The small RNA GcvB regulates sstT mRNA expression in Escherichia coli

Post-transcriptional regulation of aromatic amino acid metabolism by GcvB small RNA in Escherichia coli

Escherichia coli synthesizes aromatic amino acids (AAAs) through the common pathway to produce the precursor, chorismate, and the three terminal pathways to convert chorismate into Phe, Tyr, and Trp. E. coli also imports exogenous AAAs through five transporters. GcvB small RNA post-transcriptionally regulates more than 50 genes involved in amino acid uptake and biosynthesis in E. coli, but the full extent of GcvB regulon is still underestimated. This study examined all genes involved in AAA biosynthesis and transport using translation reporter assay and qRT-PCR analysis. In addition to previously verified targets, aroC, aroP, and trpE, we identified new target genes that were significantly repressed by GcvB primarily via the R1 seed region. Exceptionally, GcvB strongly inhibits the expression of aroG, which encodes the major isozyme of the first reaction in the common pathway, through direct base pairing between the aroG translation initiation region and the GcvB R3 seed sequence. RNase E mediates the degradation of target mRNAs except aroC and aroP via its C-terminal domain. GcvB overexpression prolongs the lag phase and reduces the growth rate in minimal media supplemented with AAAs and confers resistance to an antibiotic compound, azaserine, by repressing AAA transporters.IMPORTANCEE. coli strains have been genetically modified in relevant transcription factors and biosynthetic enzymes for industrial use in the fermentative production of aromatic amino acids (AAAs) and their derivative compounds. This study focuses on GcvB small RNA, a global regulator of amino acid metabolism in E. coli, and identifies new GcvB targets involved in AAA biosynthesis and uptake. GcvB represses the expression of the first and last enzymes of the common pathway and the first enzymes of Trp and Phe terminal pathways. GcvB also limits import of AAAs. This paper documents the impact of RNA-mediated regulation on AAA metabolism in E. coli.

Unraveling the interplay between a small RNA and RNase E in bacteria

Small RNAs (sRNAs) are major regulators of gene expression in bacteria, exerting their regulation primarily via base pairing with their target transcripts and modulating translation. Accumulating evidence suggest that sRNAs can also affect the stability of their target transcripts by altering their accessibility to endoribonucleases. Yet, the effects of sRNAs on transcript stability and the mechanisms underlying them have not been studied in wide scale. Here we employ large-scale RNA-seq-based methodologies in the model bacterium Escherichia coli to quantitatively study the functional interaction between a sRNA and an endoribonuclease in regulating gene expression, using the well-established sRNA, GcvB, and the major endoribonuclease, RNase E. Studying single and double mutants of gcvB and rne and analysing their RNA-seq results by the Double Mutant Cycle approach, we infer distinct modes of the interplay between GcvB and RNase E. Transcriptome-wide mapping of RNase E cleavage sites provides further support to the results of the RNA-seq analysis, identifying cleavage sites in targets in which the functional interaction between GcvB and RNase E is evident. Together, our results indicate that the most dominant mode of GcvB-RNase E functional interaction is GcvB enhancement of RNase E cleavage, which varies in its magnitude between different targets.

Base editor-mediated large-scale screening of functional mutations in bacteria for industrial phenotypes

Base editing, the targeted introduction of point mutations into cellular DNA, holds promise for improving genome-scale functional genome screening to single-nucleotide resolution. Current efforts in prokaryotes, however, remain confined to loss-of-function screens using the premature stop codons-mediated gene inactivation library, which falls far short of fully releasing the potential of base editors. Here, we developed a base editor-mediated functional single nucleotide variant screening pipeline in Escherichia coli. We constructed a library with 31,123 sgRNAs targeting 462 stress response-related genes in E. coli, and screened for adaptive mutations under isobutanol and furfural selective conditions. Guided by the screening results, we successfully identified several known and novel functional mutations. Our pipeline might be expanded to the optimization of other phenotypes or the strain engineering in other microorganisms.

Balanced cell division is secured by two different regulatory sites in OxyS RNA

The hydrogen peroxide-induced small RNA OxyS has been proposed to originate from the 3' UTR of a peroxide mRNA. Unexpectedly, phylogenetic OxyS targetome predictions indicate that most OxyS targets belong to the category of "cell cycle," including cell division and cell elongation. Previously, we reported that Escherichia coli OxyS inhibits cell division by repressing expression of the essential transcription termination factor nusG, thereby leading to the expression of the KilR protein, which interferes with the function of the major cell division protein, FtsZ. By interfering with cell division, OxyS brings about cell-cycle arrest, thus allowing DNA damage repair. Cell division and cell elongation are opposing functions to the extent that inhibition of cell division requires a parallel inhibition of cell elongation for the cells to survive. In this study, we report that in addition to cell division, OxyS inhibits mepS, which encodes an essential peptidoglycan endopeptidase that is responsible for cell elongation. Our study indicates that cell-cycle arrest and balancing between cell division and cell elongation are important and conserved functions of the oxidative stress-induced sRNA OxyS.

An overview of gene regulation in bacteria by small RNAs derived from mRNA 3' ends

Over the past two decades, small noncoding RNAs (sRNAs) that regulate mRNAs by short base pairing have gone from a curiosity to a major class of post-transcriptional regulators in bacteria. They are integral to many stress responses and regulatory circuits, affecting almost all aspects of bacterial life. Following pioneering sRNA searches in the early 2000s, the field quickly focused on conserved sRNA genes in the intergenic regions of bacterial chromosomes. Yet, it soon emerged that there might be another rich source of bacterial sRNAs-processed 3' end fragments of mRNAs. Several such 3' end-derived sRNAs have now been characterized, often revealing unexpected, conserved functions in diverse cellular processes. Here, we review our current knowledge of these 3' end-derived sRNAs-their biogenesis through ribonucleases, their molecular mechanisms, their interactions with RNA-binding proteins such as Hfq or ProQ and their functional scope, which ranges from acting as specialized regulators of single metabolic genes to constituting entire noncoding arms in global stress responses. Recent global RNA interactome studies suggest that the importance of functional 3' end-derived sRNAs has been vastly underestimated and that this type of cross-regulation between genes at the mRNA level is more pervasive in bacteria than currently appreciated.

GcvB Regulon Revealed by Transcriptomic and Proteomic Analysis in Vibrio alginolyticus

Vibrio alginolyticus is a widely distributed marine bacterium that is a threat to the aquaculture industry as well as human health. Evidence has revealed critical roles for small RNAs (sRNAs) in bacterial physiology and cellular processes by modulating gene expression post-transcriptionally. GcvB is one of the most conserved sRNAs that is regarded as the master regulator of amino acid uptake and metabolism in a wide range of Gram-negative bacteria. However, little information about GcvB-mediated regulation in V. alginolyticus is available. Here we first characterized GcvB in V. alginolyticus ZJ-T and determined its regulon by integrated transcriptome and quantitative proteome analysis. Transcriptome analysis revealed 40 genes differentially expressed (DEGs) between wild-type ZJ-T and gcvB mutant ZJ-T-ΔgcvB, while proteome analysis identified 50 differentially expressed proteins (DEPs) between them, but only 4 of them displayed transcriptional differences, indicating that most DEPs are the result of post-transcriptional regulation of gcvB. Among the differently expressed proteins, 21 are supposed to be involved in amino acid biosynthesis and transport, and 11 are associated with type three secretion system (T3SS), suggesting that GcvB may play a role in the virulence besides amino acid metabolism. RNA-EMSA showed that Hfq binds to GcvB, which promotes its stability.

Mining RNA-seq data reveals the massive regulon of GcvB small RNA and its physiological significance in maintaining amino acid homeostasis in Escherichia coli

Bacterial small RNAs regulate the expression of multiple genes through imperfect base-pairing with target mRNAs mediated by RNA chaperone proteins such as Hfq. GcvB is the master sRNA regulator of amino acid metabolism and transport in a wide range of Gram-negative bacteria. Recently, independent RNA-seq approaches identified a plethora of transcripts interacting with GcvB in Escherichia coli. In this study, the compilation of RIL-seq, CLASH, and MAPS data sets allowed us to identify GcvB targets with high accuracy. We validated 21 new GcvB targets repressed at the posttranscriptional level, raising the number of direct targets to >50 genes in E. coli. Among its multiple seed sequences, GcvB utilizes either R1 or R3 to regulate most of these targets. Furthermore, we demonstrated that both R1 and R3 seed sequences are required to fully repress the expression of gdhA, cstA, and sucC genes. In contrast, the ilvLXGMEDA polycistronic mRNA is targeted by GcvB through at least four individual binding sites in the mRNA. Finally, we revealed that GcvB is involved in the susceptibility of peptidase-deficient E. coli strain (Δpeps) to Ala-Gln dipeptide by regulating both Dpp dipeptide importer and YdeE dipeptide exporter via R1 and R3 seed sequences, respectively.

Small RNA GcvB Regulates Oxidative Stress Response of Escherichia coli

Small non-translated regulatory RNAs control plenty of bacterial vital activities. The small RNA GcvB has been extensively studied, indicating the multifaceted roles of GcvB beyond amino acid metabolism. However, few reported GcvB-dependent regulation in minimal medium. Here, by applying a high-resolution RNA-seq assay, we compared the transcriptomes of a wild-type Escherichia coli K-12 strain and its gcvB deletion derivative grown in minimal medium and identified putative targets responding to GcvB, including flu, a determinant gene of auto-aggregation. The following molecular studies and the enhanced auto-aggregation ability of the gcvB knockout strain further substantiated the induced expression of these genes. Intriguingly, the reduced expression of OxyR (the oxidative stress regulator) in the gcvB knockout strain was identified to account for the increased expression of flu. Additionally, GcvB was characterized to up-regulate the expression of OxyR at the translational level. Accordingly, compared to the wild type, the GcvB deletion strain was more sensitive to oxidative stress and lost some its ability to eliminate endogenous reactive oxygen species. Taken together, we reveal that GcvB regulates oxidative stress response by up-regulating OxyR expression. Our findings provide an insight into the diversity of GcvB regulation and add an additional layer to the regulation of OxyR.

The impact of Hfq-mediated sRNA-mRNA interactome on the virulence of enteropathogenic Escherichia coli

Small RNAs (sRNAs) exert their regulation posttranscriptionally by base pairing with their target mRNAs, often in association with the RNA chaperone protein Hfq. Here, integrating RNA-seq–based technologies and bioinformatics, we deciphered the Hfq-mediated sRNA-target interactome of enteropathogenic Escherichia coli (EPEC). The emerging network comprises hundreds of sRNA-mRNA pairs, including mRNAs of virulence-associated genes interacting with known sRNAs encoded within the core genome, as well as with newly found sRNAs encoded within pathogenicity islands. Some of the sRNAs affect multiple virulence genes, suggesting they function as hubs of virulence control. We further analyzed one such sRNA hub, MgrR, and one of its targets identified here, the major virulence-associated chaperon, cesT. We show that MgrR adjusts the level of EPEC cytotoxicity via regulation of CesT expression. Our results reveal an elaborate sRNA-mRNA interactome controlling the pathogenicity of EPEC and reinforce a role for sRNAs in the control of pathogen-host interaction.

The mRNA derived MalH sRNA contributes to alternative carbon source utilization by tuning maltoporin expression in E. coli

Previous high-throughput studies in Gram-negative bacteria identified a large number of 3ʹUTR fragments that potentially function as sRNAs. Here we extensively characterize the MalH sRNA. We show that MalH is a stable degradation intermediate derived from the 3ʹ end of malG, which is part of the maltose uptake operon transcript malEFG. Unlike the majority of bacterial sRNAs, MalH is transiently expressed during the transition from the exponential to the stationary growth phase, suggesting that it contributes to adaptation to changes in nutrient availability. Over-expression of MalH reduces expression of general outer membrane porins and MicA, a repressor of the high-affinity maltose/maltodextrin transporter LamB. Disrupting MalH production and function significantly reduces lamB accumulation when maltose is the only available carbon source, presumably due to the accumulation of the MicA repressor. We propose that MalH is part of a regulatory network that, during the transition phase, directly or indirectly promotes accumulation of high-affinity maltose transporters in the outer membrane by dampening competing pathways.

Genomic plasticity of pathogenic Escherichia coli mediates d-serine tolerance via multiple adaptive mechanisms

Pathogens ensure infection of favored sites in the body by responding to chemical signals. One chemical abundant in urine, the amino acid d-Ser, is toxic to EHEC and reduces expression of the machinery used for host cell attachment, making the bladder an unfavorable environment. We observed that under d-Ser stress, EHEC acquires genetic changes that lead to blocking d-Ser uptake into the cell or activating a silent enzyme for degrading d-Ser. This prevents growth inhibition and, critically, inhibits the repression of attachment machinery normally caused by d-Ser. These findings highlight the importance of pathogen evolution in determining how host molecules regulate colonization. These interactions underpin a process known as niche restriction that is important for pathogen success within the host.

The molecular environment of the host can have profound effects on the behavior of resident bacterial species. We recently established how the sensing and response of enterohemorrhagic Escherichia coli (EHEC) to d-serine (d-Ser) resulted in down-regulation of type 3 secretion system-dependent colonization, thereby avoiding unfavorable environments abundant in this toxic metabolite. However, this model ignores a key determinant of the success of bacterial pathogens, adaptive evolution. In this study, we have explored the adaptation of EHEC to d-Ser and its consequences for pathogenesis. We rapidly isolated multiple, independent, EHEC mutants whose growth was no longer compromised in the presence of d-Ser. Through a combination of whole-genome sequencing and transcriptomics, we showed that tolerance could be attributed to disruption of one of two d-Ser transporters and/or activation of a previously nonfunctional d-Ser deaminase. While the implication of cytoplasmic transport in d-Ser toxicity was unsurprising, disruption of a single transporter, CycA, was sufficient to completely overcome the repression of type 3 secretion system activity normally associated with exposure to d-Ser. Despite the fact that this reveals a mechanism by which evolution could drive a pathogen to colonize new niches, interrogation of sequenced E. coli O157:H7 genomes showed a high level of CycA conservation, highlighting a strong selective pressure for functionality. Collectively, these data show that CycA is a critically important conduit for d-Ser uptake that is central to the niche restriction of EHEC.

Spatiotemporal Organization of the E. coli Transcriptome: Translation Independence and Engagement in Regulation

RNA localization in eukaryotes is a mechanism to regulate transcripts fate. Conversely, bacterial transcripts were not assumed to be specifically localized. We previously demonstrated that E. coli mRNAs may localize to where their products localize in a translation-independent manner, thus challenging the transcription-translation coupling extent. However, the scope of RNA localization in bacteria remained unknown. Here, we report the distribution of the E. coli transcriptome between the membrane, cytoplasm, and poles by combining cell fractionation with deep-sequencing (Rloc-seq). Our results reveal asymmetric RNA distribution on a transcriptome-wide scale, significantly correlating with proteome localization and prevalence of translation-independent RNA localization. The poles are enriched with stress-related mRNAs and small RNAs, the latter becoming further enriched upon stress in an Hfq-dependent manner. Genome organization may play a role in localizing membrane protein-encoding transcripts. Our results show an unexpected level of intricacy in bacterial transcriptome organization and highlight the poles as hubs for regulation.

Regulation of metE+ mRNA expression by FnrS small RNA in Salmonella enterica serovar Typhimurium

Methionine is critical for variety of metabolic processes in biological organisms, acting as a precursor or intermediate for many final products. The last step for the synthesis of methionine is the methylation of homocysteine, which is catalyzed by MetE. Here, we use Salmonella enterica serovar Typhimurium LT2 to study the regulation of the metE⁺ gene by an anaerobically induced small non-coding RNA-FnrS, the expression of which is strictly dependent on the anaerobic regulator-FNR. The MetE-HA protein was expressed at an increased level in the fnrS^- and hfq^- deficient strains under anaerobic conditions. The Hfq protein is predicted to stabilize the binding between small RNA(s) and their target mRNA(s). A transcriptional (op) and translational (pr) metE::lacZ fusion gene were separately constructed, with the metE⁺-promoter fused to a lacZ reporter gene. In an anaerobic environment, the metE::lacZ (pr) fusion gene and reverse transcription-PCR identified that FnrS and/or FNR negatively regulate metE⁺ mRNA levels in the rich media. Analysis of FnrS revealed a sequence complementary to the 5' mRNA translational initiation region (TIR) of the metE⁺ gene. Mutation(s) predicted to disrupt base pairing between FnrS and metE⁺ TIR were constructed in fnrS, and most of those resulted in the loss of repressive activity. When compensatory mutation(s) were made in metE⁺ 5' TIR to restore base pairing with FnrS, the repressive regulation was completely restored. Therefore, in this study, we identified that in anaerobic phase, there is a repression of metE⁺ gene expression by FnrS and that base-paring, between both expressive transcripts, plays an important role for this negative regulation.

Reprogramming of gene expression in Escherichia coli cultured on pyruvate versus glucose

Previous studies revealed important roles of small RNAs (sRNAs) in regulation of bacterial metabolism, stress responses and virulence. However, only a minor fraction of sRNAs is well characterized with respect to the spectra of their targets, conditional expression profiles and actual mechanisms they use to regulate gene expression to control particular biological pathways. To learn more about the specific contribution of sRNAs to the global regulatory network controlling the Escherichia coli central carbon metabolism (CCM), we employed microarray analysis and compared transcriptome profiles of E. coli cells grown on two alternative minimal media supplemented with either pyruvate or glucose, respectively. Microarray analysis revealed that utilization of these alternative carbon sources led to profound differences in gene expression affecting all major gene clusters associated with CCM as well as expression of several known (CyaR, RyhB, GcvB and RyeA) and putative (C0652) sRNAs. To assess the impact of transcriptional reprogramming of gene expression on E. coli protein abundance, we also employed two-dimensional protein gel electrophoresis. Our experimental data made it possible to determine the major pathways for pyruvate assimilation when it is used as a sole carbon source and reveal the impact of other key processes (i.e., energy production, molecular transport and cell resistance to stress) associated with the CCM in E. coli. Moreover, some of these processes were apparently controlled by GcvB, RyhB and CyaR at the post-transcriptional level, thus indicating the complexity and interconnection of the regulatory networks that control CCM in bacteria.

GcvB small RNA uses two distinct seed regions to regulate an extensive targetome

GcvB small RNA is described as post-transcriptional regulator of 1-2% of all mRNAs in Escherichia coli and Salmonella Typhimurium. At least 24 GcvB:mRNA interactions have been validated in vivo, establishing the largest characterized sRNA targetome. By performing MS2-affinity purification coupled with RNA sequencing (MAPS) technology, we identified seven additional mRNAs negatively regulated by GcvB in E. coli. Contrary to the vast majority of previously known targets, which pair to the well-conserved GcvB R1 region, we validated four mRNAs targeted by GcvB R3 region. This indicates that base-pairing through R3 seed sequence seems relatively common. We also noticed unusual GcvB pairing sites in the coding sequence of two target mRNAs. One of these target mRNAs has a pairing site displaying a unique ACA motif, suggesting that GcvB could hijack a translational enhancer element. The second target mRNA is likely regulated via an active RNase E-mediated mRNA degradation mechanism. Remarkably, we confirmed the importance of the sRNA sponge SroC in the fine-tuning control of GcvB activity in function of growth conditions such as growth phase and nutrient availability.

Expansion of the SOS regulon of Vibrio cholerae through extensive transcriptome analysis and experimental validation

BACKGROUND

The SOS response is an almost ubiquitous response of cells to genotoxic stresses. The full complement of genes in the SOS regulon for Vibrio species has only been addressed through bioinformatic analyses predicting LexA binding box consensus and in vitro validation. Here, we perform whole transcriptome sequencing from Vibrio cholerae treated with mitomycin C as an SOS inducer to characterize the SOS regulon and other pathways affected by this treatment.

RESULTS

Comprehensive transcriptional profiling allowed us to define the full landscape of promoters and transcripts active in V. cholerae. We performed extensive transcription start site (TSS) mapping as well as detection/quantification of the coding and non-coding RNA (ncRNA) repertoire in strain N16961. To improve TSS detection, we developed a new technique to treat RNA extracted from cells grown in various conditions. This allowed for identification of 3078 TSSs with an average 5'UTR of 116 nucleotides, and peak distribution between 16 and 64 nucleotides; as well as 629 ncRNAs. Mitomycin C treatment induced transcription of 737 genes and 28 ncRNAs at least 2 fold, while it repressed 231 genes and 17 ncRNAs. Data analysis revealed that in addition to the core genes known to integrate the SOS regulon, several metabolic pathways were induced. This study allowed for expansion of the Vibrio SOS regulon, as twelve genes (ubiEJB, tatABC, smpA, cep, VC0091, VC1190, VC1369-1370) were found to be co-induced with their adjacent canonical SOS regulon gene(s), through transcriptional read-through. Characterization of UV and mitomycin C susceptibility for mutants of these newly identified SOS regulon genes and other highly induced genes and ncRNAs confirmed their role in DNA damage rescue and protection.

CONCLUSIONS

We show that genotoxic stress induces a pervasive transcriptional response, affecting almost 20% of the V. cholerae genes. We also demonstrate that the SOS regulon is larger than previously known, and its syntenic organization is conserved among Vibrio species. Furthermore, this specific co-localization is found in other γ-proteobacteria for genes recN-smpA and rmuC-tatABC, suggesting SOS regulon conservation in this phylum. Finally, we comment on the limitations of widespread NGS approaches for identification of all RNA species in bacteria.

A Recurrent Silent Mutation Implicates fecA in Ethanol Tolerance by Escherichia coli

Background

An issue associated with efficient bioethanol production is the fact that the desired product is toxic to the biocatalyst. Among other effects, ethanol has previously been found to influence the membrane of E. coli in a dose-dependent manner and induce changes in the lipid composition of the plasma membrane. We describe here the characterization of a collection of ethanol-tolerant strains derived from the ethanologenic Escherichia coli strain FBR5.

Results

Membrane permeability assays indicate that many of the strains in the collection have alterations in membrane permeability and/or responsiveness of the membrane to environmental changes such as temperature shifts or ethanol exposure. However, analysis of the strains by gas chromatography and mass spectrometry revealed no qualitative changes in the acyl chain composition of membrane lipids in response to ethanol or temperature. To determine whether these strains contain any mutations that might contribute to ethanol tolerance or changes in membrane permeability, we sequenced the entire genome of each strain. Unexpectedly, none of the strains displayed mutations in genes known to control membrane lipid synthesis, and a few strains carried no mutations at all. Interestingly, we found that four independently-isolated strains acquired an identical C → A (V244 V) silent mutation in the ferric citrate transporter gene fecA. Further, we demonstrated that either a deletion of fecA or over-expression of fecA can confer increased ethanol survival, suggesting that any misregulation of fecA expression affects the cellular response to ethanol.

Conclusions

The fact that no mutations were observed in several ethanol-tolerant strains suggested that epigenetic mechanisms play a role in E. coli ethanol tolerance and membrane permeability. Our data also represent the first direct phenotypic evidence that the fecA gene plays a role in ethanol tolerance. We propose that the recurring silent mutation may exert an effect on phenotype by altering RNA-mediated regulation of fecA expression.

Electronic supplementary material

The online version of this article (10.1186/s12866-018-1180-1) contains supplementary material, which is available to authorized users.

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.

Increased production of L-serine in Escherichia coli through Adaptive Laboratory Evolution

L-serine is a promising building block biochemical with a high theoretical production yield from glucose. Toxicity of L-serine is however prohibitive for high-titer production in E. coli. Here, E. coli lacking L-serine degradation pathways was evolved for improved tolerance by gradually increasing L-serine concentration from 3 to 100g/L using adaptive laboratory evolution (ALE). Genome sequencing of isolated clones revealed multiplication of genetic regions, as well as mutations in thrA, thereby showing a potential mechanism of serine inhibition. Additional mutations were evaluated by MAGE combined with amplicon sequencing, revealing role of rho, lrp, pykF, eno, and rpoB on tolerance and fitness in minimal medium. Production using the tolerant strains resulted in 37g/L of L-serine with a 24% mass yield. The resulting titer is similar to the highest production reported for any organism thereby highlighting the potential of ALE for industrial biotechnology.

The Small RNA GcvB Promotes Mutagenic Break Repair by Opposing the Membrane Stress Response

Microbes and human cells possess mechanisms of mutagenesis activated by stress responses. Stress-inducible mutagenesis mechanisms may provide important models for mutagenesis that drives host-pathogen interactions, antibiotic resistance, and possibly much of evolution generally. In Escherichia coli, repair of DNA double-strand breaks is switched to a mutagenic mode, using error-prone DNA polymerases, via the SOS DNA damage and general (σ^S) stress responses. We investigated small RNA (sRNA) clients of Hfq, an RNA chaperone that promotes mutagenic break repair (MBR), and found that GcvB promotes MBR by allowing a robust σ^S response, achieved via opposing the membrane stress (σ^E) response. Cells that lack gcvB were MBR deficient and displayed reduced σ^S-dependent transcription but not reduced σ^S protein levels. The defects in MBR and σ^S-dependent transcription in ΔgcvB cells were alleviated by artificially increasing σ^S levels, implying that GcvB promotes mutagenesis by allowing a normal σ^S response. ΔgcvB cells were highly induced for the σ^E response, and blocking σ^E response induction restored both mutagenesis and σ^S-promoted transcription. We suggest that GcvB may promote the σ^S response and mutagenesis indirectly, by promoting membrane integrity, which keeps σ^E levels lower. At high levels, σ^E might outcompete σ^S for binding RNA polymerase and so reduce the σ^S response and mutagenesis. The data show the delicate balance of stress response modulation of mutagenesis.

IMPORTANCE

Mutagenesis mechanisms upregulated by stress responses promote de novo antibiotic resistance and cross-resistance in bacteria, antifungal drug resistance in yeasts, and genome instability in cancer cells under hypoxic stress. This paper describes the role of a small RNA (sRNA) in promoting a stress-inducible-mutagenesis mechanism, mutagenic DNA break repair in Escherichia coli The roles of many sRNAs in E. coli remain unknown. This study shows that ΔgcvB cells, which lack the GcvB sRNA, display a hyperactivated membrane stress response and reduced general stress response, possibly because of sigma factor competition for RNA polymerase. This results in a mutagenic break repair defect. The data illuminate a function of GcvB sRNA in opposing the membrane stress response, and thus indirectly upregulating mutagenesis.

Role of Small Noncoding RNAs in Bacterial Metabolism

The study of prokaryotic small RNAs is one of the most important directions in modern molecular biology. In the last decade, multiple short regulatory transcripts have been found in prokaryotes, and for some of them functional roles have been elucidated. Bacterial small RNAs are implicated in the regulation of transcription and translation, and they affect mRNA stability and gene expression via different mechanisms, including changes in mRNA conformation and interaction with proteins. Most small RNAs are expressed in response to external factors, and they help bacteria to adapt to changing environmental conditions. Bacterial infections of various origins remain a serious medical problem, despite significant progress in fighting them. Discovery of mechanisms that bacteria employ to survive in infected organisms and ways to block these mechanisms is promising for finding new treatments for bacterial infections. Regulation of pathogenesis with small RNAs is an attractive example of such mechanisms. This review considers the role of bacterial small RNAs in adaptation to stress conditions. We pay special attention to the role of small RNAs in Mycobacterium tuberculosis infection, in particular during establishment and maintenance of latent infection.

A new custom microarray for sRNA profiling in Escherichia coli

Bacterial small RNAs (sRNAs) play essential roles in the post-transcriptional control of gene expression. To improve their detection by conventional microarrays, we designed a custom microarray containing a group of probes targeting known and some putative Escherichia coli sRNAs. To assess its potential in detection of sRNAs, RNA profiling experiments were performed with total RNA extracted from E. coli MG1655 cells exponentially grown in rich (Luria-Bertani) and minimal (M9/glucose) media. We found that many sRNAs could yield reasonably strong and statistically significant signals corresponding to nearly all sRNAs annotated in the EcoCyc database. Besides differential expression of two sRNAs (GcvB and RydB), expression of other sRNAs was less affected by the composition of the growth media. Other examples of the differentially expressed sRNAs were revealed by comparing gene expression of the wild-type strain and its isogenic mutant lacking functional poly(A) polymerase I (pcnB). Further, northern blot analysis was employed to validate these data and to assess the existence of new putative sRNAs. Our results suggest that the use of custom microarrays with improved capacities for detection of sRNAs can offer an attractive opportunity for efficient gene expression profiling of sRNAs and their target mRNAs at the whole transcriptome level.

A dual reporter system for detecting RNA interactions in bacterial cells

Detecting RNA-partner interactions in cells is often difficult due to a lack of suitable tools. Here we describe a dual reporter system capable of detecting intracellular interactions in which one of the partners is an RNA. The system utilizes two fluorescent proteins with similar maturation rates but distinct spectral properties, specifically cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP). By placing the CFP gene upstream and the YFP gene downstream of an RNA gene of interest, the production of YFP becomes sensitive to RNA-partner interaction, whereas the synthesis of CFP is not disturbed. Therefore, the RNA-partner interaction can be simply measured by the change in the ratio of fluorescence of YFP over CFP. The utility of our approach is demonstrated through verification of three known RNA-partner interactions in the model bacterium Escherichia coli. Our two-reporter strategy should be broadly useful to the study of RNA-targeted interactions in bacteria.

Coregulation of host-adapted metabolism and virulence by pathogenic yersiniae

Deciphering the principles how pathogenic bacteria adapt their metabolism to a specific host microenvironment is critical for understanding bacterial pathogenesis. The enteric pathogenic Yersinia species Yersinia pseudotuberculosis and Yersinia enterocolitica and the causative agent of plague, Yersinia pestis, are able to survive in a large variety of environmental reservoirs (e.g., soil, plants, insects) as well as warm-blooded animals (e.g., rodents, pigs, humans) with a particular preference for lymphatic tissues. In order to manage rapidly changing environmental conditions and interbacterial competition, Yersinia senses the nutritional composition during the course of an infection by special molecular devices, integrates this information and adapts its metabolism accordingly. In addition, nutrient availability has an impact on expression of virulence genes in response to C-sources, demonstrating a tight link between the pathogenicity of yersiniae and utilization of nutrients. Recent studies revealed that global regulatory factors such as the cAMP receptor protein (Crp) and the carbon storage regulator (Csr) system are part of a large network of transcriptional and posttranscriptional control strategies adjusting metabolic changes and virulence in response to temperature, ion and nutrient availability. Gained knowledge about the specific metabolic requirements and the correlation between metabolic and virulence gene expression that enable efficient host colonization led to the identification of new potential antimicrobial targets.

Physiological roles of small RNA molecules

Unlike proteins, RNA molecules have emerged lately as key players in regulation in bacteria. Most reviews hitherto focused on the experimental and/or in silico methods used to identify genes encoding small RNAs (sRNAs) or on the diverse mechanisms of these RNA regulators to modulate expression of their targets. However, less is known about their biological functions and their implications in various physiological responses. This review aims to compile what is known presently about the diverse roles of sRNA transcripts in the regulation of metabolic processes, in different growth conditions, in adaptation to stress and in microbial pathogenesis. Several recent studies revealed that sRNA molecules are implicated in carbon metabolism and transport, amino acid metabolism or metal sensing. Moreover, regulatory RNAs participate in cellular adaptation to environmental changes, e.g. through quorum sensing systems or development of biofilms, and analyses of several sRNAs under various physiological stresses and culture conditions have already been performed. In addition, recent experiments performed with Gram-positive and Gram-negative pathogens showed that regulatory RNAs play important roles in microbial virulence and during infection. The combined results show the diversity of regulation mechanisms and physiological processes in which sRNA molecules are key actors.

Roles of rpoS-activating small RNAs in pathways leading to acid resistance of Escherichia coli

Escherichia coli and related enteric bacteria can survive under extreme acid stress condition at least for several hours. RpoS is a key factor for acid stress management in many enterobacteria. Although three rpoS-activating sRNAs, DsrA, RprA, and ArcZ, have been identified in E. coli, it remains unclear how these small RNA molecules participate in pathways leading to acid resistance (AR). Here, we showed that overexpression of ArcZ, DsrA, or RprA enhances AR in a RpoS-dependent manner. Mutant strains with deletion of any of three sRNA genes showed lowered AR, and deleting all three sRNA genes led to more severe defects in protecting against acid stress. Overexpression of any of the three sRNAs fully rescued the acid tolerance defects of the mutant strain lacking all three genes, suggesting that all three sRNAs perform the same function in activating RpoS required for AR. Notably, acid stress led to the induction of DsrA and RprA but not ArcZ.

Characterization of Escherichia coli D-cycloserine transport and resistant mutants

d-Cycloserine (DCS) is a broad-spectrum antibiotic that inhibits d-alanine ligase and alanine racemase activity. When Escherichia coli K-12 or CFT073 is grown in minimal glucose or glycerol medium, CycA transports DCS into the cell. E. coli K-12 cycA and CFT073 cycA mutant strains display increased DCS resistance when grown in minimal medium. However, the cycA mutants exhibit no change in DCS sensitivity compared to their parental strains when grown in LB (CFT073 and K-12) or human urine (CFT073 only). These data suggest that cycA does not participate in DCS sensitivity when strains are grown in a non-minimal medium. The small RNA GvcB acts as a negative regulator of E. coli K-12 cycA expression when grown in LB. Three E. coli K-12 gcvB mutant strains failed to demonstrate a change in DCS sensitivity when grown in LB. This further suggests a limited role for cycA in DCS sensitivity. To aid in the identification of E. coli genes involved in DCS sensitivity when grown on complex media, the Keio K-12 mutant collection was screened for DCS-resistant strains. dadA, pnp, ubiE, ubiF, ubiG, ubiH, and ubiX mutant strains showed elevated DCS resistance. The phenotypes associated with these mutants were used to further define three previously characterized E. coli DCS-resistant strains (χ316, χ444, and χ453) isolated by Curtiss and colleagues (R. Curtiss, III, L. J. Charamella, C. M. Berg, and P. E. Harris, J. Bacteriol. 90:1238-1250, 1965). A dadA mutation was identified in both χ444 and χ453. In addition, results are presented that indicate for the first time that DCS can antagonize d-amino acid dehydrogenase (DadA) activity.

Post-transcriptional control of the Escherichia coli PhoQ-PhoP two-component system by multiple sRNAs involves a novel pairing region of GcvB

PhoQ/PhoP is a central two-component system involved in magnesium homeostasis, pathogenicity, cell envelope composition, and acid resistance in several bacterial species. The small RNA GcvB is identified here as a novel direct regulator of the synthesis of PhoQ/PhoP in Escherichia coli, and this control relies on a novel pairing region of GcvB. After MicA, this is the second Hfq-dependent small RNA that represses expression of the phoPQ operon. Both MicA and GcvB bind phoPQ mRNA in vivo and in vitro around the translation initiation region of phoP. Binding of either small RNA is sufficient to inhibit ribosome binding and induce mRNA degradation. Surprisingly, however, MicA and GcvB have different effects on the levels of the PhoP protein and therefore on the expression of the PhoP regulon. These results highlight the complex connections between small RNAs and transcriptional regulation networks in bacteria.

Regulation of bacterial gene expression participates in the ability of these microorganisms to quickly adapt to their environment. This regulation can occur at every level of gene expression. For instance, two-component systems are involved in transcriptional control, while small RNAs usually act at the post-transcriptional level. In this study, the pleiotropic small RNA GcvB is identified as the second small RNA regulator of the central PhoQ/PhoP two-component system, which highlights the connections between the different types of regulation.

Transcriptomic profiling of the oyster pathogen Vibrio splendidus opens a window on the evolutionary dynamics of the small RNA repertoire in the Vibrio genus

Work in recent years has led to the recognition of the importance of small regulatory RNAs (sRNAs) in bacterial regulation networks. New high-throughput sequencing technologies are paving the way to the exploration of an expanding sRNA world in nonmodel bacteria. In the Vibrio genus, compared to the enterobacteriaceae, still a limited number of sRNAs have been characterized, mostly in Vibrio cholerae, where they have been shown to be important for virulence, as well as in Vibrio harveyi. In addition, genome-wide approaches in V. cholerae have led to the discovery of hundreds of potential new sRNAs. Vibrio splendidus is an oyster pathogen that has been recently associated with massive mortality episodes in the French oyster growing industry. Here, we report the first RNA-seq study in a Vibrio outside of the V. cholerae species. We have uncovered hundreds of candidate regulatory RNAs, be it cis-regulatory elements, antisense RNAs, and trans-encoded sRNAs. Conservation studies showed the majority of them to be specific to V. splendidus. However, several novel sRNAs, previously unidentified, are also present in V. cholerae. Finally, we identified 28 trans sRNAs that are conserved in all the Vibrio genus species for which a complete genome sequence is available, possibly forming a Vibrio "sRNA core."

The Escherichia coli GcvB sRNA Uses Genetic Redundancy to Control cycA Expression

The Escherichia coli sRNA GcvB regulates several genes involved in transport of amino acids and peptides (sstT, oppA, dppA, and cycA). Two regions of GcvB from nt +124 to +161 and from nt +73 to +82 are complementary with essentially the same region of the cycA mRNA. Transcriptional fusions of cycA to lacZ showed the region of cycA mRNA that can pair with either region of GcvB is necessary for regulation by GcvB. However, mutations in either region of gcvB predicted to disrupt pairing between cycA mRNA and GcvB did not alter expression of a cycA-lacZ translational fusion. A genetic analysis identified nts in GcvB necessary for regulation of the cycA-lacZ fusion. The results show that either region of GcvB complementary to cycA mRNA can basepair with and independently repress cycA-lacZ and both regions need to be changed to cause a significant loss of repression.

Antagonistic Roles for GcvA and GcvB in hdeAB Expression in Escherichia coli

In E. coli, the periplasmic proteins HdeA and HdeB have chaperone-like functions, suppressing aggregation of periplasmic proteins under acidic conditions. A microarray analysis of RNA isolated from an E. coli wild type and a ΔgcvB strain grown to mid-log phase in Luria-Bertani broth indicated the hdeAB operon, encoding the HdeA and HdeB proteins, is regulated by the sRNA GcvB. We wanted to verify that GcvB and its coregulator Hfq play a role in regulation of the hdeAB operon. In this study, we show that GcvB positively regulates hdeA::lacZ and hdeB::lacZ translational fusions in cells grown in Luria-Bertani broth and in glucose minimal media + glycine. Activation also requires the Hfq protein. Although many sRNAs dependent on Hfq regulate by an antisense mechanism, GcvB regulates hdeAB either directly or indirectly at the level of transcription. GcvA, the activator of gcvB, negatively regulates hdeAB at the level of transcription. Although expression of gcvB is dependent on GcvA, activation of hdeAB by GcvB occurs independently of GcvA's ability to repress the operon. Cell survival and growth at low pH are consistent with GcvA negatively regulating and GcvB positively regulating the hdeAB operon.

Novel members of the phosphate regulon in Escherichia coli O157:H7 identified using a whole-genome shotgun approach

Escherichia coli PhoB protein is the transcriptional activator of the phosphate (pho) regulon genes involved in phosphate utilization. To gain further insight into the potential roles of PhoB in the phosphate starvation response, we attempted to identify PhoB-regulated promoters using a random shotgun library of E. coli O157:H7 genomic fragments that were fused to a promoterless lacZ reporter gene on a low-copy-number plasmid. Using this approach, numerous chromosomal regions containing phosphate-starvation-inducible (psi) promoters, including nearly all known pho regulon promoters, were identified. β-Galactosidase and electrophoretic mobility shift assays showed that transcription from the 22 identified psi promoters was directly regulated by PhoB. PhoB-binding sites within the promoter regions were identified by DNase I footprinting. The genes for yoaI, rpsG, galP, rnr, udp, sstT, ybiM, and vgrE were located downstream of these promoters, indicating that these genes are members of the pho regulon. Surprisingly, the other 14 promoters were located within sense or antisense strands of open reading frames (ORFs), and/or at a distance from ORFs. Our results suggest that PhoB has broader roles in gene regulation and RNA expression in E. coli strains than was previously supposed. Our shotgun-library cloning approach represents a powerful tool for identifying promoters activated or repressed by transcriptional regulators that respond to environmental stimuli.

Multiple factors dictate target selection by Hfq-binding small RNAs

Hfq-binding small RNAs (sRNAs) in bacteria modulate the stability and translational efficiency of target mRNAs through limited base-pairing interactions. While these sRNAs are known to regulate numerous mRNAs as part of stress responses, what distinguishes targets and non-targets among the mRNAs predicted to base pair with Hfq-binding sRNAs is poorly understood. Using the Hfq-binding sRNA Spot 42 of Escherichia coli as a model, we found that predictions using only the three unstructured regions of Spot 42 substantially improved the identification of previously known and novel Spot 42 targets. Furthermore, increasing the extent of base-pairing in single or multiple base-pairing regions improved the strength of regulation, but only for the unstructured regions of Spot 42. We also found that non-targets predicted to base pair with Spot 42 lacked an Hfq-binding site, folded into a secondary structure that occluded the Spot 42 targeting site, or had overlapping Hfq-binding and targeting sites. By modifying these features, we could impart Spot 42 regulation on non-target mRNAs. Our results thus provide valuable insights into the requirements for target selection by sRNAs.

Bacterial small RNA regulators: versatile roles and rapidly evolving variations

Small RNA regulators (sRNAs) have been identified in a wide range of bacteria and found to play critical regulatory roles in many processes. The major families of sRNAs include true antisense RNAs, synthesized from the strand complementary to the mRNA they regulate, sRNAs that also act by pairing but have limited complementarity with their targets, and sRNAs that regulate proteins by binding to and affecting protein activity. The sRNAs with limited complementarity are akin to eukaryotic microRNAs in their ability to modulate the activity and stability of multiple mRNAs. In many bacterial species, the RNA chaperone Hfq is required to promote pairing between these sRNAs and their target mRNAs. Understanding the evolution of regulatory sRNAs remains a challenge; sRNA genes show evidence of duplication and horizontal transfer but also could be evolved from tRNAs, mRNAs or random transcription.

Functional characterization of bacterial sRNAs using a network biology approach

Small RNAs (sRNAs) are important components of posttranscriptional regulation. These molecules are prevalent in bacterial and eukaryotic organisms, and involved in a variety of responses to environmental stresses. The functional characterization of sRNAs is challenging and requires highly focused and extensive experimental procedures. Here, using a network biology approach and a compendium of gene expression profiles, we predict functional roles and regulatory interactions for sRNAs in Escherichia coli. We experimentally validate predictions for three sRNAs in our inferred network: IsrA, GlmZ, and GcvB. Specifically, we validate a predicted role for IsrA and GlmZ in the SOS response, and we expand on current knowledge of the GcvB sRNA, demonstrating its broad role in the regulation of amino acid metabolism and transport. We also show, using the inferred network coupled with experiments, that GcvB and Lrp, a transcription factor, repress each other in a mutually inhibitory network. This work shows that a network-based approach can be used to identify the cellular function of sRNAs and characterize the relationship between sRNAs and transcription factors.

Combined experimental and computational strategies define an expansive regulon for GcvB small RNA

The bacterial small RNA GcvB has been known as a translational repressor of mRNAs encoding amino acid transporters and has been postulated to limit uptake of unnecessary amino acids under nutrient-rich conditions. In this issue of Molecular Microbiology, Sharma et al. (2011) provide evidence for a much broader role for GcvB as a global regulator of amino acid metabolism. Using a unique combination of experimental and biocomputational approaches, the authors triple the size of the GcvB regulon, making it the largest sRNA regulon defined to date.

Pervasive post-transcriptional control of genes involved in amino acid metabolism by the Hfq-dependent GcvB small RNA

GcvB is one of the most highly conserved Hfq-associated small RNAs in Gram-negative bacteria and was previously reported to repress several ABC transporters for amino acids. To determine the full extent of GcvB-mediated regulation in Salmonella, we combined a genome-wide experimental approach with biocomputational target prediction. Comparative pulse expression of wild-type versus mutant sRNA variants revealed that GcvB governs a large post-transcriptional regulon, impacting ~1% of all Salmonella genes via its conserved G/U-rich domain R1. Complementary predictions of C/A-rich binding sites in mRNAs and gfp reporter fusion experiments increased the number of validated GcvB targets to more than 20, and doubled the number of regulated amino acid transporters. Unlike the previously described targeting via the single R1 domain, GcvB represses the glycine transporter CycA by exceptionally redundant base-pairing. This novel ability of GcvB is focused upon the one target that could feedback-regulate the glycine-responsive synthesis of GcvB. Several newly discovered mRNA targets involved in amino acid metabolism, including the global regulator Lrp, question the previous assumption that GcvB simply acts to limit unnecessary amino acid uptake. Rather, GcvB rewires primary transcriptional control circuits and seems to act as a distinct regulatory node in amino acid metabolism.

Mutation of GOGAT prevents pea bacteroid formation and N2 fixation by globally downregulating transport of organic nitrogen sources

Mutation of gltB (encoding glutamate oxoglutarate amidotransferase or GOGAT) in RU2307 increased the intracellular Gln:Glu ratio and inhibited amino acid transport via Aap and Bra. The mechanism probably involves global post-translational inhibition independent of Ntr. Transport was separately restored by increased gene expression of Aap or heterologous transporters. Likewise, second site suppressor mutations in the RNA chaperone Hfq elevated transport by Aap and Bra by increasing mRNA levels. Microarrays showed Hfq regulates 34 ABC transporter genes, including aap, bra and opp. The genes coding for integral membrane proteins and ABC subunits aapQMP braDEFGC were more strongly elevated in the hfq mutants than solute-binding proteins (aapJ braC). aapQMP and braDEFG are immediately downstream of stem-loops, indicating Hfq attenuates downstream translation and stability of mRNA, explaining differential expression of ABC genes. RU2307 nodulated peas and bacteria grew down infection threads, but bacteroid development was arrested and N(2) was not fixed. This probably results from an inability to synthesize or transport amino acids. However, GOGAT and GOGAT/AldA double mutants carrying suppressor mutations that increased amino acid uptake fixed N(2) on pea plants. Thus de novo ammonium assimilation into amino acids is unnecessary in bacteroids demonstrating sufficient amino acids are supplied by plants.

Accessibility and evolutionary conservation mark bacterial small-rna target-binding regions

Bacterial small noncoding RNAs have attracted much interest in recent years as posttranscriptional regulators of genes involved in diverse pathways. Small RNAs (sRNAs) are 50 to 400 nucleotides long and exert their regulatory function by directly base pairing with mRNA targets to alter their stability and/or affect their translation. This base pairing is achieved through a region of about 10 to 25 nucleotides, which may be located at various positions along different sRNAs. By compiling a data set of experimentally determined target-binding regions of sRNAs and systematically analyzing their properties, we reveal that they are both more evolutionarily conserved and more accessible than random regions. We demonstrate the use of these properties for computational identification of sRNA target-binding regions with high specificity and sensitivity. Our results show that these predicted regions are likely to base pair with known targets of an sRNA, suggesting that pointing out these regions in a specific sRNA can help in searching for its targets.

Characterization of a novel riboswitch-regulated lysine transporter in Aggregatibacter actinomycetemcomitans

Aggregatibacter actinomycetemcomitans is an opportunistic pathogen that resides primarily in the mammalian oral cavity. In this environment, A. actinomycetemcomitans faces numerous host- and microbe-derived stresses, including intense competition for nutrients and exposure to the host immune system. While it is clear that A. actinomycetemcomitans responds to precise cues that allow it to adapt and proliferate in the presence of these stresses, little is currently known about the regulatory mechanisms that underlie these responses. Many bacteria use noncoding regulatory RNAs (ncRNAs) to rapidly alter gene expression in response to environmental stresses. Although no ncRNAs have been reported in A. actinomycetemcomitans, we propose that they are likely important for colonization and persistence in the oral cavity. Using a bioinformatic and experimental approach, we identified three putative metabolite-sensing riboswitches and nine small regulatory RNAs (sRNAs) in A. actinomycetemcomitans during planktonic and biofilm growth. Molecular characterization of one of the riboswitches revealed that it is a lysine riboswitch and that its target gene, lysT, encodes a novel lysine-specific transporter. Finally, we demonstrated that lysT and the lysT lysine riboswitch are conserved in over 40 bacterial species, including the phylogenetically related pathogen Haemophilus influenzae.

Is the secondary putative RNA-RNA interaction site relevant to GcvB mediated regulation of oppA mRNA in Escherichia coli?

GcvB is a non-coding RNA that regulates oppA mRNA in different bacterial species by binding a GcvB GU-rich region named R1 to oppA mRNA. A secondary putative interaction site (PS1) was identified in this study that is able to form a second nearly perfect 10 base-pair duplex between these two RNAs in Escherichia coli. In this work, we have studied whether the formation of a second interaction site could help stabilize the previously reported GcvB/oppA complex. Several mutations and the full deletion of PS1 were engineered. None of these modifications affected the ability of GcvB to control OppA expression. Therefore the second, putative, interaction site appears to be unnecessary for the regulatory function of GcvB with regard to its oppA target mRNA.

Proteomic alterations explain phenotypic changes in Sinorhizobium meliloti lacking the RNA chaperone Hfq

The ubiquitous bacterial RNA-binding protein Hfq is involved in stress resistance and pathogenicity. In Sinorhizobium meliloti, Hfq is essential for the establishment of symbiosis with Medicago sativa and for nitrogen fixation. A proteomic analysis identifies 55 proteins with significantly affected expression in the hfq mutant; most of them are involved in cell metabolism or stress resistance. Important determinants of oxidative stress resistance, such as CysK, Gsh, Bfr, SodC, KatB, KatC, and a putative peroxiredoxine (SMc00072), are downregulated in the hfq mutant. The hfq mutant is affected for H(2)O(2), menadione, and heat stress resistance. Part of these defects could result from the reductions of rpoE1, rpoE2, rpoE3, and rpoE4 expression levels in the hfq mutant. Some proteins required for efficient symbiosis are reduced in the hfq mutant, contributing to the drastic defect in nodulation observed in this mutant.

Multiple target regulation by small noncoding RNAs rewires gene expression at the post-transcriptional level

Small noncoding RNAs (sRNAs), often in conjunction with Hfq protein, have increasingly been shown to regulate multiple rather than individual mRNAs, thereby reprogramming gene expression at the post-transcriptional level. This review summarizes how and when several such regulators (CyaR, DsrA, GcvB, OmrAB, RNAIII, RybB, RyhB) act upon multiple targets.