Stress response as implemented by hibernating ribosomes: a structural overview

Recent advances in applying omic technologies for studying acetic acid bacteria in industrial vinegar production: A comprehensive review

Vinegar and related bioproducts containing acetic acid as the main component are among the most appreciated fermented foodstuffs in numerous European and Asian countries because of their exceptional organoleptic and bio-healthy properties. Regarding the acetification process and obtaining of final products, there is still a lack of knowledge on fundamental aspects, especially those related to the study of biodiversity and metabolism of the present microbiota. In this context, omic technologies currently allow for the massive analysis of macromolecules and metabolites for the identification and characterization of these microorganisms working in their natural media without the need for isolation. This review approaches comprehensive research on the application of omic tools for the identification of vinegar microbiota, mainly acetic acid bacteria, with subsequent emphasis on the study of the microbial diversity, behavior, and key molecular strategies used by the predominant groups throughout acetification. The current omics tools are enabling both the finding of new vinegar microbiota members and exploring underlying strategies during the elaboration process. The species Komagataeibacter europaeus may be a model organism for present and future research in this industry; moreover, the development of integrated meta-omic analysis may facilitate the achievement of numerous of the proposed milestones. This work might provide useful guidance for the vinegar industry establishing the first steps towards the improvement of the acetification conditions and the development of new products with sensory and bio-healthy profiles adapted to the agri-food market.

Ultrastructural insights into the microsporidian infection apparatus reveal the kinetics and morphological transitions of polar tube and cargo during host cell invasion

During host cell invasion, microsporidian spores translocate their entire cytoplasmic content through a thin, hollow superstructure known as the polar tube. To achieve this, the polar tube transitions from a compact spring-like state inside the environmental spore to a long needle-like tube capable of long-range sporoplasm delivery. The unique mechanical properties of the building blocks of the polar tube allow for an explosive transition from compact to extended state and support the rapid cargo translocation process. The molecular and structural factors enabling this ultrafast process and the structural changes during cargo delivery are unknown. Here, we employ light microscopy and in situ cryo-electron tomography to visualize multiple ultrastructural states of the Vairimorpha necatrix polar tube, allowing us to evaluate the kinetics of its germination and characterize the underlying morphological transitions. We describe a cargo-filled state with a unique ordered arrangement of microsporidian ribosomes, which cluster along the thin tube wall, and an empty post-translocation state with a reduced diameter but a thicker wall. Together with a proteomic analysis of endogenously affinity-purified polar tubes, our work provides comprehensive data on the infection apparatus of microsporidia and uncovers new aspects of ribosome regulation and transport.

Cryo- EM structure of the mycobacterial 70S ribosome in complex with ribosome hibernation promotion factor RafH

Ribosome hibernation is a key survival strategy bacteria adopt under environmental stress, where a protein, hibernation promotion factor (HPF), transitorily inactivates the ribosome. Mycobacterium tuberculosis encounters hypoxia (low oxygen) as a major stress in the host macrophages, and upregulates the expression of RafH protein, which is crucial for its survival. The RafH, a dual domain HPF, an orthologue of bacterial long HPF (HPFlong), hibernates ribosome in 70S monosome form, whereas in other bacteria, the HPFlong induces 70S ribosome dimerization and hibernates its ribosome in 100S disome form. Here, we report the cryo- EM structure of M. smegmatis, a close homolog of M. tuberculosis, 70S ribosome in complex with the RafH factor at an overall 2.8 Å resolution. The N- terminus domain (NTD) of RafH binds to the decoding center, similarly to HPFlong NTD. In contrast, the C- terminus domain (CTD) of RafH, which is larger than the HPFlong CTD, binds to a distinct site at the platform binding center of the ribosomal small subunit. The two domain-connecting linker regions, which remain mostly disordered in earlier reported HPFlong structures, interact mainly with the anti-Shine Dalgarno sequence of the 16S rRNA.

Comparative transcriptomics analysis of multidrug-resistant Acinetobacter baumannii in response to treatment with the terpenic compounds thymol and carvacrol

Acinetobacter baumannii is a gram-negative opportunistic bacterium that has become a major public health concern and a substantial medical challenge due to its ability to acquire multidrug resistance (MDR), extended-drug resistance, or pan-drug resistance. In this study, we evaluated the antibacterial activity of thymol and carvacrol alone or in combination against clinical isolates of MDR A. baumannii. Additionally, we used RNA-sequency to perform a comparative transcriptomic analysis of the effects of carvacrol and thymol on the Acb35 strain under different treatment conditions. Our results demonstrated that thymol and carvacrol alone, effectively inhibited the bacterial growth of MDR A. baumannii isolates, with a minimum inhibitory concentration (MIC) lower than 500 μg/mL. Furthermore, the combination of thymol and carvacrol exhibited either synergistic (FICI ≤ 0.5) or additive effects (0.5 < FICI ≤ 4), enhancing their antibacterial activity. Importantly, these compounds were found to be non-cytotoxic to Vero cells and did not cause hemolysis in erythrocytes at concentrations that effectively inhibited bacterial growth. Transcriptomic analysis revealed the down-regulation of mRNA associated with ribosomal subunit assemblies under all experimental conditions tested. However, the up-regulation of specific genes encoding stress response proteins and transcriptional regulators varied depending on the experimental condition, particularly in response to the treatment with carvacrol and thymol in combination. Based on our findings, thymol and carvacrol demonstrate promising potential as chemotherapeutic agents for controlling MDR A. baumannii infections. These compounds exhibit strong antibacterial activity, particularly in combination and lower cytotoxicity towards mammalian cells. The observed effects on gene expression provide insights into the underlying mechanisms of action, highlighting the regulation of stress response pathways.

Coping with stress: How bacteria fine-tune protein synthesis and protein transport

Maintaining a functional proteome under different environmental conditions is challenging for every organism, in particular for unicellular organisms, such as bacteria. In order to cope with changing environments and stress conditions, bacteria depend on strictly coordinated proteostasis networks that control protein production, folding, trafficking, and degradation. Regulation of ribosome biogenesis and protein synthesis are cornerstones of this cellular adaptation in all domains of life, which is rationalized by the high energy demand of both processes and the increased resistance of translationally silent cells against internal or external poisons. Reduced protein synthesis ultimately also reduces the substrate load for protein transport systems, which are required for maintaining the periplasmic, inner, and outer membrane subproteomes. Consequences of impaired protein transport have been analyzed in several studies and generally induce a multifaceted response that includes the upregulation of chaperones and proteases and the simultaneous downregulation of protein synthesis. In contrast, generally less is known on how bacteria adjust the protein targeting and transport machineries to reduced protein synthesis, e.g., when cells encounter stress conditions or face nutrient deprivation. In the current review, which is mainly focused on studies using Escherichia coli as a model organism, we summarize basic concepts on how ribosome biogenesis and activity are regulated under stress conditions. In addition, we highlight some recent developments on how stress conditions directly impair protein targeting to the bacterial membrane. Finally, we describe mechanisms that allow bacteria to maintain the transport of stress-responsive proteins under conditions when the canonical protein targeting pathways are impaired.

Transcription termination factor ρ polymerizes under stress

Bacterial RNA helicase ρ is a genome sentinel that terminates synthesis of damaged and junk RNAs that are not translated by the ribosome. Co-transcriptional RNA surveillance by ρ is essential for quality control of the transcriptome during optimal growth. However, it is unclear how bacteria protect their RNAs from overzealous ρ during dormancy or stress, conditions common in natural habitats. Here we used cryogenic electron microscopy, biochemical, and genetic approaches to show that residue substitutions, ADP, or ppGpp promote hyper-oligomerization of Escherichia coli ρ. Our results demonstrate that nucleotides bound at subunit interfaces control ρ switching from active hexamers to inactive higher-order oligomers and extended filaments. Polymers formed upon exposure to antibiotics or ppGpp disassemble when stress is relieved, thereby directly linking termination activity to cellular physiology. Inactivation of ρ through hyper-oligomerization is a regulatory strategy shared by RNA polymerases, ribosomes, and metabolic enzymes across all life.

Quiescence of Escherichia coli Aerosols to Survive Mechanical Stress during High-Velocity Collection

A low cutpoint wetted wall bioaerosol sampling cyclone (LCP-WWC), with an aerosol sampling flow rate of 300 L/min at 55″ H2O pressure drop and a continuous liquid outflow rate of about 0.2 mL/min, was developed by upgrading an existing system. The laboratory strain Escherichia coli MG1655 was aerosolized using a six-jet Collison Nebulizer and collected at high velocity using the LCP-WWC for 10 min with different collection liquids. Each sample was quantitated during a 15-day archiving period after aerosolization for culturable counts (CFUs) and gene copy numbers (GCNs) using microbial plating and whole-cell quantitative polymerase chain (qPCR) reaction. The samples were analyzed for protein composition and antimicrobial resistance using protein gel electrophoresis and disc diffusion susceptibility testing. Aerosolization and collection were followed by an initial period of quiescence or dormancy. After 2 days of archiving at 4 °C and RT, the bacteria exhibited increased culturability and antibiotic resistance (ABR), especially to cell wall inhibitors (ampicillin and cephalothin). The number of resistant bacteria on Day 2 increased nearly four-times compared to the number of cells at the initial time of collection. The mechanical stress of aerosolization and high-velocity sampling likely stunned the cells triggering a response of dormancy, though with continued synthesis of vital proteins for survival. This study shows that an increase in intensity in environmental conditions surrounding airborne bacteria affects their ability to grow and their potential to develop antimicrobial resistance.

Bidirectional sequestration between a bacterial hibernation factor and a glutamate metabolizing protein

Bacterial hibernating 100S ribosomes (the 70S dimers) are excluded from translation and are protected from ribonucleolytic degradation, thereby promoting long-term viability and increased regrowth. No extraribosomal target of any hibernation factor has been reported. Here, we discovered a previously unrecognized binding partner (YwlG) of hibernation-promoting factor (HPF) in the human pathogen Staphylococcus aureus. YwlG is an uncharacterized virulence factor in S. aureus. We show that the HPF-YwlG interaction is direct, independent of ribosome binding, and functionally linked to cold adaptation and glucose metabolism. Consistent with the distant resemblance of YwlG to the hexameric structures of nicotinamide adenine dinucleotide (NAD)-specific glutamate dehydrogenases (GDHs), YwlG overexpression can compensate for a loss of cellular GDH activity. The reduced abundance of 100S complexes and the suppression of YwlG-dependent GDH activity provide evidence for a two-way sequestration between YwlG and HPF. These findings reveal an unexpected layer of regulation linking the biogenesis of 100S ribosomes to glutamate metabolism.

MqsR is a noncanonical microbial RNase toxin that is inhibited by antitoxin MqsA via steric blockage of substrate binding

The MqsRA toxin-antitoxin system is a component of the Escherichia coli stress response. Free MqsR, a ribonuclease, cleaves mRNAs containing a 5′-GC-3′ sequence causing a global shutdown of translation and the cell to enter a state of dormancy. Despite a general understanding of MqsR function, the molecular mechanism(s) by which MqsR binds and cleaves RNA and how one or more of these activities is inhibited by its cognate antitoxin MqsA is still poorly understood. Here, we used NMR spectroscopy coupled with mRNA cleavage assays to identify the molecular mechanism of MqsR substrate recognition and the MqsR residues that are essential for its catalytic activity. We show that MqsR preferentially binds substrates that contain purines in the −2 and −1 position relative to the MqsR consensus cleavage sequence and that two residues of MqsR, Tyr81, and Lys56 are strictly required for mRNA cleavage. We also show that MqsA inhibits MqsR activity by sterically blocking mRNA substrates from binding while leaving the active site fully accessible to mononucleotides. Together, these data identify the residues of MqsR that mediate RNA cleavage and reveal a novel mechanism that regulates MqsR substrate specificity.

Unraveling the Role of Acetic Acid Bacteria Comparing Two Acetification Profiles From Natural Raw Materials: A Quantitative Approach in Komagataeibacter europaeus

The industrial production of vinegar is carried out by the activity of a complex microbiota of acetic acid bacteria (AAB) working, mainly, within bioreactors providing a quite specific and hard environment. The “omics” sciences can facilitate the identification and characterization analyses of these microbial communities, most of which are difficult to cultivate by traditional methods, outside their natural medium. In this work, two acetification profiles coming from the same AAB starter culture but using two natural raw materials of different alcoholic origins (fine wine and craft beer), were characterized and compared and the emphasis of this study is the effect of these raw materials. For this purpose, the composition and natural behavior of the microbiota present throughout these profiles were analyzed by metaproteomics focusing, mainly, on the quantitative protein profile of Komagataeibacter europaeus. This species provided a protein fraction significantly higher (73.5%) than the others. A submerged culture system and semi-continuous operating mode were employed for the acetification profiles and liquid chromatography with tandem mass spectrometry (LC-MS/MS) for the protein analyses. The results showed that neither of two raw materials barely modified the microbiota composition of the profiles, however, they had an effect on the protein expression changes in different biological process. A molecular strategy in which K. europaeus would prevail over other species by taking advantage of the different features offered by each raw material has been suggested. First, by assimilating the excess of inner acetic acid through the TCA cycle and supplying biosynthetic precursors to replenish the cellular material losses; second, by a previous assimilation of the excess of available glucose, mainly in the beer medium, through the glycolysis and the pentose phosphate pathway (PPP); and third, by triggering membrane mechanisms dependent on proton motive force to detoxify the cell at the final moments of acetification. This study could complement the current knowledge of these bacteria as well as to expand the use of diverse raw materials and optimize operating conditions to obtain quality vinegars.

How to save a bacterial ribosome in times of stress

Biogenesis of ribosomes is one of the most cost- and resource-intensive processes in all living cells. In bacteria, ribosome biogenesis is rate-limiting for growth and must be tightly coordinated to yield maximum fitness of the cells. Since bacteria are continuously facing environmental changes and stress conditions, they have developed sophisticated systems to sense and regulate their nutritional status. Amino acid starvation leads to the synthesis and accumulation of the nucleotide-based second messengers ppGpp and pppGpp [(p)ppGpp], which in turn function as central players of a pleiotropic metabolic adaptation mechanism named the stringent response. Here, we review our current knowledge on the multiple roles of (p)ppGpp in the stress-related modulation of the prokaryotic protein biosynthesis machinery with the ribosome as its core constituent. The alarmones ppGpp/pppGpp act as competitors of their GDP/GTP counterparts, to affect a multitude of ribosome-associated P-loop GTPases involved in the translation cycle, ribosome biogenesis and hibernation. A similar mode of inhibition has been found for the GTPases of the proteins involved in the SRP-dependent membrane-targeting machinery present in the periphery of the ribosome. In this sense, during stringent conditions, binding of (p)ppGpp restricts the membrane insertion and secretion of proteins. Altogether, we highlight the enormously resource-intensive stages of ribosome biogenesis as a critical regulatory hub of the stringent response that ultimately tunes the protein synthesis capacity and consequently the survival of the cell.

Sleeping ribosomes: Bacterial signaling triggers RaiA mediated persistence to aminoglycosides

Indole is a molecule proposed to be involved in bacterial signaling. We find that indole secretion is induced by sublethal tobramycin concentrations and increases persistence to aminoglycosides in V. cholerae. Indole transcriptomics showed increased expression of raiA, a ribosome associated factor. Deletion of raiA abolishes the appearance of indole dependent persisters to aminoglycosides, although its overexpression leads to 100-fold increase of persisters, and a reduction in lag phase, evocative of increased active 70S ribosome content, confirmed by sucrose gradient analysis. We propose that, under stress conditions, RaiA-bound inactive 70S ribosomes are stored as “sleeping ribosomes”, and are rapidly reactivated upon stress relief. Our results point to an active process of persister formation through ribosome protection during translational stress (e.g., aminoglycoside treatment) and reactivation upon antibiotic removal. Translation is a universal process, and these results could help elucidate a mechanism of persistence formation in a controlled, thus inducible way.

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Indole is produced under sub-MIC tobramycin stress in V. cholerae and upregulates raiA

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RaiA is involved in indole-dependent formation of aminoglycoside specific persisters

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RaiA overexpression allows faster growth restart and increases 70S ribosome content

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RaiA-bound inactive 70S ribosomes form intact and reactivable sleeping ribosome pools

Two P or Not Two P: Understanding Regulation by the Bacterial Second Messengers (p)ppGpp

Under stressful growth conditions and nutrient starvation, bacteria adapt by synthesizing signaling molecules that profoundly reprogram cellular physiology. At the onset of this process, called the stringent response, members of the RelA/SpoT homolog (RSH) protein superfamily are activated by specific stress stimuli to produce several hyperphosphorylated forms of guanine nucleotides, commonly referred to as (p)ppGpp. Some bifunctional RSH enzymes also harbor domains that allow for degradation of (p)ppGpp by hydrolysis. (p)ppGpp synthesis or hydrolysis may further be executed by single-domain alarmone synthetases or hydrolases, respectively. The downstream effects of (p)ppGpp rely mainly on direct interaction with specific intracellular effectors, which are widely used throughout most cellular processes. The growing number of identified (p)ppGpp targets allows us to deduce both common features of and differences between gram-negative and gram-positive bacteria. In this review, we give an overview of (p)ppGpp metabolism with a focus on the functional and structural aspects of the enzymes involved and discuss recent findings on alarmone-regulated cellular effectors. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Transcription regulates ribosome hibernation

Most bacteria are quiescent, typically as a result of nutrient limitation. In order to minimize energy consumption during this potentially prolonged state, quiescent bacteria substantially attenuate protein synthesis, the most energetically costly cellular process. Ribosomes in quiescent bacteria are present as dimers of two 70S ribosomes. Dimerization is dependent on a single protein, hibernation promoting factor (HPF), that binds the ribosome in the mRNA channel. This interaction indicates that dimers are inactive, suggesting that HPF inhibits translation. However, we observe that HPF does not significantly affect protein synthesis in vivo suggesting that dimerization is a consequence of inactivity, not the cause. The HPF-dimer interaction further implies that re-initiation of translation when the bacteria exit quiescence requires dimer resolution. We show that ribosome dimers quickly resolve in the presence of nutrients, and this resolution is dependent on transcription, indicating that mRNA synthesis is required for dimer resolution. Finally, we observe that ectopic HPF expression in growing cells where mRNA is abundant does not significantly affect protein synthesis despite stimulating dimer formation, suggesting that dimerization is dynamic. Thus, the extensive transcription that occurs in response to nutrient availability rapidly re-activates the translational apparatus of a quiescent cell and induces dimer resolution.

Hibernation-Promoting Factor Sequesters Staphylococcus aureus Ribosomes to Antagonize RNase R-Mediated Nucleolytic Degradation

Bacterial and eukaryotic hibernation factors prevent translation by physically blocking the decoding center of ribosomes, a phenomenon called ribosome hibernation that often occurs in response to nutrient deprivation. The human pathogen Staphylococcus aureus lacking the sole hibernation factor HPF undergoes massive ribosome degradation via an unknown pathway. Using genetic and biochemical approaches, we find that inactivating the 3′-to-5′ exonuclease RNase R suppresses ribosome degradation in the Δhpf mutant. In vitro cell-free degradation assays confirm that 30S and 70S ribosomes isolated from the Δhpf mutant are extremely susceptible to RNase R, in stark contrast to nucleolytic resistance of the HPF-bound 70S and 100S complexes isolated from the wild type. In the absence of HPF, specific S. aureus 16S rRNA helices are sensitive to nucleolytic cleavage. These RNase hot spots are distinct from that found in the Escherichia coli ribosomes. S. aureus RNase R is associated with ribosomes, but unlike the E. coli counterpart, it is not regulated by general stressors and acetylation. The results not only highlight key differences between the evolutionarily conserved RNase R homologs but also provide direct evidence that HPF preserves ribosome integrity beyond its role in translational avoidance, thereby poising the hibernating ribosomes for rapid resumption of translation.

Physiology of guanosine-based second messenger signaling in Bacillus subtilis

The guanosine-based second messengers (p)ppGpp and c-di-GMP are key players of the physiological regulation of the Gram-positive model organism Bacillus subtilis. Their regulatory spectrum ranges from key metabolic processes over motility to biofilm formation. Here we review our mechanistic knowledge on their synthesis and degradation in response to environmental and stress signals as well as what is known on their cellular effectors and targets. Moreover, we discuss open questions and our gaps in knowledge on these two important second messengers.

Colonisation dynamics of Listeria monocytogenes strains isolated from food production environments

Listeria monocytogenes is a ubiquitous bacterium capable of colonising and persisting within food production environments (FPEs) for many years, even decades. This ability to colonise, survive and persist within the FPEs can result in food product cross-contamination, including vulnerable products such as ready to eat food items. Various environmental and genetic elements are purported to be involved, with the ability to form biofilms being an important factor. In this study we examined various mechanisms which can influence colonisation in FPEs. The ability of isolates (n = 52) to attach and grow in biofilm was assessed, distinguishing slower biofilm formers from isolates forming biofilm more rapidly. These isolates were further assessed to determine if growth rate, exopolymeric substance production and/or the agr signalling propeptide influenced these dynamics and could promote persistence in conditions reflective of FPE. Despite no strong association with the above factors to a rapid colonisation phenotype, the global transcriptome suggested transport, energy production and metabolism genes were widely upregulated during the initial colonisation stages under nutrient limited conditions. However, the upregulation of the metabolism systems varied between isolates supporting the idea that L. monocytogenes ability to colonise the FPEs is strain-specific.

Impact of Ribosome Activity on SARS-CoV-2 LNP - Based mRNA Vaccines

Coronavirus-related Severe Acute Respiratory Syndrome-2 (SARS-CoV-2) initially was detected in Wuhan, Hubei, China. Since early 2021, World Health Organization (WHO) has declared Coronavirus Disease 2019 (COVID-19) a pandemic due to rapidly transformed to a globally massive catastrophic viral infection. In order to confront this emergency situation, many pharmaceutical companies focused on the design and development of efficient vaccines that are considered necessary for providing a level of normalization in totally affected human social-economical activity worldwide. A variety of vaccine types are under development, validation or even some of them have already completed these stages, initially approved as conditional marketing authorisation by Food and Drug Administration (FDA), European Medicines Agency (EMA), and other national health authorities for commercial purposes (in vivo use in general population), accelerating their production and distribution process. Innovative nucleoside-modified viral messenger RNA (v-mRNA)-based vaccines encapsulated within nanoparticles-specifically lipid ones (LNPs)-are now well recognized. Although this is a promising genetic engineering topic in the field of nanopharmacogenomics or targeted nucleic vaccines, there are limited but continuously enriched in vivo data in depth of time regarding their safety, efficacy, and immune response. In the current paper we expand the limited published data in the field of ribosome machinery and SARS-CoV-2 mRNA fragment vaccines interaction by describing their functional specialization and modifications. Additionally, alterations in post-transcriptional/translational molecules and mechanisms that could potentially affect the interaction between target cells and vaccines are also presented. Understanding these mechanisms is a crucial step for the next generation v-mRNA vaccines development.

Functional metaproteomic analysis of alcohol vinegar microbiota during an acetification process: A quantitative proteomic approach

Vinegar is elaborated using a semi-continuous submerged culture of a complex microbiota of acetic acid bacteria. The genus Komagataeibacter provides much of the proteins of the metaproteome, being K. europaeus the main species working in this environment. In this work, the protein profile of the vinegar microbiota, obtained by means of liquid chromatography-tandem mass spectrometry (LC-MS/MS) in samples from different cycle times of an acetification process using an alcohol medium, has been used to describe the functional metaproteome throughout the process. The analysis was focused on Komagataeibacter species which supplied about 90% of the metaproteome and particularly K. europaeus which accounts for more than 70%. According to these results, the natural behaviour of a microbial community in vinegar has been predicted at a quantitative proteomic level. The results revealed that most of the identified proteins involved in the metabolism of amino acids, biosynthesis of proteins, and energy production related-metabolic pathways increased their expression throughout the cycle loading phase and afterwards experimented a decrease coming into play other proteins acting against acetic acid stress. These findings may facilitate a better understanding of the microbiota's role and contributing to obtain a quality product.

Hibernating ribosomes exhibit chaperoning activity but can resist unfolded protein-mediated subunit dissociation

Ribosome hibernation is a prominent cellular strategy to modulate protein synthesis during starvation and the stationary phase of bacterial cell growth. Translational suppression involves the formation of either factor-bound inactive 70S monomers or dimeric 100S hibernating ribosomal complexes, the biological significance of which is poorly understood. Here, we demonstrate that the Escherichia coli 70S ribosome associated with stationary phase factors hibernation promoting factor or protein Y or ribosome-associated inhibitor A and the 100S ribosome isolated from both Gram-negative and Gram-positive bacteria are resistant to unfolded protein-mediated subunit dissociation and subsequent degradation by cellular ribonucleases. Considering that the increase in cellular stress is accompanied by accumulation of unfolded proteins, such resistance of hibernating ribosomes towards dissociation might contribute to their maintenance during the stationary phase. Analysis of existing structures provided clues on the mechanism of inhibition of the unfolded protein-mediated disassembly in case of hibernating factor-bound ribosome. Further, the factor-bound 70S and 100S ribosomes can suppress protein aggregation and assist in protein folding. The chaperoning activity of these ribosomes is the first evidence of a potential biological activity of the hibernating ribosome that might be crucial for cell survival under stress conditions.

The hibernating 100S complex is a target of ribosome-recycling factor and elongation factor G in Staphylococcus aureus

The formation of translationally inactive 70S dimers (called 100S ribosomes) by hibernation-promoting factor is a widespread survival strategy among bacteria. Ribosome dimerization is thought to be reversible, with the dissociation of the 100S complexes enabling ribosome recycling for participation in new rounds of translation. The precise pathway of 100S ribosome recycling has been unclear. We previously found that the heat-shock GTPase HflX in the human pathogen Staphylococcus aureus is a minor disassembly factor. Cells lacking hflX do not accumulate 100S ribosomes unless they are subjected to heat exposure, suggesting the existence of an alternative pathway during nonstressed conditions. Here, we provide biochemical and genetic evidence that two essential translation factors, ribosome-recycling factor (RRF) and GTPase elongation factor G (EF-G), synergistically split 100S ribosomes in a GTP-dependent but tRNA translocation-independent manner. We found that although HflX and the RRF/EF-G pair are functionally interchangeable, HflX is expressed at low levels and is dispensable under normal growth conditions. The bacterial RRF/EF-G pair was previously known to target only the post-termination 70S complexes; our results reveal a new role in the reversal of ribosome hibernation that is intimately linked to bacterial pathogenesis, persister formation, stress responses, and ribosome integrity.

Unique structural features of the Mycobacterium ribosome

Protein synthesis in all the living cells is mediated by a large protein-RNA complex called the ribosome. These macromolecular complexes can range from 2.5 (prokaryotes) to 4.2 MDa. (eukaryotes) in size and undergo various conformational transitions during protein synthesis to translate the genetic code into the nascent polypeptide chains. Recent advances in cryo-electron microscopy (cryo-EM) and image processing methods have provided numerous detailed structures of ribosomes from diverse sources and in different conformational states resolved to near-atomic resolutions. These structures have not only helped in better understanding of the translational mechanism but also revealed species-specific variations or adaptations in the ribosome structures. Structural investigations of the ribosomes from Mycobacterium smegmatis (Msm) and its closely related pathogenic Mycobacterium tuberculosis (Mtb) lead to the identification of two additional ribosomal proteins named as bS22 and bL37 and several unique extensions in ribosomal-protein and ribosomal-RNA. Hibernation Promoting Factor (HPF) bound structure of Msm ribosome, termed as the hibernating ribosome, possibly indicates a new mechanism of ribosome protection during dormancy. These studies enabled the identification of the mycobacteria-specific ribosomal features and provides an opportunity to understand their function and target them for further drug-discovery purposes. Here we review the unique structural features identified in Msm ribosome and their possible implications in comparison to a well-studied Escherichia coli (Ec) ribosome.

Coordinated Regulation of Rsd and RMF for Simultaneous Hibernation of Transcription Apparatus and Translation Machinery in Stationary-Phase Escherichia coli

Transcription and translation in growing phase of Escherichia coli, the best-studied model prokaryote, are coupled and regulated in coordinate fashion. Accordingly, the growth rate-dependent control of the synthesis of RNA polymerase (RNAP) core enzyme (the core component of transcription apparatus) and ribosomes (the core component of translation machinery) is tightly coordinated to keep the relative level of transcription apparatus and translation machinery constant for effective and efficient utilization of resources and energy. Upon entry into the stationary phase, transcription apparatus is modulated by replacing RNAP core-associated sigma (promoter recognition subunit) from growth-related RpoD to stationary-phase-specific RpoS. The anti-sigma factor Rsd participates for the efficient replacement of sigma, and the unused RpoD is stored silent as Rsd–RpoD complex. On the other hand, functional 70S ribosome is transformed into inactive 100S dimer by two regulators, ribosome modulation factor (RMF) and hibernation promoting factor (HPF). In this review article, we overview how we found these factors and what we know about the molecular mechanisms for silencing transcription apparatus and translation machinery by these factors. In addition, we provide our recent findings of promoter-specific transcription factor (PS-TF) screening of the transcription factors involved in regulation of the rsd and rmf genes. Results altogether indicate the coordinated regulation of Rsd and RMF for simultaneous hibernation of transcription apparatus and translation machinery.