canonical translation, an additional movement is observed due to the rotation of H2. This suggests that the larger movement required to resume translation on a tmRNA internal open reading frame starts during accommodation.

Interaction analysis between tmRNA and SmpB from Thermus thermophilus

Small protein B, SmpB, is a tmRNA-specific binding protein essential for trans-translation. We examined the interaction between SmpB and tmRNA from Thermus thermophilus, using biochemical and NMR methods. Chemical footprinting analyses using full-length tmRNA demonstrated that the sites protected upon SmpB binding are located exclusively in the tRNA-like domain (TLD) of tmRNA. To clarify the SmpB binding sites, we constructed several segments derived from TLD. Optical biosensor interaction analyses and melting profile analyses with mutational studies showed that SmpB efficiently binds to only a 30-nt segment that forms a stem and loop, with the 5' and 3' extensions composed of the D-loop and variable-loop analogues. The conserved sequences, 16UCGA and 319GAC, in the extensions are responsible for the SmpB binding. These results agree with the those visualized by the cocrystal structure of TLD and SmpB from Aquifex aeolicus. In addition, NMR chemical shift mapping analyses, using the 30-nt segment and (15)N-labeled SmpB, revealed the characteristic RNA binding mode. The hydrogen bond pattern around beta2 changes, with the Gly in beta2, which acts as a hinge, showing the largest chemical shift change. It appears that SmpB undergoes structural changes indicating an induced fit upon binding to the specific region of TLD.

Scaffolding as an organizing principle in trans-translation. The roles of small protein B and ribosomal protein S1

A eubacterial ribosome stalled on a defective mRNA can be released through a quality control mechanism referred to as trans-translation, which depends on the coordinating binding actions of transfer-messenger RNA, small protein B, and ribosome protein S1. By means of cryo-electron microscopy, we obtained a map of the complex composed of a stalled ribosome and small protein B, which appears near the decoding center. This result suggests that, when lacking a codon, the A-site on the small subunit is a target for small protein B. To investigate the role of S1 played in trans-translation, we obtained a cryo-electron microscopic map, including a stalled ribosome, transfer-messenger RNA, and small protein Bs but in the absence of S1. In this complex, several connections between the 30 S subunit and transfer-messenger RNA that appear in the +S1 complex are no longer found. We propose the unifying concept of scaffolding for the roles of small protein B and S1 in binding of transfer-messenger RNA to the ribosome during trans-translation, and we infer a pathway of sequential binding events in the initial phase of trans-translation.

Comparative 3-D modeling of tmRNA

BACKGROUND

Trans-translation releases stalled ribosomes from truncated mRNAs and tags defective proteins for proteolytic degradation using transfer-messenger RNA (tmRNA). This small stable RNA represents a hybrid of tRNA- and mRNA-like domains connected by a variable number of pseudoknots. Comparative sequence analysis of tmRNAs found in bacteria, plastids, and mitochondria provides considerable insights into their secondary structures. Progress toward understanding the molecular mechanism of template switching, which constitutes an essential step in trans-translation, is hampered by our limited knowledge about the three-dimensional folding of tmRNA.

RESULTS

To facilitate experimental testing of the molecular intricacies of trans-translation, which often require appropriately modified tmRNA derivatives, we developed a procedure for building three-dimensional models of tmRNA. Using comparative sequence analysis, phylogenetically-supported 2-D structures were obtained to serve as input for the program ERNA-3D. Motifs containing loops and turns were extracted from the known structures of other RNAs and used to improve the tmRNA models. Biologically feasible 3-D models for the entire tmRNA molecule could be obtained. The models were characterized by a functionally significant close proximity between the tRNA-like domain and the resume codon. Potential conformational changes which might lead to a more open structure of tmRNA upon binding to the ribosome are discussed. The method, described in detail for the tmRNAs of Escherichia coli, Bacillus anthracis, and Caulobacter crescentus, is applicable to every tmRNA.

CONCLUSION

Improved molecular models of biological significance were obtained. These models will guide in the design of experiments and provide a better understanding of trans-translation. The comparative procedure described here for tmRNA is easily adopted for the modeling the members of other RNA families.

tmRNA and associated ligands: a puzzling relationship

Translation is an efficient and accurate mechanism, needing thorough systems of control-quality to ensure the correspondence between the information carried by the messenger RNA (mRNA) and the newly synthesized protein. Among them, trans-translation ensures delivering of stalled ribosomes when translation occurs on truncated mRNAs in bacteria, followed by the degradation of the incomplete nascent proteins. This process requires transfer-messenger RNA (tmRNA), an original molecule acting as both a tRNA and an mRNA. tmRNA first enters the decoding site of stuck ribosomes and, despite the lack of any codon-anticodon interaction, acts as a tRNA by transferring its alanine to the incomplete protein. Translation then switches to a small internal coding sequence (mRNA domain), which encodes a tag directing the incomplete protein towards degradation. Although playing a central role during trans-translation, tmRNA function depends on associated proteins. Genetic, biochemical and recent structural data are starting to unravel how the process takes place, by involving three main protein partners. Small protein B (SmpB) interacts with the tRNA-like domain (TLD) of tmRNA and is indispensable and specific to the process. Elongation factor Tu (EF-Tu) binds simultaneously the TLD and brings aminoacylated tmRNA to the ribosome, as for canonical tRNAs. Ribosomal protein S1 forms complexes with tmRNA, facilitating its recruitment by the stalled ribosomes. The chronology of events, however, is poorly understood and recent data shed light on the functions attributed to the proteins involved in trans-translation. This review focuses on the puzzling relationship that tmRNA has with these three protein ligands, putting forward trans-translation as a highly dynamical process.

Variation of the acceptor-anticodon interstem angles among mitochondrial and non-mitochondrial tRNAs

A cloverleaf secondary structure and the concomitant "L"-shaped tertiary conformation are considered the paradigm for tRNA structure, since there appears to be very little deviation from either secondary or tertiary structural forms among the more than one dozen canonical (cloverleaf) tRNAs that have yielded to crystallographic structure determination. However, many metazoan mitochondrial tRNAs deviate markedly from the canonical secondary structure, and are often highly truncated (i.e. missing either a dihydrouridine or a TPsiC arm). These departures from the secondary cloverleaf form call into question the universality of the tertiary (L-shaped) conformation, suggesting that other structural constraints may be at play for the truncated tRNAs. To examine this issue, a set of 11 tRNAs, comprising mitochondrial and non-mitochondrial, and canonical and non-canonical species, has been examined in solution using the method of transient electric birefringence. Apparent interstem angles have been determined for each member of the set, represented as transcripts in which the anticodon and acceptor stems have each been extended by approximately 70 bp of duplex RNA helix. The measurements demonstrate much more variation in global structure than had been supposed on the basis of crystallographic analysis of canonical tRNAs. In particular, the apparent acceptor-anticodon interstem angles are more obtuse for the metazoan mitochondrial tRNAs that are truncated (missing either a dihydrouridine or a TPsiC arm) than for the canonical (cloverleaf) tRNAs. Furthermore, the magnesium dependence of this interstem angle differs for the set of truncated tRNAs compared to the canonical species.

tRNA slippage at the tmRNA resume codon

The bacterial ribosome does not initiate translation on the mRNA portion of tmRNA; instead translation that had begun on a separate mRNA molecule resumes at a particular triplet on tmRNA (the resume codon). For at least two tRNAs that could pair with both the resume and -2 triplets on mutant tmRNAs, UAA (stop) as the second codon induced high-frequency -2 slippage on the resume codon in the P site. The frameshift product was not detected when the -2 base was altered. Deficiency for ribosomal L9 protein, which affects other cases of frameshifting, had no significant effect. A special feature of this frameshifting is its dependence on a particular context, that of the tmRNA resume codon; it failed on the same sequence in a regular mRNA, and, more strikingly, at the second tmRNA codon. This focuses attention on the peculiar features expected of the slippage-prone state, such as unusual E-site filling, that might make the P-site resume codon:anticodon interaction especially unstable.

KEYWORDS

tmRNA; ribosome; frameshift; E site; translation

Dial tm for rescue: tmRNA engages ribosomes stalled on defective mRNAs

Ribosomes translate genetic information encoded by mRNAs into protein chains with high fidelity. Truncated mRNAs lacking a stop codon will cause the synthesis of incomplete peptide chains and stall translating ribosomes. In bacteria, a ribonucleoprotein complex composed of tmRNA, a molecule that combines the functions of tRNAs and mRNAs, and small protein B (SmpB) rescues stalled ribosomes. The SmpB-tmRNA complex binds to the stalled ribosome, allowing translation to resume at a short internal tmRNA open reading frame that encodes a protein degradation tag. The aberrant protein is released when the ribosome reaches the stop codon at the end of the tmRNA open reading frame and the fused peptide tag targets it for degradation by cellular proteases. The recently determined NMR structures of SmpB, the crystal structure of the SmpB-tmRNA complex and the cryo-EM structure of the SmpB-tmRNA-EF-Tu-ribosome complex have provided first detailed insights into the intricate mechanisms involved in ribosome rescue.

The tRNA-like domains of E coli and A.aeolicus transfer-messenger RNA: structural and functional studies

Transfer-messenger RNA (tmRNA, 10Sa RNA or ssrA) acts to rescue stalled bacterial ribosomes while encoding a peptide tag added trans-translationally to the nascent peptide, targeting it for proteolysis. The understanding at molecular level of this ubiquitous quality control system in eubacteria requires structural information. Here, we describe the purification and structural analysis of a functional fragment of both Aquifex aeolicus and Escherichia coli tmRNA, recapitulating their tRNA-like domain, which were expressed in vivo from synthetic genes. Both recombinant RNA are correctly processed at both 5' and 3' ends and are produced in quantities suitable for structural analysis by NMR and/or X-ray crystallography. The sequence and solution structure of the tRNA-like domains were analysed by various methods including structural mapping with chemical and enzymatic probes and 2D NMR spectroscopy. The minimalist RNAs contain two post-transcriptional base modifications, 5-methyluridine and pseudouridine, as the full-length tmRNA. Both RNAs fold into three stems, a D-analogue, a T-loop and a GAAA tetra-loop. 2D NMR analysis of the imino proton resonances of both RNAs allowed the assignment of the three stems and of a number of tertiary interactions. It shows the existence of interactions between the TPsiC-loop and the D-analogue, exhibiting a number of similarities and also differences with the canonical tRNA fold, indicating that RNA tertiary interactions can be modulated according to the sequence and secondary structure contexts. Furthermore, the E.coli minimalist RNA is aminoacylatable with alanine with a catalytic efficiency an order of magnitude higher than that for full-length tmRNA.

Crystal structure of the transfer-RNA domain of transfer-messenger RNA in complex with SmpB

Accurate translation of genetic information into protein sequence depends on complete messenger RNA molecules. Truncated mRNAs cause synthesis of defective proteins, and arrest ribosomes at the end of their incomplete message. In bacteria, a hybrid RNA molecule that combines the functions of both transfer and messenger RNAs (called tmRNA) rescues stalled ribosomes, and targets aberrant, partially synthesized, proteins for proteolytic degradation. Here we report the 3.2-A-resolution structure of the tRNA-like domain of tmRNA (tmRNA(Delta)) in complex with small protein B (SmpB), a protein essential for biological functions of tmRNA. We find that the flexible RNA molecule adopts an open L-shaped conformation and SmpB binds to its elbow region, stabilizing the single-stranded D-loop in an extended conformation. The most striking feature of the structure of tmRNA(Delta) is a 90 degrees rotation of the TPsiC-arm around the helical axis. Owing to this unusual conformation, the SmpB-tmRNA(Delta) complex positioned into the A-site of the ribosome orients SmpB towards the small ribosomal subunit, and directs tmRNA towards the elongation-factor binding region of the ribosome. On the basis of this structure, we propose a model for the binding of tmRNA on the ribosome.

Various effects of paromomycin on tmRNA-directed trans-translation

trans-Translation is an unusual translation in which tmRNA plays a dual function as a tRNA and an mRNA to relieve the stalled translation on the ribosome. In this study, we examined the effects of an aminoglycoside antibiotic, paromomycin, on several tmRNA-related events in vitro. The results of a chemical footprinting study indicated that paromomycin molecules bind tmRNA at G332/G333 in the tRNA domain and A316 in the middle of the long helix between tRNA and mRNA domains. Paromomycin bound at G332/G333 inhibited aminoacylation, and the inhibition was suppressed by the addition of SmpB, a tmRNA-binding protein. It was also found that paromomycin causes a shift of the translation resuming point on tmRNA by -1. The effect on initiation shift was canceled by a mutation at the paromomycin-binding site in 16 S rRNA but not by mutations in tmRNA. A high concentration of paromomycin inhibited trans-translation, whereas it enhanced the initiation-shifted trans-translation when SmpB was exogenously added or a mutation was introduced at 333. The effect of paromomycin on trans-translation differs substantially from that on canonical translation, in which it induces miscoding by modulating the A site of the decoding helix of the small subunit RNA of the ribosome.

Visualizing tmRNA entry into a stalled ribosome

Bacterial ribosomes stalled on defective messenger RNAs (mRNAs) are rescued by tmRNA, an approximately 300-nucleotide-long molecule that functions as both transfer RNA (tRNA) and mRNA. Translation then switches from the defective message to a short open reading frame on tmRNA that tags the defective nascent peptide chain for degradation. However, the mechanism by which tmRNA can enter and move through the ribosome is unknown. We present a cryo-electron microscopy study at approximately 13 to 15 angstroms of the entry of tmRNA into the ribosome. The structure reveals how tmRNA could move through the ribosome despite its complicated topology and also suggests roles for proteins S1 and SmpB in the function of tmRNA.

Competition between SsrA tagging and translational termination at weak stop codons in Escherichia coli

SsrA is a tmRNA involved in tagging polypeptides on stalled ribosomes. The resulting fusion proteins are then degraded. We purified endogenous SsrA-tagged proteins by means of a genetically engineered SsrA and identified some of them. Analysis of the proteins suggested that they are tagged at their C-terminal extremities. One of them, ribokinase, is expressed from a messenger with a poorly efficient stop codon, leading to translational recoding events. A change in the ribokinase coding sequence from a weak to a strong translational stop sequence (UGAc to UAAu) annihilated SsrA tagging. Translational termination by UGA recruits the translational release factor (RF) 2. We observed that SsrA tagging of ribokinase was inversely correlated with RF2 activity, revealing a dynamic competition between translational termination and SsrA tagging.

How does tmRNA move through the ribosome?

To test the structure of tmRNA in solution, cross-linking experiments were performed which showed two sets of cross-links in two different domains of tmRNA. Site-directed mutagenesis was used to search for tmRNA nucleotide bases that might form a functional analogue of a codon-anticodon duplex to be recognized by the ribosomal A-site. We demonstrate that nucleotide residues U85 and A86 from tmRNA are significant for tmRNA function and propose that they are involved in formation of a tmRNA element playing a central role in A-site recognition. These data are discussed in the frame of a hypothetical model that suggests a general scheme for the interaction of tmRNA with the ribosome and explains how it moves through the ribosome.

Functional evidence for D- and T-loop interactions in tmRNA

During bacterial protein synthesis, stalled ribosomes can be rescued by tmRNA, a molecule with both tRNA and mRNA features. The tRNA region of tmRNA has sequence similarity with tRNA(Ala) and also has a clover-leaf structure folded similarly as in canonical tRNAs. Here we propose the L-shape of tmRNA to be stabilized by two tertiary interactions between its D- and T-loop on the basis of phylogenetic and experimental evidence. Mutational analysis clearly demonstrates a tertiary interaction between G(13) and U(342). Strikingly, this in evolution conserved interaction is not primarily important for tmRNA alanylation and for binding to elongation factor Tu, but especially for a proper functioning of SmpB.

The tmRNA Website: invasion by an intron

tmRNA (also known as 10Sa RNA or SsrA) plays a central role in an unusual mode of translation, whereby a stalled ribosome switches from a problematic mRNA to a short reading frame within tmRNA during translation of a single polypeptide chain. Research on the mechanism, structure and biology of tmRNA is served by the tmRNA Website, a collection of sequences for tmRNA and the encoded proteolysis-inducing peptide tags, alignments, careful documentation and other information; the URL is http://www.indiana.edu/~tmrna. Four pseudoknots are usually present in each tmRNA, so the database is rich with information on pseudoknot variability. Since last year it has doubled (227 tmRNA sequences as of September 2001), a sequence alignment for the tmRNA cofactor SmpB has been included, and genomic data for Clostridium botulinum has revealed a group I (subgroup IA3) intron interrupting the tmRNA T-loop.

Emerging views on tmRNA-mediated protein tagging and ribosome rescue

Transfer-messenger RNA (tmRNA), also known as SsrA or 10Sa RNA, is a bacterial ribonucleic acid that recycles 70S ribosomes stalled on problematic messenger RNAs (mRNAs) and also contributes to the degradation of incompletely synthesized peptides. tmRNA acts initially as transfer RNA (tRNA), being aminoacylated at its 3'-end by alanyl-tRNA synthetase, to add alanine to the stalled polypeptide chain. Resumption of translation ensues not on the mRNA on which the ribosomes were stalled but at an internal position in tmRNA. Termination soon occurs, tmRNA recruiting the appropriate termination factors allowing the release of the tagged protein that is subsequently recognized and degraded by specific cytoplasmic and periplasmic proteases, and permits ribosome recycling. Recent data suggest that tmRNA tags bacterial proteins in three other instances; when ribosomes stall at internal sites; during 'readthrough' of canonical termination codons; and when ribosomes are at the termination codon of intact messages. The importance of bacterial tmRNAs for survival, growth under stress, and pathogenesis is also discussed. Recent in vivo and in vitro studies have identified novel ligands of tmRNA. Based on the available experimental evidences, an updated model of tmRNA mediated protein tagging and ribosome rescue in bacteria is presented.