Global effects of the DEAD-box RNA helicase DeaD (CsdA) on gene expression over a broad range of temperatures

Assembly of the bacterial ribosome with circularly permuted rRNA

Co-transcriptional assembly is an integral feature of the formation of RNA-protein complexes that mediate translation. For ribosome synthesis, prior studies have indicated that the strict order of transcription of rRNA domains may not be obligatory during bacterial ribosome biogenesis, since a series of circularly permuted rRNAs are viable. In this work, we report the structural insights into assembly of the bacterial ribosome large subunit (LSU) based on cryo-EM density maps of intermediates that accumulate during in vitro ribosome synthesis using a set of circularly permuted (CiPer) rRNAs. The observed ensemble of 23 resolved ribosome large subunit intermediates reveals conserved assembly routes with an underlying hierarchy among cooperative assembly blocks. There are intricate interdependencies for the formation of key structural rRNA helices revealed from the circular permutation of rRNA. While the order of domain synthesis is not obligatory, the order of domain association does appear to proceed with a particular order, likely due to the strong evolutionary pressure on efficient ribosome synthesis. This work reinforces the robustness of the known assembly hierarchy of the bacterial large ribosomal subunit and offers a coherent view of how efficient assembly of CiPer rRNAs can be understood in that context.

Assembly of the Bacterial Ribosome with Circularly Permuted rRNA

Co-transcriptional assembly is an integral feature of the formation of RNA-protein complexes that mediate translation. For ribosome synthesis, prior studies have indicated that the strict order of transcription of rRNA domains may not be obligatory during bacterial ribosome biogenesis, since a series of circularly permuted rRNAs are viable. In this work, we report the insights into assembly of the bacterial ribosome large subunit (LSU) based on cryo-EM density maps of intermediates that accumulate during in vitro ribosome synthesis using a set of circularly permuted (CiPer) rRNAs. The observed ensemble of twenty-three resolved ribosome large subunit intermediates reveals conserved assembly routes with an underlying hierarchy among cooperative assembly blocks. There are intricate interdependencies for the formation of key structural rRNA helices revealed from the circular permutation of rRNA. While the order of domain synthesis is not obligatory, the order of domain association does appear to proceed with a particular order, likely due to the strong evolutionary pressure on efficient ribosome synthesis. This work reinforces the robustness of the known assembly hierarchy of the bacterial large ribosomal subunit, and offers a coherent view of how efficient assembly of CiPer rRNAs can be understood in that context.

The DEAD-box RNA helicase RhlB is required for efficient RNA processing at low temperature in Caulobacter

IMPORTANCE

One of the most important control points in gene regulation is RNA stability, which determines the half-life of a transcript from its transcription until its degradation. Bacteria have evolved a sophisticated multi-enzymatic complex, the RNA degradosome, which is dedicated mostly to RNA turnover. The combined activity of RNase E and the other RNA degradosome enzymes provides an efficient pipeline for the complete degradation of RNAs. The DEAD-box RNA helicases are very often found in RNA degradosomes from phylogenetically distant bacteria, confirming their importance in unwinding structured RNA for subsequent degradation. This work showed that the absence of the RNA helicase RhlB in the free-living Alphaproteobacterium Caulobacter crescentus causes important changes in gene expression and cell physiology. These are probably due, at least in part, to inefficient RNA processing by the RNA degradosome, particularly at low-temperature conditions.

Interplay between bacterial 5'-NAD-RNA decapping hydrolase NudC and DEAD-box RNA helicase CsdA in stress responses

IMPORTANCE

Non-canonical 5'-caps removing RNA hydrolase NudC, along with stress-responsive RNA helicase CsdA, is crucial for 5'-NAD-RNA decapping and bacterial movement.

Assembly landscape for the bacterial large ribosomal subunit

Assembly of ribosomes in bacteria is highly efficient, taking ~2-3 min, but this makes the abundance of assembly intermediates very low, which is a challenge for mechanistic understanding. Genetic perturbations of the assembly process create bottlenecks where intermediates accumulate, facilitating structural characterization. We use cryo-electron microscopy, with iterative subclassification to identify intermediates in the assembly of the 50S ribosomal subunit from E. coli. The analysis of the ensemble of intermediates that spans the entire biogenesis pathway for the 50 S subunit was facilitated by a dimensionality reduction and cluster picking approach using PCA-UMAP-HDBSCAN. The identity of the cooperative folding units in the RNA with associated proteins is revealed, and the hierarchy of these units reveals a complete assembly map for all RNA and protein components. The assembly generally proceeds co-transcriptionally, with some flexibility in the landscape to ensure efficiency for this central cellular process under a variety of growth conditions.

Mutation in crrB encoding a sensor kinase increases expression of the RND-type multidrug efflux pump KexD in Klebsiella pneumoniae

BACKGROUND

RND-type multidrug efflux systems in Gram-negative bacteria protect them against antimicrobial agents. Gram-negative bacteria generally possess several genes which encode such efflux pumps, but these pumps sometimes fail to show expression. Generally, some multidrug efflux pumps are silent or expressed only at low levels. However, genome mutations often increase the expression of such genes, conferring the bacteria with multidrug-resistant phenotypes. We previously reported mutants with increased expression of the multidrug efflux pump KexD. We aimed to identify the cause of KexD overexpression in our isolates. Furthermore, we also examined the colistin resistant levels in our mutants.

METHODS

A transposon (Tn) was inserted into the genome of Klebsiella pneumoniae Em16-1, a KexD-overexpressing mutant, to identify the gene(s) responsible for KexD overexpression.

RESULTS

Thirty-two strains with decreased kexD expression after Tn insertion were isolated. In 12 of these 32 strains, Tn was identified in crrB, which encodes a sensor kinase of a two-component regulatory system. DNA sequencing of crrB in Em16-1 showed that the 452^nd cytosine on crrB was replaced by thymine, and this mutation changed the 151^st proline into leucine. The same mutation was found in all other KexD-overexpressing mutants. The expression of crrA increased in the mutant overexpressing kexD, and the strains in which crrA was complemented by a plasmid showed elevated expression of kexD and crrB from the genome. The complementation of the mutant-type crrB also increased the expression of kexD and crrA from the genome, but the complementation of the wild-type crrB did not. Deletion of crrB decreased antibiotic resistance levels and KexD expression. CrrB was reported as a factor of colistin resistance, and the colistin resistance of our strains was tested. However, our mutants and strains carrying kexD on a plasmid did not show increased colistin resistance.

CONCLUSION

Mutation in crrB is important for KexD overexpression. Increased CrrA may also be associated with KexD overexpression.

Peptidoglycan Remodeling by an L,D-Transpeptidase, LdtD during Cold Shock in Escherichia coli

Peptidoglycan (PG) is a unique and essential component of the bacterial cell envelope. It is made up of several linear glycan polymers cross-linked through covalently attached stem peptides making it a fortified mesh-like sacculus around the bacterial cytosolic membrane. In most bacteria, including Escherichia coli, the stem peptide is made up of l-alanine (l-Ala¹), d-glutamate (d-Glu²), meso-diaminopimelic acid (mDAP³), d-alanine (d-Ala⁴), and d-Ala⁵ with cross-links occurring either between d-ala⁴ and mDAP³ or between two mDAP³ residues. Of these, the cross-links of the 4-3 (d-Ala⁴-mDAP³) type are the most predominant and are formed by penicillin-binding D,D-transpeptidases, whereas the formation of less frequent 3-3 linkages (mDAP³-mDAP³) is catalyzed by L,D-transpeptidases. In this study, we found that the frequency of the 3-3 cross-linkages increased upon cold shock in exponentially growing E. coli and that the increase was mediated by an L,D-transpeptidase, LdtD. We found that a cold-inducible RNA helicase DeaD enhanced the cellular LdtD level by facilitating its translation resulting in an increased abundance of 3-3 cross-linkages during cold shock. However, DeaD was also required for optimal expression of LdtD during growth at ambient temperature. Overall, our study finds that E. coli undergoes PG remodeling during cold shock by altering the frequency of 3-3 cross-linkages, implying a role for these modifications in conferring fitness and survival advantage to bacteria growing in diverse environmental conditions. IMPORTANCE Most bacteria are surrounded by a protective exoskeleton called peptidoglycan (PG), an extensively cross-linked mesh-like macromolecule. In bacteria, such as Escherichia coli, the cross-links in the PG are of two types: a major fraction is of 4-3 type whereas a minor fraction is of 3-3 type. Here, we showed that E. coli exposed to cold shock had elevated levels of 3-3 cross-links due to the upregulation of an enzyme, LdtD, that catalyzed their formation. We showed that a cold-inducible RNA helicase DeaD enhanced the cellular LdtD level by facilitating its translation, resulting in increased 3-3 cross-links during cold shock. Our results suggest that PG remodeling contributes to the survival and fitness of bacteria growing in conditions of cold stress.

The absence of CsdA in Escherichia coli increases DNA replication and cell size but decreases growth rate at low temperature

The CsdA protein is a highly conserved, DEAD-box RNA helicase and assists RNA structural remodeling at low temperature. We show that the fast-growing wild-type (WT) cells contain higher number of replication origins per cell with bigger cell size and the slowly growing cells possess less number of replication origins per cell with smaller cell size. The absence of CsdA leads to production of larger cells with higher number of origins per cell but slower growth at low temperature in an independent-manner of growth media. The phenotypes in ΔcsdA mutant are reversed by ectopic expression of CsdA or RNase R. A global transcription analysis shows that the absence of CsdA leads to significant decreases in transcription of about 200 genes at low temperature. These genes are associated with essential metabolic pathways, flagger assembly and cell division (minDE). It is likely that the slow growth of ΔcsdA cell results from the decreased transcription of essential metabolic genes, and the larger ΔcsdA cell could be a result of decreased transcription of minDE. The increased transcription of the nrdHIEF genes in ΔcsdA mutant is a likely reason that promotes DNA replication. We conclude that CsdA coordinates the cell cycle to growth by stabilizing mRNA of essential metabolic and cell division genes and degrading mRNA for nucleotide metabolic genes at low temperature.

Key players in regulatory RNA realm of bacteria

Precise regulation of gene expression is crucial for living cells to adapt for survival in diverse environmental conditions. Among the common cellular regulatory mechanisms, RNA-based regulators play a key role in all domains of life. Discovery of regulatory RNAs have made a paradigm shift in molecular biology as many regulatory functions of RNA have been identified beyond its canonical roles as messenger, ribosomal and transfer RNA. In the complex regulatory RNA network, riboswitches, small RNAs, and RNA thermometers can be identified as some of the key players. Herein, we review the discovery, mechanism, and potential therapeutic use of these classes of regulatory RNAs mainly found in bacteria. Being highly adaptive organisms that inhabit a broad range of ecological niches, bacteria have adopted tight and rapid-responding gene regulation mechanisms. This review aims to highlight how bacteria utilize versatile RNA structures and sequences to build a sophisticated gene regulation network.

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The three major classes of prokaryotic ncRNAs and their characterized mechanisms of operation in gene regulation.

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sRNAs emerging as major players in global gene regulatory networks.

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Riboswitch mediated gene control mechanisms through on/off switches in response to ligand binding.

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RNA thermo sensors for temperature-dependent gene expression.

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Therapeutic importance of ncRNAs and computational approaches involved in the discovery of ncRNAs.

Expression of the DeaD RNA helicase is regulated at multiple levels through its long mRNA 5' untranslated region

DEAD-box proteins (DBPs) are a prominent class of RNA remodeling proteins that alter RNA structure, a process they typically perform through an ATP-dependent RNA helicase activity. Although many DBPs have been characterized at the structural and functional level in detail, much less is known about how they are regulated. We previously showed that the messenger RNA (mRNA) for the Escherichia coli (E. coli) DeaD DBP contains an unusually long 5' untranslated region (5' UTR) of 838 nucleotides (nts) and that it is the primary RNA determinant of DeaD autoregulation. We speculated that such a long and complex 5' UTR might regulate deaD expression in additional ways. Here we show that the deaD mRNA 5' UTR regulates deaD expression at two additional levels: temperature dependent expression and through a stem-loop structure overlapping the start codon. These results support the hypothesis that a long 5' UTR can regulate gene expression through multiple mechanisms. Importance The expression of genes is frequently regulated by determinants with the 5' UTR. Although many different regulatory mechanisms that operate via the 5' UTR have been described, the functional relevance of genes with long UTRs is less clear. Here, we show that the 838 nt long 5' UTR in the deaD mRNA regulates the expression of DeaD at multiple levels. We propose that long UTRs originate to provide precise control of gene expression through multiple regulatory mechanisms, and they are indicators of the importance of their associated gene products for cellular adaptation to different environments.

GenomegaMap: Within-Species Genome-Wide dN/dS Estimation from over 10,000 Genomes

The dN/dS ratio provides evidence of adaptation or functional constraint in protein-coding genes by quantifying the relative excess or deficit of amino acid-replacing versus silent nucleotide variation. Inexpensive sequencing promises a better understanding of parameters, such as dN/dS, but analyzing very large data sets poses a major statistical challenge. Here, I introduce genomegaMap for estimating within-species genome-wide variation in dN/dS, and I apply it to 3,979 genes across 10,209 tuberculosis genomes to characterize the selection pressures shaping this global pathogen. GenomegaMap is a phylogeny-free method that addresses two major problems with existing approaches: 1) It is fast no matter how large the sample size and 2) it is robust to recombination, which causes phylogenetic methods to report artefactual signals of adaptation. GenomegaMap uses population genetics theory to approximate the distribution of allele frequencies under general, parent-dependent mutation models. Coalescent simulations show that substitution parameters are well estimated even when genomegaMap’s simplifying assumption of independence among sites is violated. I demonstrate the ability of genomegaMap to detect genuine signatures of selection at antimicrobial resistance-conferring substitutions in Mycobacterium tuberculosis and describe a novel signature of selection in the cold-shock DEAD-box protein A gene deaD/csdA. The genomegaMap approach helps accelerate the exploitation of big data for gaining new insights into evolution within species.

Genera specific distribution of DEAD-box RNA helicases in cyanobacteria

Although RNA helicases are essentially ubiquitous and perform roles in all stages of RNA metabolism, phylogenetic analysis of the DEAD (Asp-Glu-Ala-Asp)-box RNA helicase family in a single phylum has not been performed. Here, we performed a phylogenetic analysis on DEAD-box helicases from all currently available cyanobacterial genomes, comprising a total of 362 helicase protein sequences from 280 strains. DEAD-box helicases belonging to three distinct clades were observed. Two clades, the CsdA (cold shock DEAD-box A)-like and RhlE (RNA helicase E)-like helicases, cluster with the homologous proteins from Escherichia coli. The third clade, the CrhR (cyanobacterial RNA helicase Redox)-like helicases, is unique to cyanobacteria and characterized by a conserved sequence motif in the C-terminal extension. Restricted distribution is observed across cyanobacterial diversity with respect to both helicase type and strain. CrhR-like and CsdA-like helicases essentially never occur together, while RhlE always occurs with either a CrhR-like or CsdA-like helicase. CrhR-like and RhlE-like proteins occurred in filamentous cyanobacteria of the orders Nostocales, Oscillatoriales and Synechococcales. Similarly, CsdA- and RhlE-like proteins are restricted to unicellular cyanobacteria of the genera Cyanobium and Synechococcus. In addition, the unexpected occurrence of RhlE in two Synechococcus strains suggests recent acquisition and evolutionary divergence. This study, therefore, raises physiological and evolutionary questions as to why DEAD-box RNA helicases encoded in cyanobacterial lineages display restricted distributions, suggesting niches that require either CrhR or CsdA RNA helicase activity but not both. Extensive conservation of gene synteny surrounding the previously described rimO–crhR operon is also observed, indicating a role in the maintenance of photosynthesis. The analysis provides insights into the evolution, origin and dissemination of sequences within a single gene family to yield divergent functional roles.

Diverse Mechanisms and Circuitry for Global Regulation by the RNA-Binding Protein CsrA

The carbon storage regulator (Csr) or repressor of stationary phase metabolites (Rsm) system of Gammaproteobacteria is among the most complex and best-studied posttranscriptional regulatory systems. Based on a small RNA-binding protein, CsrA and homologs, it controls metabolism, physiology, and bacterial lifestyle decisions by regulating gene expression on a vast scale. Binding of CsrA to sequences containing conserved GGA motifs in mRNAs can regulate translation, RNA stability, riboswitch function, and transcript elongation. CsrA governs the expression of dozens of transcription factors and other regulators, further expanding its influence on cellular physiology, and these factors can participate in feedback to the Csr system. Expression of csrA itself is subject to autoregulation via translational inhibition and indirect transcriptional activation. CsrA activity is controlled by small noncoding RNAs (sRNAs), CsrB and CsrC in Escherichia coli, which contain multiple high affinity CsrA binding sites that compete with those of mRNA targets. Transcription of CsrB/C is induced by certain nutrient limitations, cellular stresses, and metabolites, while these RNAs are targeted for degradation by the presence of a preferred carbon source. Consistent with these findings, CsrA tends to activate pathways and processes that are associated with robust growth and repress stationary phase metabolism and stress responses. Regulatory loops between Csr components affect the signaling dynamics of the Csr system. Recently, systems-based approaches have greatly expanded our understanding of the roles played by CsrA, while reinforcing the notion that much remains to be learned about the Csr system.

Dual-level autoregulation of the E. coli DeaD RNA helicase via mRNA stability and Rho-dependent transcription termination

DEAD-box proteins (DBPs) are RNA remodeling factors associated with RNA helicase activity that are found in nearly all organisms. Despite extensive studies on the mechanisms used by DBPs to regulate RNA function, very little is known about how DBPs themselves are regulated. In this work, we have analyzed the expression and regulation of DeaD/CsdA, the largest of the DBPs in Escherichia coli (E. coli). We show that deaD transcription initiates 838 nt upstream of the start of the coding region. We have also found that DeaD is autoregulated through a negative feedback mechanism that operates both at the level of deaD mRNA stability and Rho-dependent transcription termination, and this regulation is dependent upon its mRNA 5' untranslated region (5' UTR). These findings suggest that DeaD might be regulating the conformation of its own mRNA through its RNA helicase activity to facilitate ribonuclease and Rho access to its 5' UTR.

A DEAD-box protein regulates ribosome assembly through control of ribosomal protein synthesis

DEAD-box proteins (DBPs) comprise a large family of proteins that most commonly have been identified as regulators of ribosome assembly. The Escherichia coli DBP, SrmB, represents a model bacterial DBP whose absence impairs formation of the large ribosomal subunit (LSU). To define the basis for SrmB function, suppressors of the ribosomal defect of ΔsrmB strains were isolated. The major class of suppressors was found to map to the 5′ untranslated region (UTR) of the rplM-rpsI operon, which encodes the ribosomal proteins (r-proteins) L13 and S9. An analysis of protein abundance indicated that both r-proteins are under-produced in the ΔsrmB strain, but are increased in these suppressors, implicating r-protein underproduction as the molecular basis for the observed ribosomal defects. Reduced r-protein synthesis was determined to be caused by intrinsic transcription termination within the rplM 5′ UTR that is abrogated by SrmB. These results reveal a specific mechanism for DBP regulation of ribosomal assembly, indirectly mediated through its effects on r-protein expression.

The RNA-Binding Protein CsrA Controls Virulence in Erwinia amylovora by Regulating RelA, RcsB, and FlhD at the Posttranscriptional Level

CsrA, an RNA-binding protein, binds to target transcripts and alters their translation or stability. In Erwinia amylovora, CsrA positively regulates the expression of type III secretion system (T3SS), exopolysaccharide amylovoran, and motility. In this study, the global effect of CsrA and its noncoding small RNA (ncsRNA) csrB in E. amylovora was determined by RNA-seq, and potential molecular mechanisms of CsrA-dependent virulence regulation were examined. Transcriptomic analyses under the T3SS-inducing condition revealed that mutation in the csrA gene led to differential expression of more than 20% of genes in the genome. Among them, T3SS genes and those required for cell growth and viability were significantly downregulated. On the other hand, the csrB mutant exhibited significant upregulation of most major virulence genes, suggesting an antagonistic effect of csrB on CsrA targets. Direct interaction between CsrA protein and csrB was further confirmed through the RNA electrophoretic mobility shift assay (REMSA). However, no direct interaction between CsrA and hrpL and hrpS transcripts was detected, suggesting that HrpL and HrpS are not targets of CsrA, whereas three CsrA targets (relA, rcsB, and flhD) were identified and confirmed by REMSA, site-directed mutagenesis, and LacZ reporter gene assays. These findings might partially explain how CsrA positively controls E. amylovora virulence by targeting major regulators at the posttranscriptional level.

Posttranscription Initiation Control of Gene Expression Mediated by Bacterial RNA-Binding Proteins

RNA-binding proteins play vital roles in regulating gene expression and cellular physiology in all organisms. Bacterial RNA-binding proteins can regulate transcription termination via attenuation or antitermination mechanisms, while others can repress or activate translation initiation by affecting ribosome binding. The RNA targets for these proteins include short repeated sequences, longer single-stranded sequences, RNA secondary or tertiary structure, and a combination of these features. The activity of these proteins can be influenced by binding of metabolites, small RNAs, or other proteins, as well as by phosphorylation events. Some of these proteins regulate specific genes, while others function as global regulators. As the regulatory mechanisms, components, targets, and signaling circuitry surrounding RNA-binding proteins have become better understood, in part through rapid advances provided by systems approaches, a sense of the true nature of biological complexity is becoming apparent, which we attempt to capture for the reader of this review.

Regulation of Iron Storage by CsrA Supports Exponential Growth of Escherichia coli

The global regulatory protein CsrA coordinates gene expression in response to physiological cues reflecting cellular stress and nutrition. CsrA binding to the 5' segments of mRNA targets affects their translation, RNA stability, and/or transcript elongation. Recent studies identified probable mRNA targets of CsrA that are involved in iron uptake and storage in Escherichia coli, suggesting an unexplored role for CsrA in regulating iron homeostasis. Here, we assessed the impact of CsrA on iron-related gene expression, cellular iron, and growth under various iron levels. We investigated five new targets of CsrA regulation, including the genes for 4 ferritin or ferritin-like iron storage proteins (ISPs) and the stress-inducible Fe-S repair protein, SufA. CsrA bound with high affinity and specificity to ftnB, bfr, and dps mRNAs and inhibited their translation, while it modestly activated ftnA expression. Furthermore, CsrA was found to regulate cellular iron levels and support growth by repressing the expression of genes for ISPs, most importantly, ferritin B (FtnB) and bacterioferritin (Bfr). Iron starvation did not substantially affect cellular levels of CsrA or its small RNA (sRNA) antagonists, CsrB and CsrC. csrA disruption led to increased resistance to the lethal effects of H₂O₂ during exponential growth, consistent with a regulatory role in oxidative stress resistance. We propose that during exponential growth and under minimal stress, CsrA represses the deleterious expression of the ISPs that function under oxidative stress and stationary-phase conditions (FtnB, Bfr, and Dps), thus ensuring that cellular iron is available to processes that are required for growth.IMPORTANCE Iron is an essential micronutrient for nearly all living organisms but is toxic in excess. Consequently, the maintenance of iron homeostasis is a critical biological process, and the genes involved in this function are tightly regulated. Here, we explored a new role for the bacterial RNA binding protein CsrA in the regulation of iron homeostasis. CsrA was shown to be a key regulator of iron storage genes in Escherichia coli, with consequential effects on cellular iron levels and growth. Our findings establish a model in which robust CsrA activity during the exponential phase of growth leads to repression of genes whose products sequester iron or divert it to unnecessary stress response processes. In so doing, CsrA supports E. coli growth under iron-limiting laboratory conditions and may promote fitness in the competitive iron-limited environment of the host large intestine.

Role of ribosome assembly in Escherichia coli ribosomal RNA degradation

DEAD-Box proteins (DBPs) constitute a prominent class of RNA remodeling factors that play a role in virtually all aspects of RNA metabolism. To better define their cellular functions, deletions in the genes encoding each of the Escherichia coli DBPs were combined with mutations in genes encoding different Ribonucleases (RNases). Significantly, double-deletion strains lacking Ribonuclease R (RNase R) and either the DeaD or SrmB DBP were found to display growth defects and an enhanced accumulation of ribosomal RNA (rRNA) fragments. As RNase R is known to play a key role in removing rRNA degradation products, these observations initially suggested that these two DBPs could be directly involved in the same process. However, additional investigations indicated that DeaD and SrmB-dependent rRNA breakdown is caused by delays in ribosome assembly that increase the exposure of nascent RNAs to endonucleolytic cleavage. Consistent with this notion, mutations in factors known to be important for ribosome assembly also resulted in enhanced rRNA breakdown. Additionally, significant levels of rRNA breakdown products could be visualized in growing cells even in the absence of assembly defects. These findings reveal a hitherto unappreciated mechanism of rRNA degradation under conditions of both normal and abnormal ribosome assembly.

Suppression of Alternative Lipooligosaccharide Glycosyltransferase Activity by UDP-Galactose Epimerase Enhances Murine Lung Infection and Evasion of Serum IgM

In pathogens that produce lipooligosaccharide (LOS), sugar residues within the surface-exposed LOS outer core mediate interactions with components of the host immune system, promoting bacterial infection. Many LOS structures are controlled by phase variation mediated by random slipped-strand base mispairing, which can reversibly switch gene expression on or off. Phase variation diversifies the LOS, however its adaptive role is not well-understood. Nontypeable Haemophilus influenzae (NTHi) is an important pathogen that causes a range of illnesses in the upper and lower respiratory tract. In NTHi a phase variable galactosyltransferase encoded by lic2A initiates galactose chain extension of the LOS outer core. The donor substrate for Lic2A, UDP-galactose, is generated from UDP-glucose by UDP-galactose epimerase encoded by galE. Our previous fitness profiling of H. influenzae mutants in a murine lung model showed that the galE mutant had a severe survival defect, while the lic2A mutant's defect was modest, leading us to postulate that unidentified factors act as suppressors of potential defects in a lic2A mutant. Herein we conducted a genome-wide genetic interaction screen to identify genes epistatic on lic2A for survival in the murine lung. An unexpected finding was that galE mutants exhibited restored virulence properties in a lic2A mutant background. We identified an alternative antibody epitope generated by Lic2A in the galE mutant that increased sensitivity to classical complement mediated killing in human serum. Deletion of lic2A or restoration of UDP-galactose synthesis alleviated the galE mutant's virulence defects. These studies indicate that when deprived of its galactosyl substrate, Lic2A acquires an alternative activity leading to increased recognition of NTHi by IgM and decreased survival in the lung model. Biofilm formation was increased by deletion of galE and by increased availability of UDP-GlcNAc precursors that can compete with UDP-galactose production. NTHi's ability to reversibly inactivate lic2A by phase-variation may influence survival in niches of infection in which UDP-Galactose levels are limiting.

Identification of Hfq-binding RNAs in Caulobacter crescentus

Small RNAs are important for post-transcriptional regulation of gene expression, affecting stability and activity of their target mRNAs. The bacterial Sm-like protein Hfq is required to promote pairing between both RNAs when their sequence complementarity is limited. To provide a first global view on the post-transcriptional landscape of the α-proteobacterium Caulobacter crescentus, we have identified the Hfq-binding RNAs employing High-throughput sequencing of RNA isolated by cross-linking immunoprecipitation (HITS-CLIP). A total of 261 RNAs, including 3 unannotated RNAs, were successfully identified and classified according to putative function. Moreover, possible interactions between the identified sRNAs with mRNA targets were postulated through computational target predictions.

Global Regulation by CsrA and Its RNA Antagonists

The sequence-specific RNA binding protein CsrA is employed by diverse bacteria in the posttranscriptional regulation of gene expression. Its binding interactions with RNA have been documented at atomic resolution and shown to alter RNA secondary structure, RNA stability, translation, and/or Rho-mediated transcription termination through a growing number of molecular mechanisms. In Gammaproteobacteria, small regulatory RNAs (sRNAs) that contain multiple CsrA binding sites compete with mRNA for binding to CsrA, thereby sequestering and antagonizing this protein. Both the synthesis and turnover of these sRNAs are regulated, allowing CsrA activity to be rapidly and efficiently adjusted in response to nutritional conditions and stresses. Feedback loops between the Csr regulatory components improve the dynamics of signal response by the Csr system. The Csr system of Escherichia coli is intimately interconnected with other global regulatory systems, permitting it to contribute to regulation by those systems. In some species, a protein antagonist of CsrA functions as part of a checkpoint for flagellum biosynthesis. In other species, a protein antagonist participates in a mechanism in which a type III secretion system is used for sensing interactions with host cells. Recent transcriptomics studies reveal vast effects of CsrA on gene expression through direct binding to hundreds of mRNAs, and indirectly through its effects on the expression of dozens of transcription factors. CsrA binding to base-pairing sRNAs and novel mRNA segments, such as the 3' untranslated region and deep within coding regions, predict its participation in yet-to-be-discovered regulatory mechanisms.

The RNA chaperone Hfq is important for the virulence, motility and stress tolerance in the phytopathogen Xanthomonas campestris

The RNA chaperone, Hfq, is known to play extensive roles in bacterial growth and development. More recently, it has been shown to be required for virulence in many human and animal bacterial pathogens. Despite these studies little is known about the role Hfq plays in phytopathogenic bacteria. In this study, we show Hfq is required for full virulence of the crucifer black rot pathogen Xanthomonas campestris pv. campestris (Xcc). We demonstrate that an Xcc hfq deletion strain is highly attenuated for virulence in Chinese radish and shows a severe defect in the production of virulence factors including extracellular enzymes and extracellular polysaccharide. Furthermore, the Xcc strain lacking Hfq had significantly reduced cell motility and stress tolerance. These findings suggest that Hfq is a key regulator of important aspects of virulence and adaptation of Xcc. Taken together, our findings are suggestive of a regulatory network placing Hfq at the centre of virulence gene expression control in Xcc.

Global role of the bacterial post-transcriptional regulator CsrA revealed by integrated transcriptomics

CsrA is a post-transcriptional regulatory protein that is widely distributed among bacteria. This protein influences bacterial lifestyle decisions by binding to the 5′ untranslated and/or early coding regions of mRNA targets, causing changes in translation initiation, RNA stability, and/or transcription elongation. Here, we assess the contribution of CsrA to gene expression in Escherichia coli on a global scale. UV crosslinking immunoprecipitation and sequencing (CLIP-seq) identify RNAs that interact directly with CsrA in vivo, while ribosome profiling and RNA-seq uncover the impact of CsrA on translation, RNA abundance, and RNA stability. This combination of approaches reveals unprecedented detail about the regulatory role of CsrA, including novel binding targets and physiological roles, such as in envelope function and iron homeostasis. Our findings highlight the integration of CsrA throughout the E. coli regulatory network, where it orchestrates vast effects on gene expression.

The RNA-binding protein CsrA regulates the expression of hundreds of bacterial genes. Here, Potts et al. use several approaches to assess the contribution of CsrA to global gene expression in E. coli, revealing new binding targets and physiological roles such as in envelope function and iron homeostasis.

Integrative FourD omics approach profiles the target network of the carbon storage regulatory system

Multi-target regulators represent a largely untapped area for metabolic engineering and anti-bacterial development. These regulators are complex to characterize because they often act at multiple levels, affecting proteins, transcripts and metabolites. Therefore, single omics experiments cannot profile their underlying targets and mechanisms. In this work, we used an Integrative FourD omics approach (INFO) that consists of collecting and analyzing systems data throughout multiple time points, using multiple genetic backgrounds, and multiple omics approaches (transcriptomics, proteomics and high throughput sequencing crosslinking immunoprecipitation) to evaluate simultaneous changes in gene expression after imposing an environmental stress that accentuates the regulatory features of a network. Using this approach, we profiled the targets and potential regulatory mechanisms of a global regulatory system, the well-studied carbon storage regulatory (Csr) system of Escherichia coli, which is widespread among bacteria. Using 126 sets of proteomics and transcriptomics data, we identified 136 potential direct CsrA targets, including 50 novel ones, categorized their behaviors into distinct regulatory patterns, and performed in vivo fluorescence-based follow up experiments. The results of this work validate 17 novel mRNAs as authentic direct CsrA targets and demonstrate a generalizable strategy to integrate multiple lines of omics data to identify a core pool of regulator targets.

Translational Repression of the RpoS Antiadapter IraD by CsrA Is Mediated via Translational Coupling to a Short Upstream Open Reading Frame

CsrA is a global regulatory RNA binding protein that has important roles in regulating carbon metabolism, motility, biofilm formation, and numerous other cellular processes. IraD functions as an antiadapter protein that inhibits RssB-mediated degradation of RpoS, the general stress response and stationary-phase sigma factor of Escherichia coli. Here we identified a novel mechanism in which CsrA represses iraD translation via translational coupling. Expression studies with quantitative reverse transcriptase PCR, Western blotting, and lacZ fusions demonstrated that CsrA represses iraD expression. Gel mobility shift, footprint, and toeprint studies identified four CsrA binding sites in the iraD leader transcript, all of which are far upstream of the iraD ribosome binding site. Computational modeling and RNA structure mapping identified an RNA structure that sequesters the iraD Shine-Dalgarno (SD) sequence. Three open reading frames (ORFs), all of which are translated, were identified in the iraD leader region. Two of these ORFs do not affect iraD expression. However, the translation initiation region of the third ORF contains three of the CsrA binding sites, one of which overlaps its SD sequence. Furthermore, the ORF stop codon overlaps the iraD start codon, a sequence arrangement indicative of translational coupling. In vivo expression and in vitro translation studies with wild-type and mutant reporter fusions demonstrated that bound CsrA directly represses translation initiation of this ORF. We further established that CsrA-dependent repression of iraD translation occurs entirely via translational coupling with this ORF, leading to accelerated iraD mRNA decay.

CsrA posttranscriptionally represses gene expression associated with stationary-phase bacterial growth, often in opposition to the transcriptional effects of the stationary-phase sigma factor RpoS. We show that CsrA employs a novel regulatory mechanism to repress translation of iraD, which encodes an antiadapter protein that protects RpoS against proteolysis. CsrA binds to four sites in the iraD leader transcript but does not directly occlude ribosome binding to the iraD SD sequence. Instead, CsrA represses translation of a short open reading frame encoded upstream of iraD, causing repression of iraD translation via translational coupling. This finding offers a novel mechanism of gene regulation by the global regulator CsrA, and since RpoS can activate csrA transcription, this also highlights a new negative-feedback loop within the complex Csr and RpoS circuitry.

Identification by PCR signature-tagged mutagenesis of attenuated Salmonella Pullorum mutants and corresponding genes in a chicken embryo model

A key feature of the fowl-specific pathogen Salmonella Pullorum is its vertical transmission to progeny via the egg. In this study, PCR signature-tagged mutagenesis identified nine genes of a strain of S. Pullorum that contributed to survival in the chicken embryo during incubation. The genes were involved in invasion, cell division, metabolism and bacterial defence. The competition index in vivo and in vitro together with a virulence evaluation for chicken embryos of all nine mutant strains confirmed their attenuation.

Circuitry Linking the Catabolite Repression and Csr Global Regulatory Systems of Escherichia coli

Cyclic AMP (cAMP) and the cAMP receptor protein (cAMP-CRP) and CsrA are the principal regulators of the catabolite repression and carbon storage global regulatory systems, respectively. cAMP-CRP controls the transcription of genes for carbohydrate metabolism and other processes in response to carbon nutritional status, while CsrA binds to diverse mRNAs and regulates translation, RNA stability, and/or transcription elongation. CsrA also binds to the regulatory small RNAs (sRNAs) CsrB and CsrC, which antagonize its activity. The BarA-UvrY two-component signal transduction system (TCS) directly activates csrB and csrC (csrB/C) transcription, while CsrA does so indirectly. We show that cAMP-CRP inhibits csrB/C transcription without negatively regulating phosphorylated UvrY (P-UvrY) or CsrA levels. A crp deletion caused an elevation in CsrB/C levels in the stationary phase of growth and increased the expression of csrB-lacZ and csrC-lacZ transcriptional fusions, although modest stimulation of CsrB/C turnover by the crp deletion partially masked the former effects. DNase I footprinting and other studies demonstrated that cAMP-CRP bound specifically to three sites located upstream from the csrC promoter, two of which overlapped the P-UvrY binding site. These two proteins competed for binding at the overlapping sites. In vitro transcription-translation experiments confirmed direct repression of csrC-lacZ expression by cAMP-CRP. In contrast, cAMP-CRP effects on csrB transcription may be mediated indirectly, as it bound nonspecifically to csrB DNA. In the reciprocal direction, CsrA bound to crp mRNA with high affinity and specificity and yet exhibited only modest, conditional effects on expression. Our findings are incorporated into an emerging model for the response of Csr circuitry to carbon nutritional status.

IMPORTANCE

Csr (Rsm) noncoding small RNAs (sRNAs) CsrB and CsrC of Escherichia coli use molecular mimicry to sequester the RNA binding protein CsrA (RsmA) away from lower-affinity mRNA targets, thus eliciting major shifts in the bacterial lifestyle. CsrB/C transcription and turnover are activated by carbon metabolism products (e.g., formate and acetate) and by a preferred carbon source (glucose), respectively. We show that cAMP-CRP, a mediator of classical catabolite repression, inhibits csrC transcription by binding to the upstream region of this gene and also inhibits csrB transcription, apparently indirectly. We propose that glucose availability activates pathways for both synthesis and turnover of CsrB/C, thus shaping the dynamics of global signaling in response to the nutritional environment by poising CsrB/C sRNA levels for rapid response.

FliW antagonizes CsrA RNA binding by a noncompetitive allosteric mechanism

CsrA (carbon storage regulator A) is a widely distributed bacterial RNA binding protein that regulates translation initiation and mRNA stability of target transcripts. In γ-proteobacteria, CsrA activity is competitively antagonized by one or more small RNAs (sRNAs) containing multiple CsrA binding sites, but CsrA in bacteria outside the γ-proteobacteria is antagonized by a protein called FliW. Here we show that FliW of Bacillus subtilis does not bind to the same residues of CsrA required for RNA binding. Instead, CsrA mutants resistant to FliW antagonism (crw) altered residues of CsrA on an allosteric surface of previously unattributed function. Some crw mutants abolished CsrA-FliW binding, but others did not, suggesting that FliW and RNA interaction is not mutually exclusive. We conclude that FliW inhibits CsrA by a noncompetitive mechanism that differs dramatically from the well-established sRNA inhibitors. FliW is highly conserved with CsrA in bacteria, appears to be the ancestral form of CsrA regulation, and represents a widespread noncompetitive mechanism of CsrA control.

Antagonistic control of the turnover pathway for the global regulatory sRNA CsrB by the CsrA and CsrD proteins

The widely conserved protein CsrA (carbon storage regulator A) globally regulates bacterial gene expression at the post-transcriptional level. In many species, CsrA activity is governed by untranslated sRNAs, CsrB and CsrC in Escherichia coli, which bind to multiple CsrA dimers, sequestering them from lower affinity mRNA targets. Both the synthesis and turnover of CsrB/C are regulated. Their turnover requires the housekeeping endonuclease RNase E and is activated by the presence of a preferred carbon source via the binding of EIIA^Glc of the glucose transport system to the GGDEF-EAL domain protein CsrD. We demonstrate that the CsrB 3′ segment contains the features necessary for CsrD-mediated decay. RNase E cleavage in an unstructured segment located immediately upstream from the intrinsic terminator is necessary for subsequent degradation to occur. CsrA stabilizes CsrB against RNase E cleavage by binding to two canonical sites adjacent to the necessary cleavage site, while CsrD acts by overcoming CsrA-mediated protection. Our genetic, biochemical and structural studies establish a molecular framework for sRNA turnover by the CsrD-RNase E pathway. We propose that CsrD evolution was driven by the selective advantage of decoupling Csr sRNA decay from CsrA binding, connecting it instead to the availability of a preferred carbon source.

The ribonuclease polynucleotide phosphorylase can interact with small regulatory RNAs in both protective and degradative modes

In all bacterial species examined thus far, small regulatory RNAs (sRNAs) contribute to intricate patterns of dynamic genetic regulation. Many of the actions of these nucleic acids are mediated by well-characterized chaperones such as the Hfq protein, but genetic screens have also recently identified the 3'-to-5' exoribonuclease polynucleotide phosphorylase (PNPase) as an unexpected stabilizer and facilitator of sRNAs in vivo. To understand how a ribonuclease might mediate these effects, we tested the interactions of PNPase with sRNAs and found that the enzyme can readily degrade these nucleic acids in vitro but, nonetheless, copurifies from cell extracts with the same sRNAs without discernible degradation or modification to their 3' ends, suggesting that the associated RNA is protected against the destructive activity of the ribonuclease. In vitro, PNPase, Hfq, and sRNA can form a ternary complex in which the ribonuclease plays a nondestructive, structural role. Such ternary complexes might be formed transiently in vivo, but could help to stabilize particular sRNAs and remodel their population on Hfq. Taken together, our results indicate that PNPase can be programmed to act on RNA in either destructive or stabilizing modes in vivo and may form complex, protective ribonucleoprotein assemblies that shape the landscape of sRNAs available for action.

RNA helicases in bacteria

RNA plays a crucial role in the control of bacterial gene expression, either as carrier of information or as positive or negative regulators. Moreover, the machinery to decode the information, the ribosome, is a large ribonucleoprotein complex composed of rRNAs and many proteins. RNAs are normally single stranded but have the propensity to fold into secondary structures or anneal each other. In some instances these interactions are beneficial for the function of the RNA, but in other cases they may be deleterious. All cells have therefore developed proteins that act as chaperones or helicases to keep RNA metabolism alive.

Genomic Targets and Features of BarA-UvrY (-SirA) Signal Transduction Systems

The two-component signal transduction system BarA-UvrY of Escherichia coli and its orthologs globally regulate metabolism, motility, biofilm formation, stress resistance, virulence of pathogens and quorum sensing by activating the transcription of genes for regulatory sRNAs, e.g. CsrB and CsrC in E. coli. These sRNAs act by sequestering the RNA binding protein CsrA (RsmA) away from lower affinity mRNA targets. In this study, we used ChIP-exo to identify, at single nucleotide resolution, genomic sites for UvrY (SirA) binding in E. coli and Salmonella enterica. The csrB and csrC genes were the strongest targets of crosslinking, which required UvrY phosphorylation by the BarA sensor kinase. Crosslinking occurred at two sites, an inverted repeat sequence far upstream of the promoter and a site near the -35 sequence. DNAse I footprinting revealed specific binding of UvrY in vitro only to the upstream site, indicative of additional binding requirements and/or indirect binding to the downstream site. Additional genes, including cspA, encoding the cold-shock RNA-binding protein CspA, showed weaker crosslinking and modest or negligible regulation by UvrY. We conclude that the global effects of UvrY/SirA on gene expression are primarily mediated by activating csrB and csrC transcription. We also used in vivo crosslinking and other experimental approaches to reveal new features of csrB/csrC regulation by the DeaD and SrmB RNA helicases, IHF, ppGpp and DksA. Finally, the phylogenetic distribution of BarA-UvrY was analyzed and found to be uniquely characteristic of γ-Proteobacteria and strongly anti-correlated with fliW, which encodes a protein that binds to CsrA and antagonizes its activity in Bacillus subtilis. We propose that BarA-UvrY and orthologous TCS transcribe sRNA antagonists of CsrA throughout the γ-Proteobacteria, but rarely or never perform this function in other species.

Regulation of CsrB/C sRNA decay by EIIA(Glc) of the phosphoenolpyruvate: carbohydrate phosphotransferase system

Csr is a conserved global regulatory system, which uses the sequence-specific RNA-binding protein CsrA to activate or repress gene expression by binding to mRNA and altering translation, stability and/or transcript elongation. In Escherichia coli, CsrA activity is regulated by two sRNAs, CsrB and CsrC, which bind to multiple CsrA dimers, thereby sequestering this protein away from its mRNA targets. Turnover of CsrB/C sRNAs is tightly regulated by a GGDEF-EAL domain protein, CsrD, which targets them for cleavage by RNase E. Here, we show that EIIA(Glc) of the glucose-specific PTS system is also required for the normal decay of these sRNAs and that it acts by binding to the EAL domain of CsrD. Only the unphosphorylated form of EIIA(Glc) bound to CsrD in vitro and was capable of activating CsrB/C turnover in vivo. Genetic studies confirmed that this mechanism couples CsrB/C sRNA decay to the availability of a preferred carbon source. These findings reveal a new physiological influence on the workings of the Csr system, a novel function for the EAL domain, and an important new way in which EIIA(Glc) shapes global regulatory circuitry in response to nutritional status.

DExD-box RNA-helicases in Listeria monocytogenes are important for growth, ribosomal maturation, rRNA processing and virulence factor expression

RNA-helicases are proteins required for the unwinding of occluding secondary RNA structures, especially at low temperatures. In this work, we have deleted all 4 DExD-box RNA helicases in various combinations in the Gram-positive pathogen Listeria monocytogenes. Our results show that 3 out of 4 RNA-helicases were important for growth at low temperatures, whereas the effect was less prominent at 37°C. Over-expression of one RNA-helicase, Lmo1450, was able to overcome the reduced growth of the quadruple mutant strain at temperatures above 26°C, but not at lower temperatures. The maturation of ribosomes was affected in different degrees in the various strains at 20°C, whereas the effect was marginal at 37°C. This was accompanied by an increased level of immature 23S rRNA precursors in some of the RNA-helicase mutants at low temperatures. Although the expression of the PrfA regulated virulence factors ActA and LLO decreased in the quadruple mutant strain, this strain showed a slightly increased infection ability. Interestingly, even though the level of the virulence factor LLO was decreased in the quadruple mutant strain as compared with the wild-type strain, the hly-transcript (encoding LLO) was increased. Hence, our results could suggest a role for the RNA-helicases during translation. In this work, we show that DExD-box RNA-helicases are involved in bacterial virulence gene-expression and infection of eukaryotic cells.

Secondary Structure across the Bacterial Transcriptome Reveals Versatile Roles in mRNA Regulation and Function

Messenger RNA acts as an informational molecule between DNA and translating ribosomes. Emerging evidence places mRNA in central cellular processes beyond its major function as informational entity. Although individual examples show that specific structural features of mRNA regulate translation and transcript stability, their role and function throughout the bacterial transcriptome remains unknown. Combining three sequencing approaches to provide a high resolution view of global mRNA secondary structure, translation efficiency and mRNA abundance, we unraveled structural features in E. coli mRNA with implications in translation and mRNA degradation. A poorly structured site upstream of the coding sequence serves as an additional unspecific binding site of the ribosomes and the degree of its secondary structure propensity negatively correlates with gene expression. Secondary structures within coding sequences are highly dynamic and influence translation only within a very small subset of positions. A secondary structure upstream of the stop codon is enriched in genes terminated by UAA codon with likely implications in translation termination. The global analysis further substantiates a common recognition signature of RNase E to initiate endonucleolytic cleavage. This work determines for the first time the E. coli RNA structurome, highlighting the contribution of mRNA secondary structure as a direct effector of a variety of processes, including translation and mRNA degradation.

RNA Helicase Important for Listeria monocytogenes Hemolytic Activity and Virulence Factor Expression

RNA helicases have been shown to be important for the function of RNA molecules at several levels, although their putative involvement in microbial pathogenesis has remained elusive. We have previously shown that Listeria monocytogenes DExD-box RNA helicases are important for bacterial growth, motility, ribosomal maturation, and rRNA processing. We assessed the importance of the RNA helicase Lmo0866 (here named CshA) for expression of virulence traits. We observed a reduction in hemolytic activity in a strain lacking CshA compared to the wild type. This phenomenon was less evident in strains lacking other RNA helicases. The reduced hemolysis was accompanied by lower expression of major listerial virulence factors in the ΔcshA strain, mainly listeriolysin O, but also to some degree the actin polymerizing factor ActA. Reduced expression of these virulence factors in the strain lacking CshA did not, however, correlate with a decreased level of the virulence regulator PrfA. When combining the ΔcshA knockout with a mutation creating a constitutively active PrfA protein (PrfA*), the effect of the ΔcshA knockout on LLO expression was negated. These data suggest a role for the RNA helicase CshA in posttranslational activation of PrfA. Surprisingly, although the expression of several virulence factors was reduced, the ΔcshA strain did not demonstrate any reduced ability to infect nonphagocytic cells compared to the wild-type strain.

CsrA Participates in a PNPase Autoregulatory Mechanism by Selectively Repressing Translation of pnp Transcripts That Have Been Previously Processed by RNase III and PNPase

Csr is a conserved global regulatory system that represses or activates gene expression posttranscriptionally. CsrA of Escherichia coli is a homodimeric RNA binding protein that regulates transcription elongation, translation initiation, and mRNA stability by binding to the 5' untranslated leader or initial coding sequence of target transcripts. pnp mRNA, encoding the 3' to 5' exoribonuclease polynucleotide phosphorylase (PNPase), was previously identified as a CsrA target by transcriptome sequencing (RNA-seq). Previous studies also showed that RNase III and PNPase participate in a pnp autoregulatory mechanism in which RNase III cleavage of the untranslated leader, followed by PNPase degradation of the resulting 5' fragment, leads to pnp repression by an undefined translational repression mechanism. Here we demonstrate that CsrA binds to two sites in pnp leader RNA but only after the transcript is fully processed by RNase III and PNPase. In the absence of processing, both of the binding sites are sequestered in an RNA secondary structure, which prevents CsrA binding. The CsrA dimer bridges the upstream high-affinity site to the downstream site that overlaps the pnp Shine-Dalgarno sequence such that bound CsrA causes strong repression of pnp translation. CsrA-mediated translational repression also leads to a small increase in the pnp mRNA decay rate. Although CsrA has been shown to regulate translation and mRNA stability of numerous genes in a variety of organisms, this is the first example in which prior mRNA processing is required for CsrA-mediated regulation.

IMPORTANCE

CsrA protein represses translation of numerous mRNA targets, typically by binding to multiple sites in the untranslated leader region preceding the coding sequence. We found that CsrA represses translation of pnp by binding to two sites in the pnp leader transcript but only after it is processed by RNase III and PNPase. Processing by these two ribonucleases alters the mRNA secondary structure such that it becomes accessible to the ribosome for translation as well as to CsrA. As one of the CsrA binding sites overlaps the pnp ribosome binding site, bound CsrA prevents ribosome binding. This is the first example in which regulation by CsrA requires prior mRNA processing and should link pnp expression to conditions affecting CsrA activity.

The RNA Helicase DeaD Stimulates ExsA Translation To Promote Expression of the Pseudomonas aeruginosa Type III Secretion System

The Pseudomonas aeruginosa type III secretion system (T3SS) is a primary virulence factor important for phagocytic avoidance, disruption of host cell signaling, and host cell cytotoxicity. ExsA is the master regulator of T3SS transcription. The expression, synthesis, and activity of ExsA is tightly regulated by both intrinsic and extrinsic factors. Intrinsic regulation consists of the well-characterized ExsECDA partner-switching cascade, while extrinsic factors include global regulators that alter exsA transcription and/or translation. To identify novel extrinsic regulators of ExsA, we conducted a transposon mutagenesis screen in the absence of intrinsic control. Transposon disruptions within gene PA2840, which encodes a homolog of the Escherichia coli RNA-helicase DeaD, significantly reduced T3SS gene expression. Recent studies indicate that E. coli DeaD can promote translation by relieving inhibitory secondary structures within target mRNAs. We report here that PA2840, renamed DeaD, stimulates ExsA synthesis at the posttranscriptional level. Genetic experiments demonstrate that the activity of an exsA translational fusion is reduced in a deaD mutant. In addition, exsA expression in trans fails to restore T3SS gene expression in a deaD mutant. We hypothesized that DeaD relaxes mRNA secondary structure to promote exsA translation and found that altering the mRNA sequence of exsA or the native exsA Shine-Dalgarno sequence relieved the requirement for DeaD in vivo. Finally, we show that purified DeaD promotes ExsA synthesis using in vitro translation assays. Together, these data reveal a novel regulatory mechanism for P. aeruginosa DeaD and add to the complexity of global regulation of T3SS.

IMPORTANCE

Although members of the DEAD box family of RNA helicases are appreciated for their roles in mRNA degradation and ribosome biogenesis, an additional role in gene regulation is now emerging in bacteria. By relaxing secondary structures in mRNAs, DEAD box helicases are now thought to promote translation by enhancing ribosomal recruitment. We identify here an RNA helicase that plays a critical role in promoting ExsA synthesis, the central regulator of the Pseudomonas aeruginosa type III secretion system, and provide additional evidence that DEAD box helicases directly stimulate translation of target genes. The finding that DeaD stimulates exsA translation adds to a growing list of transcriptional and posttranscriptional regulatory mechanisms that control type III gene expression.

Regulation of bacterial virulence by Csr (Rsm) systems

Most bacterial pathogens have the remarkable ability to flourish in the external environment and in specialized host niches. This ability requires their metabolism, physiology, and virulence factors to be responsive to changes in their surroundings. It is no surprise that the underlying genetic circuitry that supports this adaptability is multilayered and exceedingly complex. Studies over the past 2 decades have established that the CsrA/RsmA proteins, global regulators of posttranscriptional gene expression, play important roles in the expression of virulence factors of numerous proteobacterial pathogens. To accomplish these tasks, CsrA binds to the 5' untranslated and/or early coding regions of mRNAs and alters translation, mRNA turnover, and/or transcript elongation. CsrA activity is regulated by noncoding small RNAs (sRNAs) that contain multiple CsrA binding sites, which permit them to sequester multiple CsrA homodimers away from mRNA targets. Environmental cues sensed by two-component signal transduction systems and other regulatory factors govern the expression of the CsrA-binding sRNAs and, ultimately, the effects of CsrA on secretion systems, surface molecules and biofilm formation, quorum sensing, motility, pigmentation, siderophore production, and phagocytic avoidance. This review presents the workings of the Csr system, the paradigm shift that it generated for understanding posttranscriptional regulation, and its roles in virulence networks of animal and plant pathogens.

Bacterial versatility requires DEAD-box RNA helicases

RNA helicases of the DEAD-box and DEAH-box families are important players in many processes involving RNA molecules. These proteins can modify RNA secondary structures or intermolecular RNA interactions and modulate RNA-protein complexes. In bacteria, they are known to be involved in ribosome biogenesis, RNA turnover and translation initiation. They thereby play an important role in the adaptation of bacteria to changing environments and to respond to stress conditions.

CsrA impacts survival of Yersinia enterocolitica by affecting a myriad of physiological activities

BACKGROUND

A previous study identified a Yersinia enterocolitica transposon mutant, GY448, that was unable to export the flagellar type three secretion system (T3SS)-dependent phospholipase, YplA. This strain was also deficient for motility and unable to form colonies on Lauria-Bertani agar medium. Preliminary analysis suggested it carried a mutation in csrA. CsrA in Escherichia coli is an RNA-binding protein that is involved in specific post-transcriptional regulation of a myriad of physiological activities. This study investigated how CsrA affects expression of the flagellar regulatory cascade that controls YplA export and motility. It also explored the effect of csrA mutation on Y. enterocolitica in response to conditions that cue physiological changes important for growth in environments found both in nature and the laboratory.

RESULTS

The precise location of the transposon insertion in GMY448 was mapped within csrA. Genetic complementation restored disruptions in motility and the YplA export phenotype (Yex), which confirmed this mutation disrupted CsrA function. Mutation of csrA affected expression of yplA and flagellar genes involved in flagellar T3SS dependent export and motility by altering expression of the master regulators flhDC. Mutation of csrA also resulted in increased sensitivity of Y. enterocolitica to various osmolytes, temperatures and antibiotics.

CONCLUSIONS

The results of this study reveal unique aspects of how CsrA functions in Y. enterocolitica to control its physiology. This provides perspective on how the Csr system is susceptible to adaptation to particular environments and bacterial lifestyles.

Effects of the global regulator CsrA on the BarA/UvrY two-component signaling system

The hybrid sensor kinase BarA and its cognate response regulator UvrY, members of the two-component signal transduction family, activate transcription of CsrB and CsrC noncoding RNAs. These two small RNAs act by sequestering the RNA binding protein CsrA, which posttranscriptionally regulates translation and/or stability of its target mRNAs. Here, we provide evidence that CsrA positively affects, although indirectly, uvrY expression, at both the transcriptional and translational levels. We also demonstrate that CsrA is required for properly switching BarA from its phosphatase to its kinase activity. Thus, the existence of a feedback loop mechanism that involves the Csr and BarA/UvrY global regulatory systems is exposed.