tmRNA-mediated trans-translation as the major ribosome rescue system in a bacterial cell

Improving polyketide biosynthesis by rescuing the translation of truncated mRNAs into functional polyketide synthase subunits

Modular polyketide synthases (mPKSs) are multidomain enzymes in bacteria that synthesize a variety of pharmaceutically important compounds. mPKS genes are usually longer than 10 kb and organized in operons. To understand the transcriptional and translational characteristics of these large genes, here we split the 13-kb busA gene, encoding a 456-kDa three-module PKS for butenyl-spinosyn biosynthesis, into three smaller separately translated genes encoding one PKS module in an operon. Expression of the native and split busA genes in Streptomyces albus reveals that the majority ( >93%) of PKS mRNAs are truncated, resulting in a greater abundance of and a higher synthesis rate for the proteins encoded by genes closer to the operon promoter. Splitting the large busA gene rescues translation of truncated mRNAs into functional PKS subunits, and increases the biosynthetic efficiency of butenyl-spinosyn PKS by 13-fold. The truncated mRNA translation rescue strategy will facilitate engineering of multi-domain proteins to enhance their functions.

A Proposal for the RNAome at the Dawn of the Last Universal Common Ancestor

From the most ancient RNAs, which followed an RNY pattern and folded into small hairpins, modern RNA molecules evolved by two different pathways, dubbed Extended Genetic Code 1 and 2, finally conforming to the current standard genetic code. Herein, we describe the evolutionary path of the RNAome based on these evolutionary routes. In general, all the RNA molecules analysed contain portions encoded by both genetic codes, but crucial features seem to be better recovered by Extended 2 triplets. In particular, the whole Peptidyl Transferase Centre, anti-Shine-Dalgarno motif, and a characteristic quadruplet of the RNA moiety of RNAse-P are clearly unveiled. Differences between bacteria and archaea are also detected; in most cases, the biological sequences are more stable than their controls. We then describe an evolutionary trajectory of the RNAome formation, based on two complementary evolutionary routes: one leading to the formation of essentials, while the other complemented the molecules, with the cooperative assembly of their constituents giving rise to modern RNAs.

CoCoNuTs are a diverse subclass of Type IV restriction systems predicted to target RNA

A comprehensive census of McrBC systems, among the most common forms of prokaryotic Type IV restriction systems, followed by phylogenetic analysis, reveals their enormous abundance in diverse prokaryotes and a plethora of genomic associations. We focus on a previously uncharacterized branch, which we denote coiled-coil nuclease tandems (CoCoNuTs) for their salient features: the presence of extensive coiled-coil structures and tandem nucleases. The CoCoNuTs alone show extraordinary variety, with three distinct types and multiple subtypes. All CoCoNuTs contain domains predicted to interact with translation system components, such as OB-folds resembling the SmpB protein that binds bacterial transfer-messenger RNA (tmRNA), YTH-like domains that might recognize methylated tmRNA, tRNA, or rRNA, and RNA-binding Hsp70 chaperone homologs, along with RNases, such as HEPN domains, all suggesting that the CoCoNuTs target RNA. Many CoCoNuTs might additionally target DNA, via McrC nuclease homologs. Additional restriction systems, such as Type I RM, BREX, and Druantia Type III, are frequently encoded in the same predicted superoperons. In many of these superoperons, CoCoNuTs are likely regulated by cyclic nucleotides, possibly, RNA fragments with cyclic termini, that bind associated CARF (CRISPR-Associated Rossmann Fold) domains. We hypothesize that the CoCoNuTs, together with the ancillary restriction factors, employ an echeloned defense strategy analogous to that of Type III CRISPR-Cas systems, in which an immune response eliminating virus DNA and/or RNA is launched first, but then, if it fails, an abortive infection response leading to PCD/dormancy via host RNA cleavage takes over.

Genome-Wide Computational Prediction and Analysis of Noncoding RNAs in Oleidesulfovibrio alaskensis G20

Noncoding RNAs (ncRNAs) play key roles in the regulation of important pathways, including cellular growth, stress management, signaling, and biofilm formation. Sulfate-reducing bacteria (SRB) contribute to huge economic losses causing microbial-induced corrosion through biofilms on metal surfaces. To effectively combat the challenges posed by SRB, it is essential to understand their molecular mechanisms of biofilm formation. This study aimed to identify ncRNAs in the genome of a model SRB, Oleidesulfovibrio alaskensis G20 (OA G20). Three in silico approaches revealed genome-wide distribution of 37 ncRNAs excluding tRNAs in the OA G20. These ncRNAs belonged to 18 different Rfam families. This study identified riboswitches, sRNAs, RNP, and SRP. The analysis revealed that these ncRNAs could play key roles in the regulation of several pathways of biosynthesis and transport involved in biofilm formation by OA G20. Three sRNAs, Pseudomonas P10, Hammerhead type II, and sX4, which were found in OA G20, are rare and their roles have not been determined in SRB. These results suggest that applying various computational methods could enrich the results and lead to the discovery of additional novel ncRNAs, which could lead to understanding the "rules of life of OA G20" during biofilm formation.

CoCoNuTs: A diverse subclass of Type IV restriction systems predicted to target RNA

A comprehensive census of McrBC systems, among the most common forms of prokaryotic Type IV restriction systems, followed by phylogenetic analysis, reveals their enormous abundance in diverse prokaryotes and a plethora of genomic associations. We focus on a previously uncharacterized branch, which we denote CoCoNuTs (coiled-coil nuclease tandems) for their salient features: the presence of extensive coiled-coil structures and tandem nucleases. The CoCoNuTs alone show extraordinary variety, with 3 distinct types and multiple subtypes. All CoCoNuTs contain domains predicted to interact with translation system components, such as OB-folds resembling the SmpB protein that binds bacterial transfer-messenger RNA (tmRNA), YTH-like domains that might recognize methylated tmRNA, tRNA, or rRNA, and RNA-binding Hsp70 chaperone homologs, along with RNases, such as HEPN domains, all suggesting that the CoCoNuTs target RNA. Many CoCoNuTs might additionally target DNA, via McrC nuclease homologs. Additional restriction systems, such as Type I RM, BREX, and Druantia Type III, are frequently encoded in the same predicted superoperons. In many of these superoperons, CoCoNuTs are likely regulated by cyclic nucleotides, possibly, RNA fragments with cyclic termini, that bind associated CARF (CRISPR-Associated Rossmann Fold) domains. We hypothesize that the CoCoNuTs, together with the ancillary restriction factors, employ an echeloned defense strategy analogous to that of Type III CRISPR-Cas systems, in which an immune response eliminating virus DNA and/or RNA is launched first, but then, if it fails, an abortive infection response leading to PCD/dormancy via host RNA cleavage takes over.

New Perspectives on Crosstalks Between Bacterial Regulatory RNAs from Outer Membrane Vesicles and Eukaryotic Cells

Regulatory small RNAs (sRNAs) help the bacteria to survive harsh environmental conditions by posttranscriptional regulation of genes involved in various biological pathways including stress responses, homeostasis, and virulence. These sRNAs can be found carried by different membrane-bound vesicles like extracellular vesicles (EVs), membrane vesicles (MVs), or outer membrane vesicles (OMVs). OMVs provide myriad functions in bacterial cells including carrying a cargo of proteins, lipids, and nucleic acids including sRNAs. A few interesting studies have shown that these sRNAs can be transported to the host cell by membrane vesicles and can regulate the host immune system. Although there is evidence that sRNAs can be exported to host cells and sometimes can even cross the blood-brain barrier, the exact mechanism is still unknown. In this review, we investigated the new techniques implemented in various studies, to elucidate the crosstalks between bacterial cells and human immune systems by membrane vesicles carrying bacterial regulatory sRNAs.

Biochemical and Cellular Characterization of the Function of Fluorophosphonate-Binding Hydrolase H (FphH) in Staphylococcus aureus Support a Role in Bacterial Stress Response

The development of new treatment options for bacterial infections requires access to new targets for antibiotics and antivirulence strategies. Chemoproteomic approaches are powerful tools for profiling and identifying novel druggable target candidates, but their functions often remain uncharacterized. Previously, we used activity-based protein profiling in the opportunistic pathogen Staphylococcus aureus to identify active serine hydrolases termed fluorophosphonate-binding hydrolases (Fph). Here, we provide the first characterization of S. aureus FphH, a conserved, putative carboxylesterase (referred to as yvaK in Bacillus subtilis) at the molecular and cellular level. First, phenotypic characterization of fphH-deficient transposon mutants revealed phenotypes during growth under nutrient deprivation, biofilm formation, and intracellular survival. Biochemical and structural investigations revealed that FphH acts as an esterase and lipase based on a fold well suited to act on a small to long hydrophobic unbranched lipid group within its substrate and can be inhibited by active site-targeting oxadiazoles. Prompted by a previous observation that fphH expression was upregulated in response to fusidic acid, we found that FphH can deacetylate this ribosome-targeting antibiotic, but the lack of FphH function did not infer major changes in antibiotic susceptibility. In conclusion, our results indicate a functional role of this hydrolase in S. aureus stress responses, and hypothetical functions connecting FphH with components of the ribosome rescue system that are conserved in the same gene cluster across Bacillales are discussed. Our atomic characterization of FphH will facilitate the development of specific FphH inhibitors and probes to elucidate its physiological role and validity as a drug target.

Insights into the structure and function of the RNA ligase RtcB

To be functional, some RNAs require a processing step involving splicing events. Each splicing event necessitates an RNA ligation step. RNA ligation is a process that can be achieved with various intermediaries such as self-catalysing RNAs, 5'-3' and 3'-5' RNA ligases. While several types of RNA ligation mechanisms occur in human, RtcB is the only 3'-5' RNA ligase identified in human cells to date. RtcB RNA ligation activity is well known to be essential for the splicing of XBP1, an essential transcription factor of the unfolded protein response; as well as for the maturation of specific intron-containing tRNAs. As such, RtcB is a core factor in protein synthesis and homeostasis. Taking advantage of the high homology between RtcB orthologues in archaea, bacteria and eukaryotes, this review will provide an introduction to the structure of RtcB and the mechanism of 3'-5' RNA ligation. This analysis is followed by a description of the mechanisms regulating RtcB activity and localisation, its known partners and its various functions from bacteria to human with a specific focus on human cancer.

Shutdown of multidrug transporter bmrCD mRNA expression mediated by the ribosome-associated endoribonuclease (Rae1) cleavage in a new cryptic ORF

Rae1 is a well-conserved endoribonuclease among Gram-positive bacteria, cyanobacteria, and the chloroplasts of higher plants. We have previously shown that Rae1 cleaves the Bacillus subtilis yrzI operon mRNA in a translation-dependent manner within a short open reading frame (ORF) called S1025, encoding a 17-amino acid (aa) peptide of unknown function. Here, we map a new Rae1 cleavage site in the bmrBCD operon mRNA encoding a multidrug transporter, within an unannotated 26-aa cryptic ORF that we have named bmrX Expression of the bmrCD portion of the mRNA is ensured by an antibiotic-dependent ribosome attenuation mechanism within the upstream ORF bmrB Cleavage by Rae1 within bmrX suppresses bmrCD expression that escapes attenuation control in the absence of antibiotics. Similar to S1025, Rae1 cleavage within bmrX is both translation- and reading frame-dependent. Consistent with this, we show that translation-dependent cleavage by Rae1 promotes ribosome rescue by the tmRNA.

Recent advances in genetic systems in obligate intracellular human-pathogenic bacteria

The ability to genetically manipulate a pathogen is fundamental to discovering factors governing host-pathogen interactions at the molecular level and is critical for devising treatment and prevention strategies. While the genetic "toolbox" for many important bacterial pathogens is extensive, approaches for modifying obligate intracellular bacterial pathogens were classically limited due in part to the uniqueness of their obligatory lifestyles. Many researchers have confronted these challenges over the past two and a half decades leading to the development of multiple approaches to construct plasmid-bearing recombinant strains and chromosomal gene inactivation and deletion mutants, along with gene-silencing methods enabling the study of essential genes. This review will highlight seminal genetic achievements and recent developments (past 5 years) for Anaplasma spp., Rickettsia spp., Chlamydia spp., and Coxiella burnetii including progress being made for the still intractable Orientia tsutsugamushi. Alongside commentary of the strengths and weaknesses of the various approaches, future research directions will be discussed to include methods for C. burnetii that should have utility in the other obligate intracellular bacteria. Collectively, the future appears bright for unraveling the molecular pathogenic mechanisms of these significant pathogens.

Detection of Streptococcus pyogenes M1UK in Australia and characterization of the mutation driving enhanced expression of superantigen SpeA

A new variant of Streptococcus pyogenes serotype M1 (designated 'M1_UK') has been reported in the United Kingdom, linked with seasonal scarlet fever surges, marked increase in invasive infections, and exhibiting enhanced expression of the superantigen SpeA. The progenitor S. pyogenes 'M1_global' and M1_UK clones can be differentiated by 27 SNPs and 4 indels, yet the mechanism for speA upregulation is unknown. Here we investigate the previously unappreciated expansion of M1_UK in Australia, now isolated from the majority of serious infections caused by serotype M1 S. pyogenes. M1_UK sub-lineages circulating in Australia also contain a novel toxin repertoire associated with epidemic scarlet fever causing S. pyogenes in Asia. A single SNP in the 5' transcriptional leader sequence of the transfer-messenger RNA gene ssrA drives enhanced SpeA superantigen expression as a result of ssrA terminator read-through in the M1_UK lineage. This represents a previously unappreciated mechanism of toxin expression and urges enhanced international surveillance.

The RNA repair proteins RtcAB regulate transcription activator RtcR via its CRISPR-associated Rossmann fold domain

CRISPR-associated Rossmann fold (CARF) domain signaling underpins modulation of CRISPR-Cas nucleases; however, the RtcR CARF domain controls expression of two conserved RNA repair enzymes, cyclase RtcA and ligase RtcB. Here, we demonstrate that RtcAB are required for RtcR-dependent transcription activation and directly bind to RtcR CARF. RtcAB catalytic activity is not required for complex formation with CARF, but is essential yet not sufficient for RtcRAB-dependent transcription activation, implying the need for an additional RNA repair-dependent activating signal. This signal differs from oligoadenylates, a known ligand of CARF domains, and instead appears to originate from the translation apparatus: RtcB repairs a tmRNA that rescues stalled ribosomes and increases translation elongation speed. Taken together, our data provide evidence for an expanded range for CARF domain signaling, including the first evidence of its control via in trans protein-protein interactions, and a feed-forward mechanism to regulate RNA repair required for a functioning translation apparatus.

Group II intron-like reverse transcriptases function in double-strand break repair

Bacteria encode reverse transcriptases (RTs) of unknown function that are closely related to group II intron-encoded RTs. We found that a Pseudomonas aeruginosa group II intron-like RT (G2L4 RT) with YIDD instead of YADD at its active site functions in DNA repair in its native host and when expressed in Escherichia coli. G2L4 RT has biochemical activities strikingly similar to those of human DNA repair polymerase θ and uses them for translesion DNA synthesis and double-strand break repair (DSBR) via microhomology-mediated end-joining (MMEJ). We also found that a group II intron RT can function similarly in DNA repair, with reciprocal active-site substitutions showing isoleucine favors MMEJ and alanine favors primer extension in both enzymes. These DNA repair functions utilize conserved structural features of non-LTR-retroelement RTs, including human LINE-1 and other eukaryotic non-LTR-retrotransposon RTs, suggesting such enzymes may have inherent ability to function in DSBR in a wide range of organisms.

Bacterial degrons in synthetic circuits

Bacterial proteases are a promising post-translational regulation strategy in synthetic circuits because they recognize specific amino acid degradation tags (degrons) that can be fine-tuned to modulate the degradation levels of tagged proteins. For this reason, recent efforts have been made in the search for new degrons. Here we review the up-to-date applications of degradation tags for circuit engineering in bacteria. In particular, we pay special attention to the effects of degradation bottlenecks in synthetic oscillators and introduce mathematical approaches to study queueing that enable the quantitative modelling of proteolytic queues.

Structural and mechanistic basis of RNA processing by protein-only ribonuclease P enzymes

Ribonuclease P (RNase P) enzymes are responsible for the 5' processing of tRNA precursors. In addition to the well-characterised ribozyme-based RNase P enzymes, an evolutionarily distinct group of protein-only RNase Ps exists. These proteinaceous RNase Ps (PRORPs) can be found in all three domains of life and can be divided into two structurally different types: eukaryotic and prokaryotic. Recent structural studies on members of both families reveal a surprising diversity of molecular architectures, but also highlight conceptual and mechanistic similarities. Here, we provide a comparison between the different types of PRORP enzymes and review how the combination of structural, biochemical, and biophysical studies has led to a molecular picture of protein-mediated tRNA processing.

Identification and characterisation of sPEPs in Cryptococcus neoformans

Short open reading frame (sORF)-encoded peptides (sPEPs) have been found across a wide range of genomic locations in a variety of species. To date, their identification, validation, and characterisation in the human fungal pathogen Cryptococcus neoformans has been limited due to a lack of standardised protocols. We have developed an enrichment process that enables sPEP detection within a protein sample from this polysaccharide-encapsulated yeast, and implemented proteogenomics to provide insights into the validity of predicted and hypothetical sORFs annotated in the C. neoformans genome. Novel sORFs were discovered within the 5' and 3' UTRs of known transcripts as well as in "non-coding" RNAs. One novel candidate, dubbed NPB1, that resided in an RNA annotated as "non-coding", was chosen for characterisation. Through the creation of both specific point mutations and a full deletion allele, the function of the new sPEP, Npb1, was shown to resemble that of the bacterial trans-translation protein SmpB.

Nanopore sequencing of RNA and cDNA molecules in Escherichia coli

High-throughput sequencing dramatically changed our view of transcriptome architectures and allowed for ground-breaking discoveries in RNA biology. Recently, sequencing of full-length transcripts based on the single-molecule sequencing platform from Oxford Nanopore Technologies (ONT) was introduced and is widely used to sequence eukaryotic and viral RNAs. However, experimental approaches implementing this technique for prokaryotic transcriptomes remain scarce. Here, we present an experimental and bioinformatic workflow for ONT RNA-seq in the bacterial model organism Escherichia coli, which can be applied to any microorganism. Our study highlights critical steps of library preparation and computational analysis and compares the results to gold standards in the field. Furthermore, we comprehensively evaluate the applicability and advantages of different ONT-based RNA sequencing protocols, including direct RNA, direct cDNA, and PCR-cDNA. We find that (PCR)-cDNA-seq offers improved yield and accuracy compared to direct RNA sequencing. Notably, (PCR)-cDNA-seq is suitable for quantitative measurements and can be readily used for simultaneous and accurate detection of transcript 5' and 3' boundaries, analysis of transcriptional units, and transcriptional heterogeneity. In summary, based on our comprehensive study, we show nanopore RNA-seq to be a ready-to-use tool allowing rapid, cost-effective, and accurate annotation of multiple transcriptomic features. Thereby nanopore RNA-seq holds the potential to become a valuable alternative method for RNA analysis in prokaryotes.

Processing of D1 Protein: A Mysterious Process Carried Out in Thylakoid Lumen

In oxygenic photosynthetic organisms, D1 protein, a core subunit of photosystem II (PSII), displays a rapid turnover in the light, in which D1 proteins are distinctively damaged and immediately removed from the PSII. In parallel, as a repair process, D1 proteins are synthesized and simultaneously assembled into the PSII. On this flow, the D1 protein is synthesized as a precursor with a carboxyl-terminal extension, and the D1 processing is defined as a step for proteolytic removal of the extension by a specific protease, CtpA. The D1 processing plays a crucial role in appearance of water-oxidizing capacity of PSII, because the main chain carboxyl group at carboxyl-terminus of the D1 protein, exposed by the D1 processing, ligates a manganese and a calcium atom in the Mn4CaO5-cluster, a special equipment for water-oxidizing chemistry of PSII. This review focuses on the D1 processing and discusses it from four angles: (i) Discovery of the D1 processing and recognition of its importance: (ii) Enzyme involved in the D1 processing: (iii) Efforts for understanding significance of the D1 processing: (iv) Remaining mysteries in the D1 processing. Through the review, I summarize the current status of our knowledge on and around the D1 processing.

Bacterial Ribosome Rescue Systems

To maintain proteostasis, the cell employs multiple ribosome rescue systems to relieve the stalled ribosome on problematic mRNA. One example of problematic mRNA is non-stop mRNA that lacks an in-frame stop codon produced by endonucleolytic cleavage or transcription error. In Escherichia coli, there are at least three ribosome rescue systems that deal with the ribosome stalled on non-stop mRNA. According to one estimation, 2-4% of translation is the target of ribosome rescue systems even under normal growth conditions. In the present review, we discuss the recent findings of ribosome rescue systems in bacteria.

The Genome Reduction Excludes the Ribosomal Rescue System in Acholeplasmataceae

The trans-translation process is a ribosomal rescue system for stalled ribosomes processing truncated mRNA. The genes ssrA and smpB fulfil the key functions in most bacteria, but some species have either lost these genes or the function of the ribosomal rescue system is taken over by other genes. To date, the ribosomal rescue system has not been analysed in detail for the Acholeplasmataceae. This family, in the Mollicutes class, comprises the genus Acholeplasma and the provisional taxon "Candidatus Phytoplasma". Despite their monophyletic origin, the two clades can be separated by traits such as not representing primary pathogens for acholeplasmas versus being phytopathogenic for the majority of phytoplasmas. Both taxa share reduced genomes, but only phytoplasma genomes are characterised by a remarkable level of instability and reduction. Despite the general relevance of the ribosomal rescue system, information is lacking on coding, the genomic context and pseudogenisation of smpB and ssrA and their possible application as a phylogenetic marker. Herein, we provide a comprehensive analysis of the ribosomal rescue system in members of Acholeplasmataceae. The examined Acholeplasmataceae genomes encode a ribosomal rescue system, which depends on tmRNA encoded by ssrA acting in combination with its binding protein SmpB. Conserved gene synteny is evident for smpB, while ssrA shows a less conserved genomic context. Analysis of the tmRNA sequences highlights the variability of proteolysis tag sequences and short conserved sites at the 5'- and 3'-ends. Analyses of smpB provided no hints regarding the coding of pseudogenes, but they did suggest its application as a phylogenetic marker of Acholeplasmataceae - in accordance with 16S rDNA topology. Sequence variability of smpB provides sufficient information for species assignment and phylogenetic analysis.

Protein synthesis in Mycobacterium tuberculosis as a potential target for therapeutic interventions

Mycobacterium tuberculosis (Mtb) causes one of humankind's deadliest diseases, tuberculosis. Mtb protein synthesis machinery possesses several unique species-specific features, including its ribosome that carries two mycobacterial specific ribosomal proteins, bL37 and bS22, and ribosomal RNA segments. Since the protein synthesis is a vital cellular process that occurs on the ribosome, a detailed knowledge of the structure and function of mycobacterial ribosomes is essential to understand the cell's proteome by translation regulation. Like in many bacterial species such as Bacillus subtilis and Streptomyces coelicolor, two distinct populations of ribosomes have been identified in Mtb. Under low-zinc conditions, Mtb ribosomal proteins S14, S18, L28, and L33 are replaced with their non-zinc binding paralogues. Depending upon the nature of physiological stress, species-specific modulation of translation by stress factors and toxins that interact with the ribosome have been reported. In addition, about one-fourth of messenger RNAs in mycobacteria have been reported to be leaderless, i.e., without 5' UTR regions. However, the mechanism by which they are recruited to the Mtb ribosome is not understood. In this review, we highlight the mycobacteria-specific features of the translation apparatus and propose exploiting these features to improve the efficacy and specificity of existing antibiotics used to treat tuberculosis.

Site-Selective Artificial Ribonucleases: Renaissance of Oligonucleotide Conjugates for Irreversible Cleavage of RNA Sequences

RNA-targeting therapeutics require highly efficient sequence-specific devices capable of RNA irreversible degradation in vivo. The most developed methods of sequence-specific RNA cleavage, such as siRNA or antisense oligonucleotides (ASO), are currently based on recruitment of either intracellular multi-protein complexes or enzymes, leaving alternative approaches (e.g., ribozymes and DNAzymes) far behind. Recently, site-selective artificial ribonucleases combining the oligonucleotide recognition motifs (or their structural analogues) and catalytically active groups in a single molecular scaffold have been proven to be a great competitor to siRNA and ASO. Using the most efficient catalytic groups, utilising both metal ion-dependent (Cu(II)-2,9-dimethylphenanthroline) and metal ion-free (Tris(2-aminobenzimidazole)) on the one hand and PNA as an RNA recognising oligonucleotide on the other, allowed site-selective artificial RNases to be created with half-lives of 0.5-1 h. Artificial RNases based on the catalytic peptide [(ArgLeu)₂Gly]₂ were able to take progress a step further by demonstrating an ability to cleave miRNA-21 in tumour cells and provide a significant reduction of tumour growth in mice.

Intermolecular Communication in Bacillus subtilis: RNA-RNA, RNA-Protein and Small Protein-Protein Interactions

In bacterial cells we find a variety of interacting macromolecules, among them RNAs and proteins. Not only small regulatory RNAs (sRNAs), but also small proteins have been increasingly recognized as regulators of bacterial gene expression. An average bacterial genome encodes between 200 and 300 sRNAs, but an unknown number of small proteins. sRNAs can be cis- or trans-encoded. Whereas cis-encoded sRNAs interact only with their single completely complementary mRNA target transcribed from the opposite DNA strand, trans-encoded sRNAs are only partially complementary to their numerous mRNA targets, resulting in huge regulatory networks. In addition to sRNAs, uncharged tRNAs can interact with mRNAs in T-box attenuation mechanisms. For a number of sRNA-mRNA interactions, the stability of sRNAs or translatability of mRNAs, RNA chaperones are required. In Gram-negative bacteria, the well-studied abundant RNA-chaperone Hfq fulfils this role, and recently another chaperone, ProQ, has been discovered and analyzed in this respect. By contrast, evidence for RNA chaperones or their role in Gram-positive bacteria is still scarce, but CsrA might be such a candidate. Other RNA-protein interactions involve tmRNA/SmpB, 6S RNA/RNA polymerase, the dual-function aconitase and protein-bound transcriptional terminators and antiterminators. Furthermore, small proteins, often missed in genome annotations and long ignored as potential regulators, can interact with individual regulatory proteins, large protein complexes, RNA or the membrane. Here, we review recent advances on biological role and regulatory principles of the currently known sRNA-mRNA interactions, sRNA-protein interactions and small protein-protein interactions in the Gram-positive model organism Bacillus subtilis. We do not discuss RNases, ribosomal proteins, RNA helicases or riboswitches.

Large-scale identification of trans-translation substrates targeted by tmRNA in Aeromonas veronii

Transfer-messenger RNA (tmRNA) is ubiquitous in bacteria, acting as the core component for the trans-translation system that contributes to label the aberrantly synthesized peptides for degradation and to release the stalled ribosomes. Deletion of tmRNA causes a variety of phenotypes related to important physiological processes in bacteria. To illustrate the molecular mechanism of the versatility of tmRNA in aquatic pathogen Aeromonas veronii, we mutated the C-terminal nucleotides of tmRNA (MutmRNA) for encoding a tag containing six histidine residues (His₆tag), so as to capture and enrich the trans-translation substrates from the cell lysates through a Ni²⁺-NTA affinity chromatograph. The results showed that the concentrated substrates were detected as distinct and specific bands in western blotting using anti-His antibody, demonstrating that specific defective mRNAs were frequently and intensively rescued by trans-translation during the translation process in A. veronii. The substrates were analyzed by LC-MS/MS and further identified by searching a theoretically constructed database specific for A. veronii. Total of 24 potential substrates were identified, with various functions involved in metabolism, as well as structure and signal-based cellular events. Among the identified substrates, PspA and AsmA were labeled by Flag, and expressed in the presence of the modified trans-translation system in E. coli. Their labelings with MutmRNA were validated by purification through Ni²⁺-NTA column followed by western blotting using anti-Flag antibody. This study provided the most abundant set of endogenous targets for tmRNA in A. veronii, and facilitated further investigations about the molecular mechanism and signal pathway of tmRNA-mediated trans-translation.

Reassembling green fluorescent protein for in vitro evaluation of trans-translation

In order to discover new antibiotics with improved activity and selectivity, we created a reliable in vitro reporter system to detect trans-translation activity, the main mechanism for recycling ribosomes stalled on problematic messenger RNA (mRNA) in bacteria. This system is based on an engineered tmRNA variant that reassembles the green fluorescent protein (GFP) when trans-translation is active. Our system is adapted for high-throughput screening of chemical compounds by fluorescence.

Non-coding RNAs: what are we missing?

Over the past two decades, the importance of small non-coding RNAs (sncRNAs) as regulatory molecules has become apparent in all three domains of life (archaea, bacteria, eukaryotes). In fact, sncRNAs play an important role in the control of gene expression at both the transcriptional and the post-transcriptional level, with crucial roles in fine-tuning cell responses during internal and external stress. Multiple pathways for sncRNA biogenesis and diverse mechanisms of regulation have been reported, and although biogenesis and mechanisms of sncRNAs in prokaryotes and eukaryotes are different, remarkable similarities exist. Here, we briefly review and compare the major sncRNA classes that act post-transcriptionally, and focus on recent discoveries regarding the ribosome as a target of regulation and the conservation of these mechanisms between prokaryotes and eukaryotes.

Dual RNA-seq in Streptococcus pneumoniae Infection Reveals Compartmentalized Neutrophil Responses in Lung and Pleural Space

The factors that regulate the passage of bacteria between different anatomical compartments are unclear. We have used an experimental model of infection with Streptococcus pneumoniae to examine the host and bacterial factors involved in the passage of bacteria from the lung to the pleural space. The transcriptional profile of host and bacterial cells within the pleural space and lung was analyzed using deep sequencing of the entire transcriptome using the technique of dual RNA-seq. We found significant differences in the host and bacterial RNA profiles in infection, which shed light on the key factors that allow passage of this bacterium into the pleural space.

Streptococcus pneumoniae is the dominant cause of community-acquired pneumonia worldwide. Invasion of the pleural space is common and results in increased mortality. We set out to determine the bacterial and host factors that influence invasion of the pleural space. In a murine model of pneumococcal infection, we isolated neutrophil-dominated samples of bronchoalveolar and pleural fluid containing bacteria 48 hours after infection. Using dual RNA sequencing (RNA-seq), we characterized bacterial and host transcripts that were differentially regulated between these compartments and bacteria in broth and resting neutrophils, respectively. Pleural and lung samples showed upregulation of genes involved in the positive regulation of neutrophil extravasation but downregulation of genes mediating bacterial killing. Compared to the lung samples, cells within the pleural space showed marked upregulation of many genes induced by type I interferons, which are cytokines implicated in preventing bacterial transmigration across epithelial barriers. Differences in the bacterial transcripts between the infected samples and bacteria grown in broth showed the upregulation of genes in the bacteriocin locus, the pneumococcal surface adhesin PsaA, and the glycopeptide resistance gene vanZ; the gene encoding the ClpP protease was downregulated in infection. One hundred sixty-nine intergenic putative small bacterial RNAs were also identified, of which 43 (25.4%) small RNAs had been previously described. Forty-two of the small RNAs were upregulated in pleura compared to broth, including many previously identified as being important in virulence. Our results have identified key host and bacterial responses to invasion of the pleural space that can be potentially exploited to develop alternative antimicrobial strategies for the prevention and treatment of pneumococcal pleural disease.

IMPORTANCE The factors that regulate the passage of bacteria between different anatomical compartments are unclear. We have used an experimental model of infection with Streptococcus pneumoniae to examine the host and bacterial factors involved in the passage of bacteria from the lung to the pleural space. The transcriptional profile of host and bacterial cells within the pleural space and lung was analyzed using deep sequencing of the entire transcriptome using the technique of dual RNA-seq. We found significant differences in the host and bacterial RNA profiles in infection, which shed light on the key factors that allow passage of this bacterium into the pleural space.

Engineered SUMO/protease system identifies Pdr6 as a bidirectional nuclear transport receptor

Cleavage of affinity tags by specific proteases can be exploited for highly selective affinity chromatography. The SUMO/SENP1 system is the most efficient for such application but fails in eukaryotic expression because it cross-reacts with endogenous proteases. Using a novel selection system, we have evolved the SUMOEu/SENP1Eu pair to orthogonality with the yeast and animal enzymes. SUMOEu fusions therefore remain stable in eukaryotic cells. Likewise, overexpressing a SENP1Eu protease is nontoxic in yeast. We have used the SUMOEu system in an affinity-capture-proteolytic-release approach to identify interactors of the yeast importin Pdr6/Kap122. This revealed not only further nuclear import substrates such as Ubc9, but also Pil1, Lsp1, eIF5A, and eEF2 as RanGTP-dependent binders and thus as export cargoes. We confirmed that Pdr6 functions as an exportin in vivo and depletes eIF5A and eEF2 from cell nuclei. Thus, Pdr6 is a bidirectional nuclear transport receptor (i.e., a biportin) that shuttles distinct sets of cargoes in opposite directions.

Small RNA AvrA Regulates IscR to Increase the Stress Tolerances in SmpB Deficiency of Aeromonas veronii

The superbacteria Aeromonas veronii displays not only a strong pathogenicity but also the resistance to nine kinds of antibiotics, resulting in the economic losses and health hazards. Small Protein B (SmpB) plays an important role in protein quality control, virulence, and stress reactions. Transcriptomic data revealed that expressions of the type IV pilus assembly and type VI secretion system (T6SS) proteins were downregulated in SmpB deficiency, indicating that the virulence of A. veronii might be attenuated. Although SmpB deletion decreased colonization in the mouse spleen and liver, LD50 of the smpB mutant was not altered as expected, compared with the wild type. Further, the transcriptomic and quantitative RT-PCR analyses showed that the combination of the downregulated AvrA and the upregulated iron-sulfur protein activator IscR, mediated the oxidative tolerance in smpB deletion. Next a reporter plasmid was constructed in which the promoter of iscR was applied to control the expression of the enhanced green fluorescent protein (eGFP) gene. When the reporter plasmid was co-expressed with the AvrA expression into E. coli, the relative fluorescence intensity was decreased significantly, suggesting that AvrA bound to iscR mRNA by base pairing, which in turn relieved the inhibition of iscR and intensified the downstream iron-sulfur proteins. Collectively, the smpB mutant exhibited an attenuated virulence in mice and enhanced tolerances to oxidative stress. This study demonstrates the complexity of gene regulation networks mediated by sRNA in systems biology, and also reflects the strong adaptability of superbacteria A. veronii in the process of evolution.

Folding up and Moving on-Nascent Protein Folding on the Ribosome

All cellular proteins are synthesized by the ribosome, an intricate molecular machine that translates the information of protein coding genes into the amino acid alphabet. The linear polypeptides synthesized by the ribosome must generally fold into specific three-dimensional structures to become biologically active. Folding has long been recognized to begin before synthesis is complete. Recently, biochemical and biophysical studies have shed light onto how the ribosome shapes the folding pathways of nascent proteins. Here, we discuss recent progress that is beginning to define the role of the ribosome in the folding of newly synthesized polypeptides.

Toxins targeting transfer RNAs: Translation inhibition by bacterial toxin-antitoxin systems

Prokaryotic toxin-antitoxin (TA) systems are composed of a protein toxin and its cognate antitoxin. These systems are abundant in bacteria and archaea and play an important role in growth regulation. During favorable growth conditions, the antitoxin neutralizes the toxin's activity. However, during conditions of stress or starvation, the antitoxin is inactivated, freeing the toxin to inhibit growth and resulting in dormancy. One mechanism of growth inhibition used by several TA systems results from targeting transfer RNAs (tRNAs), either through preventing aminoacylation, acetylating the primary amino group, or endonucleolytic cleavage. All of these mechanisms inhibit translation and result in growth arrest. Many of these toxins only act on a specific tRNA or a specific subset of tRNAs; however, more work is necessary to understand the specificity determinants of these toxins. For the toxins whose specificity has been characterized, both sequence and structural components of the tRNA appear important for recognition by the toxin. Questions also remain regarding the mechanisms used by dormant bacteria to resume growth after toxin induction. Rescue of stalled ribosomes by transfer-messenger RNAs, removal of acetylated amino groups from tRNAs, or ligation of cleaved RNA fragments have all been implicated as mechanisms for reversing toxin-induced dormancy. However, the mechanisms of resuming growth after induction of the majority of tRNA targeting toxins are not yet understood. This article is categorized under: Translation > Translation Regulation RNA in Disease and Development > RNA in Disease RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition.

The RNA Complement of Outer Membrane Vesicles From Salmonella enterica Serovar Typhimurium Under Distinct Culture Conditions

Bacterial outer membrane vesicles (OMVs), as well as OMV-associated small RNAs, have been demonstrated to play a role in host-pathogen interactions. The presence of larger RNA transcripts in OMVs has been less studied and their potential role in host-pathogen interactions remains largely unknown. Here we analyze RNA from OMVs secreted by Salmonella enterica serovar Typhimurium (S. Typhimurium) cultured under different conditions, which mimic host-pathogen interactions. S. Typhimurium was grown to exponential and stationary growth phases in minimal growth control medium (phosphate-carbon-nitrogen, PCN), as well as in acidic and phosphate-depleted PCN, comparable to the macrophage environment and inducing therefore the expression of Salmonella pathogenicity island 2 (SPI-2) genes. Moreover, Salmonella pathogenicity island 1 (SPI-1), which is required for virulence during the intestinal phase of infection, was induced by culturing S. Typhimurium to the stationary phase in Lysogeny Broth (LB). For each condition, we identified OMV-associated RNAs that are enriched in the extracellular environment relative to the intracellular space. All RNA classes could be observed, but a vast majority of rRNA was exported in all conditions in variable proportions with a notable decrease in LB SPI-1 inducing media. Several mRNAs and ncRNAs were specifically enriched in/on OMVs dependent on the growth conditions. Important to note is that some RNAs showed identical read coverage profiles intracellularly and extracellularly, whereas distinct coverage patterns were observed for other transcripts, suggesting a specific processing or degradation. Moreover, PCR experiments confirmed that distinct RNAs were present in or on OMVs as full-length transcripts (IsrB-1/2; IsrA; ffs; SsrS; CsrC; pSLT035; 10Sa; rnpB; STM0277; sseB; STM0972; STM2606), whereas others seemed to be rather present in a processed or degraded form. Finally, we show by a digestion protection assay that OMVs are able to prevent enzymatic degradation of given full-length transcripts (SsrS, CsrC, 10Sa, and rnpB). In summary, we show that OMV-associated RNA is clearly different in distinct culture conditions and that at least a fraction of the extracellular RNA is associated as a full-length transcripts with OMVs, indicating that some RNAs are protected by OMVs and thereby leaving open the possibility that those might be functionally active.

Translation Elongation Factor 4 (LepA) Contributes to Tetracycline Susceptibility by Stalling Elongating Ribosomes

Even though elongation factor 4 (EF4) is the third most conserved protein in bacteria, its physiological functions remain largely unknown and its proposed molecular mechanisms are conflicting among previous studies. In the present study, we show that the growth of an Escherichia coli strain is more susceptible to tetracycline than its EF4 knockout strain. Consistent with previous studies, our results suggested that EF4 affects ribosome biogenesis when tetracycline is present. Through ribosome profiling analysis, we discovered that EF4 causes 1-nucleotide shifting of ribosomal footprints on mRNA when cells have been exposed to tetracycline. In addition, when tetracycline is present, EF4 inhibits the elongation of protein synthesis, which leads to the accumulation of ribosomes in the early segment of mRNA. Altogether, when cells are exposed to tetracycline, EF4 alters both ribosome biogenesis and the elongation phase of protein synthesis.

Transposon Sequencing of Brucella abortus Uncovers Essential Genes for Growth In Vitro and Inside Macrophages

Brucella abortus is a class III zoonotic bacterial pathogen able to survive and replicate inside host cells, including macrophages. Here we report a multidimensional transposon sequencing analysis to identify genes essential for Brucella abortus growth in rich medium and replication in RAW 264.7 macrophages. The construction of a dense transposon mutant library and mapping of 929,769 unique mini-Tn5 insertion sites in the genome allowed identification of 491 essential coding sequences and essential segments in the B. abortus genome. Chromosome II carries a lower proportion (5%) of essential genes than chromosome I (19%), supporting the hypothesis of a recent acquisition of a megaplasmid as the origin of chromosome II. Temporally resolved transposon sequencing analysis as a function of macrophage infection stages identified 79 genes with a specific attenuation phenotype in macrophages, at either 2, 5, or 24 h postinfection, and 86 genes for which the attenuated mutant phenotype correlated with a growth defect on plates. We identified 48 genes required for intracellular growth, including the virB operon, encoding the type IV secretion system, which supports the validity of the screen. The remaining genes encode amino acid and pyrimidine biosynthesis, electron transfer systems, transcriptional regulators, and transporters. In particular, we report the need of an intact pyrimidine nucleotide biosynthesis pathway in order for B. abortus to proliferate inside RAW 264.7 macrophages.

Linear Regression Links Transcriptomic Data and Cellular Raman Spectra

Raman microscopy is an imaging technique that has been applied to assess molecular compositions of living cells to characterize cell types and states. However, owing to the diverse molecular species in cells and challenges of assigning peaks to specific molecules, it has not been clear how to interpret cellular Raman spectra. Here, we provide firm evidence that cellular Raman spectra and transcriptomic profiles of Schizosaccharomyces pombe and Escherichia coli can be computationally connected and thus interpreted. We find that the dimensions of high-dimensional Raman spectra and transcriptomes measured by RNA sequencing can be reduced and connected linearly through a shared low-dimensional subspace. Accordingly, we were able to predict global gene expression profiles by applying the calculated transformation matrix to Raman spectra, and vice versa. Highly expressed non-coding RNAs contributed to the Raman-transcriptome linear correspondence more significantly than mRNAs in S. pombe. This demonstration of correspondence between cellular Raman spectra and transcriptomes is a promising step toward establishing spectroscopic live-cell omics studies.

Horizontal Acquisition and Transcriptional Integration of Novel Genes in Mosquito-Associated Spiroplasma

Genetic differentiation among symbiotic bacteria is important in shaping biodiversity. The genus Spiroplasma contains species occupying diverse niches and is a model system for symbiont evolution. Previous studies have established that two mosquito-associated species have diverged extensively in their carbohydrate metabolism genes despite having a close phylogenetic relationship. Notably, although the commensal Spiroplasma diminutum lacks identifiable pathogenicity factors, the pathogenic Spiroplasma taiwanense was found to have acquired a virulence factor glpO and its associated genes through horizontal transfer. However, it is unclear if these acquired genes have been integrated into the regulatory network. In this study, we inferred the gene content evolution in these bacteria, as well as examined their transcriptomes in response to glucose availability. The results indicated that both species have many more gene acquisitions from the Mycoides-Entomoplasmataceae clade, which contains several important pathogens of ruminants, than previously thought. Moreover, several acquired genes have higher expression levels than the vertically inherited homologs, indicating possible functional replacement. Finally, the virulence factor and its functionally linked genes in S. taiwanense were up-regulated in response to glucose starvation, suggesting that these acquired genes are under expression regulation and the pathogenicity may be a stress response. In summary, although differential gene losses are a major process for symbiont divergence, gene gains are critical in counteracting genome degradation and driving diversification among facultative symbionts.

Complete Genome Sequence of a Ciprofloxacin-Resistant Salmonella enterica subsp. enterica Serovar Kentucky Sequence Type 198 Strain, PU131, Isolated from a Human Patient in Washington State

Strains of the ciprofloxacin-resistant (Cip^r) Salmonella enterica subsp. enterica serovar Kentucky sequence type 198 (ST198) have rapidly and extensively disseminated globally to become a major food safety and public health concern. Here, we report the complete genome sequence of a Cip^rS. Kentucky ST198 strain, PU131, isolated from a human patient in Washington State (USA).

Crosstalk between Diverse Synthetic Protein Degradation Tags in Escherichia coli

Recently, a synthetic circuit in E. coli demonstrated that two proteins engineered with LAA tags targeted to the native protease ClpXP are susceptible to crosstalk due to competition for degradation between proteins. To understand proteolytic crosstalk beyond the single protease regime, we investigated in E. coli a set of synthetic circuits designed to probe the dynamics of existing and novel degradation tags fused to fluorescent proteins. These circuits were tested using both microplate reader and single-cell assays. We first quantified the degradation rates of each tag in isolation. We then tested if there was crosstalk between two distinguishable fluorescent proteins engineered with identical or different degradation tags. We demonstrated that proteolytic crosstalk was indeed not limited to the LAA degradation tag, but was also apparent between other diverse tags, supporting the complexity of the E. coli protein degradation system.

A Genetic Tool to Quantify trans-Translation Activity in Vivo

In bacteria, trans-translation is the main quality control mechanism for rescuing ribosomes arrested during translation. This key process is universally conserved and plays a critical role in the viability and virulence of many pathogens. We developed a reliable in vivo double-fluorescence reporter system for the simultaneous quantification of both trans-translation and the associated proteolysis activities in bacteria. The assay was validated using mutant bacteria lacking tmRNA, SmpB, and the ClpP protease. Both antisense tmRNA-binding RNA and a peptide mimicking the SmpB C-terminal tail proved to be potent inhibitors of trans-translation in vivo. The double-fluorescent reporter was also tested with KKL-35, an oxadiazole derivative that is supposed to be a promising trans-translation inhibitor, and it surprisingly turns out that trans-translation is not the only target of KKL-35 in vivo.

Miscoding-induced stalling of substrate translocation on the bacterial ribosome

Directional transit of the ribosome along the messenger RNA (mRNA) template is a key determinant of the rate and processivity of protein synthesis. Imaging of the multistep translocation mechanism using single-molecule FRET has led to the hypothesis that substrate movements relative to the ribosome resolve through relatively long-lived late intermediates wherein peptidyl-tRNA enters the P site of the small ribosomal subunit via reversible, swivel-like motions of the small subunit head domain within the elongation factor G (GDP)-bound ribosome complex. Consistent with translocation being rate-limited by recognition and productive engagement of peptidyl-tRNA within the P site, we now show that base-pairing mismatches between the peptidyl-tRNA anticodon and the mRNA codon dramatically delay this rate-limiting, intramolecular process. This unexpected relationship between aminoacyl-tRNA decoding and translocation suggests that miscoding antibiotics may impact protein synthesis by impairing the recognition of peptidyl-tRNA in the small subunit P site during EF-G-catalyzed translocation. Strikingly, we show that elongation factor P (EF-P), traditionally known to alleviate ribosome stalling at polyproline motifs, can efficiently rescue translocation defects arising from miscoding. These findings help reveal the nature and origin of the rate-limiting steps in substrate translocation on the bacterial ribosome and indicate that EF-P can aid in resuming translation elongation stalled by miscoding errors.

Ribosome Rescue Inhibitors Kill Actively Growing and Nonreplicating Persister Mycobacterium tuberculosis Cells

The emergence of Mycobacterium tuberculosis (MTB) strains that are resistant to most or all available antibiotics has created a severe problem for treating tuberculosis and has spurred a quest for new antibiotic targets. Here, we demonstrate that trans-translation is essential for growth of MTB and is a viable target for development of antituberculosis drugs. We also show that an inhibitor of trans-translation, KKL-35, is bactericidal against MTB under both aerobic and anoxic conditions. Biochemical experiments show that this compound targets helix 89 of the 23S rRNA. In silico molecular docking predicts a binding pocket for KKL-35 adjacent to the peptidyl-transfer center in a region not targeted by conventional antibiotics. Computational solvent mapping suggests that this pocket is a druggable hot spot for small molecule binding. Collectively, our findings reveal a new target for antituberculosis drug development and provide critical insight on the mechanism of antibacterial action for KKL-35 and related 1,3,4-oxadiazole benzamides.

An integrated computational-experimental approach reveals Yersinia pestis genes essential across a narrow or a broad range of environmental conditions

Background

The World Health Organization has categorized plague as a re-emerging disease and the potential for Yersinia pestis to also be used as a bioweapon makes the identification of new drug targets against this pathogen a priority. Environmental temperature is a key signal which regulates virulence of the bacterium. The bacterium normally grows outside the human host at 28 °C. Therefore, understanding the mechanisms that the bacterium used to adapt to a mammalian host at 37 °C is central to the development of vaccines or drugs for the prevention or treatment of human disease.

Results

Using a library of over 1 million Y. pestis CO92 random mutants and transposon-directed insertion site sequencing, we identified 530 essential genes when the bacteria were cultured at 28 °C. When the library of mutants was subsequently cultured at 37 °C we identified 19 genes that were essential at 37 °C but not at 28 °C, including genes which encode proteins that play a role in enabling functioning of the type III secretion and in DNA replication and maintenance. Using genome-scale metabolic network reconstruction we showed that growth conditions profoundly influence the physiology of the bacterium, and by combining computational and experimental approaches we were able to identify 54 genes that are essential under a broad range of conditions.

Conclusions

Using an integrated computational-experimental approach we identify genes which are required for growth at 37 °C and under a broad range of environments may be the best targets for the development of new interventions to prevent or treat plague in humans.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-017-1073-8) contains supplementary material, which is available to authorized users.

[Role of small noncoding RNA in the regulation of bacterial virulence]

In the long-term interaction between pathogens and host, the pathogens regulate the expression of related viru-lence genes to fit the host environment in response to the changes in the host microenvironment. Gene expression was believed to be controlled mainly at the level of transcription initiation by repressors or activators. Recent studies have revealed that small noncoding RNAs (sRNAs) are key regulators in bacterial pathogenesis. sRNA in bacteria is a noncoding RNA with length ranging from 50 to 500 nucleotides. Pathogens can sense the changes in the host environment and consequently regulate the expression of virulence genes by sRNAs. This condition promotes the ability of pathogens to survive within the host, which is beneficial to the invasion and pathogenicity of pathogens. In contrast to transcriptional factors, sRNA-mediated gene regu-lation makes rapid and sensitive responses to environmental cues. Many sRNAs involved in bacterial virulence and pathogenesis have been identified. These sRNAs are key components of coordinated regulation networks, playing important roles in regulating the expression of virulence genes at post-transcriptional level. This review aims to provide an overview on the molecular mechanisms and roles of sRNAs in the regulation of bacterial virulence.

Evolution of Nucleotide Punctuation Marks: From Structural to Linear Signals

We present an evolutionary hypothesis assuming that signals marking nucleotide synthesis (DNA replication and RNA transcription) evolved from multi- to unidimensional structures, and were carried over from transcription to translation. This evolutionary scenario presumes that signals combining secondary and primary nucleotide structures are evolutionary transitions. Mitochondrial replication initiation fits this scenario. Some observations reported in the literature corroborate that several signals for nucleotide synthesis function in translation, and vice versa. (a) Polymerase-induced frameshift mutations occur preferentially at translational termination signals (nucleotide deletion is interpreted as termination of nucleotide polymerization, paralleling the role of stop codons in translation). (b) Stem-loop hairpin presence/absence modulates codon-amino acid assignments, showing that translational signals sometimes combine primary and secondary nucleotide structures (here codon and stem-loop). (c) Homopolymer nucleotide triplets (AAA, CCC, GGG, TTT) cause transcriptional and ribosomal frameshifts. Here we find in recently described human mitochondrial RNAs that systematically lack mono-, dinucleotides after each trinucleotide (delRNAs) that delRNA triplets include 2x more homopolymers than mitogenome regions not covered by delRNA. Further analyses of delRNAs show that the natural circular code X (a little-known group of 20 translational signals enabling ribosomal frame retrieval consisting of 20 codons {AAC, AAT, ACC, ATC, ATT, CAG, CTC, CTG, GAA, GAC, GAG, GAT, GCC, GGC, GGT, GTA, GTC, GTT, TAC, TTC} universally overrepresented in coding versus other frames of gene sequences), regulates frameshift in transcription and translation. This dual transcription and translation role confirms for X the hypothesis that translational signals were carried over from transcriptional signals.

NONCODEv4: Annotation of Noncoding RNAs with Emphasis on Long Noncoding RNAs

The rapid development of high-throughput sequencing technologies and bioinformatics algorithms now enables detection and profiling of a large number of noncoding transcripts. Long noncoding RNAs (lncRNAs), which are longer than 200 nucleotides, are accumulating with important roles involved in biological processes and tissue physiology. In this chapter, we describe the use of NONCODEv4, a database that provide a comprehensive catalog of noncoding RNAs with particularly detailed annotations for lncRNAs. NONCODEv4 stores more than half million transcripts, of which more than 200,000 are lncRNAs. NONCODEv4 raises the concept of lncRNA genes and explores their expression and functions based on public transcriptome data. NONCODEv4 also integrated a series of online tools and have a web interface easy to use. NONCODEv4 is available at http://www.noncode.org/ http://www.bioinfo.org/ noncode.

Origins of tmRNA: the missing link in the birth of protein synthesis?

The RNA world hypothesis refers to the early period on earth in which RNA was central in assuring both genetic continuity and catalysis. The end of this era coincided with the development of the genetic code and protein synthesis, symbolized by the apparition of the first non-random messenger RNA (mRNA). Modern transfer-messenger RNA (tmRNA) is a unique hybrid molecule which has the properties of both mRNA and transfer RNA (tRNA). It acts as a key molecule during trans-translation, a major quality control pathway of modern bacterial protein synthesis. tmRNA shares many common characteristics with ancestral RNA. Here, we present a model in which proto-tmRNAs were the first molecules on earth to support non-random protein synthesis, explaining the emergence of early genetic code. In this way, proto-tmRNA could be the missing link between the first mRNA and tRNA molecules and modern ribosome-mediated protein synthesis.

Non-canonical roles of tRNAs and tRNA mimics in bacterial cell biology

Transfer RNAs (tRNAs) are the macromolecules that transfer activated amino acids from aminoacyl-tRNA synthetases to the ribosome, where they are used for the mRNA guided synthesis of proteins. Transfer RNAs are ancient molecules, perhaps even predating the existence of the translation machinery. Albeit old, these molecules are tremendously conserved, a characteristic that is well illustrated by the fact that some bacterial tRNAs are efficient and specific substrates of eukaryotic aminoacyl-tRNA synthetases and ribosomes. Considering their ancient origin and high structural conservation, it is not surprising that tRNAs have been hijacked during evolution for functions outside of translation. These roles beyond translation include synthetic, regulatory and information functions within the cell. Here we provide an overview of the non-canonical roles of tRNAs and their mimics in bacteria, and discuss some of the common themes that arise when comparing these different functions.

Ribosome-associated protein quality control

Protein synthesis by the ribosome can fail for numerous reasons including faulty mRNA, insufficient availability of charged tRNAs and genetic errors. All organisms have evolved mechanisms to recognize stalled ribosomes and initiate pathways for recycling, quality control and stress signaling. Here we review the discovery and molecular dissection of the eukaryotic ribosome-associated quality-control pathway for degradation of nascent polypeptides arising from interrupted translation.