Hfq-dependent mRNA unfolding promotes sRNA-based inhibition of translation

Hfq mediates transcriptome-wide RNA structurome reprogramming under virulence-inducing conditions in a phytopathogen

Although RNA structures play important roles in regulating gene expression, the mechanism and function of mRNA folding in plant bacterial pathogens remain elusive. Therefore, we perform dimethyl sulfate sequencing (DMS-seq) on the Pseudomonas syringae under nutrition-rich and -deficient conditions, revealing that the mRNA structure changes substantially in the minimal medium (MM) that tunes global translation efficiency (TE), thereby inducing virulence. This process is led by the increased expression of hfq, which is directly activated by transcription regulators RpoS and CysB. The co-occurrence of Hfq and RpoS in diverse bacteria and the deep conservation of Hfq Y25 is critical for RNA-mediated regulation and implicates the wider biological importance of mRNA structure and feedback loops in the control of global gene expression.

Riboswitch and small RNAs modulate btuB translation initiation in Escherichia coli and trigger distinct mRNA regulatory mechanisms

Small RNAs (sRNAs) and riboswitches represent distinct classes of RNA regulators that control gene expression upon sensing metabolic or environmental variations. While sRNAs and riboswitches regulate gene expression by affecting mRNA and protein levels, existing studies have been limited to the characterization of each regulatory system in isolation, suggesting that sRNAs and riboswitches target distinct mRNA populations. We report that the expression of btuB in Escherichia coli, which is regulated by an adenosylcobalamin (AdoCbl) riboswitch, is also controlled by the small RNAs OmrA and, to a lesser extent, OmrB. Strikingly, we find that the riboswitch and sRNAs reduce mRNA levels through distinct pathways. Our data show that while the riboswitch triggers Rho-dependent transcription termination, sRNAs rely on the degradosome to modulate mRNA levels. Importantly, OmrA pairs with the btuB mRNA through its central region, which is not conserved in OmrB, indicating that these two sRNAs may have specific targets in addition to their common regulon. In contrast to canonical sRNA regulation, we find that OmrA repression of btuB is lost using an mRNA binding-deficient Hfq variant. Together, our study demonstrates that riboswitch and sRNAs modulate btuB expression, providing an example of cis- and trans-acting RNA-based regulatory systems maintaining cellular homeostasis.

sRNA expression profile of KPC-2-producing carbapenem-resistant Klebsiella pneumoniae: Functional role of sRNA51

The emergence of carbapenem-resistant Klebsiella pneumoniae (CRKP) has significant challenges to human health and clinical treatment, with KPC-2-producing CRKP being the predominant epidemic strain. Therefore, there is an urgent need to identify new therapeutic targets and strategies. Non-coding small RNA (sRNA) is a post-transcriptional regulator of genes involved in important biological processes in bacteria and represents an emerging therapeutic strategy for antibiotic-resistant bacteria. In this study, we analyzed the transcription profile of KPC-2-producing CRKP using RNA-seq. Of the 4693 known genes detected, the expression of 307 genes was significantly different from that of carbapenem-sensitive Klebsiella pneumoniae (CSKP), including 133 up-regulated and 174 down-regulated genes. Both the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment and Gene Ontology (GO) analysis showed that these differentially expressed genes (DEGs) were mainly related to metabolism. In addition, we identified the sRNA expression profile of KPC-2-producing CRKP for the first time and detected 115 sRNAs, including 112 newly discovered sRNAs. Compared to CSKP, 43 sRNAs were differentially expressed in KPC-2-producing CRKP, including 39 up-regulated and 4 down-regulated sRNAs. We chose sRNA51, the most significantly differentially expressed sRNA in KPC-2-producing CRKP, as our research subject. By constructing sRNA51-overexpressing KPC-2-producing CRKP strains, we found that sRNA51 overexpression down-regulated the expression of acrA and alleviated resistance to meropenem and ertapenem in KPC-2-producing CRKP, while overexpression of acrA in sRNA51-overexpressing strains restored the reduction of resistance. Therefore, we speculated that sRNA51 could affect the resistance of KPC-2-producing CRKP by inhibiting acrA expression and affecting the formation of efflux pumps. This provides a new approach for developing antibiotic adjuvants to restore the sensitivity of CRKP.

Enhancing the Efficacy of Chloramphenicol Therapy for Escherichia coli by Targeting the Secondary Resistome

The increasing prevalence of antimicrobial resistance and the limited availability of new antimicrobial agents have created an urgent need for new approaches to combat these issues. One such approach involves reevaluating the use of old antibiotics to ensure their appropriate usage and maximize their effectiveness, as older antibiotics could help alleviate the burden on newer agents. An example of such an antibiotic is chloramphenicol (CHL), which is rarely used due to its hematological toxicity. In the current study, we employed a previously published transposon mutant library in MG1655/pTF2::blaCTX-M-1, containing over 315,000 unique transposon insertions, to identify the genetic factors that play an important role during growth in the presence of CHL. The list of conditionally essential genes, collectively referred to as the secondary resistome (SR), included 67 genes. To validate our findings, we conducted gene knockout experiments on six genes: arcA, hfq, acrZ, cls, mdfA, and nlpI. Deleting these genes resulted in increased susceptibility to CHL as demonstrated by MIC estimations and growth experiments, suggesting that targeting the products encoded from these genes may reduce the dose of CHL needed for treatment and hence reduce the toxicity associated with CHL treatment. Thus, the gene products are indicated as targets for antibiotic adjuvants to favor the use of CHL in modern medicine.

How to do RNA-RNA Interaction Prediction? A Use-Case Driven Handbook Using IntaRNA

Computational prediction of RNA-RNA interactions (RRI) is a central methodology for the specific investigation of inter-molecular RNA interactions and regulatory effects of non-coding RNAs like eukaryotic microRNAs or prokaryotic small RNAs. Available methods can be classified according to their underlying prediction strategies, each implicating specific capabilities and restrictions often not transparent to the non-expert user. Within this work, we review seven classes of RRI prediction strategies and discuss the advantages and limitations of respective tools, since such knowledge is essential for selecting the right tool in the first place.Among the RRI prediction strategies, accessibility-based approaches have been shown to provide the most reliable predictions. Here, we describe how IntaRNA, as one of the state-of-the-art accessibility-based tools, can be applied in various use cases for the task of computational RRI prediction. Detailed hands-on examples for individual RRI predictions as well as large-scale target prediction scenarios are provided. We illustrate the flexibility and capabilities of IntaRNA through the examples. Each example is designed using real-life data from the literature and is accompanied by instructions on interpreting the respective results from IntaRNA output. Our use-case driven instructions enable non-expert users to comprehensively understand and utilize IntaRNA's features for effective RRI predictions.

Molecular responses during bacterial filamentation reveal inhibition methods of drug-resistant bacteria

Bacterial antimicrobial resistance (AMR) is among the most significant challenges to current human society. Exposing bacteria to antibiotics can activate their self-saving responses, e.g., filamentation, leading to the development of bacterial AMR. Understanding the molecular changes during the self-saving responses can reveal new inhibition methods of drug-resistant bacteria. Herein, we used an online microfluidics mass spectrometry system for real-time characterization of metabolic changes of bacteria during filamentation under the stimulus of antibiotics. Significant pathways, e.g., nucleotide metabolism and coenzyme A biosynthesis, correlated to the filamentation of extended-spectrum beta-lactamase-producing Escherichia coli (ESBL-E. coli) were identified. A cyclic dinucleotide, c-di-GMP, which is derived from nucleotide metabolism and reported closely related to bacterial resistance and tolerance, was observed significantly up-regulated during the bacterial filamentation. By using a chemical inhibitor, ebselen, to inhibit diguanylate cyclases which catalyzes the synthesis of c-di-GMP, the minimum inhibitory concentration of ceftriaxone against ESBL-E. coli was significantly decreased. This inhibitory effect was also verified with other ESBL-E. coli strains and other beta-lactam antibiotics, i.e., ampicillin. A mutant strain of ESBL-E. coli by knocking out the dgcM gene was used to demonstrate that the inhibition of the antibiotic resistance to beta-lactams by ebselen was mediated through the inhibition of the diguanylate cyclase DgcM and the modulation of c-di-GMP levels. Our study uncovers the molecular changes during bacterial filamentation and proposes a method to inhibit antibiotic-resistant bacteria by combining traditional antibiotics and chemical inhibitors against the enzymes involved in bacterial self-saving responses.

Dynamic Refolding of OxyS sRNA by the Hfq RNA Chaperone

The Sm protein Hfq chaperones small non-coding RNAs (sRNAs) in bacteria, facilitating sRNA regulation of target mRNAs. Hfq acts in part by remodeling the sRNA and mRNA structures, yet the basis for this remodeling activity is not understood. To understand how Hfq remodels RNA, we used single-molecule Förster resonance energy transfer (smFRET) to monitor conformational changes in OxyS sRNA upon Hfq binding. The results show that E. coli Hfq first compacts OxyS, bringing its 5' and 3 ends together. Next, Hfq destabilizes an internal stem-loop in OxyS, allowing the RNA to adopt a more open conformation that is stabilized by a conserved arginine on the rim of Hfq. The frequency of transitions between compact and open conformations depend on interactions with Hfqs flexible C-terminal domain (CTD), being more rapid when the CTD is deleted, and slower when OxyS is bound to Caulobacter crescentus Hfq, which has a shorter and more stable CTD than E. coli Hfq. We propose that the CTDs gate transitions between OxyS conformations that are stabilized by interaction with one or more arginines. These results suggest a general model for how basic residues and intrinsically disordered regions of RNA chaperones act together to refold RNA.

Synthetic Genetic Interactions Reveal a Dense and Cryptic Regulatory Network of Small Noncoding RNAs in Escherichia coli

Over the past 20 years, we have learned that bacterial small noncoding RNAs (sRNAs) can rapidly effect changes in gene expression in response to stress. However, the broader role and impact of sRNA-mediated regulation in promoting bacterial survival has remained elusive. Indeed, there are few examples where disruption of sRNA-mediated gene regulation results in a discernible change in bacterial growth or survival. The lack of phenotypes attributable to loss of sRNA function suggests that either sRNAs are wholly dispensable or functional redundancies mask the impact of deleting a single sRNA. We investigated synthetic genetic interactions among sRNA genes in Escherichia coli by constructing pairwise deletions in 54 genes, including 52 sRNAs. Some 1,373 double deletion strains were studied for growth defects under 32 different nutrient stress conditions and revealed 1,131 genetic interactions. In one example, we identified a profound synthetic lethal interaction between ArcZ and CsrC when E. coli was grown on pyruvate, lactate, oxaloacetate, or d-/l-alanine, and we provide evidence that the expression of ppsA is dysregulated in the double deletion background, causing the conditionally lethal phenotype. This work employs a unique platform for studying sRNA-mediated gene regulation and sheds new light on the genetic network of sRNAs that underpins bacterial growth.

Regulation Mechanisms of Virulence Genes in Enterohemorrhagic Escherichia coli

Enterohemorrhagic Escherichia coli (EHEC) is one of the most common E. coli pathotypes reported to cause several outbreaks of foodborne illnesses. EHEC is a zoonotic pathogen, and ruminants, especially cattle, are considered important reservoirs for the most common EHEC serotype, E. coli O157:H7. Humans are infected indirectly through the consumption of food (milk, meat, leafy vegetables, and fruits) and water contaminated by animal feces or direct contact with carrier animals or humans. E. coli O157:H7 is one of the most frequently reported causes of foodborne illnesses in developed countries. It employs two essential virulence mechanisms to trigger damage to the host. These are the development of attaching and effacing (AE) phenotypes on the intestinal mucosa of the host and the production of Shiga toxin (Stx) that causes hemorrhagic colitis and hemolytic uremic syndrome. The AE phenotype is controlled by the pathogenicity island, the locus of enterocyte effacement (LEE). The induction of both AE and Stx is under strict and highly complex regulatory mechanisms. Thus, a good understanding of these mechanisms, major proteins expressed, and environmental cues involved in the regulation of the expression of the virulence genes is vital to finding a method to control the colonization of reservoir hosts, especially cattle, and disease development in humans. This review is a concise account of the current state of knowledge of virulence gene regulation in the LEE-positive EHEC.

Comprehensive analysis of PNA-based antisense antibiotics targeting various essential genes in uropathogenic Escherichia coli

Antisense peptide nucleic acids (PNAs) that target mRNAs of essential bacterial genes exhibit specific bactericidal effects in several microbial species, but our mechanistic understanding of PNA activity and their target gene spectrum is limited. Here, we present a systematic analysis of PNAs targeting 11 essential genes with varying expression levels in uropathogenic Escherichia coli (UPEC). We demonstrate that UPEC is susceptible to killing by peptide-conjugated PNAs, especially when targeting the widely-used essential gene acpP. Our evaluation yields three additional promising target mRNAs for effective growth inhibition, i.e.dnaB, ftsZ and rpsH. The analysis also shows that transcript abundance does not predict target vulnerability and that PNA-mediated growth inhibition is not universally associated with target mRNA depletion. Global transcriptomic analyses further reveal PNA sequence-dependent but also -independent responses, including the induction of envelope stress response pathways. Importantly, we show that 9mer PNAs are generally as effective in inhibiting bacterial growth as their 10mer counterparts. Overall, our systematic comparison of a range of PNAs targeting mRNAs of different essential genes in UPEC suggests important features for PNA design, reveals a general bacterial response to PNA conjugates and establishes the feasibility of using PNA antibacterials to combat UPEC.

Bacterial RNA chaperones and chaperone-like riboregulators: behind the scenes of RNA-mediated regulation of cellular metabolism

In all domains of life, RNA chaperones safeguard and guide the fate of the cellular RNA pool. RNA chaperones comprise structurally diverse proteins that ensure proper folding, stability, and ribonuclease resistance of RNA, and they support regulatory activities mediated by RNA. RNA chaperones constitute a topologically diverse group of proteins that often present an unstructured region and bind RNA with limited nucleotide sequence preferences. In bacteria, three main proteins - Hfq, ProQ, and CsrA - have been shown to regulate numerous complex processes, including bacterial growth, stress response and virulence. Hfq and ProQ have well-studied activities as global chaperones with pleiotropic impact, while CsrA has a chaperone-like role with more defined riboregulatory function. Here, we describe relevant novel insights into their common features, including RNA binding properties, unstructured domains, and interplay with other proteins important to RNA metabolism.

Bacterial Chaperone Protein Hfq Facilitates the Annealing of Sponge RNAs to Small Regulatory RNAs

Bacterial small RNAs (sRNAs) in association with the chaperone protein Hfq regulate the expression of many target mRNAs. Since sRNAs' action is crucial to engendering a response to changing environmental conditions, their activity needs to be regulated. One such mechanism occurs at the post-transcriptional level and involves sponge RNAs, which sequester sRNAs affecting their regulatory output. Both types of RNAs were identified on Hfq, but it is not known how Hfq interacts with RNA sponges and stimulates their base-pairing with sRNAs. Here, we used biochemical methods to demonstrate that sponge RNAs resemble sRNAs by their structure and their modes of Hfq binding. Hfq facilitates the annealing of AgvB and 3'ETS^leuZ sponge RNAs to targeted sRNAs: GcvB and RybB, respectively, and each surface of the protein plays a particular role in the process. Moreover, we found that the efficiency of sponge RNA interactions with sRNAs can be improved; therefore, we propose that natural RNA sponges might not sequester sRNAs optimally.

Riboregulation in bacteria: From general principles to novel mechanisms of the trp attenuator and its sRNA and peptide products

Gene expression strategies ensuring bacterial survival and competitiveness rely on cis- and trans-acting RNA-regulators (riboregulators). Among the cis-acting riboregulators are transcriptional and translational attenuators, and antisense RNAs (asRNAs). The trans-acting riboregulators are small RNAs (sRNAs) that bind proteins or base pairs with other RNAs. This classification is artificial since some regulatory RNAs act both in cis and in trans, or function in addition as small mRNAs. A prominent example is the archetypical, ribosome-dependent attenuator of tryptophan (Trp) biosynthesis genes. It responds by transcription attenuation to two signals, Trp availability and inhibition of translation, and gives rise to two trans-acting products, the attenuator sRNA rnTrpL and the leader peptide peTrpL. In Escherichia coli, rnTrpL links Trp availability to initiation of chromosome replication and in Sinorhizobium meliloti, it coordinates regulation of split tryptophan biosynthesis operons. Furthermore, in S. meliloti, peTrpL is involved in mRNA destabilization in response to antibiotic exposure. It forms two types of asRNA-containing, antibiotic-dependent ribonucleoprotein complexes (ARNPs), one of them changing the target specificity of rnTrpL. The posttranscriptional role of peTrpL indicates two emerging paradigms: (1) sRNA reprograming by small molecules and (2) direct involvement of antibiotics in regulatory RNPs. They broaden our view on RNA-based mechanisms and may inspire new approaches for studying, detecting, and using antibacterial compounds. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Small Molecule-RNA Interactions RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes Regulatory RNAs/RNAi/Riboswitches > Regulatory RNAs.

Kinetic modeling reveals additional regulation at co-transcriptional level by post-transcriptional sRNA regulators

Small RNAs (sRNAs) are important gene regulators in bacteria. Many sRNAs act post-transcriptionally by affecting translation and degradation of the target mRNAs upon base-pairing interactions. Here we present a general approach combining imaging and mathematical modeling to determine kinetic parameters at different levels of sRNA-mediated gene regulation that contribute to overall regulation efficacy. Our data reveal that certain sRNAs previously characterized as post-transcriptional regulators can regulate some targets co-transcriptionally, leading to a revised model that sRNA-mediated regulation can occur early in an mRNA’s lifetime, as soon as the sRNA binding site is transcribed. This co-transcriptional regulation is likely mediated by Rho-dependent termination when transcription-coupled translation is reduced upon sRNA binding. Our data also reveal several important kinetic steps that contribute to the differential regulation of mRNA targets by an sRNA. Particularly, binding of sRNA to the target mRNA may dictate the regulation hierarchy observed within an sRNA regulon.

Reyer et al. use fluorescent microscopy and kinetic modeling to find that two sRNAs canonically described as post-transcriptional regulators can regulate their targets co-transcriptionally and determine the in vivo kinetic parameters that dictate differential regulation efficiency.

Impacts of Small RNAs and Their Chaperones on Bacterial Pathogenicity

Bacterial small RNAs (sRNAs) are critical post-transcriptional regulators that exert broad effects on cell physiology. One class of sRNAs, referred to as trans-acting sRNAs, base-pairs with mRNAs to cause changes in their stability or translation. Another class of sRNAs sequesters RNA-binding proteins that in turn modulate mRNA expression. RNA chaperones play key roles in these regulatory events by promoting base-pairing of sRNAs to mRNAs, increasing the stability of sRNAs, inducing conformational changes on mRNA targets upon binding, or by titrating sRNAs away from their primary targets. In pathogenic bacteria, sRNAs and their chaperones exert broad impacts on both cell physiology and virulence, highlighting the central role of these systems in pathogenesis. This review provides an overview of the growing number and roles of these chaperone proteins in sRNA regulation, highlighting how these proteins contribute to bacterial pathogenesis.

Next-Generation Antibiotics, Bacteriophage Endolysins, and Nanomaterials for Combating Pathogens

This review presents various strategies to fight causative agents of infectious diseases. Species-specific programmable RNA-containing antibiotics open up new possibilities for creating next-generation of personalized drugs based on microbiome editing and can serve as a new tool for selective elimination of pathogenic bacterial species while keeping intact the rest of microbiota. Another promising approach in combating bacterial infections is genome editing using the CRISPR-Cas systems. Expanding knowledge on the molecular mechanisms of innate immunity has been actively used for developing new antimicrobials. However, obvious risks of using antibiotic adjuvants aimed at activation of the host immune system include development of the autoimmune response with subsequent organ damage. To avoid these risks, it is essential to elucidate action mechanisms of the specific ligands and signal molecules used as components of the hybrid antibiotics. Bacteriophage endolysins are also considered as effective antimicrobials against antibiotic-resistant bacteria, metabolically inactive persisters, and microbial biofilms. Despite significant advances in the design of implants with antibacterial properties, the problem of postoperative infections still remains. Different nanomodifications of the implant surface have been designed to reduce bacterial contamination. Here, we review bactericidal, fungicidal, and immunomodulating properties of compounds used for the implant surface nanomodifications, such as silver, boron nitride nanomaterials, nanofibers, and nanogalvanic materials.

Three Ribosomal Operons of Escherichia coli Contain Genes Encoding Small RNAs That Interact With Hfq and CsrA in vitro

Three out of the seven ribosomal RNA operons in Escherichia coli end in dual terminator structures. Between the two terminators of each operon is a short sequence that we report here to be an sRNA gene, transcribed as part of the ribosomal RNA primary transcript by read-through of the first terminator. The sRNA genes (rrA, rrB and rrF) from the three operons (rrnA, rrnB and rrnD) are more than 98% identical, and pull-down experiments show that their transcripts interact with Hfq and CsrA. Deletion of rrA, B, F, as well as overexpression of rrB, only modestly affect known CsrA-regulated phenotypes like biofilm formation, pgaA translation and glgC translation, and the role of the sRNAs in vivo may not yet be fully understood. Since RrA, B, F are short-lived and transcribed along with the ribosomal RNA components, their concentration reflect growth-rate regulation at the ribosomal RNA promoters and they could function to fine-tune other growth-phase-dependent processes in the cell. The primary and secondary structure of these small RNAs are conserved among species belonging to different genera of Enterobacteriales.

Recent Research Advances in Small Regulatory RNAs in Streptococcus

Small non-coding RNAs (sRNAs) are a class of regulatory RNAs 20-500 nucleotides in length, which have recently been discovered in prokaryotic organisms. sRNAs are key regulators in many biological processes, such as sensing various environmental changes and regulating intracellular gene expression through binding target mRNAs or proteins. Bacterial sRNAs have recently been rapidly mined, thus providing new insights into the regulatory network of biological functions in prokaryotes. Although most bacterial sRNAs have been discovered and studied in Escherichia coli and other Gram-negative bacteria, sRNAs have increasingly been predicted and verified in Gram-positive bacteria in the past decade. The genus Streptococcus includes many commensal and pathogenic Gram-positive bacteria. However, current understanding of sRNA-mediated regulation in Streptococcus is limited. Most known sRNAs in Streptococcus are associated with the regulation of virulence. In this review, we summarize recent advances in understanding of the functions and mechanisms of sRNAs in Streptococcus, and we discuss the RNA chaperone protein and synthetic sRNA-mediated gene regulation, with the aim of providing a reference for the study of microbial sRNAs.

MD simulations reveal the basis for dynamic assembly of Hfq-RNA complexes

The conserved protein Hfq is a key factor in the RNA-mediated control of gene expression in most known bacteria. The transient intermediates Hfq forms with RNA support intricate and robust regulatory networks. In Pseudomonas, Hfq recognizes repeats of adenine–purine–any nucleotide (ARN) in target mRNAs via its distal binding side, and together with the catabolite repression control (Crc) protein, assembles into a translation–repression complex. Earlier experiments yielded static, ensemble-averaged structures of the complex, but details of its interface dynamics and assembly pathway remained elusive. Using explicit solvent atomistic molecular dynamics simulations, we modeled the extensive dynamics of the Hfq–RNA interface and found implications for the assembly of the complex. We predict that syn/anti flips of the adenine nucleotides in each ARN repeat contribute to a dynamic recognition mechanism between the Hfq distal side and mRNA targets. We identify a previously unknown binding pocket that can accept any nucleotide and propose that it may serve as a ‘status quo’ staging point, providing nonspecific binding affinity, until Crc engages the Hfq–RNA binary complex. The dynamical components of the Hfq–RNA recognition can speed up screening of the pool of the surrounding RNAs, participate in rapid accommodation of the RNA on the protein surface, and facilitate competition among different RNAs. The register of Crc in the ternary assembly could be defined by the recognition of a guanine-specific base–phosphate interaction between the first and last ARN repeats of the bound RNA. This dynamic substrate recognition provides structural rationale for the stepwise assembly of multicomponent ribonucleoprotein complexes nucleated by Hfq–RNA binding.

Stepwise sRNA targeting of structured bacterial mRNAs leads to abortive annealing

Bacterial small RNAs (sRNAs) regulate the expression of hundreds of transcripts via base pairing mediated by the Hfq chaperone protein. sRNAs and the mRNA sites they target are heterogeneous in sequence, length, and secondary structure. To understand how Hfq can flexibly match diverse sRNA and mRNA pairs, we developed a single-molecule Förster resonance energy transfer (smFRET) platform that visualizes the target search on timescales relevant in cells. Here we show that unfolding of target secondary structure on Hfq creates a kinetic energy barrier that determines whether target recognition succeeds or aborts before a stable anti-sense complex is achieved. Premature dissociation of the sRNA can be alleviated by strong RNA-Hfq interactions, explaining why sRNAs have different target recognition profiles. We propose that the diverse sequences and structures of Hfq substrates create an additional layer of information that tunes the efficiency and selectivity of non-coding RNA regulation in bacteria.

An RNA pseudoknot is essential for standby-mediated translation of the tisB toxin mRNA in Escherichia coli

In response to DNA damage, Escherichia coli cells activate the expression of the toxin gene tisB of the toxin-antitoxin system tisB-istR1. Of three isoforms, only the processed, highly structured +42 tisB mRNA is active. Translation requires a standby site, composed of two essential elements: a single-stranded region located 100 nucleotides upstream of the sequestered RBS, and a structure near the 5'-end of the active mRNA. Here, we propose that this 5'-structure is an RNA pseudoknot which is required for 30S and protein S1-alone binding to the mRNA. Point mutations that prevent formation of this pseudoknot inhibit formation of translation initiation complexes, impair S1 and 30S binding to the mRNA, and render the tisB mRNA non-toxic in vivo. A set of mutations created in either the left or right arm of stem 2 of the pseudoknot entailed loss of toxicity upon overexpression of the corresponding mRNA variants. Combining the matching right-left arm mutations entirely restored toxicity levels to that of the wild-type, active mRNA. Finally, since many pseudoknots have high affinity for S1, we predicted similar pseudoknots in non-homologous type I toxin-antitoxin systems that exhibit features similar to that of tisB-IstR1, suggesting a shared requirement for standby acting at great distances.

Regulatory RNAs: A Universal Language for Inter-Domain Communication

In eukaryotes, microRNAs (miRNAs) have roles in development, homeostasis, disease and the immune response. Recent work has shown that plant and mammalian miRNAs also mediate cross-kingdom and cross-domain communications. However, these studies remain controversial and are lacking critical mechanistic explanations. Bacteria do not produce miRNAs themselves, and therefore it is unclear how these eukaryotic RNA molecules could function in the bacterial recipient. In this review, we compare and contrast the biogenesis and functions of regulatory RNAs in eukaryotes and bacteria. As a result, we discovered several conserved features and homologous components in these distinct pathways. These findings enabled us to propose novel mechanisms to explain how eukaryotic miRNAs could function in bacteria. Further understanding in this area is necessary to validate the findings of existing studies and could facilitate the use of miRNAs as novel tools for the directed remodelling of the human microbiota.

An RNA biology perspective on species-specific programmable RNA antibiotics

Our body is colonized by a vast array of bacteria the sum of which forms our microbiota. The gut alone harbors >1,000 bacterial species. An understanding of their individual or synergistic contributions to human health and disease demands means to interfere with their functions on the species level. Most of the currently available antibiotics are broad-spectrum, thus too unspecific for a selective depletion of a single species of interest from the microbiota. Programmable RNA antibiotics in the form of short antisense oligonucleotides (ASOs) promise to achieve precision manipulation of bacterial communities. These ASOs are coupled to small peptides that carry them inside the bacteria to silence mRNAs of essential genes, for example, to target antibiotic-resistant pathogens as an alternative to standard antibiotics. There is already proof-of-principle with diverse bacteria, but many open questions remain with respect to true species specificity, potential off-targeting, choice of peptides for delivery, bacterial resistance mechanisms and the host response. While there is unlikely a one-fits-all solution for all microbiome species, I will discuss how recent progress in bacterial RNA biology may help to accelerate the development of programmable RNA antibiotics for microbiome editing and other applications.

Keeping Up with RNA-Based Regulation in Bacteria: New Roles for RNA Binding Proteins

RNA binding proteins (RBPs) are ubiquitously found in all kingdoms of life. They are involved in a plethora of regulatory events, ranging from direct regulation of gene expression to guiding modification of RNA molecules. As bacterial regulators, RBPs can act alone or in concert with RNA-based regulators, such as small regulatory RNAs (sRNAs), riboswitches, or clustered regularly interspaced short palindromic repeats (CRISPR) RNAs. Various functions of RBPs, whether dependent or not on an RNA regulator, have been described in the past. However, the past decade has been a fertile ground for the development of novel high-throughput methods. These methods acted as stepping-stones for the discovery of new functions of RBPs and helped in the understanding of the molecular mechanisms behind previously described regulatory events. Here, we present an overview of the recently identified roles of major bacterial RBPs from different model organisms. Moreover, the tight relationship between RBPs and RNA-based regulators will be explored.

The Bacterial Ro60 Protein and Its Noncoding Y RNA Regulators

Ro60 ribonucleoproteins (RNPs), composed of the ring-shaped Ro 60-kDa (Ro60) protein and noncoding RNAs called Y RNAs, are present in all three domains of life. Ro60 was first described as an autoantigen in patients with rheumatic disease, and Ro60 orthologs have been identified in 3% to 5% of bacterial genomes, spanning the majority of phyla. Their functions have been characterized primarily in Deinococcus radiodurans, the first sequenced bacterium with a recognizable ortholog. In D. radiodurans, the Ro60 ortholog enhances the ability of 3'-to-5' exoribonucleases to degrade structured RNA during several forms of environmental stress. Y RNAs are regulators that inhibit or allow the interactions of Ro60 with other proteins and RNAs. Studies of Ro60 RNPs in other bacteria hint at additional functions, since the most conserved Y RNA contains a domain that is a close tRNA mimic and Ro60 RNPs are often encoded adjacent to components of RNA repair systems.

Small RNAs as Fundamental Players in the Transference of Information During Bacterial Infectious Diseases

Communication shapes life on Earth. Transference of information has played a paramount role on the evolution of all living or extinct organisms since the appearance of life. Success or failure in this process will determine the prevalence or disappearance of a certain set of genes, the basis of Darwinian paradigm. Among different molecules used for transmission or reception of information, RNA plays a key role. For instance, the early precursors of life were information molecules based in primitive RNA forms. A growing field of research has focused on the contribution of small non-coding RNA forms due to its role on infectious diseases. These are short RNA species that carry out regulatory tasks in cis or trans. Small RNAs have shown their relevance in fine tuning the expression and activity of important regulators of essential genes for bacteria. Regulation of targets occurs through a plethora of mechanisms, including mRNA stabilization/destabilization, driving target mRNAs to degradation, or direct binding to regulatory proteins. Different studies have been conducted during the interplay of pathogenic bacteria with several hosts, including humans, animals, or plants. The sRNAs help the invader to quickly adapt to the change in environmental conditions when it enters in the host, or passes to a free state. The adaptation is achieved by direct targeting of the pathogen genes, or subversion of the host immune system. Pathogens trigger also an immune response in the host, which has been shown as well to be regulated by a wide range of sRNAs. This review focuses on the most recent host-pathogen interaction studies during bacterial infectious diseases, providing the perspective of the pathogen.

RNA-Binding Proteins Driving the Regulatory Activity of Small Non-coding RNAs in Bacteria

Small non-coding RNAs (sRNAs) are critical post-transcriptional regulators of gene expression. Distinct RNA-binding proteins (RBPs) influence the processing, stability and activity of bacterial small RNAs. The vast majority of bacterial sRNAs interact with mRNA targets, affecting mRNA stability and/or its translation rate. The assistance of RNA-binding proteins facilitates and brings accuracy to sRNA-mRNA basepairing and the RNA chaperones Hfq and ProQ are now recognized as the most prominent RNA matchmakers in bacteria. These RBPs exhibit distinct high affinity RNA-binding surfaces, promoting RNA strand interaction between a trans-encoding sRNA and its mRNA target. Nevertheless, some organisms lack ProQ and/or Hfq homologs, suggesting the existence of other RBPs involved in sRNA function. Along this line of thought, the global regulator CsrA was recently shown to facilitate the access of an sRNA to its target mRNA and may represent an additional factor involved in sRNA function. Ribonucleases (RNases) can be considered a class of RNA-binding proteins with nucleolytic activity that are responsible for RNA maturation and/or degradation. Presently RNase E, RNase III, and PNPase appear to be the main players not only in sRNA turnover but also in sRNA processing. Here we review the current knowledge on the most important bacterial RNA-binding proteins affecting sRNA activity and sRNA-mediated networks.

Translation inhibition from a distance: The small RNA SgrS silences a ribosomal protein S1-dependent enhancer

Many bacterial small RNAs (sRNAs) efficiently inhibit translation of target mRNAs by forming a duplex that sequesters the Shine-Dalgarno (SD) sequence or start codon and prevents formation of the translation initiation complex. There are a growing number of examples of sRNA-mRNA binding interactions distant from the SD region, but how these mediate translational regulation remains unclear. Our previous work in Escherichia coli and Salmonella identified a mechanism of translational repression of manY mRNA by the sRNA SgrS through a binding interaction upstream of the manY SD. Here, we report that SgrS forms a duplex with a uridine-rich translation-enhancing element in the manY 5' untranslated region. Notably, we show that the enhancer is ribosome-dependent and that the small ribosomal subunit protein S1 interacts with the enhancer to promote translation of manY. In collaboration with the chaperone protein Hfq, SgrS interferes with the interaction between the translation enhancer and ribosomal protein S1 to repress translation of manY mRNA. Since bacterial translation is often modulated by enhancer-like elements upstream of the SD, sRNA-mediated enhancer silencing could be a common mode of gene regulation.

sRNA-mediated control in bacteria: An increasing diversity of regulatory mechanisms

Small regulatory RNAs (sRNAs) act as post-transcriptional regulators controlling bacterial adaptation to environmental changes. Our current understanding of the mechanisms underlying sRNA-mediated control is mainly based on studies in Escherichia coli and Salmonella. Ever since the discovery of sRNAs decades ago, these Gram-negative species have served as excellent model organisms in the field of sRNA biology. More recently, the role of sRNAs in gene regulation has become the center of attention in a broader range of species, including Gram-positive model organisms. Here, we highlight some of the most apparent similarities and differences between Gram-negative and Gram-positive bacteria with respect to the mechanisms underlying sRNA-mediated control. Although key aspects of sRNA regulation appear to be highly conserved, novel themes are arising from studies in Gram-positive species, such as a clear abundance of sRNAs acting through multiple C-rich motifs, and an apparent lack of RNA-binding proteins with chaperone activity.

Small RNAs OmrA and OmrB promote class III flagellar gene expression by inhibiting the synthesis of anti-Sigma factor FlgM

Bacteria can move by a variety of mechanisms, the best understood being flagella-mediated motility. Flagellar genes are organized in a three-tiered cascade allowing for temporally regulated expression that involves both transcriptional and post-transcriptional control. The class I operon encodes the master regulator FlhDC that drives class II gene transcription. Class II genes include fliA and flgM, which encode the Sigma factor σ²⁸, required for class III transcription, and the anti-Sigma factor FlgM, which inhibits σ²⁸ activity, respectively. The flhDC mRNA is regulated by several small regulatory RNAs (sRNAs). Two of these, the sequence-related OmrA and OmrB RNAs, inhibit FlhD synthesis. Here, we report on a second layer of sRNA-mediated control downstream of FhlDC in the flagella pathway. By mutational analysis, we confirm that a predicted interaction between the conserved 5ʹ seed sequences of OmrA/B and the early coding sequence in flgM mRNA reduces FlgM expression. Regulation is dependent on the global RNA-binding protein Hfq. In vitro experiments support a canonical mechanism: binding of OmrA/B prevents ribosome loading and decreases FlgM protein synthesis. Simultaneous inhibition of both FlhD and FlgM synthesis by OmrA/B complicated an assessment of how regulation of FlgM alone impacts class III gene transcription. Using a combinatorial mutation strategy, we were able to uncouple these two targets and demonstrate that OmrA/B-dependent inhibition of FlgM synthesis liberates σ²⁸ to ultimately promote higher expression of the class III flagellin gene fliC.

Trans-Acting Small RNAs and Their Effects on Gene Expression in Escherichia coli and Salmonella enterica

The last few decades have led to an explosion in our understanding of the major roles that small regulatory RNAs (sRNAs) play in regulatory circuits and the responses to stress in many bacterial species. Much of the foundational work was carried out with Escherichia coli and Salmonella enterica serovar Typhimurium. The studies of these organisms provided an overview of how the sRNAs function and their impact on bacterial physiology, serving as a blueprint for sRNA biology in many other prokaryotes. They also led to the development of new technologies. In this chapter, we first summarize how these sRNAs were identified, defining them in the process. We discuss how they are regulated and how they act and provide selected examples of their roles in regulatory circuits and the consequences of this regulation. Throughout, we summarize the methodologies that were developed to identify and study the regulatory RNAs, most of which are applicable to other bacteria. Newly updated databases of the known sRNAs in E. coli K-12 and S. enterica Typhimurium SL1344 serve as a reference point for much of the discussion and, hopefully, as a resource for readers and for future experiments to address open questions raised in this review.

Bacterial Cyclopropane Fatty Acid Synthase mRNA Is Targeted by Activating and Repressing Small RNAs

Altering membrane protein and lipid composition is an important strategy for maintaining membrane integrity during environmental stress. Many bacterial small RNAs (sRNAs) control membrane protein production, but sRNA-mediated regulation of membrane fatty acid composition is less well understood. The sRNA RydC was previously shown to stabilize cfa (cyclopropane fatty acid synthase) mRNA, resulting in higher levels of cyclopropane fatty acids in the cell membrane. Here, we report that additional sRNAs, ArrS and CpxQ, also directly regulate cfa posttranscriptionally. RydC and ArrS act through masking an RNase E cleavage site in the cfa mRNA 5' untranslated region (UTR), and both sRNAs posttranscriptionally activate cfa In contrast, CpxQ binds to a different site in the cfa mRNA 5' UTR and represses cfa expression. Alteration of membrane lipid composition is a key mechanism for bacteria to survive low-pH environments, and we show that cfa translation increases in an sRNA-dependent manner when cells are subjected to mild acid stress. This work suggests an important role for sRNAs in the acid stress response through regulation of cfa mRNA.IMPORTANCE Small RNAs (sRNAs) in bacteria are abundant and play important roles in posttranscriptional regulation of gene expression, particularly under stress conditions. Some mRNAs are targets for regulation by multiple sRNAs, each responding to different environmental signals. Uncovering the regulatory mechanisms governing sRNA-mRNA interactions and the relevant conditions for these interactions is an ongoing challenge. In this study, we discovered that multiple sRNAs control membrane lipid composition by regulating stability of a single mRNA target. The sRNA-dependent regulation occurred in response to changing pH and was important for cell viability under acid stress conditions. This work reveals yet another aspect of bacterial physiology controlled at the posttranscriptional level by sRNA regulators.

Atractylodes lancea volatile oils attenuated helicobacter pylori NCTC11637 growth and biofilm

Atractylodes lancea is a traditional Chinese perennial herb, which has been used for treating gastrointestinal diseases in traditional medicine. The aim of this study was to investigate the effect of Atractylodes lancea volatile oils on the planktonic growth and biofilm formation of Helicobacter pylori (H. pylori). Firstly, the minimal inhibitory concentration (MIC) of the volatile oils against H. pylori were determined using broth dilution method. SPSS17.0 was used to account 50% inhibiting concentration (IC50). Moreover, the anti-biofilm activity of the volatile oils was determined by crystal violet measurement and fluorescence microscope. Finally, gastric epithelial cells (GES-1 cells) were co-incubated with H. pylori with or without volatile oils treated. Real-time PCR and western blot were performed to detect the translocation of virulence factor Cag A. We found that Atractylodes lancea volatile oils inhibited the growth of H. pylori in a concentration dependent manner. The MIC and IC50 of volatile oils against H. pylori were 7.5 mg/mL and 2.181 mg/mL respectively. Fluorescence microscopy and crystal violet measurement indicated that volatile oils at sub-MIC concentration could reduce biofilm formation of H. pylori. In addition, volatile oils decreased the translocation of Cag A and reduced inflammatory cytokine IL-8 in GES-1 cells. Our results suggested that Atractylodes lancea volatile oils could be a potential compound of a novel class of H. pylori inhibitors with anti-H. pylori effects.