A role for the 30S subunit E site in maintenance of the translational reading frame

RNA elements required for the high efficiency of West Nile virus-induced ribosomal frameshifting

West Nile virus (WNV) requires programmed -1 ribosomal frameshifting for translation of the viral genome. The efficiency of WNV frameshifting is among the highest known. However, it remains unclear why WNV exhibits such a high frameshifting efficiency. Here, we employed dual-luciferase reporter assays in multiple human cell lines to probe the RNA requirements for highly efficient frameshifting by the WNV genome. We find that both the sequence and structure of a predicted RNA pseudoknot downstream of the slippery sequence-the codons in the genome on which frameshifting occurs-are required for efficient frameshifting. We also show that multiple proposed RNA secondary structures downstream of the slippery sequence are inconsistent with efficient frameshifting. We also find that the base of the pseudoknot structure likely is unfolded prior to frameshifting. Finally, we show that many mutations in the WNV slippery sequence allow efficient frameshifting, but often result in aberrant shifting into other reading frames. Mutations in the slippery sequence also support a model in which frameshifting occurs concurrent with or after ribosome translocation. These results provide a comprehensive analysis of the molecular determinants of WNV-programmed ribosomal frameshifting and provide a foundation for the development of new antiviral strategies targeting viral gene expression.

RNA elements required for the high efficiency of West Nile Virus-induced ribosomal frameshifting

West Nile Virus (WNV), a member of the Flaviviridae family, requires programmed -1 ribosomal frameshifting (PRF) for translation of the viral genome. The efficiency of WNV frameshifting is among the highest observed to date. Despite structural similarities to frameshifting sites in other viruses, it remains unclear why WNV exhibits such a high frameshifting efficiency. Here we employed dual-luciferase reporter assays in multiple human cell lines to probe the RNA requirements for highly efficient frameshifting by the WNV genome. We find that both the sequence and structure of a predicted RNA pseudoknot downstream of the slippery sequence-the codons in the genome on which frameshifting occurs-are required for efficient frameshifting. We also show that multiple proposed RNA secondary structures downstream of the slippery sequence are inconsistent with efficient frameshifting. We mapped the most favorable distance between the slippery site and the pseudoknot essential for optimal frameshifting, and found the base of the pseudoknot structure likely is unfolded prior to frameshifting. Finally, we find that many mutations in the WNV slippery sequence allow efficient frameshifting, but often result in aberrant shifting into other reading frames. Mutations in the slippery sequence also support a model in which frameshifting occurs concurrent with or after translocation of the mRNA and tRNA on the ribosome. These results provide a comprehensive analysis of the molecular determinants of WNV-programmed ribosomal frameshifting and provide a foundation for the development of new antiviral strategies targeting viral gene expression.

Genome-wide screening reveals metabolic regulation of stop-codon readthrough by cyclic AMP

Translational fidelity is critical for microbial fitness, survival and stress responses. Much remains unknown about the genetic and environmental control of translational fidelity and its single-cell heterogeneity. In this study, we used a high-throughput fluorescence-based assay to screen a knock-out library of Escherichia coli and identified over 20 genes critical for stop-codon readthrough. Most of these identified genes were not previously known to affect translational fidelity. Intriguingly, we show that several genes controlling metabolism, including cyaA and crp, enhance stop-codon readthrough. CyaA catalyzes the synthesis of cyclic adenosine monophosphate (cAMP). Combining RNA sequencing, metabolomics and biochemical analyses, we show that deleting cyaA impairs amino acid catabolism and production of ATP, thus repressing the transcription of rRNAs and tRNAs to decrease readthrough. Single-cell analyses further show that cAMP is a major driver of heterogeneity in stop-codon readthrough and rRNA expression. Our results highlight that carbon metabolism is tightly coupled with stop-codon readthrough.

Structural analysis of mitochondrial rRNA gene variants identified in patients with deafness

The last few years have witnessed dramatic advances in our understanding of the structure and function of the mammalian mito-ribosome. At the same time, the first attempts to elucidate the effects of mito-ribosomal fidelity (decoding accuracy) in disease have been made. Hence, the time is right to push an important frontier in our understanding of mitochondrial genetics, that is, the elucidation of the phenotypic effects of mtDNA variants affecting the functioning of the mito-ribosome. Here, we have assessed the structural and functional role of 93 mitochondrial (mt-) rRNA variants thought to be associated with deafness, including those located at non-conserved positions. Our analysis has used the structural description of the human mito-ribosome of the highest quality currently available, together with a new understanding of the phenotypic manifestation of mito-ribosomal-associated variants. Basically, any base change capable of inducing a fidelity phenotype may be considered non-silent. Under this light, out of 92 previously reported mt-rRNA variants thought to be associated with deafness, we found that 49 were potentially non-silent. We also dismissed a large number of reportedly pathogenic mtDNA variants, 41, as polymorphisms. These results drastically update our view on the implication of the primary sequence of mt-rRNA in the etiology of deafness and mitochondrial disease in general. Our data sheds much-needed light on the question of how mt-rRNA variants located at non-conserved positions may lead to mitochondrial disease and, most notably, provide evidence of the effect of haplotype context in the manifestation of some mt-rRNA variants.

Variance in translational fidelity of different bacterial species is affected by pseudouridines in the tRNA anticodon stem-loop

Delicate variances in the translational machinery affect how efficiently different organisms approach protein synthesis. Determining the scale of this effect, however, requires knowledge on the differences of mistranslation levels. Here, we used a dual-luciferase reporter assay cloned into a broad host range plasmid to reveal the translational fidelity profiles of Pseudomonas putida, Pseudomonas aeruginosa and Escherichia coli. We observed that these profiles are surprisingly different, whereas species more prone to translational frameshifting are not necessarily more prone to stop codon readthrough. As tRNA modifications are among the factors that have been implicated to affect translation accuracy, we also show that translational fidelity is context-specifically influenced by pseudouridines in the anticodon stem-loop of tRNA, but the effect is not uniform between species.

Altered tRNA dynamics during translocation on slippery mRNA as determinant of spontaneous ribosome frameshifting

When reading consecutive mRNA codons, ribosomes move by exactly one triplet at a time to synthesize a correct protein. Some mRNA tracks, called slippery sequences, are prone to ribosomal frameshifting, because the same tRNA can read both 0- and –1-frame codon. Using smFRET we show that during EF-G-catalyzed translocation on slippery sequences a fraction of ribosomes spontaneously switches from rapid, accurate translation to a slow, frameshifting-prone translocation mode where the movements of peptidyl- and deacylated tRNA become uncoupled. While deacylated tRNA translocates rapidly, pept-tRNA continues to fluctuate between chimeric and posttranslocation states, which slows down the re-locking of the small ribosomal subunit head domain. After rapid release of deacylated tRNA, pept-tRNA gains unconstrained access to the –1-frame triplet, resulting in slippage followed by recruitment of the –1-frame aa-tRNA into the A site. Our data show how altered choreography of tRNA and ribosome movements reduces the translation fidelity of ribosomes translocating in a slow mode.

Hyper-swivel head domain motions are required for complete mRNA-tRNA translocation and ribosome resetting

Translocation of messenger RNA (mRNA) and transfer RNA (tRNA) substrates through the ribosome during protein synthesis, an exemplar of directional molecular movement in biology, entails a complex interplay of conformational, compositional, and chemical changes. The molecular determinants of early translocation steps have been investigated rigorously. However, the elements enabling the ribosome to complete translocation and reset for subsequent protein synthesis reactions remain poorly understood. Here, we have combined molecular simulations with single-molecule fluorescence resonance energy transfer imaging to gain insights into the rate-limiting events of the translocation mechanism. We find that diffusive motions of the ribosomal small subunit head domain to hyper-swivelled positions, governed by universally conserved rRNA, can maneuver the mRNA and tRNAs to their fully translocated positions. Subsequent engagement of peptidyl-tRNA and disengagement of deacyl-tRNA from mRNA, within their respective small subunit binding sites, facilitate the ribosome resetting mechanism after translocation has occurred to enable protein synthesis to resume.

Plasticity and conditional essentiality of modification enzymes for domain V of Escherichia coli 23S ribosomal RNA

Escherichia coli rRNAs are post-transcriptionally modified at 36 positions but their modification enzymes are dispensable individually for growth, bringing into question their significance. However, a major growth defect was reported for deletion of the RlmE enzyme, which abolished a 2'O methylation near the peptidyl transferase center (PTC) of the 23S rRNA. Additionally, an adjacent 80-nt "critical region" around the PTC had to be modified to yield significant peptidyl transferase activity in vitro. Surprisingly, we discovered that an absence of just two rRNA modification enzymes is conditionally lethal (at 20°C): RlmE and RluC. At a permissive temperature (37°C), this double knockout was shown to abolish four modifications and be defective in ribosome assembly, though not more so than the RlmE single knockout. However, the double knockout exhibited an even lower rate of tripeptide synthesis than did the single knockout, suggesting an even more defective ribosomal translocation. A combination knockout of the five critical-region-modifying enzymes RluC, RlmKL, RlmN, RlmM, and RluE (not RlmE), which synthesize five of the seven critical-region modifications and 14 rRNA and tRNA modifications altogether, was viable (minor growth defect at 37°C, major at 20°C). This was surprising based on prior in vitro studies. This five-knockout combination had minimal effects on ribosome assembly and frameshifting at 37°C, but greater effects on ribosome assembly and in vitro peptidyl transferase activity at cooler temperatures. These results establish the conditional essentiality of bacterial rRNA modification enzymes and also reveal unexpected plasticity of modification of the PTC region in vivo.

Frameshifting at collided ribosomes is modulated by elongation factor eEF3 and by integrated stress response regulators Gcn1 and Gcn20

Ribosome stalls can result in ribosome collisions that elicit quality control responses, one function of which is to prevent ribosome frameshifting, an activity that entails the interaction of the conserved yeast protein Mbf1 with uS3 on colliding ribosomes. However, the full spectrum of factors that mediate frameshifting during ribosome collisions is unknown. To delineate such factors in the yeast Saccharomyces cerevisiae, we used genetic selections for mutants that affect frameshifting from a known ribosome stall site, CGA codon repeats. We show that the general translation elongation factor eEF3 and the integrated stress response (ISR) pathway components Gcn1 and Gcn20 modulate frameshifting in opposing manners. We found a mutant form of eEF3 that specifically suppressed frameshifting, but not translation inhibition by CGA codons. Thus, we infer that frameshifting at collided ribosomes requires eEF3, which facilitates tRNA-mRNA translocation and E-site tRNA release in yeast and other single cell organisms. In contrast, we found that removal of either Gcn1 or Gcn20, which bind collided ribosomes with Mbf1, increased frameshifting. Thus, we conclude that frameshifting is suppressed by Gcn1 and Gcn20, although these effects are not mediated primarily through activation of the ISR. Furthermore, we examined the relationship between eEF3-mediated frameshifting and other quality control mechanisms, finding that Mbf1 requires either Hel2 or Gcn1 to suppress frameshifting with wild-type eEF3. Thus, these results provide evidence of a direct link between translation elongation and frameshifting at collided ribosomes, as well as evidence that frameshifting is constrained by quality control mechanisms that act on collided ribosomes.

Optimization of a fluorescent-mRNA based real-time assay for precise kinetic measurements of ribosomal translocation

Kinetic characterization of ribosomal translocation is important for understanding the mechanism of elongation in protein synthesis. Here we have optimized a popular fluorescent-mRNA based translocation assay conducted in stopped-flow, by calibrating it with the functional tripeptide formation assay in quench-flow. We found that a fluorescently labelled mRNA, ten bases long from position +1 (mRNA+10), is best suited for both assays as it forms tripeptide at a fast rate equivalent to the longer mRNAs, and yet produces a large fluorescence change upon mRNA movement. Next, we compared the commonly used peptidyl tRNA analog, N-acetyl-Phe-tRNA^Phe, with the natural dipeptidyl fMet-Phe-tRNA^Phe in the stopped-flow assay. This analog translocates about two times slower than the natural dipeptidyl tRNA and produces biphasic kinetics. The rates reduce further at lower temperatures and with higher Mg²⁺ concentration, but improve with higher elongation factor G (EF-G) concentration, which increase both rate and amplitude of the fast phase significantly. In summary, we present here an improved real time assay for monitoring mRNA-translocation with the natural- and an N-Ac-analog of dipeptidyl tRNA.

Phenotypic effects of paralogous ribosomal proteins bL31A and bL31B in E. coli

Ribosomes are essential macromolecular complexes conducting protein biosynthesis in all domains of life. Cells can have heterogeneous ribosomes, i.e. ribosomes with various ribosomal RNA and ribosomal protein (r-protein) composition. However, the functional importance of heterogeneous ribosomes has remained elusive. One of the possible sources for ribosome heterogeneity is provided by paralogous r-proteins. In E. coli, ribosomal protein bL31 has two paralogs: bL31A encoded by rpmE and bL31B encoded by ykgM. This study investigates phenotypic effects of these ribosomal protein paralogs using bacterial strains expressing only bL31A or bL31B. We show that bL31A confers higher fitness to E. coli under lower temperatures. In addition, bL31A and bL31B have different effects on translation reading frame maintenance and apparent translation processivity in vivo as demonstrated by dual luciferase assay. In general, this study demonstrates that ribosomal protein paralog composition (bL31A versus bL31B) can affect cell growth and translation outcome.

Global analysis of protein synthesis in Flavobacterium johnsoniae reveals the use of Kozak-like sequences in diverse bacteria

In all cells, initiation of translation is tuned by intrinsic features of the mRNA. Here, we analyze translation in Flavobacterium johnsoniae, a representative of the Bacteroidetes. Members of this phylum naturally lack Shine-Dalgarno (SD) sequences in their mRNA, and yet their ribosomes retain the conserved anti-SD sequence. Translation initiation is tuned by mRNA secondary structure and by the identities of several key nucleotides upstream of the start codon. Positive determinants include adenine at position -3, reminiscent of the Kozak sequence of Eukarya. Comparative analysis of Escherichia coli reveals use of the same Kozak-like sequence to enhance initiation, suggesting an ancient and widespread mechanism. Elimination of contacts between A-3 and the conserved β-hairpin of ribosomal protein uS7 fails to diminish the contribution of A-3 to initiation, suggesting an indirect mode of recognition. Also, we find that, in the Bacteroidetes, the trinucleotide AUG is underrepresented in the vicinity of the start codon, which presumably helps compensate for the absence of SD sequences in these organisms.

Ribosome collisions alter frameshifting at translational reprogramming motifs in bacterial mRNAs

Ribosomes move along mRNAs in 3-nucleotide steps as they interpret codons that specify which amino acid is required at each position in the protein. There are multiple examples of genes with DNA sequences that do not match the produced proteins because ribosomes move to a new reading frame in the message before finishing translation (so-called frameshifting). This report shows that, when ribosomes stall at mRNA regions prone to cause frameshifting events, trailing ribosomes that collide with them can significantly change the outcome and potentially regulate protein production. This work highlights the principle that biological macromolecules do not function in isolation, and it provides an example of how physical interactions between neighboring complexes can be used to augment their performance.

Translational frameshifting involves the repositioning of ribosomes on their messages into decoding frames that differ from those dictated during initiation. Some messenger RNAs (mRNAs) contain motifs that promote deliberate frameshifting to regulate production of the encoded proteins. The mechanisms of frameshifting have been investigated in many systems, and the resulting models generally involve single ribosomes responding to stimulator sequences in their engaged mRNAs. We discovered that the abundance of ribosomes on messages containing the IS3, dnaX, and prfB frameshift motifs significantly influences the levels of frameshifting. We show that this phenomenon results from ribosome collisions that occur during translational stalling, which can alter frameshifting in both the stalled and trailing ribosomes. Bacteria missing ribosomal protein bL9 are known to exhibit a reduction in reading frame maintenance and to have a strong dependence on elongation factor P (EFP). We discovered that ribosomes lacking bL9 become compacted closer together during collisions and that the E-sites of the stalled ribosomes appear to become blocked, which suggests subsequent transpeptidation in transiently stalled ribosomes may become compromised in the absence of bL9. In addition, we determined that bL9 can suppress frameshifting of its host ribosome, likely by regulating E-site dynamics. These findings provide mechanistic insight into the behavior of colliding ribosomes during translation and suggest naturally occurring frameshift elements may be regulated by the abundance of ribosomes relative to an mRNA pool.

Evolutionary compaction and adaptation visualized by the structure of the dormant microsporidian ribosome

Microsporidia are eukaryotic parasites that infect essentially all animal species, including many of agricultural importance^1-3, and are significant opportunistic parasites of humans⁴. They are characterized by having a specialized infection apparatus, an obligate intracellular lifestyle⁵, rudimentary mitochondria and the smallest known eukaryotic genomes^5-7. Extreme genome compaction led to minimal gene sizes affecting even conserved ancient complexes such as the ribosome^8-10. In the present study, the cryo-electron microscopy structure of the ribosome from the microsporidium Vairimorpha necatrix is presented, which illustrates how genome compaction has resulted in the smallest known eukaryotic cytoplasmic ribosome. Selection pressure led to the loss of two ribosomal proteins and removal of essentially all eukaryote-specific ribosomal RNA (rRNA) expansion segments, reducing the rRNA to a functionally conserved core. The structure highlights how one microsporidia-specific and several repurposed existing ribosomal proteins compensate for the extensive rRNA reduction. The microsporidian ribosome is kept in an inactive state by two previously uncharacterized dormancy factors that specifically target the functionally important E-site, P-site and polypeptide exit tunnel. The present study illustrates the distinct effects of evolutionary pressure on RNA and protein-coding genes, provides a mechanism for ribosome inhibition and can serve as a structural basis for the development of inhibitors against microsporidian parasites.

Roles of specific aminoglycoside-ribosome interactions in the inhibition of translation

Aminoglycosides containing a 2-deoxystreptamine core (AGs) represent a large family of antibiotics that target the ribosome. These compounds promote miscoding, inhibit translocation, and inhibit ribosome recycling. AG binding to helix h44 of the small subunit induces rearrangement of A-site nucleotides A1492 and A1493, which promotes a key open-to-closed conformational change of the subunit and thereby increases miscoding. Mechanisms by which AGs inhibit translocation and recycling remain less clear. Structural studies have revealed a secondary AG binding site in H69 of the large subunit, and it has been proposed that interaction at this site is crucial for inhibition of translocation and recycling. Here, we analyze ribosomes with mutations targeting either or both AG binding sites. Assaying translocation, we find that ablation of the h44 site increases the IC₅₀ values for AGs dramatically, while removal of the H69 site increases these values modestly. This suggests that the AG-h44 interaction is primarily responsible for inhibition, with H69 playing a minor role. Assaying recycling, we find that mutation of h44 has no effect on AG inhibition, consistent with a primary role for AG-H69 interaction. Collectively, these findings help clarify the roles of the two AG binding sites in mechanisms of inhibition by these compounds.

Hepatitis C virus: Enslavement of host factors

Hepatitis C virus (HCV) has infected over 170 million people world-wide. This infection causes severe liver damage that can progress to hepatocellular carcinoma leading to death of the infected patients. Development of a cell culture model system for the study of HCV infection in the recent past has helped the researchers world-wide to understand the biology of this virus. Studies over the past decade have revealed the tricks played by the virus to sustain itself, for as long as 40 years, in the host setup without being eliminated by the immune system. Today we understand that the host organelles and different cellular proteins are affected during HCV infection. This cytoplasmic virus has all the cellular organelles at its disposal to successfully replicate, from ribosomes and intracellular membranous structures to the nucleus. It modulates these organelles at both the structural and the functional levels. The vast knowledge about the viral genome and viral proteins has also helped in the development of drugs against the virus. Despite the achieved success rate to cure the infected patients, we struggle to eliminate the cases of recurrence and the non-responders. Such cases might emerge owing to the property of the viral genome to accumulate mutations during its succeeding replication cycles which favours its survival. The current situation calls an urgent need for alternate therapeutic strategies to counter this major problem of human health. © 2017 IUBMB Life, 70(1):41-49, 2018.

Errors during Gene Expression: Single-Cell Heterogeneity, Stress Resistance, and Microbe-Host Interactions

Gene expression has been considered a highly accurate process, and deviation from such fidelity has been shown previously to be detrimental for the cell. More recently, increasing evidence has supported the notion that the accuracy of gene expression is indeed flexibly variable. The levels of errors during gene expression differ from condition to condition and even from cell to cell within genetically identical populations grown under the same conditions. The different levels of errors resulting from inaccurate gene expression are now known to play key roles in regulating microbial stress responses and host interactions. This minireview summarizes the recent development in understanding the level, regulation, and physiological impact of errors during gene expression.

Gene expression has been considered a highly accurate process, and deviation from such fidelity has been shown previously to be detrimental for the cell. More recently, increasing evidence has supported the notion that the accuracy of gene expression is indeed flexibly variable.

Decoding on the ribosome depends on the structure of the mRNA phosphodiester backbone

During translation, the ribosome plays an active role in ensuring that mRNA is decoded accurately and rapidly. Recently, biochemical studies have also implicated certain accessory factors in maintaining decoding accuracy. However, it is currently unclear whether the mRNA itself plays an active role in the process beyond its ability to base pair with the tRNA. Structural studies revealed that the mRNA kinks at the interface of the P and A sites. A magnesium ion appears to stabilize this structure through electrostatic interactions with the phosphodiester backbone of the mRNA. Here we examined the role of the kink structure on decoding using a well-defined in vitro translation system. Disruption of the kink structure through site-specific phosphorothioate modification resulted in an acute hyperaccurate phenotype. We measured rates of peptidyl transfer for near-cognate tRNAs that were severely diminished and in some instances were almost 100-fold slower than unmodified mRNAs. In contrast to peptidyl transfer, the modifications had little effect on GTP hydrolysis by elongation factor thermal unstable (EF-Tu), suggesting that only the proofreading phase of tRNA selection depends critically on the kink structure. Although the modifications appear to have no effect on typical cognate interactions, peptidyl transfer for a tRNA that uses atypical base pairing is compromised. These observations suggest that the kink structure is important for decoding in the absence of Watson-Crick or G-U wobble base pairing at the third position. Our findings provide evidence for a previously unappreciated role for the mRNA backbone in ensuring uniform decoding of the genetic code.

Heterogeneity of Stop Codon Readthrough in Single Bacterial Cells and Implications for Population Fitness

Gene expression noise (heterogeneity) leads to phenotypic diversity among isogenic individual cells. Our current understanding of gene expression noise is mostly limited to transcription, as separating translational noise from transcriptional noise has been challenging. It also remains unclear how translational heterogeneity originates. Using a transcription-normalized reporter system, we discovered that stop codon readthrough is heterogeneous among single cells, and individual cells with higher UGA readthrough grow faster from stationary phase. Our work also revealed that individual cells with lower protein synthesis levels exhibited higher UGA readthrough, which was confirmed with ribosome-targeting antibiotics (e.g., chloramphenicol). Further experiments and mathematical modeling suggest that varied competition between ternary complexes and release factors perturbs the UGA readthrough level. Our results indicate that fluctuations in the concentrations of translational components lead to UGA readthrough heterogeneity among single cells, which enhances phenotypic diversity of the genetically identical population and facilitates its adaptation to changing environments.

TrmD: A Methyl Transferase for tRNA Methylation With m1G37

TrmD is an S-adenosyl methionine (AdoMet)-dependent methyl transferase that synthesizes the methylated m¹G37 in tRNA. TrmD is specific to and essential for bacterial growth, and it is fundamentally distinct from its eukaryotic and archaeal counterpart Trm5. TrmD is unusual by using a topological protein knot to bind AdoMet. Despite its restricted mobility, the TrmD knot has complex dynamics necessary to transmit the signal of AdoMet binding to promote tRNA binding and methyl transfer. Mutations in the TrmD knot block this intramolecular signaling and decrease the synthesis of m¹G37-tRNA, prompting ribosomes to +1-frameshifts and premature termination of protein synthesis. TrmD is unique among AdoMet-dependent methyl transferases in that it requires Mg²⁺ in the catalytic mechanism. This Mg²⁺ dependence is important for regulating Mg²⁺ transport to Salmonella for survival of the pathogen in the host cell. The strict conservation of TrmD among bacterial species suggests that a better characterization of its enzymology and biology will have a broad impact on our understanding of bacterial pathogenesis.

The Intersubunit Bridge B1b of the Bacterial Ribosome Facilitates Initiation of Protein Synthesis and Maintenance of Translational Fidelity

In bacteria, ribosomal subunits are connected via 12 intersubunit bridges involving RNA-RNA, RNA-protein, and protein-protein interactions. The only protein-protein bridge in the ribosome is ribosomal intersubunit bridge 1b (B1b), which is mainly formed by the bacterial protein L31 (bL31) and connects the head domain of 30S subunit and the central protuberance of the 50S subunit. It is known to be the most dynamic intersubunit bridge. Here, we have evaluated the role of bL31 and thereby the bridge B1b in the working cycle of the ribosome. First, bL31-deficient ribosomes are severely compromised in their ability to ensure translational fidelity particularly in reading frame maintenance in vivo. Second, in the absence of bL31, the rate of initiation is significantly reduced both in vivo and in vitro. Third, polysome profile and subunit reassociation assays demonstrate that bL31 is important for stabilizing subunit joining in vivo and in vitro. Together, our results demonstrate that bL31 is important for determining translational fidelity and stabilizing subunit association. We conclude that the only protein-protein intersubunit bridge of the bacterial ribosome facilitates translation initiation and is essential for maintaining the reading frame of mRNA translation.

Ribosomal frameshifting and transcriptional slippage: From genetic steganography and cryptography to adventitious use

Genetic decoding is not ‘frozen’ as was earlier thought, but dynamic. One facet of this is frameshifting that often results in synthesis of a C-terminal region encoded by a new frame. Ribosomal frameshifting is utilized for the synthesis of additional products, for regulatory purposes and for translational ‘correction’ of problem or ‘savior’ indels. Utilization for synthesis of additional products occurs prominently in the decoding of mobile chromosomal element and viral genomes. One class of regulatory frameshifting of stable chromosomal genes governs cellular polyamine levels from yeasts to humans. In many cases of productively utilized frameshifting, the proportion of ribosomes that frameshift at a shift-prone site is enhanced by specific nascent peptide or mRNA context features. Such mRNA signals, which can be 5′ or 3′ of the shift site or both, can act by pairing with ribosomal RNA or as stem loops or pseudoknots even with one component being 4 kb 3′ from the shift site. Transcriptional realignment at slippage-prone sequences also generates productively utilized products encoded trans-frame with respect to the genomic sequence. This too can be enhanced by nucleic acid structure. Together with dynamic codon redefinition, frameshifting is one of the forms of recoding that enriches gene expression.

Intersubunit Bridges of the Bacterial Ribosome

The ribosome is a large two-subunit ribonucleoprotein machine that translates the genetic code in all cells, synthesizing proteins according to the sequence of the mRNA template. During translation, the primary substrates, transfer RNAs, pass through binding sites formed between the two subunits. Multiple interactions between the ribosomal subunits, termed intersubunit bridges, keep the ribosome intact and at the same time govern dynamics that facilitate the various steps of translation such as transfer RNA-mRNA movement. Here, we review the molecular nature of these intersubunit bridges, how they change conformation during translation, and their functional roles in the process.

MMB-GUI: a fast morphing method demonstrates a possible ribosomal tRNA translocation trajectory

Easy-to-use macromolecular viewers, such as UCSF Chimera, are a standard tool in structural biology. They allow rendering and performing geometric operations on large complexes, such as viruses and ribosomes. Dynamical simulation codes enable modeling of conformational changes, but may require considerable time and many CPUs. There is an unmet demand from structural and molecular biologists for software in the middle ground, which would allow visualization combined with quick and interactive modeling of conformational changes, even of large complexes. This motivates MMB-GUI. MMB uses an internal-coordinate, multiscale approach, yielding as much as a 2000-fold speedup over conventional simulation methods. We use Chimera as an interactive graphical interface to control MMB. We show how this can be used for morphing of macromolecules that can be heterogeneous in biopolymer type, sequence, and chain count, accurately recapitulating structural intermediates. We use MMB-GUI to create a possible trajectory of EF-G mediated gate-passing translocation in the ribosome, with all-atom structures. This shows that the GUI makes modeling of large macromolecules accessible to a wide audience. The morph highlights similarities in tRNA conformational changes as tRNA translocates from A to P and from P to E sites and suggests that tRNA flexibility is critical for translocation completion.

Molecular architecture of the ribosome-bound Hepatitis C Virus internal ribosomal entry site RNA

Internal ribosomal entry sites (IRESs) are structured cis-acting RNAs that drive an alternative, cap-independent translation initiation pathway. They are used by many viruses to hijack the translational machinery of the host cell. IRESs facilitate translation initiation by recruiting and actively manipulating the eukaryotic ribosome using only a subset of canonical initiation factor and IRES transacting factors. Here we present cryo-EM reconstructions of the ribosome 80S- and 40S-bound Hepatitis C Virus (HCV) IRES. The presence of four subpopulations for the 80S•HCV IRES complex reveals dynamic conformational modes of the complex. At a global resolution of 3.9 Å for the most stable complex, a derived atomic model reveals a complex fold of the IRES RNA and molecular details of its interaction with the ribosome. The comparison of obtained structures explains how a modular architecture facilitates mRNA loading and tRNA binding to the P-site. This information provides the structural foundation for understanding the mechanism of HCV IRES RNA-driven translation initiation.

The integrity of the G2421-C2395 base pair in the ribosomal E-site is crucial for protein synthesis

During the elongation cycle of protein biosynthesis, tRNAs traverse through the ribosome by consecutive binding to the 3 ribosomal binding sites (A-, P-, and E- sites). While the ribosomal A- and P-sites have been functionally well characterized in the past, the contribution of the E-site to protein biosynthesis is still poorly understood in molecular terms. Previous studies suggested an important functional interaction of the terminal residue A76 of E-tRNA with the nucleobase of the universally conserved 23S rRNA residue C2394. Using an atomic mutagenesis approach to introduce non-natural nucleoside analogs into the 23S rRNA, we could show that removal of the nucleobase or the ribose 2'-OH at C2394 had no effect on protein synthesis. On the other hand, our data disclose the importance of the highly conserved E-site base pair G2421-C2395 for effective translation. Ribosomes with a disrupted G2421-C2395 base pair are defective in tRNA binding to the E-site. This results in an impaired translation of genuine mRNAs, while homo-polymeric templates are not affected. Cumulatively our data emphasize the importance of E-site tRNA occupancy and in particular the intactness of the 23S rRNA base pair G2421-C2395 for productive protein biosynthesis.

A dynamic RNA loop in an IRES affects multiple steps of elongation factor-mediated translation initiation

Internal ribosome entry sites (IRESs) are powerful model systems to understand how the translation machinery can be manipulated by structured RNAs and for exploring inherent features of ribosome function. The intergenic region (IGR) IRESs from the Dicistroviridae family of viruses are structured RNAs that bind directly to the ribosome and initiate translation by co-opting the translation elongation cycle. These IRESs require an RNA pseudoknot that mimics a codon-anticodon interaction and contains a conformationally dynamic loop. We explored the role of this loop and found that both the length and sequence are essential for translation in different types of IGR IRESs and from diverse viruses. We found that loop 3 affects two discrete elongation factor-dependent steps in the IRES initiation mechanism. Our results show how the IRES directs multiple steps after 80S ribosome placement and highlights the often underappreciated significance of discrete conformationally dynamic elements within the context of structured RNAs.

DOI:http://dx.doi.org/10.7554/eLife.08146.001

Many viruses store their genetic information in the form of strands of ribonucleic acid (RNA), which contain building blocks called nucleotides. Once inside an infected cell, the virus hijacks the cellular structures that build proteins (called ribosomes), which forces the cell to start making viral proteins.

Many RNA viruses manipulate the cell’s ribosomes using RNA elements called Internal Ribosome Entry Sites, or IRESs. In a family of viruses called Dicistroviridae, which infect a number of insects, a section of the IRES RNA binds directly to the ribosome. Proteins called elongation factors then trigger a series of events that lead to the cell starting to make the viral proteins.

By mutating the RNA of many different Dicistroviridae viruses that infect a variety of invertebrates, Ruehle et al. have now investigated how a particular loop in the structure of the IRES helps to make cells build the viral proteins. This loop is flexible, and interacts with the ribosome to enable the IRES to move through the ribosome. Mutations that shorten the loop or alter the sequence of nucleotides in the loop prevent the occurrence of two of the steps that need to occur for the cell to make viral proteins. Both of these steps depend on elongation factors. Determining how the entire IRES might change shape as it moves through the ribosome is an important next step, since the ribosome is exquisitely sensitive to the shape and motions of its binding partners.

DOI:http://dx.doi.org/10.7554/eLife.08146.002

The β-hairpin of 40S exit channel protein Rps5/uS7 promotes efficient and accurate translation initiation in vivo

The eukaryotic 43S pre-initiation complex bearing tRNAi(Met) scans the mRNA leader for an AUG start codon in favorable context. Structural analyses revealed that the β-hairpin of 40S protein Rps5/uS7 protrudes into the 40S mRNA exit-channel, contacting the eIF2∙GTP∙Met-tRNAi ternary complex (TC) and mRNA context nucleotides; but its importance in AUG selection was unknown. We identified substitutions in β-strand-1 and C-terminal residues of yeast Rps5 that reduced bulk initiation, conferred 'leaky-scanning' of AUGs; and lowered initiation fidelity by exacerbating the effect of poor context of the eIF1 AUG codon to reduce eIF1 abundance. Consistently, the β-strand-1 substitution greatly destabilized the 'PIN' conformation of TC binding to reconstituted 43S·mRNA complexes in vitro. Other substitutions in β-hairpin loop residues increased initiation fidelity and destabilized PIN at UUG, but not AUG start codons. We conclude that the Rps5 β-hairpin is as crucial as soluble initiation factors for efficient and accurate start codon recognition.

The UGG Isoacceptor of tRNAPro Is Naturally Prone to Frameshifts

Native tRNAs often contain post-transcriptional modifications to the wobble position to expand the capacity of reading the genetic code. Some of these modifications, due to the ability to confer imperfect codon-anticodon pairing at the wobble position, can induce a high propensity for tRNA to shift into alternative reading frames. An example is the native UGG isoacceptor of E. coli tRNA^Pro whose wobble nucleotide U34 is post-transcriptionally modified to cmo⁵U34 to read all four proline codons (5ʹ-CCA, 5ʹ-CCC, 5ʹ-CCG, and 5ʹ-CCU). Because the pairing of the modified anticodon to CCC codon is particularly weak relative to CCA and CCG codons, this tRNA can readily shift into both the +1 and +2-frame on the slippery mRNA sequence CCC-CG. We show that the shift to the +2-frame is more dominant, driven by the higher stability of the codon-anticodon pairing at the wobble position. Kinetic analysis suggests that both types of shifts can occur during stalling of the tRNA in a post-translocation complex or during translocation from the A to the P-site. Importantly, while the +1-frame post complex is active for peptidyl transfer, the +2-frame complex is a poor peptidyl donor. Together with our recent work, we draw a mechanistic distinction between +1 and +2-frameshifts, showing that while the +1-shifts are suppressed by the additional post-transcriptionally modified m¹G37 nucleotide in the anticodon loop, the +2-shifts are suppressed by the ribosome, supporting a role of the ribosome in the overall quality control of reading-frame maintenance.

Maintenance of protein synthesis reading frame by EF-P and m(1)G37-tRNA

Maintaining the translational reading frame poses difficulty for the ribosome. Slippery mRNA sequences such as CC[C/U]-[C/U], read by isoacceptors of tRNA(Pro), are highly prone to +1 frameshift (+1FS) errors. Here we show that +1FS errors occur by two mechanisms, a slow mechanism when tRNA(Pro) is stalled in the P-site next to an empty A-site and a fast mechanism during translocation of tRNA(Pro) into the P-site. Suppression of +1FS errors requires the m(1)G37 methylation of tRNA(Pro) on the 3' side of the anticodon and the translation factor EF-P. Importantly, both m(1)G37 and EF-P show the strongest suppression effect when CC[C/U]-[C/U] are placed at the second codon of a reading frame. This work demonstrates that maintaining the reading frame immediately after the initiation of translation by the ribosome is an essential aspect of protein synthesis.

The highly conserved codon following the slippery sequence supports -1 frameshift efficiency at the HIV-1 frameshift site

HIV-1 utilises -1 programmed ribosomal frameshifting to translate structural and enzymatic domains in a defined proportion required for replication. A slippery sequence, U UUU UUA, and a stem-loop are well-defined RNA features modulating -1 frameshifting in HIV-1. The GGG glycine codon immediately following the slippery sequence (the 'intercodon') contributes structurally to the start of the stem-loop but has no defined role in current models of the frameshift mechanism, as slippage is inferred to occur before the intercodon has reached the ribosomal decoding site. This GGG codon is highly conserved in natural isolates of HIV. When the natural intercodon was replaced with a stop codon two different decoding molecules-eRF1 protein or a cognate suppressor tRNA-were able to access and decode the intercodon prior to -1 frameshifting. This implies significant slippage occurs when the intercodon is in the (perhaps distorted) ribosomal A site. We accommodate the influence of the intercodon in a model of frame maintenance versus frameshifting in HIV-1.

Programmed -1 frameshifting by kinetic partitioning during impeded translocation

Programmed –1 ribosomal frameshifting (−1PRF) is an mRNA recoding event utilized by cells to enhance the information content of the genome and to regulate gene expression. The mechanism of –1PRF and its timing during translation elongation are unclear. Here, we identified the steps that govern –1PRF by following the stepwise movement of the ribosome through the frameshifting site of a model mRNA derived from the IBV 1a/1b gene in a reconstituted in vitro translation system from Escherichia coli. Frameshifting occurs at a late stage of translocation when the two tRNAs are bound to adjacent slippery sequence codons of the mRNA. The downstream pseudoknot in the mRNA impairs the closing movement of the 30S subunit head, the dissociation of EF-G, and the release of tRNA from the ribosome. The slippage of the ribosome into the –1 frame accelerates the completion of translocation, thereby further favoring translation in the new reading frame.

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Kinetics of –1 ribosomal frameshifting are monitored at single-codon resolution

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Frameshifting occurs when the backward movement of the 30S subunit head is impeded

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Two tRNAs at the XXXYYYZ mRNA sequence are stalled in chimeric POST states

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The shift to the –1 reading frame occurs prior to EF-G release from the ribosome

Crystal structure of 70S ribosome with both cognate tRNAs in the E and P sites representing an authentic elongation complex

During the translation cycle, a cognate deacylated tRNA can only move together with the codon into the E site. We here present the first structure of a cognate tRNA bound to the ribosomal E site resulting from translocation by EF-G, in which an entire L1 stalk (L1 protein and L1 rRNA) interacts with E-site tRNA (E-tRNA), representing an authentic ribosome elongation complex. Our results revealed that the Watson-Crick base pairing is formed at the first and second codon-anticodon positions in the E site in the ribosome elongation complex, whereas the codon-anticodon interaction in the third position is indirect. Analysis of the observed conformations of mRNA and E-tRNA suggests that the ribosome intrinsically has the potential to form codon-anticodon interaction in the E site, independently of the mRNA configuration. We also present a detailed description of the biologically relevant position of the entire L1 stalk and its interacting cognate E-tRNA, which provides a better understanding of the structural basis for translation elongation. Furthermore, to gain insight into translocation, we report the positioning of protein L6 contacting EF-G, as well as the conformational change of the C-terminal tail of protein S13 in the decoding center.

HCV IRES manipulates the ribosome to promote the switch from translation initiation to elongation

The hepatitis C virus (HCV) internal ribosome entry site (IRES) drives non-canonical initiation of protein synthesis necessary for viral replication. HCV IRES functional studies have focused on 80S ribosome formation, but have not explored roles after the 80S ribosome is poised at the start codon. Here, we report that mutations of an IRES domain that docks in the 40S subunit’s decoding groove and cause only a local perturbation in IRES structure result in conformational changes in the IRES-rabbit 40S subunit complex. Functionally, we find the mutation decreases IRES activity by inhibiting the first ribosome translocation event, and modeling suggests that this effect is through an interaction with a single ribosomal protein. The HCV IRES’ ability to manipulate the ribosome provides insight into how the ribosome’s structure and function can be altered by bound RNAs, including those derived from cellular invaders.

Ribosomal proteins: structure, function, and evolution

The question concerning reasons for the variety of ribosomal proteins that arose for more than 40 years ago is still open. Ribosomes of modern organisms contain 50-80 individual proteins. Some are characteristic for all domains of life (universal ribosomal proteins), whereas others are specific for bacteria, archaea, or eucaryotes. Extensive information about ribosomal proteins has been obtained since that time. However, the role of the majority of ribosomal proteins in the formation and functioning of the ribosome is still not so clear. Based on recent data of experiments and bioinformatics, this review presents a comprehensive evaluation of structural conservatism of ribosomal proteins from evolutionarily distant organisms. Considering the current knowledge about features of the structural organization of the universal proteins and their intermolecular contacts, a possible role of individual proteins and their structural elements in the formation and functioning of ribosomes is discussed. The structural and functional conservatism of the majority of proteins of this group suggests that they should be present in the ribosome already in the early stages of its evolution.

Contribution of intersubunit bridges to the energy barrier of ribosomal translocation

In every round of translation elongation, EF-G catalyzes translocation, the movement of tRNAs (and paired codons) to their adjacent binding sites in the ribosome. Previous kinetic studies have shown that the rate of tRNA–mRNA movement is limited by a conformational change in the ribosome termed ‘unlocking’. Although structural studies offer some clues as to what unlocking might entail, the molecular basis of this conformational change remains an open question. In this study, the contribution of intersubunit bridges to the energy barrier of translocation was systematically investigated. Unlike those targeting B2a and B3, mutations that disrupt bridges B1a, B4, B7a and B8 increased the maximal rate of both forward (EF-G dependent) and reverse (spontaneous) translocation. As bridge B1a is predicted to constrain 30S head movement and B4, B7a and B8 are predicted to constrain intersubunit rotation, these data provide evidence that formation of the unlocked (transition) state involves both 30S head movement and intersubunit rotation.

Tricks an IRES uses to enslave ribosomes

In eukaryotes, mRNAs are primarily translated through a cap-dependent mechanism whereby initiation factors recruit the 40S ribosomal subunit to a cap structure at the 5' end of the mRNA. However, some viral and cellular messages initiate protein synthesis without a cap. They use a structured RNA element termed an internal ribosome entry site (IRES) to recruit the 40S ribosomal subunit. IRESs were discovered over 20 years ago, but only recently have studies using a model IRES from dicistroviruses expanded our understanding of how a 3D RNA structure can capture and manipulate the ribosome to initiate translation.

Ribosomal history reveals origins of modern protein synthesis

The origin and evolution of the ribosome is central to our understanding of the cellular world. Most hypotheses posit that the ribosome originated in the peptidyl transferase center of the large ribosomal subunit. However, these proposals do not link protein synthesis to RNA recognition and do not use a phylogenetic comparative framework to study ribosomal evolution. Here we infer evolution of the structural components of the ribosome. Phylogenetic methods widely used in morphometrics are applied directly to RNA structures of thousands of molecules and to a census of protein structures in hundreds of genomes. We find that components of the small subunit involved in ribosomal processivity evolved earlier than the catalytic peptidyl transferase center responsible for protein synthesis. Remarkably, subunit RNA and proteins coevolved, starting with interactions between the oldest proteins (S12 and S17) and the oldest substructure (the ribosomal ratchet) in the small subunit and ending with the rise of a modern multi-subunit ribosome. Ancestral ribonucleoprotein components show similarities to in vitro evolved RNA replicase ribozymes and protein structures in extant replication machinery. Our study therefore provides important clues about the chicken-or-egg dilemma associated with the central dogma of molecular biology by showing that ribosomal history is driven by the gradual structural accretion of protein and RNA structures. Most importantly, results suggest that functionally important and conserved regions of the ribosome were recruited and could be relics of an ancient ribonucleoprotein world.

Further in vitro exploration fails to support the allosteric three-site model

Ongoing debate in the ribosome field has focused on the role of bound E-site tRNA and the Shine-Dalgarno-anti-Shine-Dalgarno (SD-aSD) interaction on A-site tRNA interactions and the fidelity of tRNA selection. Here we use an in vitro reconstituted Escherichia coli translation system to explore the reported effects of E-site-bound tRNA and SD-aSD interactions on tRNA selection events and find no evidence for allosteric coupling. A large set of experiments exploring the role of the E-site tRNA in miscoding failed to recapitulate the observations of earlier studies (Di Giacco, V., Márquez, V., Qin, Y., Pech, M., Triana-Alonso, F. J., Wilson, D. N., and Nierhaus, K. H. (2008) Proc. Natl. Acad. Sci. U.S.A. 105, 10715-10720 and Geigenmüller, U., and Nierhaus, K. H. (1990) EMBO J. 9, 4527-4533); the frequency of miscoding was unaffected by the presence of E-site-bound cognate tRNA. Moreover, our data provide clear evidence that the reported effects of the SD-aSD interaction on fidelity can be attributed to the binding of ribosomes to an unanticipated site on the mRNA (in the absence of the SD sequence) that provides a cognate pairing codon leading naturally to incorporation of the purported "noncognate" amino acid.

Circular code motifs in transfer and 16S ribosomal RNAs: a possible translation code in genes

In 1996, a common trinucleotide circular code, called X, is identified in genes of eukaryotes and prokaryotes (Arquès and Michel, 1996). This circular code X is a set of 20 trinucleotides allowing the reading frames in genes to be retrieved locally, i.e. anywhere in genes and in particular without start codons. This reading frame retrieval needs a window length l of 12 nucleotides (l ≥ 12). With a window length strictly less than 12 nucleotides (l < 12), some words of X, called ambiguous words, are found in the shifted frames (the reading frame shifted by one or two nucleotides) preventing the reading frame in genes to be retrieved. Since 1996, these ambiguous words of X were never studied. In the first part of this paper, we identify all the ambiguous words of the common trinucleotide circular code X. With a length l varying from 1 to 11 nucleotides, the type and the occurrence number (multiplicity) of ambiguous words of X are given in each shifted frame. Maximal ambiguous words of X, words which are not factors of another ambiguous words, are also determined. Two probability definitions based on these results show that the common trinucleotide circular code X retrieves the reading frame in genes with a probability of about 90% with a window length of 6 nucleotides, and a probability of 99.9% with a window length of 9 nucleotides (100% with a window length of 12 nucleotides, by definition of a circular code). In the second part of this paper, we identify X circular code motifs (shortly X motifs) in transfer RNA and 16S ribosomal RNA: a tRNA X motif of 26 nucleotides including the anticodon stem-loop and seven 16S rRNA X motifs of length greater or equal to 15 nucleotides. Window lengths of reading frame retrieval with each trinucleotide of these X motifs are also determined. Thanks to the crystal structure 3I8G (Jenner et al., 2010), a 3D visualization of X motifs in the ribosome shows several spatial configurations involving mRNA X motifs, A-tRNA and E-tRNA X motifs, and four 16S rRNA X motifs. Another identified 16S rRNA X motif is involved in the decoding center which recognizes the codon-anticodon helix in A-tRNA. From a code theory point of view, these identified X circular code motifs and their mathematical properties may constitute a translation code involved in retrieval, maintenance and synchronization of reading frames in genes.

Allosteric vs. spontaneous exit-site (E-site) tRNA dissociation early in protein synthesis

During protein synthesis, deacylated transfer RNAs leave the ribosome via an exit (E) site after mRNA translocation. How the ribosome regulates tRNA dissociation and whether functional linkages between the aminoacyl (A) and E sites modulate the dynamics of protein synthesis have long been debated. Using single molecule fluorescence resonance energy transfer experiments, we find that, during early cycles of protein elongation, tRNAs are often held in the E site until being allosterically released when the next aminoacyl tRNA binds to the A site. This process is regulated by the length and sequence of the nascent peptide and by the conformational state, detected by tRNA proximity, prior to translocation. In later cycles, E-site tRNA dissociates spontaneously. Our results suggest that the distribution of pretranslocation tRNA states and posttranslocation pathways are correlated within each elongation cycle via communication between distant subdomains in the ribosome, but that this correlation between elongation cycle intermediates does not persist into succeeding cycles.

Ribosome-dependent ATPase interacts with conserved membrane protein in Escherichia coli to modulate protein synthesis and oxidative phosphorylation

Elongation factor RbbA is required for ATP-dependent deacyl-tRNA release presumably after each peptide bond formation; however, there is no information about the cellular role. Proteomic analysis in Escherichia coli revealed that RbbA reciprocally co-purified with a conserved inner membrane protein of unknown function, YhjD. Both proteins are also physically associated with the 30S ribosome and with members of the lipopolysaccharide transport machinery. Genome-wide genetic screens of rbbA and yhjD deletion mutants revealed aggravating genetic interactions with mutants deficient in the electron transport chain. Cells lacking both rbbA and yhjD exhibited reduced cell division, respiration and global protein synthesis as well as increased sensitivity to antibiotics targeting the ETC and the accuracy of protein synthesis. Our results suggest that RbbA appears to function together with YhjD as part of a regulatory network that impacts bacterial oxidative phosphorylation and translation efficiency.

Cellular mechanisms that control mistranslation

Mistranslation broadly encompasses the introduction of errors during any step of protein synthesis, leading to the incorporation of an amino acid that is different from the one encoded by the gene. Recent research has vastly enhanced our understanding of the mechanisms that control mistranslation at the molecular level and has led to the discovery that the rates of mistranslation in vivo are not fixed but instead are variable. In this Review we describe the different steps in translation quality control and their variations under different growth conditions and between species though a comparison of in vitro and in vivo findings. This provides new insights as to why mistranslation can have both positive and negative effects on growth and viability.

Deletion of the RluD pseudouridine synthase promotes SsrA peptide tagging of ribosomal protein S7

RluD catalyses formation of three pseudouridine residues within helix 69 of the 50S ribosome subunit. Helix 69 makes important contacts with the decoding centre on the 30S subunit and deletion of rluD was recently shown to interfere with translation termination in Escherichia coli. Here, we show that deletion of rluD increases tmRNA activity on ribosomes undergoing release factor 2 (RF2)-mediated termination at UGA stop codons. Strikingly, tmRNA-mediated SsrA peptide tagging of two proteins, ribosomal protein S7 and LacI, was dramatically increased in ΔrluD cells. S7 tagging was due to a unique C-terminal peptide extension found in E. coli K-12 strains. Introduction of the rpsG gene (encoding S7) from an E. coli B strain abrogated S7 tagging in the ΔrluD background, and partially complemented the mutant's slow-growth phenotype. Additionally, exchange of the K-12 prfB gene (encoding RF2) with the B strain allele greatly reduced tagging in ΔrluD cells. In contrast to E. coli K-12 cells, deletion of rluD in an E. coli B strain resulted in no growth phenotype. These findings indicate that the originally observed rluD phenotypes result from synthetic interactions with rpsG and prfB alleles found within E. coli K-12 strains.

Mapping the ribosomal protein S7 regulatory binding site on mRNA of the E. coli streptomycin operon

In this work it is shown by deletion analysis that an intercistronic region (ICR) approximately 80 nucleotides in length is necessary for interaction with recombinant E. coli S7 protein (r6hEcoS7). A model is proposed for the interaction of S7 with two ICR sites-region of hairpin bifurcations and Shine-Dalgarno sequence of cistron S7. A de novo RNA binding site for heterologous S7 protein of Thermus thermophilus (r6hTthS7) was constructed by selection of a combinatorial RNA library based on E. coli ICR: it has only a single supposed protein recognition site in the region of bifurcation. The SERW technique was used for selection of two intercistronic RNA libraries in which five nucleotides of a double-stranded region, adjacent to the bifurcation, had the randomized sequence. One library contained an authentic AG (-82/-20) pair, while in the other this pair was replaced by AU. A serwamer capable of specific binding to r6hTthS7 was selected; it appeared to be the RNA68 mutant with eight nucleotide mutations. The serwamer binds to r6hTthS7 with the same affinity as homologous authentic ICR of str mRNA binds to r6hEcoS7; apparent dissociation constants are 89 +/- 43 and 50 +/- 24 nM, respectively.

Short spacing between the Shine-Dalgarno sequence and P codon destabilizes codon-anticodon pairing in the P site to promote +1 programmed frameshifting

Programmed frameshifting in the RF2 gene (prfB) involves an intragenic Shine-Dalgarno (SD) sequence. To investigate the role of SD-ASD pairing in the mechanism of frameshifting, we have analysed the effect of spacing between the SD sequence and P codon on P-site tRNA binding and RF2-dependent termination. When the spacing between an extended SD sequence and the P codon is decreased from 4 to 1 nucleotide (nt), the dissociation rate (k(off) ) for P-site tRNA increases by > 100-fold. Toeprinting analysis shows that pre-translocation complexes cannot be formed when the spacer sequence is ≤ 2 nt. Instead, the tRNA added secondarily to fill the A site and its corresponding codon move spontaneously into the P site, resulting in a complex with a 3 nt longer spacer between the SD-ASD helix and the P codon. While close proximity of the SD clearly destabilizes P-site tRNA, RF2-dependent termination and EF-Tu-dependent decoding are largely unaffected in analogous complexes. These data support a model in which formation of the SD-ASD helix in ribosomes stalled at the in-frame UGA codon of prfB generates tension on the mRNA that destabilizes codon-anticodon pairing in the P site and promotes slippage of the mRNA in the 5' direction.

A local role for the small ribosomal subunit primary binder rpS5 in final 18S rRNA processing in yeast

In vivo depletion of the yeast small ribosomal subunit (SSU) protein S5 (rpS5) leads to nuclear degradation of nascent SSUs and to a perturbed global assembly state of the SSU head domain. Here, we report that rpS5 plays an additional local role at the head/platform interface in efficient SSU maturation. We find that yeast small ribosomal subunits which incorporated an rpS5 variant lacking the seven C-terminal amino acids have a largely assembled head domain and are exported to the cytoplasm. On the other hand, 3' processing of 18S rRNA precursors is inhibited in these ribosomal particles, although they associate with the putative endonuclease Nob1p and other late acting 40S biogenesis factors. We suggest that the SSU head component rpS5 and platform components as rpS14 are crucial constituents of a highly defined spatial arrangement in the head-platform interface of nascent SSUs, which is required for efficient processing of the therein predicted SSU rRNA 3' end. Positioning of rpS5 in nascent SSUs, including its relative orientation towards platform components in the head-platform cleft, will depend on the general assembly and folding state of the head domain. Therefore, the suggested model can explain 18S precursor rRNA 3' processing phenotypes observed in many eukaryotic SSU head assembly mutants.

Functional interactions by transfer RNAs in the ribosome

Recent X-ray crystal structures of the ribosome have revolutionized the field by providing a much-needed structural framework to understand ribosome function. Indeed, the crystal structures rationalize much of the genetic and biochemical data that have been meticulously gathered over 50 years. Here, we focus on the interactions between tRNAs and the ribosome and describe some of the insights that the structures provide about the mechanism of translation. Both high-resolution structures and functional studies are essential for fully appreciating the complex process of protein synthesis.