Structural dynamics of the ribosome

1H, 13C, and 15N backbone and methyl group resonance assignments of ricin toxin A subunit

Ricin is a potent plant toxin that targets the eukaryotic ribosome by depurinating an adenine from the sarcin-ricin loop (SRL), a highly conserved stem-loop of the rRNA. As a category-B agent for bioterrorism it is a prime target for therapeutic intervention with antibodies and enzyme blocking inhibitors since no effective therapy exists for ricin. Ricin toxin A subunit (RTA) depurinates the SRL by binding to the P-stalk proteins at a remote site. Stimulation of the N-glycosidase activity of RTA by the P-stalk proteins has been studied extensively by biochemical methods and by X-ray crystallography. The current understanding of RTA's depurination mechanism relies exclusively on X-ray structures of the enzyme in the free state and complexed with transition state analogues. To date we have sparse evidence of conformational dynamics and allosteric regulation of RTA activity that can be exploited in the rational design of inhibitors. Thus, our primary goal here is to apply solution NMR techniques to probe the residue specific structural and dynamic coupling active in RTA as a prerequisite to understand the functional implications of an allosteric network. In this report we present de novo sequence specific amide and sidechain methyl chemical shift assignments of the 267 residue RTA in the free state and in complex with an 11-residue peptide (P11) representing the identical C-terminal sequence of the ribosomal P-stalk proteins. These assignments will facilitate future studies detailing the propagation of binding induced conformational changes in RTA complexed with inhibitors, antibodies, and biologically relevant targets.

The energy landscape of the ribosome

The ribosome is a prototypical assembly that can be used to establish general principles and techniques for the study of biological molecular machines. Motivated by the fact that the dynamics of every biomolecule is governed by an underlying energy landscape, there has been great interest to understand and quantify ribosome energetics. In the present review, we will focus on theoretical and computational strategies for probing the interactions that shape the energy landscape of the ribosome, with an emphasis on more recent studies of the elongation cycle. These efforts include the application of quantum mechanical methods for describing chemical kinetics, as well as classical descriptions to characterize slower (microsecond to millisecond) large-scale (10-100 Å) rearrangements, where motion is described in terms of diffusion across an energy landscape. Together, these studies provide broad insights into the factors that control a diverse range of dynamical processes in this assembly.

Cryo-EM captures a unique conformational rearrangement in 23S rRNA helices of the Mycobacterium 50S subunit

Structural investigations of the ribosomes isolated from pathogenic and non-pathogenic Mycobacterium species have identified several mycobacteria-specific structural features of ribosomal RNA and proteins. Here, we report structural evidence of a hitherto unknown conformational switch of mycobacterium 23S rRNA helices (H54a and H67-H71). Cryo-electron microscopy (cryo-EM) structures (~3-4 Å) of the M. smegmatis (Msm) log-phase 50S ribosomal subunit revealed conformational variability in H67-H71 region of the 23S rRNA, and manifested that, while H68 possesses the usual stretched conformation in one class of the maps, another one exhibits a bulge-out, fused density of H68-H69 at the inter-subunit surface, indicating an intrinsic dynamics of these rRNA helices. Remarkably, altered conformation of H68 forming a more prominent bulge-out structure at the inter-subunit surface of the 50S subunit due to the conformational rearrangements of 23S rRNA H67-H71 region was clearly visualized in a 3 Å cryo-EM map of the 50S subunit obtained from the stationary phase ribosome dataset. The Msm50S subunit having such bulge-out conformation at the intersubunit surface would be incompatible for associating with the 30S subunit due to its inability to form major inter-subunit bridges. Evidently, availability of active 70S ribosome pool can be modulated by stabilizing either one of the H68 conformation.

Utilizing functional cell-free extracts to dissect ribonucleoprotein complex biology at single-molecule resolution

Cellular machineries that drive and regulate gene expression often rely on the coordinated assembly and interaction of a multitude of proteins and RNA together called ribonucleoprotein complexes (RNPs). As such, it is challenging to fully reconstitute these cellular machines recombinantly and gain mechanistic understanding of how they operate and are regulated within the complex environment that is the cell. One strategy for overcoming this challenge is to perform single molecule fluorescence microscopy studies within crude or recombinantly supplemented cell extracts. This strategy enables elucidation of the interaction and kinetic behavior of specific fluorescently labeled biomolecules within RNPs under conditions that approximate native cellular environments. In this review, we describe single molecule fluorescence microcopy approaches that dissect RNP-driven processes within cellular extracts, highlighting general strategies used in these methods. We further survey biological advances in the areas of pre-mRNA splicing and transcription regulation that have been facilitated through this approach. Finally, we conclude with a summary of practical considerations for the implementation of the featured approaches to facilitate their broader future implementation in dissecting the mechanisms of RNP-driven cellular processes. This article is categorized under: RNA Structure and Dynamics > RNA Structure, Dynamics and Chemistry RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes RNA Structure and Dynamics > Influence of RNA Structure in Biological Systems.

Quality control ensures fidelity in ribosome assembly and cellular health

The coordinated integration of ribosomal RNA and protein into two functional ribosomal subunits is safeguarded by quality control checkpoints that ensure ribosomes are correctly assembled and functional before they engage in translation. Quality control is critical in maintaining the integrity of ribosomes and necessary to support healthy cell growth and prevent diseases associated with mistakes in ribosome assembly. Its importance is demonstrated by the finding that bypassing quality control leads to misassembled, malfunctioning ribosomes with altered translation fidelity, which change gene expression and disrupt protein homeostasis. In this review, we outline our understanding of quality control within ribosome synthesis and how failure to enforce quality control contributes to human disease. We first provide a definition of quality control to guide our investigation, briefly present the main assembly steps, and then examine stages of assembly that test ribosome function, establish a pass-fail system to evaluate these functions, and contribute to altered ribosome performance when bypassed, and are thus considered "quality control."

R, de Oliveira R, Dunham C, Whitford P. Ratchet, swivel, tilt and roll: a complete description of subunit rotation in the ribosome

Protein synthesis by the ribosome requires large-scale rearrangements of the 'small' subunit (SSU; ∼1 MDa), including inter- and intra-subunit rotational motions. However, with nearly 2000 structures of ribosomes and ribosomal subunits now publicly available, it is exceedingly difficult to design experiments based on analysis of all known rotation states. To overcome this, we developed an approach where the orientation of each SSU head and body is described in terms of three angular coordinates (rotation, tilt and tilt direction) and a single translation. By considering the entire RCSB PDB database, we describe 1208 fully-assembled ribosome complexes and 334 isolated small subunits, which span >50 species. This reveals aspects of subunit rearrangements that are universal, and others that are organism/domain-specific. For example, we show that tilt-like rearrangements of the SSU body (i.e. 'rolling') are pervasive in both prokaryotic and eukaryotic (cytosolic and mitochondrial) ribosomes. As another example, domain orientations associated with frameshifting in bacteria are similar to those found in eukaryotic ribosomes. Together, this study establishes a common foundation with which structural, simulation, single-molecule and biochemical efforts can more precisely interrogate the dynamics of this prototypical molecular machine.

Precise Steric Features Control Aminoacyl-tRNA Accommodation on the Ribosome

Protein synthesis involves a complex series of large-scale conformational changes in the ribosome. While long-lived intermediate states of these processes can be characterized by experiments, computational methods can be used to identify the interactions that contribute to the rate-limiting free-energy barriers. To this end, we use a simplified energetic model to perform molecular dynamics (MD) simulations of aminoacyl-tRNA (aa-tRNA) accommodation on the ribosome. While numerous studies have probed the energetics of the early stages of accommodation, we focus on the final stage of accommodation, where the 3'-CCA tail of aa-tRNA enters the peptidyl transferase center (PTC). These simulations show how a distinct intermediate is induced by steric confinement of the tail, immediately before it completes accommodation. Multiple pathways for 3'-CCA tail accommodation can be quantitatively distinguished, where the tail enters the PTC by moving past a pocket enclosed by Helix 89, 90, and 92, or through an alternate route formed by Helix 93 and the P-site tRNA. C2573, located within Helix 90, is shown to provide the largest contribution to this late-accommodation steric barrier, such that sub-Å perturbations to this residue can alter the time scale of tail accommodation by nearly an order of magnitude. In terms of biological function, these calculations suggest how this late-stage sterically induced barrier may contribute to tRNA proofreading by the ribosome.

Y98 Mutation Leads to the Loss of RsfS Anti-Association Activity in Staphylococcus aureus

Ribosomal silencing factor S (RsfS) is a conserved protein that plays a role in the mechanisms of ribosome shutdown and cell survival during starvation. Recent studies demonstrated the involvement of RsfS in the biogenesis of the large ribosomal subunit. RsfS binds to the uL14 ribosomal protein on the large ribosomal subunit and prevents its association with the small subunit. Here, we estimated the contribution of RsfS amino acid side chains at the interface between RsfS and uL14 to RsfS anti-association function in Staphylococcus aureus through in vitro experiments: centrifugation in sucrose gradient profiles and an S. aureus cell-free system assay. The detected critical Y98 amino acid on the RsfS surface might become a new potential target for pharmacological drug development and treatment of S. aureus infections.

Is RsfS a Hibernation Factor or a Ribosome Biogenesis Factor?

Solving the structures of bacterial, archaeal, and eukaryotic ribosomes by crystallography and cryo-electron microscopy has given an impetus for studying intracellular regulatory proteins affecting various stages of protein translation. Among them are ribosome hibernation factors, which have been actively investigated during the last decade. These factors are involved in the regulation of protein biosynthesis under stressful conditions. The main role of hibernation factors is the reduction of energy consumption for protein biosynthesis and preservation of existing functional ribosomes from degradation, which increases cell survival under unfavorable conditions. Despite a broad interest in this topic, only a few articles have been published on the ribosomal silencing factor S (RsfS). According to the results of these studies, RsfS can be assigned to the group of hibernation factors. However, recent structural studies of the 50S ribosomal subunit maturation demonstrated that RsfS has the features inherent to biogenesis factors for example, ability to bind to the immature ribosomal subunit (similar to the RsfS mitochondrial ortholog MALSU1, mitochondrial assembly of ribosomal large subunit 1). In this review, we summarized the information on the function and structural features RsfS, as well as compared RsfS with MALSU1 in order to answer the emerging question on whether RsfS is a hibernation factor or a ribosome biogenesis factor. We believe that this review might promote future studies of the RsfS-involving molecular mechanisms, which so far remain completely unknown.

Structural Studies Reveal the Role of Helix 68 in the Elongation Step of Protein Biosynthesis

The ribosome, a multicomponent assembly consisting of RNA and proteins, is a pivotal macromolecular machine that translates the genetic code into proteins. The large ribosomal subunit rRNA helix 68 (H68) is a key element in the protein synthesis process, as it coordinates the coupled movements of the actors involved in translocation, including the tRNAs and L1 stalk. Examination of cryo-electron microscopy (cryo-EM) structures of ribosomes incubated for various time durations at physiological temperatures led to the identification of functionally relevant H68 movements. These movements assist the transition of the L1 stalk between its open and closed states. H68 spatial flexibility and its significance to the protein synthesis process were confirmed through its effective targeting with antisense PNA oligomers. Our results suggest that H68 is actively involved in ribosome movements that are central to the elongation process. IMPORTANCE The mechanism that regulates the translocation step in ribosomes during protein synthesis is not fully understood. In this work, cryo-EM techniques used to image ribosomes from Staphylococcus aureus after incubation at physiological temperature allowed the identification of a conformation of the helix 68 that has never been observed so far. We then propose a mechanism in which such helix, switching between two different conformations, actively coordinates the translocation step, shedding light on the dynamics of ribosomal components. In addition, the relevance of helix 68 to ribosome function and its potential as an antibiotic target was proved by inhibiting Staphylococcus aureus ribosomes activity in vitro using oligomers with sequence complementarity.

CryoEM structures of pseudouridine-free ribosome suggest impacts of chemical modifications on ribosome conformations

Pseudouridine, the most abundant form of RNA modification, is known to play important roles in ribosome function. Mutations in human DKC1, the pseudouridine synthase responsible for catalyzing the ribosome RNA modification, cause translation deficiencies and are associated with a complex cancer predisposition. The structural basis for how pseudouridine impacts ribosome function remains uncharacterized. Here, we characterized structures and conformations of a fully modified and a pseudouridine-free ribosome from Saccharomyces cerevisiae in the absence of ligands or when bound with translocation inhibitor cycloheximide by electron cryomicroscopy. In the modified ribosome, the rearranged N1 atom of pseudouridine is observed to stabilize key functional motifs by establishing predominately water-mediated close contacts with the phosphate backbone. The pseudouridine-free ribosome, however, is devoid of such interactions and displays conformations reflective of abnormal inter-subunit movements. The erroneous motions of the pseudouridine-free ribosome may explain its observed deficiencies in translation.

Release of the ribosome biogenesis factor Bud23 from small subunit precursors in yeast

The two subunits of the eukaryotic ribosome are produced through quasi-independent pathways involving the hierarchical actions of numerous trans-acting biogenesis factors and the incorporation of ribosomal proteins. The factors work together to shape the nascent subunits through a series of intermediate states into their functional architectures. One of the earliest intermediates of the small subunit (SSU or 40S) is the SSU processome which is subsequently transformed into the pre-40S intermediate. This transformation is, in part, facilitated by the binding of the methyltransferase Bud23. How Bud23 is released from the resultant pre-40S is not known. The ribosomal proteins Rps0, Rps2, and Rps21, termed the Rps0-cluster proteins, and several biogenesis factors bind the pre-40S around the time that Bud23 is released, suggesting that one or more of these factors could induce Bud23 release. Here, we systematically examined the requirement of these factors for the release of Bud23 from pre-40S particles. We found that the Rps0-cluster proteins are needed but not sufficient for Bud23 release. The atypical kinase/ATPase Rio2 shares a binding site with Bud23 and is thought to be recruited to pre-40S after the Rps0-cluster proteins. Depletion of Rio2 prevented the release of Bud23 from the pre-40S. More importantly, the addition of recombinant Rio2 to pre-40S particles affinity-purified from Rio2-depleted cells was sufficient for Bud23 release in vitro. The ability of Rio2 to displace Bud23 was independent of nucleotide hydrolysis. We propose a novel role for Rio2 in which its binding to the pre-40S actively displaces Bud23 from the pre-40S.

The Structural Dynamics of Translation

Accurate protein synthesis (translation) relies on translation factors that rectify ribosome fluctuations into a unidirectional process. Understanding this process requires structural characterization of the ribosome and translation-factor dynamics. In the 2000s, crystallographic studies determined high-resolution structures of ribosomes stalled with translation factors, providing a starting point for visualizing translation. Recent progress in single-particle cryogenic electron microscopy (cryo-EM) has enabled near-atomic resolution of numerous structures sampled in heterogeneous complexes (ensembles). Ensemble and time-resolved cryo-EM have now revealed unprecedented views of ribosome transitions in the three principal stages of translation: initiation, elongation, and termination. This review focuses on how translation factors help achieve high accuracy and efficiency of translation by monitoring distinct ribosome conformations and by differentially shifting the equilibria of ribosome rearrangements for cognate and near-cognate substrates. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Towards the Idea of Molecular Brains

How can single cells without nervous systems perform complex behaviours such as habituation, associative learning and decision making, which are considered the hallmark of animals with a brain? Are there molecular systems that underlie cognitive properties equivalent to those of the brain? This review follows the development of the idea of molecular brains from Darwin’s “root brain hypothesis”, through bacterial chemotaxis, to the recent discovery of neuron-like r-protein networks in the ribosome. By combining a structural biology view with a Bayesian brain approach, this review explores the evolutionary labyrinth of information processing systems across scales. Ribosomal protein networks open a window into what were probably the earliest signalling systems to emerge before the radiation of the three kingdoms. While ribosomal networks are characterised by long-lasting interactions between their protein nodes, cell signalling networks are essentially based on transient interactions. As a corollary, while signals propagated in persistent networks may be ephemeral, networks whose interactions are transient constrain signals diffusing into the cytoplasm to be durable in time, such as post-translational modifications of proteins or second messenger synthesis. The duration and nature of the signals, in turn, implies different mechanisms for the integration of multiple signals and decision making. Evolution then reinvented networks with persistent interactions with the development of nervous systems in metazoans. Ribosomal protein networks and simple nervous systems display architectural and functional analogies whose comparison could suggest scale invariance in information processing. At the molecular level, the significant complexification of eukaryotic ribosomal protein networks is associated with a burst in the acquisition of new conserved aromatic amino acids. Knowing that aromatic residues play a critical role in allosteric receptors and channels, this observation suggests a general role of π systems and their interactions with charged amino acids in multiple signal integration and information processing. We think that these findings may provide the molecular basis for designing future computers with organic processors.

Two alternative conformations of mRNA in the human ribosome during elongation and termination of translation as revealed by EPR spectroscopy

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DEER reveals the conformational variability of mRNA at the certain translation steps.

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Elongation and termination complexes exist in 2 conformations in dynamic equilibrium.

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The conformations of mRNA in 40S channel undergo no major change during termination.

The conformation of mRNA in the region of the human 80S ribosome decoding site was monitored using 11-mer mRNA analogues that bore nitroxide spin labels attached to the terminal nucleotide bases. Intramolecular spin–spin distances were measured by DEER/PELDOR spectroscopy in model complexes mimicking different states of the 80S ribosome during elongation and termination of translation. The measurements revealed that in all studied complexes, mRNA exists in two alternative conformations, whose ratios are different in post-translocation, pre-translocation and termination complexes. We found that the presence of a tRNA molecule at the ribosomal A site decreases the relative share of the more extended mRNA conformation, whereas the binding of eRF1 (alone or in a complex with eRF3) results in the opposite effect. In the termination complexes, the ratios of mRNA conformations are practically the same, indicating that a part of mRNA bound in the ribosome channel does not undergo significant structural alterations in the course of completion of the translation. Our results contribute to the understanding of mRNA molecular dynamics in the mammalian ribosome channel during translation.

Ribosome as a Translocase and Helicase

During protein synthesis, ribosome moves along mRNA to decode one codon after the other. Ribosome translocation is induced by a universally conserved protein, elongation factor G (EF-G) in bacteria and elongation factor 2 (EF-2) in eukaryotes. EF-G-induced translocation results in unwinding of the intramolecular secondary structures of mRNA by three base pairs at a time that renders the translating ribosome a processive helicase. Professor Alexander Sergeevich Spirin has made numerous seminal contributions to understanding the molecular mechanism of translocation. Here, we review Spirin's insights into the ribosomal translocation and recent advances in the field that stemmed from Spirin's pioneering work. We also discuss key remaining challenges in studies of translocase and helicase activities of the ribosome.

Comparative Analysis of Structural and Dynamical Features of Ribosome Upon Association With mRNA Reveals Potential Role of Ribosomal Proteins

Ribosomes play a critical role in maintaining cellular proteostasis. The binding of messenger RNA (mRNA) to the ribosome regulates kinetics of protein synthesis. To generate an understanding of the structural, mechanistic, and dynamical features of mRNA recognition in the ribosome, we have analysed mRNA-protein interactions through a structural comparison of the ribosomal complex in the presence and absence of mRNA. To do so, we compared the 3-Dimensional (3D) structures of components of the two assembly structures and analysed their structural differences because of mRNA binding, using elastic network models and structural network-based analysis. We observe that the head region of 30S ribosomal subunit undergoes structural displacement and subunit rearrangement to accommodate incoming mRNA. We find that these changes are observed in proteins that lie far from the mRNA-protein interface, implying allostery. Further, through perturbation response scanning, we show that the proteins S13, S19, and S20 act as universal sensors that are sensitive to changes in the inter protein network, upon binding of 30S complex with mRNA and other initiation factors. Our study highlights the significance of mRNA binding in the ribosome complex and identifies putative allosteric sites corresponding to alterations in structure and/or dynamics, in regions away from mRNA binding sites in the complex. Overall, our work provides fresh insights into mRNA association with the ribosome, highlighting changes in the interactions and dynamics of the ribosome assembly because of the binding.

Functional Sites of Ribosome Modulation Factor (RMF) Involved in the Formation of 100S Ribosome

One of the important cellular events in all organisms is protein synthesis, which is catalyzed by ribosomes. The ribosomal activity is dependent on the environmental situation of the cell. Bacteria form 100S ribosomes, lacking translational activity, to survive under stress conditions such as nutrient starvation. The 100S ribosome is a dimer of two 70S ribosomes bridged through the 30S subunits. In some pathogens of gammaproteobacteria, such as Escherichia coli, Yersinia pestis, and Vibrio cholerae, the key factor for ribosomal dimerization is the small protein, ribosome modulation factor (RMF). When ribosomal dimerization by RMF is impaired, long-term bacterial survival is abolished. This shows that the interconversion system between active 70S ribosomes and inactive 100S ribosomes is an important survival strategy for bacteria. According to the results of several structural analyses, RMF does not directly connect two ribosomes, but binds to them and changes the conformation of their 30S subunits, thus promoting ribosomal dimerization. In this study, conserved RMF amino acids among 50 bacteria were selectively altered by mutagenesis to identify the residues involved in ribosome binding and dimerization. The activities of mutant RMF for ribosome binding and ribosome dimerization were measured using the sucrose density gradient centrifugation (SDGC) and western blotting methods. As a result, some essential amino acids of RMF for the ribosomal binding and dimerization were elucidated. Since the induction of RMF expression inhibits bacterial growth, the data on this protein could serve as information for the development of antibiotic or bacteriostatic agents.

The World of Stable Ribonucleoproteins and Its Mapping With Grad-Seq and Related Approaches

Macromolecular complexes of proteins and RNAs are essential building blocks of cells. These stable supramolecular particles can be viewed as minimal biochemical units whose structural organization, i.e., the way the RNA and the protein interact with each other, is directly linked to their biological function. Whether those are dynamic regulatory ribonucleoproteins (RNPs) or integrated molecular machines involved in gene expression, the comprehensive knowledge of these units is critical to our understanding of key molecular mechanisms and cell physiology phenomena. Such is the goal of diverse complexomic approaches and in particular of the recently developed gradient profiling by sequencing (Grad-seq). By separating cellular protein and RNA complexes on a density gradient and quantifying their distributions genome-wide by mass spectrometry and deep sequencing, Grad-seq charts global landscapes of native macromolecular assemblies. In this review, we propose a function-based ontology of stable RNPs and discuss how Grad-seq and related approaches transformed our perspective of bacterial and eukaryotic ribonucleoproteins by guiding the discovery of new RNA-binding proteins and unusual classes of noncoding RNAs. We highlight some methodological aspects and developments that permit to further boost the power of this technique and to look for exciting new biology in understudied and challenging biological models.

Evolution of ribosomal protein network architectures

To perform an accurate protein synthesis, ribosomes accomplish complex tasks involving the long-range communication between its functional centres such as the peptidyl transfer centre, the tRNA bindings sites and the peptide exit tunnel. How information is transmitted between these sites remains one of the major challenges in current ribosome research. Many experimental studies have revealed that some r-proteins play essential roles in remote communication and the possible involvement of r-protein networks in these processes have been recently proposed. Our phylogenetic, structural and mathematical study reveals that of the three kingdom's r-protein networks converged towards non-random graphs where r-proteins collectively coevolved to optimize interconnection between functional centres. The massive acquisition of conserved aromatic residues at the interfaces and along the extensions of the newly connected eukaryotic r-proteins also highlights that a strong selective pressure acts on their sequences probably for the formation of new allosteric pathways in the network.

Sequence variation, common tissue expression patterns and learning models: a genome-wide survey of vertebrate ribosomal proteins

Ribosomal genes produce the constituents of the ribosome, one of the most conserved subcellular structures of all cells, from bacteria to eukaryotes, including animals. There are notions that some protein-coding ribosomal genes vary in their roles across species, particularly vertebrates, through the involvement of some in a number of genetic diseases. Based on extensive sequence comparisons and systematic curation, we establish a reference set for ribosomal proteins (RPs) in eleven vertebrate species and quantify their sequence conservation levels. Moreover, we correlate their coordinated gene expression patterns within up to 33 tissues and assess the exceptional role of paralogs in tissue specificity. Importantly, our analysis supported by the development and use of machine learning models strongly proposes that the variation in the observed tissue-specific gene expression of RPs is rather species-related and not due to tissue-based evolutionary processes. The data obtained suggest that RPs exhibit a complex relationship between their structure and function that broadly maintains a consistent expression landscape across tissues, while most of the variation arises from species idiosyncrasies. The latter may be due to evolutionary change and adaptation, rather than functional constraints at the tissue level throughout the vertebrate lineage.

Local and Global Motions Underlying Antibiotic Binding in Bacterial Ribosome

The bacterial ribosome is one of the most important targets in the treatment of infectious diseases. As antibiotic resistance in bacteria poses a growing threat, a significant amount of effort is concentrated on exploring new drug-binding sites where testable predictions are of significance. Here, we study the dynamics of a ribosomal complex and 67 small and large subunits of the ribosomal crystal structures (64 antibiotic-bound, 3 antibiotic-free) from Deinococcus radiodurans, Escherichia coli, Haloarcula marismortui, and Thermus thermophilus by the Gaussian network model. Interestingly, a network of nucleotides coupled in high-frequency fluctuations reveals known antibiotic-binding sites. These sites are seen to locate at the interface of dynamic domains that have an intrinsic dynamic capacity to interfere with functional globular motions. The nucleotides and the residues fluctuating in the fast and slow modes of motion thus have promise for plausible antibiotic-binding and allosteric sites that can alter antibiotic binding and resistance. Overall, the present analysis brings a new dynamic perspective to the long-discussed link between small-molecule binding and large conformational changes of the supramolecule.

Selective inhibition of human translation termination by a drug-like compound

Methods to directly inhibit gene expression using small molecules hold promise for the development of new therapeutics targeting proteins that have evaded previous attempts at drug discovery. Among these, small molecules including the drug-like compound PF-06446846 (PF846) selectively inhibit the synthesis of specific proteins, by stalling translation elongation. These molecules also inhibit translation termination by an unknown mechanism. Using cryo-electron microscopy (cryo-EM) and biochemical approaches, we show that PF846 inhibits translation termination by arresting the nascent chain (NC) in the ribosome exit tunnel. The arrested NC adopts a compact α-helical conformation that induces 28 S rRNA nucleotide rearrangements that suppress the peptidyl transferase center (PTC) catalytic activity stimulated by eukaryotic release factor 1 (eRF1). These data support a mechanism of action for a small molecule targeting translation that suppresses peptidyl-tRNA hydrolysis promoted by eRF1, revealing principles of eukaryotic translation termination and laying the foundation for new therapeutic strategies.

Structural basis of elongation factor 2 switching

Archaebacterial and eukaryotic elongation factor 2 (EF-2) and bacterial elongation factor G (EF-G) are five domain GTPases that catalyze the ribosomal translocation of tRNA and mRNA. In the classical mechanism of activation, GTPases are switched on through GDP/GTP exchange, which is accompanied by the ordering of two flexible segments called switch I and II. However, crystal structures of EF-2 and EF-G have thus far not revealed the conformations required by the classical mechanism. Here, we describe crystal structures of Methanoperedens nitroreducens EF-2 (MnEF-2) and MnEF-2-H595N bound to GMPPCP (GppCp) and magnesium displaying previously unreported compact conformations. Domain III forms interfaces with the other four domains and the overall conformations resemble that of SNU114, the eukaryotic spliceosomal GTPase. The gamma phosphate of GMPPCP is detected through interactions with switch I and a P-loop structural element. Switch II is highly ordered whereas switch I shows a variable degree of ordering. The ordered state results in a tight interdomain arrangement of domains I-III and the formation of a portion of a predicted monovalent cation site involving the P-loop and switch I. The side chain of an essential histidine residue in switch II is placed in the inactive conformation observed for the “on” state of elongation factor EF-Tu. The compact conformations of MnEF-2 and MnEF-2-H595N suggest an “on” ribosome-free conformational state.

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Crystal structures of ribosome-free elongation factor 2 (EF-2) bound to GTP analog and magnesium.

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Compact conformation and P-loop, switch I, and switch II structures suggest “on” state.

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Arrangement of domains I-III similar to that of ribosome-bound EF-2/EF-G complexed with GTP analog.

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Switch II histidine shows inactive conformation observed for “on” state of ribosome-free EF-Tu.

Rescuing stalled mammalian mitoribosomes - what can we learn from bacteria?

In the canonical process of translation, newly completed proteins escape from the ribosome following cleavage of the ester bond that anchors the polypeptide to the P-site tRNA, after which the ribosome can be recycled to initiate a new round of translation. Not all protein synthesis runs to completion as various factors can impede the progression of ribosomes. Rescuing of stalled ribosomes in mammalian mitochondria, however, does not share the same mechanisms that many bacteria use. The classic method for rescuing bacterial ribosomes is trans-translation. The key components of this system are absent from mammalian mitochondria; however, four members of a translation termination factor family are present, with some evidence of homology to members of a bacterial back-up rescue system. To date, there is no definitive demonstration of any other member of this family functioning in mitoribosome rescue. Here, we provide an overview of the processes and key players of canonical translation termination in both bacteria and mammalian mitochondria, followed by a perspective of the bacterial systems used to rescue stalled ribosomes. We highlight any similarities or differences with the mitochondrial translation release factors, and suggest potential roles for these proteins in ribosome rescue in mammalian mitochondria.

Diffusion of tRNA inside the ribosome is position-dependent

In recent years, there has been a growing interest to quantify the energy landscape that governs ribosome dynamics. However, in order to quantitatively integrate theoretical predictions and experimental measurements, it is essential that one has a detailed understanding of the associated diffusive properties. Here, all-atom explicit-solvent simulations (50 μs of aggregate sampling) predict that the diffusion coefficient of a tRNA molecule will depend on its position within the ribosome. Specifically, during aa-tRNA accommodation (i.e., the process by which tRNA enters the ribosome), the apparent diffusion coefficient decreases by approximately an order of magnitude. By comparing these to values obtained with an energetically "smooth" model, we show that the observed nonuniform behavior likely arises from electrostatic and solvation interactions between the tRNA and ribosome. These calculations also reveal the hierarchical character of ribosomal energetics, where steric interactions induce a large-scale free-energy barrier, and short-scale roughness determines the rate of diffusive movement across the landscape.

Translocation kinetics and structural dynamics of ribosomes are modulated by the conformational plasticity of downstream pseudoknots

Downstream stable mRNA secondary structures can stall elongating ribosomes by impeding the concerted movements of tRNAs and mRNA on the ribosome during translocation. The addition of a downstream mRNA structure, such as a stem-loop or a pseudoknot, is essential to induce -1 programmed ribosomal frameshifting (-1 PRF). Interestingly, previous studies revealed that -1 PRF efficiencies correlate with conformational plasticity of pseudoknots, defined as their propensity to form incompletely folded structures, rather than with the mechanical properties of pseudoknots. To elucidate the detailed molecular mechanisms of translocation and -1 PRF, we applied several smFRET assays to systematically examine how translocation rates and conformational dynamics of ribosomes were affected by different pseudoknots. Our results show that initial pseudoknot-unwinding significantly inhibits late-stage translocation and modulates conformational dynamics of ribosomal post-translocation complexes. The effects of pseudoknots on the structural dynamics of ribosomes strongly correlate with their abilities to induce -1 PRF. Our results lead us to propose a kinetic scheme for translocation which includes an initial power-stroke step and a following thermal-ratcheting step. This scheme provides mechanistic insights on how selective modulation of late-stage translocation by pseudoknots affects -1 PRF. Overall our findings advance current understanding of translocation and ribosome-induced mRNA structure unwinding.

Modeling RNA secondary structure folding ensembles using SHAPE mapping data

RNA secondary structure prediction is widely used for developing hypotheses about the structures of RNA sequences, and structure can provide insight about RNA function. The accuracy of structure prediction is known to be improved using experimental mapping data that provide information about the pairing status of single nucleotides, and these data can now be acquired for whole transcriptomes using high-throughput sequencing. Prior methods for using these experimental data focused on predicting structures for sequences assuming that they populate a single structure. Most RNAs populate multiple structures, however, where the ensemble of strands populates structures with different sets of canonical base pairs. The focus on modeling single structures has been a bottleneck for accurately modeling RNA structure. In this work, we introduce Rsample, an algorithm for using experimental data to predict more than one RNA structure for sequences that populate multiple structures at equilibrium. We demonstrate, using SHAPE mapping data, that we can accurately model RNA sequences that populate multiple structures, including the relative probabilities of those structures. This program is freely available as part of the RNAstructure software package.

Nervous-Like Circuits in the Ribosome Facts, Hypotheses and Perspectives

In the past few decades, studies on translation have converged towards the metaphor of a “ribosome nanomachine”; they also revealed intriguing ribosome properties challenging this view. Many studies have shown that to perform an accurate protein synthesis in a fluctuating cellular environment, ribosomes sense, transfer information and even make decisions. This complex “behaviour” that goes far beyond the skills of a simple mechanical machine has suggested that the ribosomal protein networks could play a role equivalent to nervous circuits at a molecular scale to enable information transfer and processing during translation. We analyse here the significance of this analogy and establish a preliminary link between two fields: ribosome structure-function studies and the analysis of information processing systems. This cross-disciplinary analysis opens new perspectives about the mechanisms of information transfer and processing in ribosomes and may provide new conceptual frameworks for the understanding of the behaviours of unicellular organisms.

How to benchmark RNA secondary structure prediction accuracy

RNA secondary structure prediction is widely used. As new methods are developed, these are often benchmarked for accuracy against existing methods. This review discusses good practices for performing these benchmarks, including the choice of benchmarking structures, metrics to quantify accuracy, the importance of allowing flexibility for pairs in the accepted structure, and the importance of statistical testing for significance.

Dissecting the Energetics of Subunit Rotation in the Ribosome

The accurate expression of proteins requires the ribosome to efficiently undergo elaborate conformational rearrangements. The most dramatic of these motions is subunit rotation, which is necessary for tRNA molecules to transition between ribosomal binding sites. While rigid-body descriptions provide a qualitative picture of the process, obtaining quantitative mechanistic insights requires one to account for the relationship between molecular flexibility and collective dynamics. Using simulated rotation events, we assess the quality of experimentally accessible measures for describing the collective displacement of the ∼4000-residue small subunit. For this, we ask whether each coordinate is able to identify the underlying free-energy barrier and transition state ensemble (TSE). We find that intuitive structurally motivated coordinates (e.g., rotation angle, interprotein distances) can distinguish between the endpoints, though they are poor indicators of barrier-crossing events, and they underestimate the free-energy barrier. In contrast, coordinates based on intersubunit bridges can identify the TSE. We additionally verify that the committor probability for the putative TSE configurations is 0.5, a hallmark feature of any transition state. In terms of structural properties, these calculations implicate a transition state in which flexibility allows for asynchronous rearrangements of the bridges, as the ribosome adopts a partially rotated orientation. This provides a theoretical foundation, upon which experimental techniques may precisely quantify the energy landscape of the ribosome.

RNA Structural Dynamics As Captured by Molecular Simulations: A Comprehensive Overview

With both catalytic and genetic functions, ribonucleic acid (RNA) is perhaps the most pluripotent chemical species in molecular biology, and its functions are intimately linked to its structure and dynamics. Computer simulations, and in particular atomistic molecular dynamics (MD), allow structural dynamics of biomolecular systems to be investigated with unprecedented temporal and spatial resolution. We here provide a comprehensive overview of the fast-developing field of MD simulations of RNA molecules. We begin with an in-depth, evaluatory coverage of the most fundamental methodological challenges that set the basis for the future development of the field, in particular, the current developments and inherent physical limitations of the atomistic force fields and the recent advances in a broad spectrum of enhanced sampling methods. We also survey the closely related field of coarse-grained modeling of RNA systems. After dealing with the methodological aspects, we provide an exhaustive overview of the available RNA simulation literature, ranging from studies of the smallest RNA oligonucleotides to investigations of the entire ribosome. Our review encompasses tetranucleotides, tetraloops, a number of small RNA motifs, A-helix RNA, kissing-loop complexes, the TAR RNA element, the decoding center and other important regions of the ribosome, as well as assorted others systems. Extended sections are devoted to RNA-ion interactions, ribozymes, riboswitches, and protein/RNA complexes. Our overview is written for as broad of an audience as possible, aiming to provide a much-needed interdisciplinary bridge between computation and experiment, together with a perspective on the future of the field.

Quantifying the Relationship between Single-Molecule Probes and Subunit Rotation in the Ribosome

A major challenge in the study of biomolecular assemblies is to identify reaction coordinates that precisely monitor conformational rearrangements. This is central to the interpretation of single-molecule fluorescence resonance energy transfer measurements, where the observed dynamics depends on the labeling strategy. As an example, different probes of subunit rotation in the ribosome have provided qualitatively distinct descriptions. In one study, changes in fluorescence suggested that the 30S body undergoes a single rotation/back-rotation cycle during the process of mRNA-tRNA translocation. In contrast, an alternate assay implicated the presence of reversible rotation events before completing translocation. For future single-molecule experiments to unambiguously measure the relationship between subunit rotation and translocation, it is necessary to rationalize these conflicting descriptions. To this end, we have simulated hundreds of spontaneous subunit rotation events (≈8°) using a residue-level coarse-grained model of the ribosome. We analyzed nine different reaction coordinates and found that the apparently inconsistent measurements are likely a consequence of ribosomal flexibility. Further, we propose a metric for quantifying the degree of energetic coupling between experimentally measured degrees of freedom and subunit rotation. This analysis provides a physically grounded framework that can guide the development of more precise single-molecule techniques.

Three tRNAs on the ribosome slow translation elongation

During protein synthesis, the ribosome simultaneously binds up to three different transfer RNA (tRNA) molecules. Among the three tRNA binding sites, the regulatory role of the exit (E) site, where deacylated tRNA spontaneously dissociates from the translational complex, has remained elusive. Here we use two donor-quencher pairs to observe and correlate both the conformation of ribosomes and tRNAs as well as tRNA occupancy. Our results reveal a partially rotated state of the ribosome wherein all three tRNA sites are occupied during translation elongation. The appearance and lifetime of this state depend on the E-site tRNA dissociation kinetics, which may vary among tRNA species and depends on temperature and ionic strength. The 3-tRNA partially rotated state is not a proper substrate for elongation factor G (EF-G), thus inhibiting translocation until the E-site tRNA dissociates. Our result presents two parallel kinetic pathways during translation elongation, underscoring the ability of E-site codons to modulate the dynamics of protein synthesis.

The translation elongation cycle-capturing multiple states by cryo-electron microscopy

During the work cycle of elongation, the ribosome, a molecular machine of vast complexity, exists in a large number of states distinguished by constellation of its subunits, its subunit domains and binding partners. Single-particle cryogenic electron microscopy (cryo-EM), developed over the past 40 years, is uniquely suited to determine the structure of molecular machines in their native states. With the emergence, 10 years ago, of unsupervised clustering techniques in the analysis of single-particle data, it has been possible to determine multiple structures from a sample containing ribosomes equilibrating in different thermally accessible states. In addition, recent advances in detector technology have made it possible to reach near-atomic resolution for some of these states. With these capabilities, single-particle cryo-EM has been at the forefront of exploring ribosome dynamics during its functional cycle, along with single-molecule fluorescence resonance energy transfer and molecular dynamics computations, offering insights into molecular architecture uniquely honed by evolution to capitalize on thermal energy in the ambient environment.This article is part of the themed issue 'Perspectives on the ribosome'.

Cryo-EM structure of a late pre-40S ribosomal subunit from Saccharomyces cerevisiae

Mechanistic understanding of eukaryotic ribosome formation requires a detailed structural knowledge of the numerous assembly intermediates, generated along a complex pathway. Here, we present the structure of a late pre-40S particle at 3.6 Å resolution, revealing in molecular detail how assembly factors regulate the timely folding of pre-18S rRNA. The structure shows that, rather than sterically blocking 40S translational active sites, the associated assembly factors Tsr1, Enp1, Rio2 and Pno1 collectively preclude their final maturation, thereby preventing untimely tRNA and mRNA binding and error prone translation. Moreover, the structure explains how Pno1 coordinates the 3’end cleavage of the 18S rRNA by Nob1 and how the late factor’s removal in the cytoplasm ensures the structural integrity of the maturing 40S subunit.

Anisotropic Fluctuations in the Ribosome Determine tRNA Kinetics

The ribosome is a large ribonucleoprotein complex that is responsible for the production of proteins in all organisms. Accommodation is the process by which an incoming aminoacyl-tRNA (aa-tRNA) molecule binds the ribosomal A site, and its kinetics has been implicated in the accuracy of tRNA selection. In addition to rearrangements in the aa-tRNA molecule, the L11 stalk can undergo large-scale anisotropic motions during translation. To explore the potential impact of this protruding region on the rate of aa-tRNA accommodation, we used molecular dynamics simulations with a simplified model to evaluate the free energy as a function of aa-tRNA position. Specifically, these calculations describe the transition between A/T and elbow-accommodated (EA) configurations (∼20 Å displacement). We find that the free-energy barrier associated with elbow accommodation is proportional to the degree of mobility exhibited by the L11 stalk. That is, when L11 is more rigid, the free-energy barrier height is decreased. This effect arises from the ability of L11 to confine, and thereby destabilize, the A/T ensemble. In addition, when elongation factor Tu (EF-Tu) is present, the A/T ensemble is further destabilized in an L11-dependent manner. These results provide a framework that suggests how next-generation experiments may precisely control the dynamics of the ribosome.

Tracking fluctuation hotspots on the yeast ribosome through the elongation cycle

Chemical modification was used to quantitatively determine the flexibility of nearly the entire rRNA component of the yeast ribosome through 8 discrete stages of translational elongation, revealing novel observations at the gross and fine-scales. These include (i) the bulk transfer of energy through the intersubunit bridges from the large to the small subunit after peptidyltransfer, (ii) differences in the interaction of the sarcin ricin loop with the two elongation factors and (iii) networked information exchange pathways that may functionally facilitate intra- and intersubunit coordination, including the 5.8S rRNA. These analyses reveal hot spots of fluctuations that set the stage for large-scale conformational changes essential for translocation and enable the first molecular dynamics simulation of an 80S complex. Comprehensive datasets of rRNA base flexibilities provide a unique resource to the structural biology community that can be computationally mined to complement ongoing research toward the goal of understanding the dynamic ribosome.

Recurring RNA structural motifs underlie the mechanics of L1 stalk movement

The L1 stalk of the large ribosomal subunit undergoes large-scale movements coupled to the translocation of deacylated tRNA during protein synthesis. We use quantitative comparative structural analysis to localize the origins of L1 stalk movement and to understand its dynamic interactions with tRNA and other structural elements of the ribosome. Besides its stacking interactions with the tRNA elbow, stalk movement is directly linked to intersubunit rotation, rotation of the 30S head domain and contact of the acceptor arm of deacylated tRNA with helix 68 of 23S rRNA. Movement originates from pivoting at stacked non-canonical base pairs in a Family A three-way junction and bending in an internal G-U-rich zone. Use of these same motifs as hinge points to enable such dynamic events as rotation of the 30S subunit head domain and in flexing of the anticodon arm of tRNA suggests that they represent general strategies for movement of functional RNAs.

Translocation of the tRNA on the ribosome is associated with large-scale molecular movements of the ribosomal L1 stalk. Here the authors identify the key determinants that allow these dramatic movements, and suggest they represent general strategies used to enable large-scale motions in functional RNAs.

Translocation as continuous movement through the ribosome

In each round of translation elongation, tRNAs and mRNA move within the ribosome by one codon at a time. tRNA-mRNA translocation is promoted by elongation factor G (EF-G) at the cost of GTP hydrolysis. The key questions for understanding translocation are how and when the tRNAs move and how EF-G coordinates motions of the ribosomal subunits with tRNA movement. Here we present 2 recent papers which describe the choreography of movements over the whole trajectory of translocation. We present the view that EF-G accelerates translocation by promoting the steps that lead to GTPase-dependent ribosome unlocking. EF-G facilitates the formation of the rotated state of the ribosome and uncouples the backward motions of the ribosomal subunits, forming an open conformation in which the tRNAs can rapidly move. Ribosome dynamics are important not only in translocation, but also in recoding events, such as frameshifting and bypassing, and mediate sensitivity to antibiotics.

How EF-Tu can contribute to efficient proofreading of aa-tRNA by the ribosome

It has long been recognized that the thermodynamics of mRNA–tRNA base pairing is insufficient to explain the high fidelity and efficiency of aminoacyl-tRNA (aa-tRNA) selection by the ribosome. To rationalize this apparent inconsistency, Hopfield proposed that the ribosome may improve accuracy by utilizing a multi-step kinetic proofreading mechanism. While biochemical, structural and single-molecule studies have provided a detailed characterization of aa-tRNA selection, there is a limited understanding of how the physical–chemical properties of the ribosome enable proofreading. To this end, we probe the role of EF-Tu during aa-tRNA accommodation (the proofreading step) through the use of energy landscape principles, molecular dynamics simulations and kinetic models. We find that the steric composition of EF-Tu can reduce the free-energy barrier associated with the first step of accommodation: elbow accommodation. We interpret this effect within an extended kinetic model of accommodation and show how EF-Tu can contribute to efficient and accurate proofreading.

The translation of mRNA by the ribosome is governed by a series of large-scale conformational transitions. Here the authors use MD simulations to demonstrate how the rate of dissociation of elongation factor Tu affects the dynamics of tRNA accommodation and proofreading.

Shortening the HIV-1 TAR RNA Bulge by a Single Nucleotide Preserves Motional Modes over a Broad Range of Time Scales

Helix-junction-helix (HJH) motifs are flexible building blocks of RNA architecture that help define the orientation and dynamics of helical domains. They are also frequently involved in adaptive recognition of proteins and small molecules and in the formation of tertiary contacts. Here, we use a battery of nuclear magnetic resonance techniques to examine how deleting a single bulge residue (C24) from the human immunodeficiency virus type 1 (HIV-1) transactivation response element (TAR) trinucleotide bulge (U23-C24-U25) affects dynamics over a broad range of time scales. Shortening the bulge has an effect on picosecond-to-nanosecond interhelical and local bulge dynamics similar to that casued by increasing the Mg(2+) and Na(+) concentration, whereby a preexisting two-state equilibrium in TAR is shifted away from a bent flexible conformation toward a coaxial conformation, in which all three bulge residues are flipped out and flexible. Surprisingly, the point deletion minimally affects microsecond-to-millisecond conformational exchange directed toward two low-populated and short-lived excited conformational states that form through reshuffling of bases pairs throughout TAR. The mutant does, however, adopt a slightly different excited conformational state on the millisecond time scale, in which U23 is intrahelical, mimicking the expected conformation of residue C24 in the excited conformational state of wild-type TAR. Thus, minor changes in HJH topology preserve motional modes in RNA occurring over the picosecond-to-millisecond time scales but alter the relative populations of the sampled states or cause subtle changes in their conformational features.

Preparation of ribosomes for smFRET studies: A simplified approach

During the past decade, single-molecule studies of the ribosome have significantly advanced our understanding of protein synthesis. The broadest application of these methods has been towards the investigation of ribosome conformational dynamics using single-molecule Förster resonance energy transfer (smFRET). The recent advances in fluorescently labeled ribosomes and translation components have resulted in success of smFRET experiments. Various methods have been employed to target fluorescent dyes to specific locations within the ribosome. Primarily, these methods have involved additional steps including subunit dissociation and/or full reconstitution, which could result in ribosomes of reduced activity and translation efficiency. In addition, substantial time and effort are required to produce limited quantities of material. To enable rapid and large-scale production of highly active, fluorescently labeled ribosomes, we have developed a procedure that combines partial reconstitution with His-tag purification. This allows for a homogeneous single-step purification of mutant ribosomes and subsequent integration of labeled proteins. Ribosomes produced with this method are shown to be as active as ribosomes purified using classical methods. While we have focused on two labeling sites in this report, the method is generalizable and can in principle be extended to any non-essential ribosomal protein.

Ensemble cryo-EM uncovers inchworm-like translocation of a viral IRES through the ribosome

Internal ribosome entry sites (IRESs) mediate cap-independent translation of viral mRNAs. Using electron cryo-microscopy of a single specimen, we present five ribosome structures formed with the Taura syndrome virus IRES and translocase eEF2•GTP bound with sordarin. The structures suggest a trajectory of IRES translocation, required for translation initiation, and provide an unprecedented view of eEF2 dynamics. The IRES rearranges from extended to bent to extended conformations. This inchworm-like movement is coupled with ribosomal inter-subunit rotation and 40S head swivel. eEF2, attached to the 60S subunit, slides along the rotating 40S subunit to enter the A site. Its diphthamide-bearing tip at domain IV separates the tRNA-mRNA-like pseudoknot I (PKI) of the IRES from the decoding center. This unlocks 40S domains, facilitating head swivel and biasing IRES translocation via hitherto-elusive intermediates with PKI captured between the A and P sites. The structures suggest missing links in our understanding of tRNA translocation.

Structural insights into ribosome translocation

During protein synthesis, tRNA and mRNA are translocated from the A to P to E sites of the ribosome thus enabling the ribosome to translate one codon of mRNA after the other. Ribosome translocation along mRNA is induced by the universally conserved ribosome GTPase, elongation factor G (EF‐G) in bacteria and elongation factor 2 (EF‐2) in eukaryotes. Recent structural and single‐molecule studies revealed that tRNA and mRNA translocation within the ribosome is accompanied by cyclic forward and reverse rotations between the large and small ribosomal subunits parallel to the plane of the intersubunit interface. In addition, during ribosome translocation, the ‘head’ domain of small ribosomal subunit undergoes forward‐ and back‐swiveling motions relative to the rest of the small ribosomal subunit around the axis that is orthogonal to the axis of intersubunit rotation. tRNA/mRNA translocation is also coupled to the docking of domain IV of EF‐G into the A site of the small ribosomal subunit that converts the thermally driven motions of the ribosome and tRNA into the forward translocation of tRNA/mRNA inside the ribosome. Despite recent and enormous progress made in the understanding of the molecular mechanism of ribosome translocation, the sequence of structural rearrangements of the ribosome, EF‐G and tRNA during translocation is still not fully established and awaits further investigation. WIREs RNA 2016, 7:620–636. doi: 10.1002/wrna.1354

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Continuous changes in structure mapped by manifold embedding of single-particle data in cryo-EM

Cryo-electron microscopy, when combined with single-particle reconstruction, is a powerful method for studying macromolecular structure. Recent developments in detector technology have pushed the resolution into a range comparable to that of X-ray crystallography. However, cryo-EM is able to separate and thus recover the structure of each of several discrete structures present in the sample. For the more general case involving continuous structural changes, a novel technique employing manifold embedding has been recently demonstrated. Potentially, the entire work-cycle of a molecular machine may be observed as it passes through a continuum of states, and its free-energy landscape may be mapped out. This technique will be outlined and discussed in the context of its application to a large single-particle dataset of yeast ribosomes.

E. coli metabolic protein aldehyde-alcohol dehydrogenase-E binds to the ribosome: a unique moonlighting action revealed

It is becoming increasingly evident that a high degree of regulation is involved in the protein synthesis machinery entailing more interacting regulatory factors. A multitude of proteins have been identified recently which show regulatory function upon binding to the ribosome. Here, we identify tight association of a metabolic protein aldehyde-alcohol dehydrogenase E (AdhE) with the E. coli 70S ribosome isolated from cell extract under low salt wash conditions. Cryo-EM reconstruction of the ribosome sample allows us to localize its position on the head of the small subunit, near the mRNA entrance. Our study demonstrates substantial RNA unwinding activity of AdhE which can account for the ability of ribosome to translate through downstream of at least certain mRNA helices. Thus far, in E. coli, no ribosome-associated factor has been identified that shows downstream mRNA helicase activity. Additionally, the cryo-EM map reveals interaction of another extracellular protein, outer membrane protein C (OmpC), with the ribosome at the peripheral solvent side of the 50S subunit. Our result also provides important insight into plausible functional role of OmpC upon ribosome binding. Visualization of the ribosome purified directly from the cell lysate unveils for the first time interactions of additional regulatory proteins with the ribosome.