How do base-pairing small RNAs evolve?

An sRNA overexpression library reveals AbnZ as a negative regulator of an essential translocation module in Caulobacter crescentus

Small RNAs (sRNAs) play a crucial role in modulating target gene expression through short base-pairing interactions and serve as integral components of many stress response pathways and regulatory circuits in bacteria. Transcriptome analyses have facilitated the annotation of dozens of sRNA candidates in the ubiquitous environmental model bacterium Caulobacter crescentus, but their physiological functions have not been systematically investigated so far. To address this gap, we have established CauloSOEP, a multi-copy plasmid library of C. crescentus sRNAs, which can be studied in a chosen genetic background and under select conditions. Demonstrating the power of CauloSOEP, we identified sRNA AbnZ to impair cell viability and morphology. AbnZ is processed from the 3' end of the polycistronic abn mRNA encoding the tripartite envelope-spanning efflux pump AcrAB-NodT. A combinatorial approach revealed the essential membrane translocation module TamAB as a target of AbnZ, implying that growth inhibition by AbnZ is linked to repression of this system.

Exploring the transcription start sites and other genomic features facilitates the accurate identification and annotation of small RNAs across multiple stress conditions in Mycobacterium tuberculosis

Mycobacterium tuberculosis (MTB) is a pathogen that is known for its ability to persist in harsh environments and cause chronic infections. Understanding the regulatory networks of MTB is crucial for developing effective treatments. Small regulatory RNAs (sRNAs) play important roles in gene expression regulation in all kingdoms of life, and their classification based solely on genomic location can be imprecise due to the computational-based prediction of protein-coding genes in bacteria, which often neglects segments of mRNA such as 5'UTRs, 3'UTRs, and intercistronic regions of operons. To address this issue, our study simultaneously discovered genomic features such as TSSs, UTRs, and operons together with sRNAs in the M. tuberculosis H37Rv strain (ATCC 27294) across multiple stress conditions. Our analysis identified 1,376 sRNA candidates and 8,173 TSSs in MTB, providing valuable insights into its complex regulatory landscape. TSS mapping enabled us to classify these sRNAs into more specific categories, including promoter-associated sRNAs, 5'UTR-derived sRNAs, 3'UTR-derived sRNAs, true intergenic sRNAs, and antisense sRNAs. Three of these sRNA candidates were experimentally validated using 3'-RACE-PCR: predictedRNA_0240, predictedRNA_0325, and predictedRNA_0578. Future characterization and validation are necessary to fully elucidate the functions and roles of these sRNAs in MTB. Our study is the first to simultaneously unravel TSSs and sRNAs in MTB and demonstrate that the identification of other genomic features, such as TSSs, UTRs, and operons, allows for more accurate and specific classification of sRNAs.

Exploring the targetome of IsrR, an iron-regulated sRNA controlling the synthesis of iron-containing proteins in Staphylococcus aureus

Staphylococcus aureus is a common colonizer of the skin and nares of healthy individuals, but also a major cause of severe human infections. During interaction with the host, pathogenic bacteria must adapt to a variety of adverse conditions including nutrient deprivation. In particular, they encounter severe iron limitation in the mammalian host through iron sequestration by haptoglobin and iron-binding proteins, a phenomenon called "nutritional immunity." In most bacteria, including S. aureus, the ferric uptake regulator (Fur) is the key regulator of iron homeostasis, which primarily acts as a transcriptional repressor of genes encoding iron acquisition systems. Moreover, Fur can control the expression of trans-acting small regulatory RNAs that play an important role in the cellular iron-sparing response involving major changes in cellular metabolism under iron-limiting conditions. In S. aureus, the sRNA IsrR is controlled by Fur, and most of its predicted targets are iron-containing proteins and other proteins related to iron metabolism and iron-dependent pathways. To characterize the IsrR targetome on a genome-wide scale, we combined proteomics-based identification of potential IsrR targets using S. aureus strains either lacking or constitutively expressing IsrR with an in silico target prediction approach, thereby suggesting 21 IsrR targets, of which 19 were negatively affected by IsrR based on the observed protein patterns. These included several Fe-S cluster- and heme-containing proteins, such as TCA cycle enzymes and catalase encoded by katA. IsrR affects multiple metabolic pathways connected to the TCA cycle as well as the oxidative stress response of S. aureus and links the iron limitation response to metabolic remodeling. In contrast to the majority of target mRNAs, the IsrR-katA mRNA interaction is predicted upstream of the ribosome binding site, and further experiments including mRNA half-life measurements demonstrated that IsrR, in addition to inhibiting translation initiation, can downregulate target protein levels by affecting mRNA stability.

Unveiling the orchestration: mycobacterial small RNAs as key mediators in host-pathogen interactions

Small RNA (sRNA) molecules, a class of non-coding RNAs, have emerged as pivotal players in the regulation of gene expression and cellular processes. Mycobacterium tuberculosis and other pathogenic mycobacteria produce diverse small RNA species that modulate bacterial physiology and pathogenesis. Recent advances in RNA sequencing have enabled identification of novel small RNAs and characterization of their regulatory functions. This review discusses the multifaceted roles of bacterial small RNAs, covering their biogenesis, classification, and functional diversity. Small RNAs (sRNAs) play pivotal roles in orchestrating diverse cellular processes, ranging from gene silencing to epigenetic modifications, across a broad spectrum of organisms. While traditionally associated with eukaryotic systems, recent research has unveiled their presence and significance within bacterial domains as well. Unlike their eukaryotic counterparts, which primarily function within the context of RNA interference (RNAi) pathways, bacterial sRNAs predominantly act through base-pairing interactions with target mRNAs, leading to post-transcriptional regulation. This fundamental distinction underscores the necessity of elucidating the unique roles and regulatory mechanisms of bacterial sRNAs in bacterial adaptation and survival. By doing these myriad functions, they regulate bacterial growth, metabolism, virulence, and drug resistance. In Mycobacterium tuberculosis, apart from having various roles in the bacillus itself, small RNA molecules have emerged as key regulators of gene expression and mediators of host-pathogen interactions. Understanding sRNA regulatory networks in mycobacteria can drive our understanding of significant role they play in regulating virulence and adaptation to the host environment. Detailed functional characterization of Mtb sRNAs at the host-pathogen interface is required to fully elucidate the complex sRNA-mediated gene regulatory networks deployed by Mtb, to manipulate the host. A deeper understanding of this aspect could pave the development of novel diagnostic and therapeutic strategies for tuberculosis.

ChimericFragments: computation, analysis and visualization of global RNA networks

RNA-RNA interactions are a key feature of post-transcriptional gene regulation in all domains of life. While ever more experimental protocols are being developed to study RNA duplex formation on a genome-wide scale, computational methods for the analysis and interpretation of the underlying data are lagging behind. Here, we present ChimericFragments, an analysis framework for RNA-seq experiments that produce chimeric RNA molecules. ChimericFragments implements a novel statistical method based on the complementarity of the base-pairing RNAs around their ligation site and provides an interactive graph-based visualization for data exploration and interpretation. ChimericFragments detects true RNA-RNA interactions with high precision and is compatible with several widely used experimental procedures such as RIL-seq, LIGR-seq or CLASH. We further demonstrate that ChimericFragments enables the systematic detection of novel RNA regulators and RNA-target pairs with crucial roles in microbial physiology and virulence. ChimericFragments is written in Julia and available at: https://github.com/maltesie/ChimericFragments.

Small RNAs direct attack and defense mechanisms in a quorum sensing phage and its host

Many, if not all, bacteria use quorum sensing (QS) to control collective behaviors, and more recently, QS has also been discovered in bacteriophages (phages). Phages can produce communication molecules of their own, or "listen in" on the host's communication processes, to switch between lytic and lysogenic modes of infection. Here, we study the interaction of Vibrio cholerae with the lysogenic phage VP882, which is activated by the QS molecule DPO. We discover that induction of VP882 results in the binding of phage transcripts to the major RNA chaperone Hfq, which in turn outcompetes and downregulates host-encoded small RNAs (sRNAs). VP882 itself also encodes Hfq-binding sRNAs, and we demonstrate that one of these sRNAs, named VpdS, promotes phage replication by regulating host and phage mRNA levels. We further show that host-encoded sRNAs can antagonize phage replication by downregulating phage mRNA expression and thus might be part of the host's phage defense arsenal.

Beyond membrane permeability: A role for the small RNA MicF in regulation of chromosome replication and partitioning

Small regulatory RNAs (sRNA) have been shown to play a large role in the management of stress responses in Escherichia coli and other bacteria. sRNAs act post-transcriptionally on target mRNA through an imperfect base pairing mechanism to regulate downstream protein expression. The imperfect base pairing allows a single sRNA to bind and regulate a variety mRNA targets which can form intricate regulatory networks that connect different physiological processes for the cell's response. Upon exposure to antimicrobials and superoxide generating agents, the MicF sRNA in E. coli has been shown to regulate a small set of genes involved in the management of membrane permeability. Currently, it is unknown whether MicF acts on other processes to mediate the response to these agents. Using an sRNA interaction prediction tool, we identified genes in E. coli that are potentially regulated by MicF. Through subsequent analysis using a sfGFP-based reporter-gene fusion, we have validated two novel targets of MicF regulation: SeqA, a negative modulator of DNA replication, and ObgE, a GTPase crucial for chromosome partitioning. Importantly, the interaction between MicF and these target mRNAs is contingent upon the presence of the RNA chaperone protein, Hfq. Furthermore, our findings affirm the role of MicF's conserved 5' seed pairing region in initiating these regulatory interactions. Our study suggests that, beyond its established role in membrane permeability management, MicF exerts control over chromosome dynamics in response to distinct environmental cues, implicating a more multifaceted regulatory function in bacterial stress adaptation.

Small RNA SmsR1 modulates acidogenicity and cariogenic virulence by affecting protein acetylation in Streptococcus mutans

Post-transcriptional regulation by small RNAs and post-translational modifications (PTM) such as lysine acetylation play fundamental roles in physiological circuits, offering rapid responses to environmental signals with low energy consumption. Yet, the interplay between these regulatory systems remains underexplored. Here, we unveil the cross-talk between sRNAs and lysine acetylation in Streptococcus mutans, a primary cariogenic pathogen known for its potent acidogenic virulence. Through systematic overexpression of sRNAs in S. mutans, we identified sRNA SmsR1 as a critical player in modulating acidogenicity, a key cariogenic virulence feature in S. mutans. Furthermore, combined with the analysis of predicted target mRNA and transcriptome results, potential target genes were identified and experimentally verified. A direct interaction between SmsR1 and 5'-UTR region of pdhC gene was determined by in vitro binding assays. Importantly, we found that overexpression of SmsR1 reduced the expression of pdhC mRNA and increased the intracellular concentration of acetyl-CoA, resulting in global changes in protein acetylation levels. This was verified by acetyl-proteomics in S. mutans, along with an increase in acetylation level and decreased activity of LDH. Our study unravels a novel regulatory paradigm where sRNA bridges post-transcriptional regulation with post-translational modification, underscoring bacterial adeptness in fine-tuning responses to environmental stress.

Small RNAs, Large Networks: Posttranscriptional Regulons in Gram-Negative Bacteria

Small regulatory RNA (sRNAs) are key mediators of posttranscriptional gene control in bacteria. Assisted by RNA-binding proteins, a single sRNA often modulates the expression of dozens of genes, and thus sRNAs frequently adopt central roles in regulatory networks. Posttranscriptional regulation by sRNAs comes with several unique features that cannot be achieved by transcriptional regulators. However, for optimal network performance, transcriptional and posttranscriptional control mechanisms typically go hand-in-hand. This view is reflected by the ever-growing class of mixed network motifs involving sRNAs and transcription factors, which are ubiquitous in biology and whose regulatory properties we are beginning to understand. In addition, sRNA activity can be antagonized by base-pairing with sponge RNAs, adding yet another layer of complexity to these networks. In this article, we summarize the regulatory concepts underlying sRNA-mediated gene control in bacteria and discuss how sRNAs shape the output of a network, focusing on several key examples.

Identification of the active mechanism of aminoglycoside entry in V. cholerae through characterization of sRNA ctrR, regulating carbohydrate utilization and transport

The possible active entry of aminoglycosides in bacterial cells has been debated since the development of this antibiotic family. Here we report the identification of their active transport mechanism in Vibrio species. We combined genome-wide transcriptional analysis and fitness screens to identify alterations driven by treatment of V. cholerae with sub-minimum inhibitory concentrations (sub-MIC) of the aminoglycoside tobramycin. RNA-seq data showed downregulation of the small non-coding RNA ncRNA586 during such treatment, while Tn-seq revealed that inactivation of this sRNA was associated with improved fitness in the presence of tobramycin. This sRNA is located near sugar transport genes and previous work on a homologous region in Vibrio tasmaniensis suggested that this sRNA stabilizes gene transcripts for carbohydrate transport and utilization, as well as phage receptors. The role for ncRNA586, hereafter named ctrR, in the transport of both carbohydrates and aminoglycosides, was further investigated. Flow cytometry on cells treated with a fluorescent aminoglycoside confirmed the role of ctrR and of carbohydrate transporters in differential aminoglycoside entry. Despite sequence diversity, ctrR showed functional conservation across the Vibrionales. This system in directly modulated by carbon sources, suggesting regulation by carbon catabolite repression, a widely conserved mechanism in Gram-negative bacteria, priming future research on aminoglycoside uptake by sugar transporters in other bacterial species.

Biocomputational Identification of sRNAs in Leptospira interrogans Serovar Lai

Regulatory small RNAs (sRNA) are RNA transcripts that are not translated into proteins but act as functional RNAs. Pathogenic Leptospira cause an epidemic spirochaetal zoonosis, Leptospirosis. It is speculated that Leptospiral sRNAs are involved in orchestrating their pathogenicity. In this study, biocomputational approach was adopted to identify Leptospiral sRNAs. In this study, two sRNA prediction programs, i.e., RNAz and nocoRNAc, were employed to screen the reference genome of Leptospira interrogans serovar Lai. Out of 126 predicted sRNAs, there are 96 cis-antisense sRNAs, 28 trans-encoded sRNAs and 2 sRNAs that partially overlap with protein-coding genes in a sense orientation. To determine whether these candidates are expressed in the pathogen, they were compared with the coverage files generated from our RNA-seq datasets. It was found out that 7 predicted sRNAs are expressed in mid-log phase, stationary phase, serum stress, temperature stress and iron stress while 2 sRNAs are expressed in mid-log phase, stationary phase, serum stress, and temperature stress. Besides, their expressions were also confirmed experimentally via RT-PCR. These experimentally validated candidates were also subjected to mRNA target prediction using TargetRNA2. Taken together, our study demonstrated that biocomputational strategy can serve as an alternative or as a complementary strategy to the laborious and expensive deep sequencing methods not only to uncover putative sRNAs but also to predict their targets in bacteria. In fact, this is the first study that integrates computational approach to predict putative sRNAs in L. interrogans serovar Lai.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12088-022-01050-9.

Identification of small RNAs associated with RNA chaperone Hfq reveals a new stress response regulator in Actinobacillus pleuropneumoniae

The RNA chaperone Hfq promotes the association of small RNAs (sRNAs) with cognate mRNAs, controlling the expression of bacterial phenotype. Actinobacillus pleuropneumoniae hfq mutants strains are attenuated for virulence in pigs, impaired in the ability to form biofilms, and more susceptible to stress, but knowledge of the extent of sRNA involvement is limited. Here, using A. pleuropneumoniae strain MIDG2331 (serovar 8), 14 sRNAs were identified by co-immunoprecipitation with Hfq and the expression of eight, identified as trans-acting sRNAs, were confirmed by Northern blotting. We focused on one of these sRNAs, named Rna01, containing a putative promoter for RpoE (stress regulon) recognition. Knockout mutants of rna01 and a double knockout mutant of rna01 and hfq, both had decreased biofilm formation and hemolytic activity, attenuation for virulence in Galleria mellonella, altered stress susceptibility, and an altered outer membrane protein profile. Rna01 affected extracellular vesicle production, size and toxicity in G. mellonella. qRT-PCR analysis of rna01 and putative cognate mRNA targets indicated that Rna01 is associated with the extracytoplasmic stress response. This work increases our understanding of the multilayered and complex nature of the influence of Hfq-dependent sRNAs on the physiology and virulence of A. pleuropneumoniae.

A 3' UTR-derived small RNA connecting nitrogen and carbon metabolism in enteric bacteria

Increasing numbers of small, regulatory RNAs (sRNAs) corresponding to 3' untranslated regions (UTR) are being discovered in bacteria. One such sRNA, denoted GlnZ, corresponds to the 3' UTR of the Escherichia coli glnA mRNA encoding glutamine synthetase. Several forms of GlnZ, processed from the glnA mRNA, are detected in cells growing with limiting ammonium. GlnZ levels are regulated transcriptionally by the NtrC transcription factor and post-transcriptionally by RNase III. Consistent with the expression, E. coli cells lacking glnZ show delayed outgrowth from nitrogen starvation compared to wild type cells. Transcriptome-wide RNA-RNA interactome datasets indicated that GlnZ binds to multiple target RNAs. Immunoblots and assays of fusions confirmed GlnZ-mediated repression of glnP and sucA, encoding proteins that contribute to glutamine transport and the citric acid cycle, respectively. Although the overall sequences of GlnZ from E. coli K-12, Enterohemorrhagic E. coli and Salmonella enterica have significant differences due to various sequence insertions, all forms of the sRNA were able to regulate the two targets characterized. Together our data show that GlnZ impacts growth of E. coli under low nitrogen conditions by modulating genes that affect carbon and nitrogen flux.

Small RNAs in the Antarctic bacterium Pseudomonas extremaustralis responsive to oxygen availability and oxidative stress

Bacterial small non-coding RNAs (sRNAs) play key roles as genetic regulators, mediating in the adaptability to changing environmental conditions and stress responses. In this work, we analysed putative sRNAs identified by RNA-seq experiments in different aeration conditions in the extremophile bacterium P. extremaustralis. These analyses allowed the identification of 177 putative sRNAs under aerobiosis (A), microaerobiosis (M) and microaerobiosis after H₂ O₂ exposure (m-OS). The size and transcription profile of eight sRNAs with differential expression were verified by Northern blot. sRNA40, with unknown function but conserved in other Pseudomonas species, was selected to perform overexpression experiments followed by RNA-seq analysis. The overexpression of sRNA40 in P. extremaustralis resulted in significant expression changes of 19 genes with 14 differentially upregulated and five downregulated. Among the upregulated genes, eight transcripts corresponded to components of secretion systems, such as gspH, gspK, and gspM, belonging to the Type II secretion system, and rspO and rspP from Type III secretion system. Our results showed a novel sRNA which expression was triggered by low oxygen levels, and whose overexpression was associated with upregulation of selected components of protein secretion systems.

Dual-function Spot 42 RNA encodes a 15-amino acid protein that regulates the CRP transcription factor

SignificanceDual-function RNAs base pair with target messenger RNAs as small regulatory RNAs and encode small protein regulators. However, only a limited number of these dual-function regulators have been identified. In this study, we show that a well-characterized base-pairing small RNA surprisingly also encodes a 15-amino acid protein. The very small protein binds the cyclic adenosine monophosphate receptor protein transcription factor to block activation of some promoters, raising the question of how many other transcription factors are modulated by unidentified small proteins.

A Peroxide-Responding sRNA Evolved from a Peroxidase mRNA

Small RNAs (sRNAs) are important gene regulators in bacteria, but it is unclear how new sRNAs originate and become part of regulatory networks that coordinate bacterial response to environmental stimuli. Using a covariance modeling-based approach, we analyzed the presence of hundreds of sRNAs in more than a thousand genomes across Enterobacterales, a bacterial order with a confluence of factors that allows robust genome-scale sRNA analyses: several well-studied organisms with fairly conserved genome structures, an established phylogeny, and substantial nucleotide diversity within a narrow evolutionary space. We discovered that a majority of sRNAs arose recently, and uncovered protein-coding genes as a potential source from which new sRNAs arise. A detailed investigation of the emergence of OxyS, a peroxide-responding sRNA, revealed that it evolved from a fragment of a peroxidase messenger RNA. Importantly, although it replaced the ancestral peroxidase, OxyS continues to be part of the ancestral peroxide-response regulon, indicating that an sRNA that arises from a protein-coding gene would inherently be part of the parental protein's regulatory network. This new insight provides a fresh framework for understanding sRNA origin and regulatory integration in bacteria.

RNase III-mediated processing of a trans-acting bacterial sRNA and its cis-encoded antagonist

Bacterial small RNAs (sRNAs) are important post-transcriptional regulators in stress responses and virulence. They can be derived from an expanding list of genomic contexts, such as processing from parental transcripts by RNase E. The role of RNase III in sRNA biogenesis is less well understood despite its well-known roles in rRNA processing, RNA decay, and cleavage of sRNA-mRNA duplexes. Here, we show that RNase III processes a pair of cis-encoded sRNAs (CJnc190 and CJnc180) of the food-borne pathogen Campylobacter jejuni. While CJnc180 processing by RNase III requires CJnc190, RNase III processes CJnc190 independent of CJnc180 via cleavage of an intramolecular duplex. We also show that CJnc190 directly represses translation of the colonization factor PtmG by targeting a G-rich ribosome-binding site, and uncover that CJnc180 is a cis-acting antagonist of CJnc190, indirectly affecting ptmG regulation. Our study highlights a role for RNase III in sRNA biogenesis and adds cis-encoded RNAs to the expanding diversity of transcripts that can antagonize bacterial sRNAs.

Small RNAs that target G-rich sequences are generated by diverse biogenesis pathways in Epsilonproteobacteria

Bacterial small RNAs (sRNAs) are widespread post-transcriptional regulators controlling bacterial stress responses and virulence. Nevertheless, little is known about how they arise and evolve. Homologues can be difficult to identify beyond the strain level using sequence-based approaches, and similar functionalities can arise by convergent evolution. Here, we found that the virulence-associated CJnc190 sRNA of the foodborne pathogen Campylobacter jejuni resembles the RepG sRNA from the gastric pathogen Helicobacter pylori. However, while both sRNAs bind G-rich sites in their target mRNAs using a C/U-rich loop, they largely differ in their biogenesis. RepG is transcribed from a stand-alone gene and does not require processing, whereas CJnc190 is transcribed from two promoters as precursors that are processed by RNase III and also has a cis-encoded antagonist, CJnc180. By comparing CJnc190 homologues in diverse Campylobacter species, we show that RNase III-dependent processing of CJnc190 appears to be a conserved feature even outside of C. jejuni. We also demonstrate the CJnc180 antisense partner is expressed in C. coli, yet here might be derived from the 3'UTR of the upstream flagella-related gene. Our analysis of G-tract targeting sRNAs in Epsilonproteobacteria demonstrates that similar sRNAs can have markedly different biogenesis pathways.

A small RNA is functional in Escherichia fergusonii despite containing a large insertion

Bacterial small RNAs (sRNAs) are important regulators of gene expression; however, the impact of natural mutations on sRNA functions has not been studied extensively. Here we show that the sRNA MgrR contains a unique 53 bp insertion in Escherichia fergusonii, a close relative of Escherichia coli and Salmonella enterica. The insertion is a repetitive extragenic palindromic (REP) sequence that could block transcription, but full-length MgrR is produced in E. fergusonii, showing that the insertion has not affected sRNA production. Additionally, despite containing the large insertion, the sRNA appears to be functional because deletion of mgrR made E. fergusonii more susceptible to H₂O₂. The molecular details of MgrR's roles in H₂O₂defence are yet to be defined, but our results suggest that having an alternative function allowed the sRNA to be retained in E. fergusonii despite it sustaining a large, potentially disruptive mutation.

Post-Transcriptional Control in the Regulation of Polyhydroxyalkanoates Synthesis

The large production of non-degradable petrol-based plastics has become a major global issue due to its environmental pollution. Biopolymers produced by microorganisms such as polyhydroxyalkanoates (PHAs) are gaining potential as a sustainable alternative, but the high cost associated with their industrial production has been a limiting factor. Post-transcriptional regulation is a key step to control gene expression in changing environments and has been reported to play a major role in numerous cellular processes. However, limited reports are available concerning the regulation of PHA accumulation in bacteria, and many essential regulatory factors still need to be identified. Here, we review studies where the synthesis of PHA has been reported to be regulated at the post-transcriptional level, and we analyze the RNA-mediated networks involved. Finally, we discuss the forthcoming research on riboregulation, synthetic, and metabolic engineering which could lead to improved strategies for PHAs synthesis in industrial production, thereby reducing the costs currently associated with this procedure.

Burning the Candle at Both Ends: Have Exoribonucleases Driven Divergence of Regulatory RNA Mechanisms in Bacteria?

Regulatory RNAs have emerged as ubiquitous gene regulators in all bacterial species studied to date. The combination of sequence-specific RNA interactions and malleable RNA structure has allowed regulatory RNA to adopt different mechanisms of gene regulation in a diversity of genetic backgrounds. In the model GammaproteobacteriaEscherichia coli and Salmonella, the regulatory RNA chaperone Hfq appears to play a global role in gene regulation, directly controlling ∼20 to 25% of the entire transcriptome. While the model FirmicutesBacillus subtilis and Staphylococcus aureus encode a Hfq homologue, its role has been significantly depreciated. These bacteria also have marked differences in RNA turnover. E. coli and Salmonella degrade RNA through internal endonucleolytic and 3′→5′ exonucleolytic cleavage that appears to allow transient accumulation of mRNA 3′ UTR cleavage fragments that contain stabilizing 3′ structures. In contrast, B. subtilis and S. aureus are able to exonucleolytically attack internally cleaved RNA from both the 5′ and 3′ ends, efficiently degrading mRNA 3′ UTR fragments. Here, we propose that the lack of 5′→3′ exoribonuclease activity in Gammaproteobacteria has allowed the accumulation of mRNA 3′ UTR ends as the “default” setting. This in turn may have provided a larger pool of unconstrained RNA sequences that has fueled the expansion of Hfq function and small RNA (sRNA) regulation in E. coli and Salmonella. Conversely, the exoribonuclease RNase J may be a significant barrier to the evolution of 3′ UTR sRNAs in B. subtilis and S. aureus that has limited the pool of RNA ligands available to Hfq and other sRNA chaperones, depreciating their function in these model Firmicutes.

Inverse regulation of light harvesting and photoprotection is mediated by a 3'-end-derived sRNA in cyanobacteria

Phycobilisomes (PBSs), the principal cyanobacterial antenna, are among the most efficient macromolecular structures in nature, and are used for both light harvesting and directed energy transfer to the photosynthetic reaction center. However, under unfavorable conditions, excess excitation energy needs to be rapidly dissipated to avoid photodamage. The orange carotenoid protein (OCP) senses light intensity and induces thermal energy dissipation under stress conditions. Hence, its expression must be tightly controlled; however, the molecular mechanism of this regulation remains to be elucidated. Here, we describe the discovery of a posttranscriptional regulatory mechanism in Synechocystis sp. PCC 6803 in which the expression of the operon encoding the allophycocyanin subunits of the PBS is directly and in an inverse fashion linked to the expression of OCP. This regulation is mediated by ApcZ, a small regulatory RNA that is derived from the 3'-end of the tetracistronic apcABC-apcZ operon. ApcZ inhibits ocp translation under stress-free conditions. Under most stress conditions, apc operon transcription decreases and ocp translation increases. Thus, a key operon involved in the collection of light energy is functionally connected to the expression of a protein involved in energy dissipation. Our findings support the view that regulatory RNA networks in bacteria evolve through the functionalization of mRNA 3'-UTRs.

Modulation of Bacterial Fitness and Virulence Through Antisense RNAs

Regulatory RNAs contribute to gene expression control in bacteria. Antisense RNAs (asRNA) are a class of regulatory RNAs that are transcribed from opposite strands of their target genes. Typically, these untranslated transcripts bind to cognate mRNAs and rapidly regulate gene expression at the post-transcriptional level. In this article, we review asRNAs that modulate bacterial fitness and increase virulence. We chose examples that underscore the variety observed in nature including, plasmid- and chromosome-encoded asRNAs, a riboswitch-regulated asRNA, and asRNAs that require other RNAs or RNA-binding proteins for stability and activity. We explore how asRNAs improve bacterial fitness and virulence by modulating plasmid acquisition and maintenance, regulating transposon mobility, increasing resistance against bacteriophages, controlling flagellar production, and regulating nutrient acquisition. We conclude with a brief discussion on how this knowledge is helping to inform current efforts to develop new therapeutics.

Coupled Transcriptomics for Differential Expression Analysis and Determination of Transcription Start Sites: Design and Bioinformatics

The term coupled transcriptomics is coined to describe a design of an RNA-seq experiment intended for both differential expression analysis and genome-wide determination of the transcription start sites (TSS). The minimal requirements for the first analysis are two experimental conditions with at least two biological replicates enabling statistical tests. The second analysis involves the bioinformatics comparison of the data generated from a control RNA-seq library with another library enriched in primary transcripts using Terminator^™ 5'-phosphate-dependent exonuclease, in an experiment denominated differential RNA-seq (dRNA-seq). Usually, dRNA-seq is carried out with specific protocols for library construction, different of those used for common differential expression analysis. Our experimental design allows to use the same data for both analyses, reducing the number of libraries to be generated and sequenced. This is a guide for designing a coupled transcriptomics experiment and for the subsequent bioinformatics procedures. The proposed methods can be applied to the detection and study of small RNA genes.

The intergenic small non-coding RNA ittA is required for optimal infectivity and tissue tropism in Borrelia burgdorferi

Post-transcriptional regulation via small regulatory RNAs (sRNAs) has been implicated in diverse regulatory processes in bacteria, including virulence. One class of sRNAs, termed trans-acting sRNAs, can affect the stability and/or the translational efficiency of regulated transcripts. In this study, we utilized a collaborative approach that employed data from infection with the Borrelia burgdorferi Tn library, coupled with Tn-seq, together with borrelial sRNA and total RNA transcriptomes, to identify an intergenic trans-acting sRNA, which we designate here as ittA for infectivity-associated and tissue-tropic sRNA locus A. The genetic inactivation of ittA resulted in a significant attenuation in infectivity, with decreased spirochetal load in ear, heart, skin and joint tissues. In addition, the ittA mutant did not disseminate to peripheral skin sites or heart tissue, suggesting a role for ittA in regulating a tissue-tropic response. RNA-Seq analysis determined that 19 transcripts were differentially expressed in the ittA mutant relative to its genetic parent, including vraA, bba66, ospD and oms28 (bba74). Subsequent proteomic analyses also showed a significant decrease of OspD and Oms28 (BBA74) proteins. To our knowledge this is the first documented intergenic sRNA that alters the infectivity potential of B. burgdorferi.

Switching fatty acid metabolism by an RNA-controlled feed forward loop

Hfq (host factor for phage Q beta) is key for posttranscriptional gene regulation in many bacteria. Hfq's function is to stabilize sRNAs and to facilitate base-pairing with trans-encoded target mRNAs. Loss of Hfq typically results in pleiotropic phenotypes, and, in the major human pathogen Vibrio cholerae, Hfq inactivation has been linked to reduced virulence, failure to produce biofilms, and impaired intercellular communication. However, the RNA ligands of Hfq in V. cholerae are currently unknown. Here, we used RIP-seq (RNA immunoprecipitation followed by high-throughput sequencing) analysis to identify Hfq-bound RNAs in V. cholerae Our work revealed 603 coding and 85 noncoding transcripts associated with Hfq, including 44 sRNAs originating from the 3' end of mRNAs. Detailed investigation of one of these latter transcripts, named FarS (fatty acid regulated sRNA), showed that this sRNA is produced by RNase E-mediated maturation of the fabB 3'UTR, and, together with Hfq, inhibits the expression of two paralogous fadE mRNAs. The fabB and fadE genes are antagonistically regulated by the major fatty acid transcription factor, FadR, and we show that, together, FadR, FarS, and FadE constitute a mixed feed-forward loop regulating the transition between fatty acid biosynthesis and degradation in V. cholerae Our results provide the molecular basis for studies on Hfq in V. cholerae and highlight the importance of a previously unrecognized sRNA for fatty acid metabolism in this major human pathogen.

Differential evolution in 3'UTRs leads to specific gene expression in Staphylococcus

The evolution of gene expression regulation has contributed to species differentiation. The 3' untranslated regions (3'UTRs) of mRNAs include regulatory elements that modulate gene expression; however, our knowledge of their implications in the divergence of bacterial species is currently limited. In this study, we performed genome-wide comparative analyses of mRNAs encoding orthologous proteins from the genus Staphylococcus and found that mRNA conservation was lost mostly downstream of the coding sequence (CDS), indicating the presence of high sequence diversity in the 3'UTRs of orthologous genes. Transcriptomic mapping of different staphylococcal species confirmed that 3'UTRs were also variable in length. We constructed chimeric mRNAs carrying the 3'UTR of orthologous genes and demonstrated that 3'UTR sequence variations affect protein production. This suggested that species-specific functional 3'UTRs might be specifically selected during evolution. 3'UTR variations may occur through different processes, including gene rearrangements, local nucleotide changes, and the transposition of insertion sequences. By extending the conservation analyses to specific 3'UTRs, as well as the entire set of Escherichia coli and Bacillus subtilis mRNAs, we showed that 3'UTR variability is widespread in bacteria. In summary, our work unveils an evolutionary bias within 3'UTRs that results in species-specific non-coding sequences that may contribute to bacterial diversity.

RNA-Sequencing Analyses of Small Bacterial RNAs and their Emergence as Virulence Factors in Host-Pathogen Interactions

Proteins have long been considered to be the most prominent factors regulating so-called invasive genes involved in host-pathogen interactions. The possible role of small non-coding RNAs (sRNAs), either intracellular, secreted or packaged in outer membrane vesicles (OMVs), remained unclear until recently. The advent of high-throughput RNA-sequencing (RNA-seq) techniques has accelerated sRNA discovery. RNA-seq radically changed the paradigm on bacterial virulence and pathogenicity to the point that sRNAs are emerging as an important, distinct class of virulence factors in both gram-positive and gram-negative bacteria. The potential of OMVs, as protectors and carriers of these functional, gene regulatory sRNAs between cells, has also provided an additional layer of complexity to the dynamic host-pathogen relationship. Using a non-exhaustive approach and through examples, this review aims to discuss the involvement of sRNAs, either free or loaded in OMVs, in the mechanisms of virulence and pathogenicity during bacterial infection. We provide a brief overview of sRNA origin and importance, and describe the classical and more recent methods of identification that have enabled their discovery, with an emphasis on the theoretical lower limit of RNA sizes considered for RNA sequencing and bioinformatics analyses.

In silico prediction and expression profile analysis of small non-coding RNAs in Herbaspirillum seropedicae SmR1

BACKGROUND

Herbaspirillum seropedicae is a diazotrophic bacterium from the β-proteobacteria class that colonizes endophytically important gramineous species, promotes their growth through phytohormone-dependent stimulation and can express nif genes and fix nitrogen inside plant tissues. Due to these properties this bacterium has great potential as a commercial inoculant for agriculture. The H. seropedicae SmR1 genome is completely sequenced and annotated but despite the availability of diverse structural and functional analysis of this genome, studies involving small non-coding RNAs (sRNAs) has not yet been done. We have conducted computational prediction and RNA-seq analysis to select and confirm the expression of sRNA genes in the H. seropedicae SmR1 genome, in the presence of two nitrogen independent sources and in presence of naringenin, a flavonoid secreted by some plants.

RESULTS

This approach resulted in a set of 117 sRNAs distributed in riboswitch, cis-encoded and trans-encoded categories and among them 20 have Rfam homologs. The housekeeping sRNAs tmRNA, ssrS and 4.5S were found and we observed that a large number of sRNAs are more expressed in the nitrate condition rather than the control condition and in the presence of naringenin. Some sRNAs expression were confirmed in vitro and this work contributes to better understand the post transcriptional regulation in this bacterium.

CONCLUSIONS

H. seropedicae SmR1 express sRNAs in the presence of two nitrogen sources and/or in the presence of naringenin. The functions of most of these sRNAs remains unknown but their existence in this bacterium confirms the evidence that sRNAs are involved in many different cellular activities to adapt to nutritional and environmental changes.

On a stake-out: Mycobacterial small RNA identification and regulation

Persistence of mycobacteria in the hostile environment of human macrophage is pivotal for its successful pathogenesis. Rapid adaptation to diverse stresses is the key aspect for their survival in the host cells. A range of heterogeneous mechanisms operate in bacteria to retaliate stress conditions. Small RNAs (sRNA) have been implicated in many of those mechanisms in either a single or multiple regulatory networks to post-transcriptionally modulate bacterial gene expression. Although small RNA profiling in mycobacteria by advanced technologies like deep sequencing, tilling microarray etc. have identified hundreds of sRNA, however, a handful of those small RNAs have been unearthed with precise regulatory mechanism. Extensive investigations on sRNA-mediated gene regulations in eubacteria like Escherichia coli revealed the existence of a plethora of distinctive sRNA mechanisms e.g. base pairing, protein sequestration, RNA decoy etc. Increasing studies on mycobacterial sRNA also discovered several eccentric mechanisms where sRNAs act at the posttranscriptional stage to either activate or repress target gene expression that lead to promote mycobacterial survival in stresses. Several intrinsic features like high GC content, absence of any homologue of abundant RNA chaperones, Hfq and ProQ, isolate sRNA mechanisms of mycobacteria from that of other bacteria. An insightful approach has been taken in this review to describe sRNA identification and its regulations in mycobacterial species especially in Mycobacterium tuberculosis.

Functional expansion of a TCA cycle operon mRNA by a 3' end-derived small RNA

Global RNA profiling studies in bacteria have predicted the existence of many of small noncoding RNAs (sRNAs) that are processed off mRNA 3′ ends to regulate other mRNAs via the RNA chaperones Hfq and ProQ. Here, we present targets of SdhX (RybD), an Hfq-dependent sRNA that is generated by RNase E mediated 3′ processing of the ∼10 000-nt mRNA of the TCA cycle operon sdhCDAB-sucABCD in enteric bacteria. An in silico search predicted ackA mRNA, which encodes acetate kinase, as a conserved primary target of SdhX. Through base pairing, SdhX represses AckA synthesis during growth of Salmonella on acetate. Repression can be achieved by a naturally occurring 38-nucleotide SdhX variant, revealing the shortest functional Hfq-associated sRNA yet. Salmonella SdhX also targets the mRNAs of fumB (anaerobic fumarase) and yfbV, a gene of unknown function adjacent to ackA. Instead, through a slightly different seed sequence, SdhX can repress other targets in Escherichia coli, namely katG (catalase) and fdoG (aerobic formate dehydrogenase). This study illustrates how a key operon from central metabolism is functionally connected to other metabolic pathways through a 3′ appended sRNA, and supports the notion that mRNA 3′UTRs are a playground for the evolution of regulatory RNA networks in bacteria.

A conserved RNA seed-pairing domain directs small RNA-mediated stress resistance in enterobacteria

Small regulatory RNAs (sRNAs) are crucial components of many stress response systems. The envelope stress response (ESR) of Gram‐negative bacteria is a paradigm for sRNA‐mediated stress management and involves, among other factors, the alternative sigma factor E (σ^E) and one or more sRNAs. In this study, we identified the MicV sRNA as a new member of the σ^E regulon in Vibrio cholerae. We show that MicV acts redundantly with another sRNA, VrrA, and that both sRNAs share a conserved seed‐pairing domain allowing them to regulate multiple target mRNAs. V. cholerae lacking σ^E displayed increased sensitivity toward antimicrobials, and over‐expression of either of the sRNAs suppressed this phenotype. Laboratory selection experiments using a library of synthetic sRNA regulators revealed that the seed‐pairing domain of σ^E‐dependent sRNAs is strongly enriched among sRNAs identified under membrane‐damaging conditions and that repression of OmpA is crucial for sRNA‐mediated stress relief. Together, our work shows that MicV and VrrA act as global regulators in the ESR of V. cholerae and provides evidence that bacterial sRNAs can be functionally annotated by their seed‐pairing sequences.

Application of Transcriptomics to Enhance Early Diagnostics of Mycobacterial Infections, with an Emphasis on Mycobacterium avium ssp. paratuberculosis

Mycobacteria cause a wide variety of disease in human and animals. Species that infect ruminants include M. bovis and M. avium ssp. paratuberculosis (MAP). MAP is the causative agent of Johne’s disease in ruminants, which is a chronic granulomatous enteric infection that leads to severe economic losses worldwide. Characteristic of MAP infection is the long, latent phase in which intermittent shedding can take place, while diagnostic tests are unable to reliably detect an infection in this stage. This leads to unnoticed dissemination within herds and the presence of many undetected, silent carriers, which makes the eradication of Johne’s disease difficult. To improve the control of MAP infection, research is aimed at improving early diagnosis. Transcriptomic approaches can be applied to characterize host-pathogen interactions during infection, and to develop novel biomarkers using transcriptional profiles. Studies have focused on the identification of specific RNAs that are expressed in different infection stages, which will assist in the development and clinical implementation of early diagnostic tests.

The Many Facets of the Small Non-coding RNA RsaE (RoxS) in Metabolic Niche Adaptation of Gram-Positive Bacteria

Small regulatory RNAs (sRNAs) are increasingly recognized as players in the complex regulatory networks governing bacterial gene expression. RsaE (synonym RoxS) is an sRNA that is highly conserved in bacteria of the Bacillales order. Recent analyses in Bacillus subtilis, Staphylococcus aureus and Staphylococcus epidermidis identified RsaE/RoxS as a potent riboregulator of central carbon metabolism and energy balance with many molecular RsaE/RoxS functions and targets being shared across species. Similarities and species-specific differences in cellular processes modulated by RsaE/RoxS suggest that this sRNA plays a prominent role in the adaptation of Gram-positive bacteria to niches with varying nutrient availabilities and environmental cues. This review summarizes recent findings on the molecular function of RsaE/RoxS and its interaction with mRNA targets. Special emphasis will be on the integration of RsaE/RoxS into metabolic regulatory circuits and, derived from this, the role of RsaE/RoxS as a putative driver to generate phenotypic heterogeneity in bacterial populations. In this respect, we will particularly discuss heterogeneous RsaE expression in S. epidermidis biofilms and its possible contribution to metabolic niche diversification, programmed bacterial lysis and biofilm matrix production.

Transcriptional noise and exaptation as sources for bacterial sRNAs

Understanding how new genes originate and integrate into cellular networks is key to understanding evolution. Bacteria present unique opportunities for both the natural history and experimental study of gene origins, due to their large effective population sizes, rapid generation times, and ease of genetic manipulation. Bacterial small non-coding RNAs (sRNAs), in particular, many of which operate through a simple antisense regulatory logic, may serve as tractable models for exploring processes of gene origin and adaptation. Understanding how and on what timescales these regulatory molecules arise has important implications for understanding the evolution of bacterial regulatory networks, in particular, for the design of comparative studies of sRNA function. Here, we introduce relevant concepts from evolutionary biology and review recent work that has begun to shed light on the timescales and processes through which non-functional transcriptional noise is co-opted to provide regulatory functions. We explore possible scenarios for sRNA origin, focusing on the co-option, or exaptation, of existing genomic structures which may provide protected spaces for sRNA evolution.

Guidelines for Inferring and Characterizing a Family of Bacterial trans-Acting Small Noncoding RNAs

So far, every sequenced bacterial transcriptome encompasses hundreds of small regulatory noncoding RNAs (sRNAs). From those sRNAs that have been already characterized, we learned that their regulatory functions could span over almost every bacterial process, mostly acting at the posttranscriptional control of gene expression (Wagner and Romby, Adv Genet 90:133-208, 2015). Canonical molecular mechanisms of sRNA action have been described to rely on both sequence and/or structural traits of the RNA molecule. As for protein-coding genes, the conservation of sRNAs among species suggests conserved and adjusted functions across evolution. Knowing the phylogenetic distribution of an sRNA gene and how its functional traits have evolved may help to get a broad picture of its biological role in each single species. Here, we present a simple computational workflow to identify close and distant sRNA homologs present in sequenced bacterial genomes, which allows defining novel sRNA families. This strategy is based on the use of Covariance Models (CM) and assumes the conservation of sequence and structure of functional sRNA genes throughout evolution. Moreover, by carefully inspecting the conservation of the close genomic context of every member of the RNA family and how the patterns of microsynteny follow the path of species evolution, it is possible to define subgroups of sRNA orthologs, which in turn enables the definition of RNA subfamilies.

Cross-Regulation between Bacteria and Phages at a Posttranscriptional Level

The study of bacteriophages (phages) and prophages has provided key insights into almost every cellular process as well as led to the discovery of unexpected new mechanisms and the development of valuable tools. This is exemplified for RNA-based regulation. For instance, the characterization and exploitation of the antiphage CRISPR (clustered regularly interspaced short palindromic repeat) systems is revolutionizing molecular biology. Phage-encoded proteins such as the RNA-binding MS2 protein, which is broadly used to isolate tagged RNAs, also have been developed as valuable tools. Hfq, the RNA chaperone protein central to the function of many base-pairing small RNAs (sRNAs), was first characterized as a bacterial host factor required for Qβ phage replication. The ongoing studies of RNAs are continuing to reveal regulatory connections between infecting phages, prophages, and bacteria and to provide novel insights. There are bacterial and prophage sRNAs that regulate prophage genes, which impact bacterial virulence as well as bacterial cell killing. Conversely, phage- and prophage-encoded sRNAs modulate the expression of bacterial genes modifying metabolism. An interesting subcategory of the prophage-encoded sRNAs are sponge RNAs that inhibit the activities of bacterial-encoded sRNAs. Phages also affect posttranscriptional regulation in bacteria through proteins that inhibit or alter the activities of key bacterial proteins involved in posttranscriptional regulation. However, what is most exciting about phage and prophage research, given the millions of phage-encoded genes that have not yet been characterized, is the vast potential for discovering new RNA regulators and novel mechanisms and for gaining insight into the evolution of regulatory RNAs.

Origin, Evolution, and Loss of Bacterial Small RNAs

Despite the central role of bacterial noncoding small RNAs (sRNAs) in posttranscriptional regulation, little is understood about their evolution. Here we compile what has been studied to date and trace a life cycle of sRNAs-from their mechanisms of emergence, through processes of change and frequent neofunctionalization, to their loss from bacterial lineages. Because they possess relatively unrestrictive structural requirements, we find that sRNA origins are varied, and include de novo emergence as well as formation from preexisting genetic elements via duplication events and horizontal gene transfer. The need for only partial complementarity to their mRNA targets facilitates apparent rapid change, which also contributes to significant challenges in tracing sRNAs across broad evolutionary distances. We document that recently emerged sRNAs in particular evolve quickly, mirroring dynamics observed in microRNAs, their functional analogs in eukaryotes. Mutations in mRNA-binding regions, transcriptional regulator or sigma factor binding sites, and protein-binding regions are all likely sources of shifting regulatory roles of sRNAs. Finally, using examples from the few evolutionary studies available, we examine cases of sRNA loss and describe how these may be the result of adaptive in addition to neutral processes. We highlight the need for more-comprehensive analyses of sRNA evolutionary patterns as a means to improve novel sRNA detection, enhance genome annotation, and deepen our understanding of regulatory networks in bacteria.

The binding of Class II sRNA MgrR to two different sites on matchmaker protein Hfq enables efficient competition for Hfq and annealing to regulated mRNAs

MgrR is an Hfq-dependent sRNA, whose transcription is controlled by the level of Mg²⁺ ions in Escherichia coli MgrR belongs to Class II sRNAs because its stability in the cell is affected by mutations in Hfq differently than canonical, Class I sRNAs. Here, we examined the effect of mutations in RNA binding sites of Hfq on the kinetics of the annealing of MgrR to two different target mRNAs, eptB and ygdQ, by global data fitting of the reaction kinetics monitored by gel electrophoresis of intermediates and products. The data showed that the mutation on the rim of the Hfq ring trapped MgrR on Hfq preventing the annealing of MgrR to either mRNA. The mutation in the distal face slowed the ternary complex formation and affected the release of MgrR-mRNA complexes from Hfq, while the mutation in the proximal face weakened the MgrR binding to Hfq and in this way affected the annealing. Moreover, competition assays established that MgrR bound to both faces of Hfq and competed against other sRNAs. Further studies showed that uridine-rich sequences located in less structurally stable regions served as Hfq binding sites in each mRNA. Overall, the data show that the binding of MgrR sRNA to both faces of the Hfq ring enables it to efficiently anneal to target mRNAs. It also confers on MgrR a competitive advantage over other sRNAs, which could contribute to efficient cellular response to changes in magnesium homeostasis.

A Small Regulatory RNA Controls Cell Wall Biosynthesis and Antibiotic Resistance

Small regulatory RNAs play an important role in the adaptation to changing conditions. Here, we describe a differentially expressed small regulatory RNA (sRNA) that affects various cellular processes in the plant pathogen Agrobacterium tumefaciens Using a combination of bioinformatic predictions and comparative proteomics, we identified nine targets, most of which are positively regulated by the sRNA. According to these targets, we named the sRNA PmaR for peptidoglycan biosynthesis, motility, and ampicillin resistance regulator. Agrobacterium spp. are long known to be naturally resistant to high ampicillin concentrations, and we can now explain this phenotype by the positive PmaR-mediated regulation of the beta-lactamase gene ampC Structure probing revealed a spoon-like structure of the sRNA, with a single-stranded loop that is engaged in target interaction in vivo and in vitro Several riboregulators have been implicated in antibiotic resistance mechanisms, such as uptake and efflux transporters, but PmaR represents the first example of an sRNA that directly controls the expression of an antibiotic resistance gene.IMPORTANCE The alphaproteobacterium Agrobacterium tumefaciens is able to infect various eudicots causing crown gall tumor formation. Based on its unique ability of interkingdom gene transfer, Agrobacterium serves as a crucial biotechnological tool for genetic manipulation of plant cells. The presence of hundreds of putative sRNAs in this organism suggests a considerable impact of riboregulation on A. tumefaciens physiology. Here, we characterized the biological function of the sRNA PmaR that controls various processes crucial for growth, motility, and virulence. Among the genes directly targeted by PmaR is ampC coding for a beta-lactamase that confers ampicillin resistance, suggesting that the sRNA is crucial for fitness in the competitive microbial composition of the rhizosphere.

Hfq chaperone brings speed dating to bacterial sRNA

Hfq is a ubiquitous, Sm-like RNA binding protein found in most bacteria and some archaea. Hfq binds small regulatory RNAs (sRNAs), facilitates base pairing between sRNAs and their mRNA targets, and directly binds and regulates translation of certain mRNAs. Because sRNAs regulate many stress response pathways in bacteria, Hfq is essential for adaptation to different environments and growth conditions. The chaperone activities of Hfq arise from multipronged RNA binding by three different surfaces of the Hfq hexamer. The manner in which the structured Sm core of Hfq binds RNA has been well studied, but recent work shows that the intrinsically disordered C-terminal domain of Hfq modulates sRNA binding, creating a kinetic hierarchy of RNA competition for Hfq and ensuring the release of double-stranded sRNA-mRNA complexes. A combination of structural, biophysical, and genetic experiments reveals how Hfq recognizes its RNA substrates and plays matchmaker for sRNAs and mRNAs in the cell. The interplay between structured and disordered domains of Hfq optimizes sRNA-mediated post-transcriptional regulation, and is a common theme in RNA chaperones. This article is categorized under: Regulatory RNAs/RNAi/Riboswitches > Regulatory RNAs RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes RNA Structure and Dynamics > RNA Structure, Dynamics, and Chemistry.

Regulatory RNAs in Virulence and Host-Microbe Interactions

Bacterial regulatory RNAs are key players in adaptation to changing environmental conditions and response to diverse cellular stresses. However, while regulatory RNAs of bacterial pathogens have been intensely studied under defined conditions in vitro, characterization of their role during the infection of eukaryotic host organisms is lagging behind. This review summarizes our current understanding of the contribution of the different classes of regulatory RNAs and RNA-binding proteins to bacterial virulence and illustrates their role in infection by reviewing the mechanisms of some prominent representatives of each class. Emerging technologies are described that bear great potential for global, unbiased studies of virulence-related RNAs in bacterial model and nonmodel pathogens in the future. The review concludes by deducing common principles of RNA-mediated gene expression control of virulence programs in different pathogens, and by defining important open questions for upcoming research in the field.

Regulatory RNA in Mycobacterium tuberculosis, back to basics

Since the turn of the millenium, RNA-based control of gene expression has added an extra dimension to the central dogma of molecular biology. Still, the roles of Mycobacterium tuberculosis regulatory RNAs and the proteins that facilitate their functions remain elusive, although there can be no doubt that RNA biology plays a central role in the baterium's adaptation to its many host environments. In this review, we have presented examples from model organisms and from M. tuberculosis to showcase the abundance and versatility of regulatory RNA, in order to emphasise the importance of these 'fine-tuners' of gene expression.

Small RNA-mediated regulation in bacteria: A growing palette of diverse mechanisms

Small RNAs (sRNAs) in bacteria have evolved with diverse mechanisms to balance their target gene expression in response to changes in the environment. Accumulating studies on bacterial regulatory processes firmly established that sRNAs modulate their target gene expression generally at the posttranscriptional level. Identification of large number of sRNAs by advanced technologies, like deep sequencing, tilling microarray, indicates the existence of a plethora of distinctive sRNA-mediated regulatory mechanisms in bacteria. Types of the novel mechanisms are increasing with the discovery of new sRNAs. Complementary base pairing between sRNAs and target RNAs assisted by RNA chaperones like Hfq and ProQ, in many occasions, to regulate the cognate gene expression is prevalent in sRNA mechanisms. sRNAs, in most studied cases, can directly base pair with target mRNA to remodel its expression. Base pairing can happen either in the untranslated regions or in the coding regions of mRNA to activate/repress its translation. sRNAs also act as target mimic to titrate away different regulatory RNAs from its target. Other mechanism includes the sequestration of regulatory proteins, especially transcription factors, by sRNAs. Numerous sRNAs, following analogous mechanism, are widespread in bacteria, and thus, has drawn immense attention for the development of RNA-based technologies. Nevertheless, typical sRNA mechanisms are also discovered to be confined in some bacteria. Analysis of the sRNA mechanisms unravels their existence in both the single step processes and the complex regulatory networks with a global effect on cell physiology. This review deals with the diverse array of mechanisms, which sRNAs follow to maintain bacterial lifestyle.

When eukaryotes and prokaryotes look alike: the case of regulatory RNAs

The discovery that all living entities express many RNAs beyond mRNAs, tRNAs and rRNAs has been a surprise in the past two decades. In fact, regulatory RNAs (regRNAs) are plentiful, and we report stunning parallels between their mechanisms and functions in prokaryotes and eukaryotes. For instance, prokaryotic CRISPR (clustered regularly interspaced short palindromic repeats) defense systems are functional analogs to eukaryotic RNA interference processes that preserve the cell against foreign nucleic acid elements. Regulatory RNAs shape the genome in many ways: by controlling mobile element transposition in both domains, via regulation of plasmid counts in prokaryotes, or by directing epigenetic modifications of DNA and associated proteins in eukaryotes. RegRNAs control gene expression extensively at transcriptional and post-transcriptional levels, with crucial roles in fine-tuning cell environmental responses, including intercellular interactions. Although the lengths, structures and outcomes of the regRNAs in all life kingdoms are disparate, they act through similar patterns: by guiding effectors to target molecules or by sequestering macromolecules to hamper their functions. In addition, their biogenesis processes have a lot in common. This unifying vision of regRNAs in all living cells from bacteria to humans points to the possibility of fruitful exchanges between fundamental and applied research in both domains.

Unraveling the evolution and coevolution of small regulatory RNAs and coding genes in Listeria

BACKGROUND

Small regulatory RNAs (sRNAs) are widely found in bacteria and play key roles in many important physiological and adaptation processes. Studying their evolution and screening for events of coevolution with other genomic features is a powerful way to better understand their origin and assess a common functional or adaptive relationship between them. However, evolution and coevolution of sRNAs with coding genes have been sparsely investigated in bacterial pathogens.

RESULTS

We designed a robust and generic phylogenomics approach that detects correlated evolution between sRNAs and protein-coding genes using their observed and inferred patterns of presence-absence in a set of annotated genomes. We applied this approach on 79 complete genomes of the Listeria genus and identified fifty-two accessory sRNAs, of which most were present in the Listeria common ancestor and lost during Listeria evolution. We detected significant coevolution between 23 sRNA and 52 coding genes and inferred the Listeria sRNA-coding genes coevolution network. We characterized a main hub of 12 sRNAs that coevolved with genes encoding cell wall proteins and virulence factors. Among them, an sRNA specific to L. monocytogenes species, rli133, coevolved with genes involved either in pathogenicity or in interaction with host cells, possibly acting as a direct negative post-transcriptional regulation.

CONCLUSIONS

Our approach allowed the identification of candidate sRNAs potentially involved in pathogenicity and host interaction, consistent with recent findings on known pathogenicity actors. We highlight four sRNAs coevolving with seven internalin genes, some of which being important virulence factors in Listeria.

A conserved stem of the Myxococcus xanthus sRNA Pxr controls sRNA accumulation and multicellular development

The small RNA (sRNA) Pxr negatively controls multicellular fruiting body formation in the bacterium Myxococcus xanthus, inhibiting the transition from growth to development when nutrients are abundant. Like many other prokaryotic sRNAs, Pxr is predicted to fold into three stem loops (SL1-SL3). SL1 and SL2 are highly conserved across the myxobacteria, whereas SL3 is much more variable. SL1 is necessary for the regulatory function of Pxr but the importance of SL3 in this regard is unknown. To test for cis genetic elements required for Pxr function, we deleted the entire pxr gene from a developmentally defective strain that fails to remove Pxr-mediated blockage of development and reintroduced variably truncated fragments of the pxr region to test for their ability to block development. These truncations demonstrated that SL3 is necessary for Pxr function in the defective strain. We further show that a highly conserved eight-base-pair segment of SL3 is not only necessary for Pxr to block development in the defective strain under starvation conditions, but is also required for Pxr to prevent fruiting body development by a developmentally proficient wild-type strain under high-nutrient conditions. This conserved segment of SL3 is also necessary for detectable levels of Pxr to accumulate, suggesting that this segment either stabilizes Pxr against premature degradation during vegetative growth or positively regulates its transcription.

The RNA targetome of Staphylococcus aureus non-coding RNA RsaA: impact on cell surface properties and defense mechanisms

The virulon of Staphyloccocus aureus is controlled by intricate connections between transcriptional and post-transcriptional regulators including proteins and small non-coding RNAs (sRNAs). Many of the sRNAs regulate gene expression through base-pairings with mRNAs. However, characterization of the direct sRNA targets in Gram-positive bacteria remained a difficult challenge. Here, we have applied and adapted the MS2-affinity purification approach coupled to RNA sequencing (MAPS) to determine the targetome of RsaA sRNA of S. aureus, known to repress the synthesis of the transcriptional regulator MgrA. Several mRNAs were enriched with RsaA expanding its regulatory network. Besides mgrA, several of these mRNAs encode a family of SsaA-like enzymes involved in peptidoglycan metabolism and the secreted anti-inflammatory FLIPr protein. Using a combination of in vivo and in vitro approaches, these mRNAs were validated as direct RsaA targets. Quantitative differential proteomics of wild-type and mutant strains corroborated the MAPS results. Additionally, it revealed that RsaA indirectly activated the synthesis of surface proteins supporting previous data that RsaA stimulated biofilm formation and favoured chronic infections. All together, this study shows that MAPS could also be easily applied in Gram-positive bacteria for identification of sRNA targetome.