Translation initiation determines both the quantity and identity of the protein that is encoded in an mRNA by establishing the reading frame for protein synthesis. In eukaryotic cells, numerous translation initiation factors prepare ribosomes for polypeptide synthesis; however, the underlying dynamics of this process remain unclear1,2. A central question is how eukaryotic ribosomes transition from translation initiation to elongation. Here we use in vitro single-molecule fluorescence microscopy approaches in a purified yeast Saccharomyces cerevisiae translation system to monitor directly, in real time, the pathways of late translation initiation and the transition to elongation. This transition was slower in our eukaryotic system than that reported for Escherichia coli3-5. The slow entry to elongation was defined by a long residence time of eukaryotic initiation factor 5B (eIF5B) on the 80S ribosome after the joining of individual ribosomal subunits-a process that is catalysed by this universally conserved initiation factor. Inhibition of the GTPase activity of eIF5B after the joining of ribosomal subunits prevented the dissociation of eIF5B from the 80S complex, thereby preventing elongation. Our findings illustrate how the dissociation of eIF5B serves as a kinetic checkpoint for the transition from initiation to elongation, and how its release may be governed by a change in the conformation of the ribosome complex that triggers GTP hydrolysis.
How biological systems such as proteins achieve robustness to ubiquitous perturbations is a fundamental biological question. Such perturbations include errors that introduce phenotypic mutations into nascent proteins during the translation of mRNA. These errors are remarkably frequent. They are also costly, because they reduce protein stability and help create toxic misfolded proteins. Adaptive evolution might reduce these costs of protein mistranslation by two principal mechanisms. The first increases the accuracy of translation via synonymous “high fidelity” codons at especially sensitive sites. The second increases the robustness of proteins to phenotypic errors via amino acids that increase protein stability. To study how these mechanisms are exploited by populations evolving in the laboratory, we evolved the antibiotic resistance gene TEM-1 in Escherichia coli hosts with either normal or high rates of mistranslation. We analyzed TEM-1 populations that evolved under relaxed and stringent selection for antibiotic resistance by single molecule real-time sequencing. Under relaxed selection, mistranslating populations reduce mistranslation costs by reducing TEM-1 expression. Under stringent selection, they efficiently purge destabilizing amino acid changes. More importantly, they accumulate stabilizing amino acid changes rather than synonymous changes that increase translational accuracy. In the large populations we study, and on short evolutionary timescales, the path of least resistance in TEM-1 evolution consists of reducing the consequences of translation errors rather than the errors themselves.
At what point during translation do proteins fold? It is well established that proteins can fold cotranslationally outside the ribosome exit tunnel, whereas studies of folding inside the exit tunnel have so far detected only the formation of helical secondary structure and collapsed or partially structured folding intermediates. Here, using a combination of cotranslational nascent chain force measurements, inter-subunit fluorescence resonance energy transfer studies on single translating ribosomes, molecular dynamics simulations, and cryoelectron microscopy, we show that a small zinc-finger domain protein can fold deep inside the vestibule of the ribosome exit tunnel. Thus, for small protein domains, the ribosome itself can provide the kind of sheltered folding environment that chaperones provide for larger proteins. Copyright © 2015 The Authors. Published by Elsevier Inc. All rights reserved.
Coupling of mRNA structure rearrangement to ribosome movement during bypassing of non-coding regions.
Nearly half of the ribosomes translating a particular bacteriophage T4 mRNA bypass a region of 50 nt, resuming translation 3′ of this gap. How this large-scale, specific hop occurs and what determines whether a ribosome bypasses remain unclear. We apply single-molecule fluorescence with zero-mode waveguides to track individual Escherichia coli ribosomes during translation of T4’s gene 60 mRNA. Ribosomes that bypass are characterized by a 10- to 20-fold longer pause in a non-canonical rotated state at the take-off codon. During the pause, mRNA secondary structure rearrangements are coupled to ribosome forward movement, facilitated by nascent peptide interactions that disengage the ribosome anticodon-codon interactions for slippage. Close to the landing site, the ribosome then scans mRNA in search of optimal base-pairing interactions. Our results provide a mechanistic and conformational framework for bypassing, highlighting a non-canonical ribosomal state to allow for mRNA structure refolding to drive large-scale ribosome movements. Copyright © 2015 Elsevier Inc. All rights reserved.
Spontaneous changes in the reading frame of translation are rare (frequency of 10(-3) to 10(-4) per codon), but can be induced by specific features in the messenger RNA (mRNA). In the presence of mRNA secondary structures, a heptanucleotide ‘slippery sequence’ usually defined by the motif X XXY YYZ, and (in some prokaryotic cases) mRNA sequences that base pair with the 3′ end of the 16S ribosomal rRNA (internal Shine-Dalgarno sequences), there is an increased probability that a specific programmed change of frame occurs, wherein the ribosome shifts one nucleotide backwards into an overlapping reading frame (-1 frame) and continues by translating a new sequence of amino acids. Despite extensive biochemical and genetic studies, there is no clear mechanistic description for frameshifting. Here we apply single-molecule fluorescence to track the compositional and conformational dynamics of individual ribosomes at each codon during translation of a frameshift-inducing mRNA from the dnaX gene in Escherichia coli. Ribosomes that frameshift into the -1 frame are characterized by a tenfold longer pause in elongation compared to non-frameshifted ribosomes, which translate through unperturbed. During the pause, interactions of the ribosome with the mRNA stimulatory elements uncouple EF-G catalysed translocation from normal ribosomal subunit reverse-rotation, leaving the ribosome in a non-canonical intersubunit rotated state with an exposed codon in the aminoacyl-tRNA site (A site). tRNA(Lys) sampling and accommodation to the empty A site and EF-G action either leads to the slippage of the tRNAs into the -1 frame or maintains the ribosome into the 0 frame. Our results provide a general mechanistic and conformational framework for -1 frameshifting, highlighting multiple kinetic branchpoints during elongation.
SecM is an E. coli secretion monitor capable of stalling translation on the prokaryotic ribosome without cofactors. Biochemical and structural studies have demonstrated that the SecM nascent chain interacts with the 50S subunit exit tunnel to inhibit peptide bond formation. However, the timescales and pathways of stalling on an mRNA remain undefined. To provide a dynamic mechanism for stalling, we directly tracked the dynamics of elongation on ribosomes translating the SecM stall sequence (FSTPVWISQAQGIRAGP) using single-molecule fluorescence techniques. Within 1 min, three peptide-ribosome interactions work cooperatively over the last five codons of the SecM sequence, leading to severely impaired elongation rates beginning from the terminal proline and lasting four codons. Our results suggest that stalling is tightly linked to the dynamics of elongation and underscore the roles that the exit tunnel and nascent chain play in controlling fundamental steps in translation. opyright © 2014 The Authors. Published by Elsevier Inc. All rights reserved.
The traditional view of macrolide antibiotics as plugs inside the ribosomal nascent peptide exit tunnel (NPET) has lately been challenged in favor of a more complex, heterogeneous mechanism, where drug-peptide interactions determine the fate of a translating ribosome. To investigate these highly dynamic processes, we applied single-molecule tracking of elongating ribosomes during inhibition of elongation by erythromycin of several nascent chains, including ErmCL and H-NS, which were shown to be, respectively, sensitive and resistant to erythromycin. Peptide sequence-specific changes were observed in translation elongation dynamics in the presence of a macrolide-obstructed NPET. Elongation rates were not severely inhibited in general by the presence of the drug; instead, stalls or pauses were observed as abrupt events. The dynamic pathways of nascent-chain-dependent elongation pausing in the presence of macrolides determine the fate of the translating ribosome stalling or readthrough. Copyright © 2014 The Authors. Published by Elsevier Inc. All rights reserved.
Inferring antibiotic mechanisms on translation through static structures has been challenging, as biological systems are highly dynamic. Dynamic single-molecule methods are also limited to few simultaneously measurable parameters. We have circumvented these limitations with a multifaceted approach to investigate three structurally distinct aminoglycosides that bind to the aminoacyl-transfer RNA site (A site) in the prokaryotic 30S ribosomal subunit: apramycin, paromomycin, and gentamicin. Using several single-molecule fluorescence measurements combined with structural and biochemical techniques, we observed distinct changes to translational dynamics for each aminoglycoside. While all three drugs effectively inhibit translation elongation, their actions are structurally and mechanistically distinct. Apramycin does not displace A1492 and A1493 at the decoding center, as demonstrated by a solution nuclear magnetic resonance structure, causing only limited miscoding; instead, it primarily blocks translocation. Paromomycin and gentamicin, which displace A1492 and A1493, cause significant miscoding, block intersubunit rotation, and inhibit translocation. Our results show the power of combined dynamics, structural, and biochemical approaches to elucidate the complex mechanisms underlying translation and its inhibition. Copyright © 2013 The Authors. Published by Elsevier Inc. All rights reserved.
The initiation of translation establishes the reading frame for protein synthesis and is a key point of regulation. Initiation involves factor-driven assembly at a start codon of a messenger RNA of an elongation-competent 70S ribosomal particle (in bacteria) from separated 30S and 50S subunits and initiator transfer RNA. Here we establish in Escherichia coli, using direct single-molecule tracking, the timing of initiator tRNA, initiation factor 2 (IF2; encoded by infB) and 50S subunit joining during initiation. Our results show multiple pathways to initiation, with orders of arrival of tRNA and IF2 dependent on factor concentration and composition. IF2 accelerates 50S subunit joining and stabilizes the assembled 70S complex. Transition to elongation is gated by the departure of IF2 after GTP hydrolysis, allowing efficient arrival of elongator tRNAs to the second codon presented in the aminoacyl-tRNA binding site (A site). These experiments highlight the power of single-molecule approaches to delineate mechanisms in complex multicomponent systems.
Translation by the ribosome occurs by a complex mechanism involving the coordinated interaction of multiple nucleic acid and protein ligands. Here we use zero-mode waveguides (ZMWs) and sophisticated detection instrumentation to allow real-time observation of translation at physiologically relevant micromolar ligand concentrations. Translation at each codon is monitored by stable binding of transfer RNAs (tRNAs)-labelled with distinct fluorophores-to translating ribosomes, which allows direct detection of the identity of tRNA molecules bound to the ribosome and therefore the underlying messenger RNA (mRNA) sequence. We observe the transit of tRNAs on single translating ribosomes and determine the number of tRNA molecules simultaneously bound to the ribosome, at each codon of an mRNA molecule. Our results show that ribosomes are only briefly occupied by two tRNA molecules and that release of deacylated tRNA from the exit (E) site is uncoupled from binding of aminoacyl-tRNA site (A-site) tRNA and occurs rapidly after translocation. The methods outlined here have broad application to the study of mRNA sequences, and the mechanism and regulation of translation.
In order to coordinate the complex biochemical and structural feat of converting triple-nucleotide codons into their corresponding amino acids, the ribosome must physically manipulate numerous macromolecules including the mRNA, tRNAs, and numerous translation factors. The ribosome choreographs binding, dissociation, physical movements, and structural rearrangements so that they synergistically harness the energy from biochemical processes, including numerous GTP hydrolysis steps and peptide bond formation. Due to the dynamic and complex nature of translation, the large cast of ligands involved, and the large number of possible configurations, tracking the global time evolution or dynamics of the ribosome complex in translation has proven to be challenging for bulk methods. Conventional single-molecule fluorescence experiments on the other hand require low concentrations of fluorescent ligands to reduce background noise. The significantly reduced bimolecular association rates under those conditions limit the number of steps that can be observed within the time window available to a fluorophore. The advent of zero-mode waveguide (ZMW) technology has allowed the study of translation at near-physiological concentrations of labeled ligands, moving single-molecule fluorescence microscopy beyond focused model systems into studying the global dynamics of translation in realistic setups. This chapter reviews the recent works using the ZMW technology to dissect the mechanism of translation initiation and elongation in prokaryotes, including complex processes such as translational stalling and frameshifting. Given the success of the technology, similarly complex biological processes could be studied in near-physiological conditions with the controllability of conventional in vitro experiments. Copyright © 2016 Elsevier Inc. All rights reserved.
Genomic and transcriptomic analysis of the streptomycin-dependent Mycobacterium tuberculosis strain 18b.
The ability of Mycobacterium tuberculosis to establish a latent infection (LTBI) in humans confounds the treatment of tuberculosis. Consequently, there is a need to discover new therapeutic agents that can kill M. tuberculosis both during active disease and LTBI. The streptomycin-dependent strain of M. tuberculosis, 18b, provides a useful tool for this purpose since upon removal of streptomycin (STR) it enters a non-replicating state that mimics latency both in vitro and in animal models.The 4.41 Mb genome sequence of M. tuberculosis 18b was determined and this revealed the strain to belong to clade 3 of the ancient ancestral lineage of the Beijing family. STR-dependence was attributable to insertion of a single cytosine in the 530 loop of the 16S rRNA and to a single amino acid insertion in the N-terminal domain of initiation factor 3. RNA-seq was used to understand the genetic programme activated upon STR-withdrawal and hence to gain insight into LTBI. This revealed reconfiguration of gene expression and metabolic pathways showing strong similarities between non-replicating 18b and M. tuberculosis residing within macrophages, and with the core stationary phase and microaerophilic responses.The findings of this investigation confirm the validity of 18b as a model for LTBI, and provide insight into both the evolution of tubercle bacilli and the functioning of the ribosome.
Genome sequence and analysis of Escherichia coli MRE600, a colicinogenic, nonmotile strain that lacks RNase I and the type I methyltransferase, EcoKI.
Escherichia coli strain MRE600 was originally identified for its low RNase I activity and has therefore been widely adopted by the biomedical research community as a preferred source for the expression and purification of transfer RNAs and ribosomes. Despite its widespread use, surprisingly little information about its genome or genetic content exists. Here, we present the first de novo assembly and description of the MRE600 genome and epigenome. To provide context to these studies of MRE600, we include comparative analyses with E. coli K-12 MG1655 (K12). Pacific Biosciences Single Molecule, Real-Time sequencing reads were assembled into one large chromosome (4.83 Mb) and three smaller plasmids (89.1, 56.9, and 7.1 kb). Interestingly, the 7.1-kb plasmid possesses genes encoding a colicin E1 protein and its associated immunity protein. The MRE600 genome has a G + C content of 50.8% and contains a total of 5,181 genes, including 4,913 protein-encoding genes and 268 RNA genes. We identified 41,469 modified DNA bases (0.83% of total) and found that MRE600 lacks the gene for type I methyltransferase, EcoKI. Phylogenetic, taxonomic, and genetic analyses demonstrate that MRE600 is a divergent E. coli strain that displays features of the closely related genus, Shigella. Nevertheless, comparative analyses between MRE600 and E. coli K12 show that these two strains exhibit nearly identical ribosomal proteins, ribosomal RNAs, and highly homologous tRNA species. Substantiating prior suggestions that MRE600 lacks RNase I activity, the RNase I-encoding gene, rna, contains a single premature stop codon early in its open-reading frame. © The Author 2016. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution.
The complexity of eukaryotic translation allows fine-tuned regulation of protein synthesis. Viruses use internal ribosome entry sites (IRESs) to minimize or, like the CrPV IRES, eliminate the need for initiation factors. Here, by exploiting the CrPV IRES, we observed the entire process of initiation and transition to elongation in real time. We directly tracked the CrPV IRES, 40S and 60S ribosomal subunits, and tRNA using single-molecule fluorescence spectroscopy and identified multiple parallel initiation pathways within the system. Our results distinguished two pathways of 80S:CrPV IRES complex assembly that produce elongation-competent complexes. Following 80S assembly, the requisite eEF2-mediated translocation results in an unstable intermediate that is captured by binding of the elongator tRNA. Whereas initiation can occur in the 0 and +1 frames, the arrival of the first tRNA defines the reading frame and strongly favors 0 frame initiation. Overall, even in the simplest system, an intricate reaction network regulates translation initiation. Copyright © 2016 Elsevier Inc. All rights reserved.
Amino acid sequence repertoire of the bacterial proteome and the occurrence of untranslatable sequences.
Bioinformatic analysis of Escherichia coli proteomes revealed that all possible amino acid triplet sequences occur at their expected frequencies, with four exceptions. Two of the four underrepresented sequences (URSs) were shown to interfere with translation in vivo and in vitro. Enlarging the URS by a single amino acid resulted in increased translational inhibition. Single-molecule methods revealed stalling of translation at the entrance of the peptide exit tunnel of the ribosome, adjacent to ribosomal nucleotides A2062 and U2585. Interaction with these same ribosomal residues is involved in regulation of translation by longer, naturally occurring protein sequences. The E. coli exit tunnel has evidently evolved to minimize interaction with the exit tunnel and maximize the sequence diversity of the proteome, although allowing some interactions for regulatory purposes. Bioinformatic analysis of the human proteome revealed no underrepresented triplet sequences, possibly reflecting an absence of regulation by interaction with the exit tunnel.