Non-canonical Binding Site for Bacterial Initiation Factor 3 on the Large Ribosomal Subunit

Cooperation of regulatory RNA and the RNA degradosome in transcript surveillance

The ompD transcript, encoding an outer membrane porin in Salmonella, harbors a controlling element in its coding region that base-pairs imperfectly with a 'seed' region of the small regulatory RNA (sRNA) MicC. When tagged with the sRNA, the ompD mRNA is cleaved downstream of the pairing site by the conserved endoribonuclease RNase E, leading to transcript destruction. We observe that the sRNA-induced cleavage site is accessible to RNase E in vitro upon recruitment of ompD into the 30S translation pre-initiation complex (PIC) in the presence of the degradosome components. Evaluation of substrate accessibility suggests that the paused 30S PIC presents the mRNA for targeted recognition and degradation. Ribonuclease activity on PIC-bound ompD is critically dependent on the recruitment of RNase E into the multi-enzyme RNA degradosome, and our data suggest a process of substrate capture and handover to catalytic sites within the degradosome, in which sequential steps of seed matching and duplex remodelling contribute to cleavage efficiency. Our findings support a putative mechanism of surveillance at translation that potentially terminates gene expression efficiently and rapidly in response to signals provided by regulatory RNA.

The Role of the Universally Conserved ATPase YchF/Ola1 in Translation Regulation during Cellular Stress

The ability to respond to metabolic or environmental changes is an essential feature in all cells and involves both transcriptional and translational regulators that adjust the metabolic activity to fluctuating conditions. While transcriptional regulation has been studied in detail, the important role of the ribosome as an additional player in regulating gene expression is only beginning to emerge. Ribosome-interacting proteins are central to this translational regulation and include universally conserved ribosome interacting proteins, such as the ATPase YchF (Ola1 in eukaryotes). In both eukaryotes and bacteria, the cellular concentrations of YchF/Ola1 determine the ability to cope with different stress conditions and are linked to several pathologies in humans. The available data indicate that YchF/Ola1 regulates the stress response via controlling non-canonical translation initiation and via protein degradation. Although the molecular mechanisms appear to be different between bacteria and eukaryotes, increased non-canonical translation initiation is a common consequence of YchF/Ola1 regulated translational control in E. coli and H. sapiens. In this review, we summarize recent insights into the role of the universally conserved ATPase YchF/Ola1 in adapting translation to unfavourable conditions.

The dynamic cycle of bacterial translation initiation factor IF3

Initiation factor IF3 is an essential protein that enhances the fidelity and speed of bacterial mRNA translation initiation. Here, we describe the dynamic interplay between IF3 domains and their alternative binding sites using pre-steady state kinetics combined with molecular modelling of available structures of initiation complexes. Our results show that IF3 accommodates its domains at velocities ranging over two orders of magnitude, responding to the binding of each 30S ligand. IF1 and IF2 promote IF3 compaction and the movement of the C-terminal domain (IF3C) towards the P site. Concomitantly, the N-terminal domain (IF3N) creates a pocket ready to accept the initiator tRNA. Selection of the initiator tRNA is accompanied by a transient accommodation of IF3N towards the 30S platform. Decoding of the mRNA start codon displaces IF3C away from the P site and rate limits translation initiation. 70S initiation complex formation brings IF3 domains in close proximity to each other prior to dissociation and recycling of the factor for a new round of translation initiation. Altogether, our results describe the kinetic spectrum of IF3 movements and highlight functional transitions of the factor that ensure accurate mRNA translation initiation.

The Universally Conserved ATPase YchF Regulates Translation of Leaderless mRNA in Response to Stress Conditions

The universally conserved P-loop GTPases control diverse cellular processes, like signal transduction, ribosome assembly, cell motility, and intracellular transport and translation. YchF belongs to the Obg-family of P-loop GTPases and is one of the least characterized member of this family. It is unique because it preferentially hydrolyses ATP rather than GTP, but its physiological role is largely unknown. Studies in different organisms including humans suggest a possible role of YchF in regulating the cellular adaptation to stress conditions. In the current study, we explored the role of YchF in the model organism Escherichia coli. By western blot and promoter fusion experiments, we demonstrate that YchF levels decrease during stress conditions or when cells enter stationary phase. The decline in YchF levels trigger increased stress resistance and cells lacking YchF are resistant to multiple stress conditions, like oxidative stress, replication stress, or translational stress. By in vivo site directed cross-linking we demonstrate that YchF interacts with the translation initiation factor 3 (IF3) and with multiple ribosomal proteins at the surface of the small ribosomal subunit. The absence of YchF enhances the anti-association activity of IF3, stimulates the translation of leaderless mRNAs, and increases the resistance against the endoribonuclease MazF, which generates leaderless mRNAs during stress conditions. In summary, our data identify YchF as a stress-responsive regulator of leaderless mRNA translation.

Mechanism of ribosome rescue by alternative ribosome-rescue factor B

Alternative ribosome-rescue factor B (ArfB) rescues ribosomes stalled on non-stop mRNAs by releasing the nascent polypeptide from the peptidyl-tRNA. By rapid kinetics we show that ArfB selects ribosomes stalled on short truncated mRNAs, rather than on longer mRNAs mimicking pausing on rare codon clusters. In combination with cryo-electron microscopy we dissect the multistep rescue pathway of ArfB, which first binds to ribosomes very rapidly regardless of the mRNA length. The selectivity for shorter mRNAs arises from the subsequent slow engagement step, as it requires longer mRNA to shift to enable ArfB binding. Engagement results in specific interactions of the ArfB C-terminal domain with the mRNA entry channel, which activates peptidyl-tRNA hydrolysis by the N-terminal domain. These data reveal how protein dynamics translate into specificity of substrate recognition and provide insights into the action of a putative rescue factor in mitochondria.

Rescue of ribosomes stalled on non-stop mRNA is essential for cell viability, and several rescue systems to resolve stalling exist in bacteria. Here, the authors use rapid kinetics and cryo-EM to reveal the pathway and selectivity mechanism of ArfB-mediated ribosome rescue.

1H, 13C and 15N resonance assignments of translation initiation factor 3 from Pseudomonas aeruginosa

Translation initiation factor 3 (IF3) is one of the three protein factors that bind to the small ribosomal subunit and it is required for the initiation of protein biosynthesis in bacteria. IF3 contains two independent domains, N- and C-terminal domains, which are connected by a lysine-rich interdomain linker. IF3 undergoes large-scale movements and conformational changes upon binding to the 30S subunit and also during the functional regulation of initiation. However, the precise dynamic interplay of the two domains and the molecular mechanism of IF3 is not well understood. A high-resolution 3D structure of a complete IF3 in bacteria has not been solved. Pseudomonas aeruginosa, a gram-negative opportunistic pathogen, is a primary cause of nosocomial infections in humans. Here we report the NMR chemical shift assignments of IF3 from P. aeruginosa as the first step toward NMR structure determination and interaction studies. Secondary structure analyses deduced from the NMR chemical shift data identified nine β-strands and four α-helices arranged in the sequential order β1-β2-α1-β3-β4-α2-β5-α3-β6-α4-β7-β8-β9.

Translation initiation in mammalian mitochondria- a prokaryotic perspective

ATP is generated in mitochondria of eukaryotic cells by oxidative phosphorylation (OXPHOS). The OXPHOS complex, which is crucial for cellular metabolism, comprises of both nuclear and mitochondrially encoded subunits. Also, the occurrence of several pathologies because of mutations in the mitochondrial translation apparatus indicates the importance of mitochondrial translation and its regulation. The mitochondrial translation apparatus is similar to its prokaryotic counterpart due to a common origin of evolution. However, mitochondrial translation has diverged from prokaryotic translation in many ways by reductive evolution. In this review, we focus on mammalian mitochondrial translation initiation, a highly regulated step of translation, and present a comparison with prokaryotic translation.

Unraveling the hidden universe of small proteins in bacterial genomes

Identification of small open reading frames (smORFs) encoding small proteins (≤ 100 amino acids; SEPs) is a challenge in the fields of genome annotation and protein discovery. Here, by combining a novel bioinformatics tool (RanSEPs) with “‐omics” approaches, we were able to describe 109 bacterial small ORFomes. Predictions were first validated by performing an exhaustive search of SEPs present in Mycoplasma pneumoniae proteome via mass spectrometry, which illustrated the limitations of shotgun approaches. Then, RanSEPs predictions were validated and compared with other tools using proteomic datasets from different bacterial species and SEPs from the literature. We found that up to 16 ± 9% of proteins in an organism could be classified as SEPs. Integration of RanSEPs predictions with transcriptomics data showed that some annotated non‐coding RNAs could in fact encode for SEPs. A functional study of SEPs highlighted an enrichment in the membrane, translation, metabolism, and nucleotide‐binding categories. Additionally, 9.7% of the SEPs included a N‐terminus predicted signal peptide. We envision RanSEPs as a tool to unmask the hidden universe of small bacterial proteins.

Unravelling Ribosome Function Through Structural Studies

Ribosomes are biological nanomachine that synthesise all proteins within a cell. It took decades to reveal the architecture of this essential cellular component. To understand the structure -function relationship of this nanomachine needed the utilisisation of different biochemical, biophysical and structural techniques. Structural studies combined with mutagenesis of the different ribosomal complexes comprising various RNAs and proteins enabled us to understand how this machine works inside a cell. Nowadays quite a number of ribosomal structures were published that confirmed biochemical studies on particular steps of protein synthesis by the ribosome . Four major steps were identified: initiation , elongation, termination and recycling. These steps lead us to the important question how the ribosome function can be regulated. Advances in technology for cryo electron microscopy: sample preparations, image recording, developments in algorithms for image analysis and processing significantly helped in revelation of structural details of the ribosome . We now have a library of ribosome structures from prokaryotes to eukaryotes that enable us to understand the complex mechanics of this nanomachine. As this structural library continues to grow, we gradually improve our understanding of this process and how it can be regulated and how the specific ribosomes can be stalled or activated, or completely disabled. This article provides a comprehensive overview of ribosomal structures that represent structural snapshots of the ribosome at its different functional states. Better understanding rises more particular questions that have to be addressed by determination structures of more complexes.Synopsis: Structural biology of the ribosome.

60S dynamic state of bacterial ribosome is fixed by yeast mitochondrial initiation factor 3

The processes of association and dissociation of ribosomal subunits are of great importance for the protein biosynthesis. The mechanistic details of these processes, however, are not well known. In bacteria, upon translation termination, the ribosome dissociates into subunits which is necessary for its further involvement into new initiation step. The dissociated state of the ribosome is maintained by initiation factor 3 (IF3) which binds to free small subunits and prevents their premature association with large subunits. In this work, we have exchanged IF3 in Escherichia coli cells by its ortholog from Saccharomyces cerevisiae mitochondria (Aim23p) and showed that yeast protein cannot functionally substitute the bacterial one and is even slightly toxic for bacterial cells. Our in vitro experiments have demonstrated that Aim23p does not split E. coli ribosomes into subunits. Instead, it fixes a state of ribosomes characterized by sedimentation coefficient about 60S which is not a stable structure but rather reflects a shift of dynamic equilibrium between associated and dissociated states of the ribosome. Mitochondria-specific terminal extensions of Aim23p are necessary for “60S state” formation, and molecular modeling results point out that these extensions might stabilize the position of the protein on the bacterial ribosome.

Translation in Prokaryotes

This review summarizes our current understanding of translation in prokaryotes, focusing on the mechanistic and structural aspects of each phase of translation: initiation, elongation, termination, and ribosome recycling. The assembly of the initiation complex provides multiple checkpoints for messenger RNA (mRNA) and start-site selection. Correct codon-anticodon interaction during the decoding phase of elongation results in major conformational changes of the small ribosomal subunit and shapes the reaction pathway of guanosine triphosphate (GTP) hydrolysis. The ribosome orchestrates proton transfer during peptide bond formation, but requires the help of elongation factor P (EF-P) when two or more consecutive Pro residues are to be incorporated. Understanding the choreography of transfer RNA (tRNA) and mRNA movements during translocation helps to place the available structures of translocation intermediates onto the time axis of the reaction pathway. The nascent protein begins to fold cotranslationally, in the constrained space of the polypeptide exit tunnel of the ribosome. When a stop codon is reached at the end of the coding sequence, the ribosome, assisted by termination factors, hydrolyzes the ester bond of the peptidyl-tRNA, thereby releasing the nascent protein. Following termination, the ribosome is dissociated into subunits and recycled into another round of initiation. At each step of translation, the ribosome undergoes dynamic fluctuations between different conformation states. The aim of this article is to show the link between ribosome structure, dynamics, and function.