The poly(A) binding protein Hfq protects RNA from RNase E and exoribonucleolytic degradation

The RNA chaperone Hfq is a novel regulator of catalase expression and hydrogen peroxide-induced oxidative stress response in Listeria monocytogenes EGD-e

The RNA chaperone Hfq plays a pivotal role in many bacteria, acting as a regulator of gene expression and promoting interaction between mRNA-sRNA pairs in Gram-negative bacteria. However, in Gram-positive bacteria this protein is expendable for riboregulation, and the main function of Hfq remains elusive. This work unveils a novel function for Hfq in the oxidative stress response of the human pathogen Listeria monocytogenes, a Gram-positive bacterium responsible for the infectious disease listeriosis. Disruption of hfq gene (Δhfq) results in a hypersensitive phenotype towards hydrogen peroxide (H2O2), in which sub-inhibitory concentrations of this reactive oxygen species (ROS) severely impair growth and viability of L. monocytogenes EGD-e. A Δhfq-complemented strain does not show this phenotype. This Hfq-dependent regulation of oxidative stress seems specific for H2O2, as exposure to superoxides caused no differences. We demonstrate that Hfq has a dual regulatory role in the expression of catalase (kat), the key enzyme involved in H2O2 detoxification. Hfq influences kat transcription under non-stress conditions by modulating the levels of the transcriptional repressor PerR, and also acts post-transcriptionally by stabilizing kat mRNA under H2O2-induced stress. Indeed, enzymatic assays revealed reduced catalase activity in Δhfq cell extracts, a result unrelated to differences in cellular iron content. Bacterial infection triggers immune cells to produce massive amounts of ROS, like H2O2. We show that inactivation of Hfq increases susceptibility to macrophage killing, connecting Hfq with the stress resistance and virulence of L. monocytogenes EGD-e. Overall, these findings advance the understanding of Hfq function within Gram-positive bacteria, revealing for the first time that Hfq is a novel regulator of catalase expression. This paves the way for the study of yet unknown oxidative stress response pathways regulated by Hfq in other pathogens.

Rational Design of High-Efficiency Synthetic Small Regulatory RNAs and Their Application in Robust Genetic Circuit Performance Through Tight Control of Leaky Gene Expression

Synthetic sRNAs show promise as tools for targeted and programmable gene expression manipulation. However, the design of high-efficiency synthetic sRNAs is a challenging task that necessitates careful consideration of multiple factors. Therefore, this study aims to investigate rational design strategies that significantly and robustly enhance the efficiency of synthetic sRNAs. This is achieved by optimizing the following parameters: the sRNA scaffold, mRNA binding affinity, Hfq protein expression level, and mRNA secondary structure. By utilizing optimized synthetic sRNAs within a positive feedback circuit, we effectively addressed the issue of gene expression leakage─an enduring challenge in synthetic biology that undermines the reliability of genetic circuits in bacteria. Our designed synthetic sRNAs successfully prevented gene expression leakage, thus averting unintended circuit activation caused by initial expression noise, even in the absence of signal molecules. This result shows that high-efficiency synthetic sRNAs not only enable precise gene knockdown for metabolic engineering but also ensure the robust performance of synthetic circuits. The strategies developed here hold significant promise for broad applications across diverse biotechnological fields, establishing synthetic sRNAs as pivotal tools in advancing synthetic biology and gene regulation.

Targeting nucleic acid phase transitions as a mechanism of action for antimicrobial peptides

Antimicrobial peptides (AMPs), which combat bacterial infections by disrupting the bacterial cell membrane or interacting with intracellular targets, are naturally produced by a number of different organisms, and are increasingly also explored as therapeutics. However, the mechanisms by which AMPs act on intracellular targets are not well understood. Using machine learning-based sequence analysis, we identified a significant number of AMPs that have a strong tendency to form liquid-like condensates in the presence of nucleic acids through phase separation. We demonstrate that this phase separation propensity is linked to the effectiveness of the AMPs in inhibiting transcription and translation in vitro, as well as their ability to compact nucleic acids and form clusters with bacterial nucleic acids in bacterial cells. These results suggest that the AMP-driven compaction of nucleic acids and modulation of their phase transitions constitute a previously unrecognised mechanism by which AMPs exert their antibacterial effects. The development of antimicrobials that target nucleic acid phase transitions may become an attractive route to finding effective and long-lasting antibiotics.

A compensatory RNase E variation increases Iron Piracy and Virulence in multidrug-resistant Pseudomonas aeruginosa during Macrophage infection

During chronic cystic fibrosis (CF) infections, evolved Pseudomonas aeruginosa antibiotic resistance is linked to increased pulmonary exacerbations, decreased lung function, and hospitalizations. However, the virulence mechanisms underlying worse outcomes caused by antibiotic resistant infections are poorly understood. Here, we investigated evolved aztreonam resistant P. aeruginosa virulence mechanisms. Using a macrophage infection model combined with genomic and transcriptomic analyses, we show that a compensatory mutation in the rne gene, encoding RNase E, increased pyoverdine and pyochelin siderophore gene expression, causing macrophage ferroptosis and lysis. We show that iron-bound pyochelin was sufficient to cause macrophage ferroptosis and lysis, however, apo-pyochelin, iron-bound pyoverdine, or apo-pyoverdine were insufficient to kill macrophages. Macrophage killing could be eliminated by treatment with the iron mimetic gallium. RNase E variants were abundant in clinical isolates, and CF sputum gene expression data show that clinical isolates phenocopied RNase E variant functions during macrophage infection. Together these data show how P. aeruginosa RNase E variants can cause host damage via increased siderophore production and host cell ferroptosis but may also be targets for gallium precision therapy.

5'UTR sequences influence protein levels in Escherichia coli by regulating translation initiation and mRNA stability

A set of 41 synthetic 5'UTRs with different theoretical translation initiation rates were generated to explore the role of 5'UTRs in the regulation of protein levels in Escherichia coli. The roles of the synthetic 5'UTRs in regulating the expression of different reporter genes were analyzed in vivo. Protein levels varied substantially between the different constructs but for most of the 5'UTRs, protein levels were not correlated with theoretical translation initiation rates. Large variations in mRNA concentrations were measured with the different 5'UTRs even though the same concentration of transcription inducer was used in each case. 5'UTRs were also found to strongly affect mRNA stability, and these changes in mRNA stability often contributed to observed differences in mRNA concentration. Unexpectedly, the effect of the 5'UTRs on mRNA half-lives was found to vary depending on the downstream reporter gene. These results clearly demonstrate that 5'UTRs contribute to gene expression regulation at the level of translation initiation and of mRNA stability, to an extent that depends on the nature of the downstream gene.

Transgenesis of mammalian PABP reveals mRNA polyadenylation as a general stress response mechanism in bacteria

In eukaryotes, mRNA 3′-polyadenylation triggers poly(A) binding protein (PABP) recruitment and stabilization. In a stark contrast, polyadenylation marks mRNAs for degradation in bacteria. To study this difference, we trans-express the mammalian nuclear PABPN1 chromosomally and extra-chromosomally in Escherichia coli. Expression of PABPN1 but not the mutant PABPN1 stabilizes polyadenylated mRNAs and improves their half-lives. In the presence of PABPN1, 3′-exonuclease PNPase is not detected on PA-tailed mRNAs compromising the degradation. We show that PABPN1 trans-expression phenocopies pcnB (that encodes poly(A) polymerase, PAPI) mutation and regulates plasmid copy number. Genome-wide RNA-seq analysis shows a general up-regulation of polyadenylated mRNAs on PABPN1 expression, the largest subset of which are those involved in general stress response. However, major global stress regulators are unaffected on PABPN1 expression. Concomitantly, PABPN1 expression or pcnB mutation imparts cellular tolerance to multiple stresses. This study establishes mRNA 3′-polyadenylation as a general stress response mechanism in E. coli.

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Trans expression of mammalian PABPN1 stabilizes polyadenyated mRNAs in E. coli

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PABPN1 expression phenocopies pcnB mutation and regulates plasmid copy number

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3′-polyadenylation acts as a general stress response mechanism in bacteria

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This study indicates an evolutionary significance of PABP in mRNA metabolism

Enhanced Prodigiosin Production in Serratia marcescens JNB5-1 by Introduction of a Polynucleotide Fragment into the pigN 3' Untranslated Region and Disulfide Bonds into O-Methyl Transferase (PigF)

In Serratia marcescens JNB5-1, prodigiosin was highly produced at 30°C, but it was noticeably repressed at ≥37°C. Our initial results demonstrated that both the production and the stability of the O-methyl transferase (PigF) and oxidoreductase (PigN) involved in the prodigiosin pathway in S. marcescens JNB5-1 sharply decreased at ≥37°C. Therefore, in this study, we improved mRNA stability and protein production using de novo polynucleotide fragments (PNFs) and the introduction of disulfide bonds, respectively, and observed their effects on prodigiosin production. Our results demonstrate that adding PNFs at the 3' untranslated regions of pigF and pigN significantly improved the mRNA half-lives of these genes, leading to an increase in the transcript and expression levels. Subsequently, the introduction of disulfide bonds in pigF improved the thermal stability, pH stability, and copper ion resistance of PigF. Finally, shake flask fermentation showed that the prodigiosin titer with the engineered S. marcescens was increased by 61.38% from 5.36 to 8.65 g/liter compared to the JNB5-1 strain at 30°C and, significantly, the prodigiosin yield increased 2.05-fold from 0.38 to 0.78 g/liter at 37°C. In this study, we revealed that the introduction of PNFs and disulfide bonds greatly improved the expression and stability of pigF and pigN, hence efficiently enhancing prodigiosin production with S. marcescens at 30 and 37°C. IMPORTANCE This study highlights a promising strategy to improve mRNA/enzyme stability and to increase production using de novo PNF libraries and the introduction of disulfide bonds into the protein. PNFs could increase the half-life of target gene mRNA and effectively prevent its degradation. Moreover, PNFs could increase the relative intensity of target genes without affecting the expression of other genes; as a result, it could alleviate the cellular burden compared to other regulatory elements such as promoters. In addition, we obtained a PigF variant with improved activity and stability by the introduction of disulfide bonds into PigF. Collectively, we demonstrate here a novel approach for improving mRNA/enzyme stability using PNFs, which results in enhanced prodigiosin production in S. marcescens at 30°C.

The essential role of mRNA degradation in understanding and engineering E. coli metabolism

Metabolic engineering strategies are crucial for the development of bacterial cell factories with improved performance. Until now, optimal metabolic networks have been designed based on systems biology approaches integrating large-scale data on the steady-state concentrations of mRNA, protein and metabolites, sometimes with dynamic data on fluxes, but rarely with any information on mRNA degradation. In this review, we compile growing evidence that mRNA degradation is a key regulatory level in E. coli that metabolic engineering strategies should take into account. We first discuss how mRNA degradation interacts with transcription and translation, two other gene expression processes, to balance transcription regulation and remove poorly translated mRNAs. The many reciprocal interactions between mRNA degradation and metabolism are also highlighted: metabolic activity can be controlled by changes in mRNA degradation and in return, the activity of the mRNA degradation machinery is controlled by metabolic factors. The mathematical models of the crosstalk between mRNA degradation dynamics and other cellular processes are presented and discussed with a view towards novel mRNA degradation-based metabolic engineering strategies. We show finally that mRNA degradation-based strategies have already successfully been applied to improve heterologous protein synthesis. Overall, this review underlines how important mRNA degradation is in regulating E. coli metabolism and identifies mRNA degradation as a key target for innovative metabolic engineering strategies in biotechnology.

Optimized expression of Hfq protein increases Escherichia coli growth

Escherichia coli is a widely used platform for metabolic engineering due to its fast growth and well-established engineering techniques. However, there has been a demand for faster-growing E. coli for higher production of desired substances. Here, to increase the growth of E. coli cells, we optimized the expression level of Hfq protein, which plays an essential role in stress responses. Six variants of the hfq gene with a different ribosome binding site sequence and thereby a different expression level were constructed. When the Hfq expression level was optimized in DH5α, its growth rate was increased by 12.1% and its cell density was also increased by 4.5%. RNA-seq and network analyses revealed the upregulation of stress response genes and metabolic genes, which increases the tolerance against pH changes. When the same strategy was applied to five other E. coli strains (BL21 (DE3), JM109, TOP10, W3110, and MG1655), all their growth rates were increased by 18-94% but not all their densities were increased (- 12 - + 32%). In conclusion, the Hfq expression optimization can increase cell growth rate and probably their cell densities as well. Since the hfq gene is highly conserved across bacterial species, the same strategy could be applied to other bacterial species to construct faster-growing strains.

Coupled Transcription-Translation in Prokaryotes: An Old Couple With New Surprises

Coupled transcription-translation (CTT) is a hallmark of prokaryotic gene expression. CTT occurs when ribosomes associate with and initiate translation of mRNAs whose transcription has not yet concluded, therefore forming "RNAP.mRNA.ribosome" complexes. CTT is a well-documented phenomenon that is involved in important gene regulation processes, such as attenuation and operon polarity. Despite the progress in our understanding of the cellular signals that coordinate CTT, certain aspects of its molecular architecture remain controversial. Additionally, new information on the spatial segregation between the transcriptional and the translational machineries in certain species, and on the capability of certain mRNAs to localize translation-independently, questions the unanimous occurrence of CTT. Furthermore, studies where transcription and translation were artificially uncoupled showed that transcription elongation can proceed in a translation-independent manner. Here, we review studies supporting the occurrence of CTT and findings questioning its extent, as well as discuss mechanisms that may explain both coupling and uncoupling, e.g., chromosome relocation and the involvement of cis- or trans-acting elements, such as small RNAs and RNA-binding proteins. These mechanisms impact RNA localization, stability, and translation. Understanding the two options by which genes can be expressed and their consequences should shed light on a new layer of control of bacterial transcripts fate.

Identification and characterization of the Hfq bacterial amyloid region DNA interactions

Nucleic acid amyloid proteins interactions have been observed in the past few years. These interactions often promote protein aggregation. Nevertheless, molecular basis and physiological consequences of these interactions are still poorly understood. Additionally, it is unknown whether the nucleic acid promotes the formation of self-assembly due to direct interactions or indirectly via sequences surrounding the amyloid region. Here we focus our attention on a bacterial amyloid, Hfq. This protein is a pleiotropic bacterial regulator that mediates many aspects of nucleic acids metabolism. The protein notably mediates mRNA stability and translation efficiency by using stress-related small non coding regulatory RNA. In addition, Hfq, thanks to its amyloid C-terminal region, binds and compacts DNA. A combination of experimental methodologies, including synchrotron radiation circular dichroism (SRCD), gel shift assay and infrared (FTIR) spectroscopy have been used to probe the interaction of Hfq C-terminal region with DNA. We clearly identify important amino acids in this region involved in DNA binding and polymerization properties. This allows to understand better how this bacterial amyloid interacts with DNA. Possible functional consequence to answer to stresses are discussed.

Hfq and RNase R Mediate rRNA Processing and Degradation in a Novel RNA Quality Control Process

RNA quality control pathways are critical for cell survival. Here, we describe a new surveillance process involved in the degradation of highly structured and stable ribosomal RNAs. The results demonstrated that the RNA chaperone Hfq and the 3'-5' exoribonuclease R mediate the elimination of detrimental rRNA fragments and are required for the correct processing of rRNA precursors. Escherichia coli cells lacking both Hfq and RNase R accumulate a high level of 16S- and 23S-derived rRNA fragments. Hfq and RNase R were also shown to participate in the maturation of 16S and 23S rRNA precursors. This correlates with the fact that in the absence of Hfq and RNase R, there are severe ribosome assembly defects and a sharp reduction in 70S ribosome levels. Hfq and RNase R may act independently or in a complex, as protein interaction studies revealed that these RNA-binding proteins can associate. This is the first demonstration that the well-conserved Hfq and RNase R proteins act on common regulatory pathways, unraveling previously unknown mechanisms of rRNA surveillance with important consequences for translation and cell survival.IMPORTANCE Quality control pathways that oversee the quality of stable RNA molecules are critical for the cell. In this work, we demonstrate, for the first time, a functional link between Hfq and RNase R in the processing and degradation of the highly structured rRNAs. These RNA-binding proteins are required for the maturation of 16S and 23S rRNAs and correct ribosome assembly. Furthermore, they participate in the degradation of rRNAs and clearance of toxic rRNA fragments from the cell. Our studies have also shown that Hfq and RNase R can form a complex. In summary, the cooperation between Hfq and RNase R in metabolic pathways of stable RNAs may represent a broader mechanism of RNA quality control, given the high conservation of these RNA-binding proteins throughout evolution.

Regulatory interplay between small RNAs and transcription termination factor Rho

The largest and best studied group of regulatory small RNAs (sRNAs) in bacteria act by modulating translation or turnover of messenger RNAs (mRNAs) through base-pairing interactions that typically take place near the 5' end of the mRNA. This allows the sRNA to bind the complementary target sequence while the remainder of the mRNA is still being made, creating conditions whereby the action of the sRNA can extend to transcriptional steps, most notably transcription termination. Increasing evidence corroborates the existence of a functional interplay between sRNAs and termination factor Rho. Two general mechanisms have emerged. One mechanism operates in translated regions subjected to sRNA repression. By inhibiting ribosome binding co-transcriptionally, the sRNA uncouples translation from transcription, allowing Rho to bind the nascent RNA and promote termination. In the second mechanism, which functions in 5' untranslated regions, the sRNA antagonizes termination directly by interfering with Rho binding to the RNA or the subsequent translocation along the RNA. Here, we review the above literature in the context of other mechanisms that underlie the participation of Rho-dependent transcription termination in gene regulation. This article is part of a Special Issue entitled: RNA and gene control in bacteria edited by Dr. M. Guillier and F. Repoila.

Bacterial growth physiology and RNA metabolism

Bacteria are sophisticated systems with high capacity and flexibility to adapt to various environmental conditions. Each prokaryote however possesses a defined metabolic network, which sets its overall metabolic capacity, and therefore the maximal growth rate that can be reached. To achieve optimal growth, bacteria adopt various molecular strategies to optimally adjust gene expression and optimize resource allocation according to the nutrient availability. The resulting physiological changes are often accompanied by changes in the growth rate, and by global regulation of gene expression. The growth-rate-dependent variation of the abundances in the cellular machineries, together with condition-specific regulatory mechanisms, affect RNA metabolism and fate and pose a challenge for rational gene expression reengineering of synthetic circuits. This article is part of a Special Issue entitled: RNA and gene control in bacteria, edited by Dr. M. Guillier and F. Repoila.

Identification of protein-protein and ribonucleoprotein complexes containing Hfq

Hfq is a RNA-binding protein that plays a pivotal role in the control of gene expression in bacteria by stabilizing sRNAs and facilitating their pairing with multiple target mRNAs. It has already been shown that Hfq, directly or indirectly, interacts with many proteins: RNase E, Rho, poly(A)polymerase, RNA polymerase… In order to detect more Hfq-related protein-protein interactions we have used two approaches, TAP-tag combined with RNase A treatment to access the role of RNA in these complexes, and protein-protein crosslinking, which freezes protein-protein complexes formed in vivo. In addition, we have performed microscale thermophoresis to evaluate the role of RNA in some of the complexes detected and used far-western blotting to confirm some protein-protein interactions. Taken together, the results show unambiguously a direct interaction between Hfq and EF-Tu. However a very large number of the interactions of proteins with Hfq in E. coli involve RNAs. These RNAs together with the interacting protein, may play an active role in the formation of Hfq-containing complexes with previously unforeseen implications for the riboregulatory functions of Hfq.

Mechanisms for Hfq-Independent Activation of rpoS by DsrA, a Small RNA, in Escherichia coli

Many small RNAs (sRNAs) regulate gene expression by base pairing to their target messenger RNAs (mRNAs) with the help of Hfq in Escherichia coli. The sRNA DsrA activates translation of the rpoS mRNA in an Hfq-dependent manner, but this activation ability was found to partially bypass Hfq when DsrA is overproduced. The precise mechanism by which DsrA bypasses Hfq is unknown. In this study, we constructed strains lacking all three rpoS-activating sRNAs (i.e., ArcZ, DsrA, and RprA) in hfq+ and Hfq- backgrounds, and then artificially regulated the cellular DsrA concentration in these strains by controlling its ectopic expression. We then examined how the expression level of rpoS was altered by a change in the concentration of DsrA. We found that the translation and stability of the rpoS mRNA are both enhanced by physiological concentrations of DsrA regardless of Hfq, but that depletion of Hfq causes a rapid degradation of DsrA and thereby decreases rpoS mRNA stability. These results suggest that the observed Hfq dependency of DsrA-mediated rpoS activation mainly results from the destabilization of DsrA in the absence of Hfq, and that DsrA itself contributes to the translational activation and stability of the rpoS mRNA in an Hfq-independent manner.

New molecular interactions broaden the functions of the RNA chaperone Hfq

The RNA chaperone Hfq is an important bacterial post-transcriptional regulator. Most studies on Hfq are focused on the role of this protein on small non-coding RNAs (sRNAs) and messenger RNAs (mRNAs). The most well-characterized function of Hfq is its role as RNA matchmaker, promoting the base-pairing between sRNAs and their mRNA targets. However, novel substrates and previous unrecognized functions of Hfq have now been identified, which expanded the regulatory spectrum of this protein. Hfq was recently found to bind rRNA and act as a new ribosome biogenesis factor, affecting rRNA processing, ribosome assembly, translational efficiency and translational fidelity. Hfq was also found to bind tRNAs, which could provide an additional mechanism for its role on the accuracy of protein synthesis. The list of substrates does not include RNA exclusively since Hfq was shown to bind DNA, playing an important role in DNA compaction. Additionally, Hfq is also capable to establish many protein-protein interactions. Overall, the functions of the RNA-binding protein Hfq have been expanded beyond its function in small RNA-mediated regulation. The identification of additional substrates and new functions provides alternative explanations for the importance of the chaperone Hfq as a global regulator.

Producing Hfq/Sm Proteins and sRNAs for Structural and Biophysical Studies of Ribonucleoprotein Assembly

Hfq is a bacterial RNA-binding protein that plays key roles in the post-transcriptional regulation of gene expression. Like other Sm proteins, Hfq assembles into toroidal discs that bind RNAs with varying affinities and degrees of sequence specificity. By simultaneously binding to a regulatory small RNA (sRNA) and an mRNA target, Hfq hexamers facilitate productive RNA∙∙∙RNA interactions; the generic nature of this chaperone-like functionality makes Hfq a hub in many sRNA-based regulatory networks. That Hfq is crucial in diverse cellular pathways-including stress response, quorum sensing, and biofilm formation-has motivated genetic and "RNAomic" studies of its function and physiology (in vivo), as well as biochemical and structural analyses of Hfq∙∙∙RNA interactions (in vitro). Indeed, crystallographic and biophysical studies first established Hfq as a member of the phylogenetically conserved Sm superfamily. Crystallography and other biophysical methodologies enable the RNA-binding properties of Hfq to be elucidated in atomic detail, but such approaches have stringent sample requirements, viz.: reconstituting and characterizing an Hfq·RNA complex requires ample quantities of well-behaved (sufficient purity, homogeneity) specimens of Hfq and RNA (sRNA, mRNA fragments, short oligoribonucleotides, or even single nucleotides). The production of such materials is covered in this chapter, with a particular focus on recombinant Hfq proteins for crystallization experiments.

RNA polyadenylation and its consequences in prokaryotes

Post-transcriptional addition of poly(A) tails to the 3' end of RNA is one of the fundamental events controlling the functionality and fate of RNA in all kingdoms of life. Although an enzyme with poly(A)-adding activity was discovered in Escherichia coli more than 50 years ago, its existence and role in prokaryotic RNA metabolism were neglected for many years. As a result, it was not until 1992 that E. coli poly(A) polymerase I was purified to homogeneity and its gene was finally identified. Further work revealed that, similar to its role in surveillance of aberrant nuclear RNAs of eukaryotes, the addition of poly(A) tails often destabilizes prokaryotic RNAs and their decay intermediates, thus facilitating RNA turnover. Moreover, numerous studies carried out over the last three decades have shown that polyadenylation greatly contributes to the control of prokaryotic gene expression by affecting the steady-state level of diverse protein-coding and non-coding transcripts including antisense RNAs involved in plasmid copy number control, expression of toxin-antitoxin systems and bacteriophage development. Here, we review the main findings related to the discovery of polyadenylation in prokaryotes, isolation, and characterization and regulation of bacterial poly(A)-adding activities, and discuss the impact of polyadenylation on prokaryotic mRNA metabolism and gene expression.This article is part of the theme issue '5' and 3' modifications controlling RNA degradation'.

Hfq chaperone brings speed dating to bacterial sRNA

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

Translational regulation by bacterial small RNAs via an unusual Hfq-dependent mechanism

In bacteria, the canonical mechanism of translational repression by small RNAs (sRNAs) involves sRNA-mRNA base pairing that occludes the ribosome binding site (RBS), directly preventing translation. In this mechanism, the sRNA is the direct regulator, while the RNA chaperone Hfq plays a supporting role by stabilizing the sRNA. There are a few examples where the sRNA does not directly interfere with ribosome binding, yet translation of the target mRNA is still inhibited. Mechanistically, this non-canonical regulation by sRNAs is poorly understood. Our previous work demonstrated repression of the mannose transporter manX mRNA by the sRNA SgrS, but the regulatory mechanism was unknown. Here, we report that manX translation is controlled by a molecular role-reversal mechanism where Hfq, not the sRNA, is the direct repressor. Hfq binding adjacent to the manX RBS is required for sRNA-mediated translational repression. Translation of manX is also regulated by another sRNA, DicF, via the same non-canonical Hfq-dependent mechanism. Our results suggest that the sRNAs recruit Hfq to its binding site or stabilize the mRNA-Hfq complex. This work adds to the growing number of examples of diverse mechanisms of translational regulation by sRNAs in bacteria.

Modeling the therapeutic efficacy of NFκB synthetic decoy oligodeoxynucleotides (ODNs)

BACKGROUND

Transfection of NF κB synthetic decoy Oligodeoxynucleotides (ODNs) has been proposed as a promising therapeutic strategy for a variety of diseases arising from constitutive activation of the eukaryotic transcription factor NF κB. The decoy approach faces some limitations under physiological conditions notably nuclease-induced degradation.

RESULTS

In this work, we show how a systems pharmacology model of NF κB regulatory networks displaying oscillatory temporal dynamics, can be used to predict quantitatively the dependence of therapeutic efficacy of NF κB synthetic decoy ODNs on dose, unbinding kinetic rates and nuclease-induced degradation rates. Both deterministic mass action simulations and stochastic simulations of the systems biology model show that the therapeutic efficacy of synthetic decoy ODNs is inversely correlated with unbinding kinetic rates, nuclease-induced degradation rates and molecular stripping rates, but is positively correlated with dose. We show that the temporal coherence of the stochastic dynamics of NF κB regulatory networks is most sensitive to adding NF κB synthetic decoy ODNs having unbinding time-scales that are in-resonance with the time-scale of the limit cycle of the network.

CONCLUSIONS

The pharmacokinetics/pharmacodynamics (PK/PD) predicted by the systems-level model should provide quantitative guidance for in-depth translational research of optimizing the thermodynamics/kinetic properties of synthetic decoy ODNs.

Compaction and condensation of DNA mediated by the C-terminal domain of Hfq

Hfq is a bacterial protein that is involved in several aspects of nucleic acids metabolism. It has been described as one of the nucleoid associated proteins shaping the bacterial chromosome, although it is better known to influence translation and turnover of cellular RNAs. Here, we explore the role of Escherichia coli Hfq's C-terminal domain in the compaction of double stranded DNA. Various experimental methodologies, including fluorescence microscopy imaging of single DNA molecules confined inside nanofluidic channels, atomic force microscopy, isothermal titration microcalorimetry and electrophoretic mobility assays have been used to follow the assembly of the C-terminal and N-terminal regions of Hfq on DNA. Results highlight the role of Hfq's C-terminal arms in DNA binding, change in mechanical properties of the double helix and compaction of DNA into a condensed form. The propensity for bridging and compaction of DNA by the C-terminal domain might be related to aggregation of bound protein and may have implications for protein binding related gene regulation.

Small RNAs in Bacterial Virulence and Communication

Bacterial pathogens must endure or adapt to different environments and stresses during transmission and infection. Posttranscriptional gene expression control by regulatory RNAs, such as small RNAs and riboswitches, is now considered central to adaptation in many bacteria, including pathogens. The study of RNA-based regulation (riboregulation) in pathogenic species has provided novel insight into how these bacteria regulate virulence gene expression. It has also uncovered diverse mechanisms by which bacterial small RNAs, in general, globally control gene expression. Riboregulators as well as their targets may also prove to be alternative targets or provide new strategies for antimicrobials. In this article, we present an overview of the general mechanisms that bacteria use to regulate with RNA, focusing on examples from pathogens. In addition, we also briefly review how deep sequencing approaches have aided in opening new perspectives in small RNA identification and the study of their functions. Finally, we discuss examples of riboregulators in two model pathogens that control virulence factor expression or survival-associated phenotypes, such as stress tolerance, biofilm formation, or cell-cell communication, to illustrate how riboregulation factors into regulatory networks in bacterial pathogens.

Crystal structure and RNA-binding properties of an Hfq homolog from the deep-branching Aquificae: conservation of the lateral RNA-binding mode

The host factor Hfq, as the bacterial branch of the Sm family, is an RNA-binding protein involved in the post-transcriptional regulation of mRNA expression and turnover. Hfq facilitates pairing between small regulatory RNAs (sRNAs) and their corresponding mRNA targets by binding both RNAs and bringing them into close proximity. Hfq homologs self-assemble into homo-hexameric rings with at least two distinct surfaces that bind RNA. Recently, another binding site, dubbed the `lateral rim', has been implicated in sRNA·mRNA annealing; the RNA-binding properties of this site appear to be rather subtle, and its degree of evolutionary conservation is unknown. An Hfq homolog has been identified in the phylogenetically deep-branching thermophile Aquifex aeolicus (Aae), but little is known about the structure and function of Hfq from basal bacterial lineages such as the Aquificae. Therefore, Aae Hfq was cloned, overexpressed, purified, crystallized and biochemically characterized. Structures of Aae Hfq were determined in space groups P1 and P6, both to 1.5 Å resolution, and nanomolar-scale binding affinities for uridine- and adenosine-rich RNAs were discovered. Co-crystallization with U6 RNA reveals that the outer rim of the Aae Hfq hexamer features a well defined binding pocket that is selective for uracil. This Aae Hfq structure, combined with biochemical and biophysical characterization of the homolog, reveals deep evolutionary conservation of the lateral RNA-binding mode, and lays a foundation for further studies of Hfq-associated RNA biology in ancient bacterial phyla.

Effect of Crc and Hfq proteins on the transcription, processing, and stability of the Pseudomonas putida CrcZ sRNA

In Pseudomonas putida, the Hfq and Crc proteins regulate the expression of many genes in response to nutritional and environmental cues, by binding to mRNAs that bear specific target motifs and inhibiting their translation. The effect of these two proteins is antagonized by the CrcZ and CrcY small RNAs (sRNAs), the levels of which vary greatly according to growth conditions. The crcZ and crcY genes are transcribed from promoters PcrcZ and PcrcY, respectively, a process that relies on the CbrB transcriptional activator and the RpoN σ factor. Here we show that crcZ can also be transcribed from the promoter of the immediate upstream gene, cbrB, a weak constitutive promoter. The cbrB-crcZ transcript was processed to render a sRNA very similar in size to the CrcZ produced from promoter PcrcZ The processed sRNA, termed CrcZ*, was able to antagonize Hfq/Crc because, when provided in trans, it relieved the deregulated Hfq/Crc-dependent hyperrepressing phenotype of a ΔcrcZΔcrcY strain. CrcZ* may help in attaining basal levels of CrcZ/CrcZ* that are sufficient to protect the cell from an excessive Hfq/Crc-dependent repression. Since a functional sRNA can be produced from PcrcZ, an inducible strong promoter, or by cleavage of the cbrB-crcZ mRNA, crcZ can be considered a 3'-untranslated region of the cbrB-crcZ mRNA. In the absence of Hfq, the processed form of CrcZ was not observed. In addition, we show that Crc and Hfq increase CrcZ stability, which supports the idea that these proteins can form a complex with CrcZ and protect it from degradation by RNases.

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.

The small RNA SraG participates in PNPase homeostasis

The rpsO-pnp operon encodes ribosomal protein S15 and polynucleotide phosphorylase, a major 3'-5' exoribonuclease involved in mRNA decay in Escherichia coli The gene for the SraG small RNA is located between the coding regions of the rpsO and pnp genes, and it is transcribed in the opposite direction relative to the two genes. No function has been assigned to SraG. Multiple levels of post-transcriptional regulation have been demonstrated for the rpsO-pnp operon. Here we show that SraG is a new factor affecting pnp expression. SraG overexpression results in a reduction of pnp expression and a destabilization of pnp mRNA; in contrast, inhibition of SraG transcription results in a higher level of the pnp transcript. Furthermore, in vitro experiments indicate that SraG inhibits translation initiation of pnp Together, these observations demonstrate that SraG participates in the post-transcriptional control of pnp by a direct antisense interaction between SraG and PNPase RNAs. Our data reveal a new level of regulation in the expression of this major exoribonuclease.

The Escherichia Coli Hfq Protein: An Unattended DNA-Transactions Regulator

The Hfq protein was discovered in Escherichia coli as a host factor for bacteriophage Qβ RNA replication. Subsequent studies indicated that Hfq is a pleiotropic regulator of bacterial gene expression. The regulatory role of Hfq is ascribed mainly to its function as an RNA-chaperone, facilitating interactions between bacterial non-coding RNA and its mRNA target. Thus, it modulates mRNA translation and stability. Nevertheless, Hfq is able to interact with DNA as well. Its role in the regulation of DNA-related processes has been demonstrated. In this mini-review, it is discussed how Hfq interacts with DNA and what is the role of this protein in regulation of DNA transactions. Particularly, Hfq has been demonstrated to be involved in the control of ColE1 plasmid DNA replication, transposition, and possibly also transcription. Possible mechanisms of these Hfq-mediated regulations are described and discussed.

The RNA-Binding Chaperone Hfq Is an Important Global Regulator of Gene Expression in Pasteurella multocida and Plays a Crucial Role in Production of a Number of Virulence Factors, Including Hyaluronic Acid Capsule

The Gram-negative bacterium Pasteurella multocida is the causative agent of a number of economically important animal diseases, including avian fowl cholera. Numerous P. multocida virulence factors have been identified, including capsule, lipopolysaccharide (LPS), and filamentous hemagglutinin, but little is known about how the expression of these virulence factors is regulated. Hfq is an RNA-binding protein that facilitates riboregulation via interaction with small noncoding RNA (sRNA) molecules and their mRNA targets. Here, we show that a P. multocida hfq mutant produces significantly less hyaluronic acid capsule during all growth phases and displays reduced in vivo fitness. Transcriptional and proteomic analyses of the hfq mutant during mid-exponential-phase growth revealed altered transcript levels for 128 genes and altered protein levels for 78 proteins. Further proteomic analyses of the hfq mutant during the early exponential growth phase identified 106 proteins that were produced at altered levels. Both the transcript and protein levels for genes/proteins involved in capsule biosynthesis were reduced in the hfq mutant, as were the levels of the filamentous hemagglutinin protein PfhB2 and its secretion partner LspB2. In contrast, there were increased expression levels of three LPS biosynthesis genes, encoding proteins involved in phosphocholine and phosphoethanolamine addition to LPS, suggesting that these genes are negatively regulated by Hfq-dependent mechanisms. Taken together, these data provide the first evidence that Hfq plays a crucial role in regulating the global expression of P. multocida genes, including the regulation of key P. multocida virulence factors, capsule, LPS, and filamentous hemagglutinin.

Cloning and sequencing of hfq (host factor required for synthesis of bacteriophage Q beta RNA) gene of Salmonella Typhimurium isolated from poultry

AIM

The aim was to clone and sequence hfq gene of Salmonella Typhimurium strain PM-45 and compare its sequence with hfq gene of other serovar of Salmonella.

MATERIALS AND METHODS

Salmonella Typhimurium strain PM-45 was procured from the G. B. Pant University of Agriculture and Technology, Pantnagar, India. The genomic DNA was isolated from Salmonella Typhimurium. Hfq gene was polymerase chain reaction (PCR) amplified from the DNA using specific primers, which was subsequently cloned into pET32a vector and transformed into Escherichia coli BL21 pLys cells. The recombinant plasmid was isolated and subjected to restriction enzyme digestion as well as PCR. The clone was then sequenced. The sequence was analyzed and submitted in GenBank.

RESULTS

PCR produced an amplicon of 309 bp. Restriction digestion of the recombinant plasmid released the desired insert. The hfq sequence shows 100% homology with similar sequences from other Salmonella Typhimurium isolates. Both nucleotide and amino acid sequences are highly conserved. The submitted sequence is having Genbank accession no KM998764.

CONCLUSION

Hfq, the hexameric RNA binding protein is one of the most important post-transcriptional regulator of bacteria. The sequence of hfq gene of Salmonella Typhimurium is highly conserved within and between Salmonella enterica serovars. This gene sequence is probably under heavy selection pressure to maintain the conformational integrity of its product in spite of its being not a survival gene.

A stress-induced small RNA modulates alpha-rhizobial cell cycle progression

Mechanisms adjusting replication initiation and cell cycle progression in response to environmental conditions are crucial for microbial survival. Functional characterization of the trans-encoded small non-coding RNA (trans-sRNA) EcpR1 in the plant-symbiotic alpha-proteobacterium Sinorhizobium meliloti revealed a role of this class of riboregulators in modulation of cell cycle regulation. EcpR1 is broadly conserved in at least five families of the Rhizobiales and is predicted to form a stable structure with two defined stem-loop domains. In S. meliloti, this trans-sRNA is encoded downstream of the divK-pleD operon. ecpR1 belongs to the stringent response regulon, and its expression was induced by various stress factors and in stationary phase. Induced EcpR1 overproduction led to cell elongation and increased DNA content, while deletion of ecpR1 resulted in reduced competitiveness. Computationally predicted EcpR1 targets were enriched with cell cycle-related mRNAs. Post-transcriptional repression of the cell cycle key regulatory genes gcrA and dnaA mediated by mRNA base-pairing with the strongly conserved loop 1 of EcpR1 was experimentally confirmed by two-plasmid differential gene expression assays and compensatory changes in sRNA and mRNA. Evidence is presented for EcpR1 promoting RNase E-dependent degradation of the dnaA mRNA. We propose that EcpR1 contributes to modulation of cell cycle regulation under detrimental conditions.

Microorganisms frequently encounter adverse conditions unfavorable for cell proliferation. They have evolved diverse mechanisms, including transcriptional control and targeted protein degradation, to adjust cell cycle progression in response to environmental cues. Non-coding RNAs are widespread regulators of various cellular processes in all domains of life. In prokaryotes, trans-encoded small non-coding RNAs (trans-sRNAs) contribute to a rapid cellular response to changing environments, but so far have not been directly related to cell cycle regulation. Here, we report the first example of a trans-sRNA (EcpR1) with two experimentally confirmed targets in the core of cell cycle regulation and demonstrate that in the plant-symbiotic alpha-proteobacterium Sinorhizobium meliloti the regulatory mechanism involves base-pairing of this sRNA with the dnaA and gcrA mRNAs. Most trans-sRNAs are restricted to closely related species, but the stress-induced EcpR1 is broadly conserved in the order of Rhizobiales suggesting an evolutionary advantage conferred by ecpR1. It broadens the functional diversity of prokaryotic sRNAs and adds a new regulatory level to the mechanisms that contribute to interlinking stress responses with the cell cycle machinery.

RNA-binding proteins involved in post-transcriptional regulation in bacteria

Post-transcriptional regulation is a very important mechanism to control gene expression in changing environments. In the past decade, a lot of interest has been directed toward the role of small RNAs (sRNAs) in bacterial post-transcriptional regulation. However, sRNAs are not the only molecules controlling gene expression at this level, RNA-binding proteins (RBPs) play an important role as well. CsrA and Hfq are the two best studied bacterial proteins of this type, but recently, additional proteins involved in post-transcriptional control have been identified. This review focuses on the general working mechanisms of post-transcriptionally active RBPs, which include (i) adaptation of the susceptibility of mRNAs and sRNAs to RNases, (ii) modulating the accessibility of the ribosome binding site of mRNAs, (iii) recruiting and assisting in the interaction of mRNAs with other molecules and (iv) regulating transcription terminator/antiterminator formation, and gives an overview of both the well-studied and the newly identified proteins that are involved in post-transcriptional regulatory processes. Additionally, the post-transcriptional mechanisms by which the expression or the activity of these proteins is regulated, are described. For many of the newly identified proteins, however, mechanistic questions remain. Most likely, more post-transcriptionally active proteins will be identified in the future.

Clostridium difficile Hfq can replace Escherichia coli Hfq for most of its function

A gene for the Hfq protein is present in the majority of sequenced bacterial genomes. Its characteristic hexameric ring-like core structure is formed by the highly conserved N-terminal regions. In contrast, the C-terminal forms an extension, which varies in length, lacks homology, and is predicted to be unstructured. In Gram-negative bacteria, Hfq facilitates the pairing of sRNAs with their mRNA target and thus affects gene expression, either positively or negatively, and modulates sRNA degradation. In Gram-positive bacteria, its role is still poorly characterized. Numerous sRNAs have been detected in many Gram-positive bacteria, but it is not yet known whether these sRNAs act in association with Hfq. Compared with all other Hfqs, the C. difficile Hfq exhibits an unusual C-terminal sequence with 75% asparagine and glutamine residues, while the N-terminal core part is more conserved. To gain insight into the functionality of the C. difficile Hfq (Cd-Hfq) protein in processes regulated by sRNAs, we have tested the ability of Cd-Hfq to fulfill the functions of the E. coli Hfq (Ec-Hfq) by examining various functions associated with Hfq in both positive and negative controls of gene expression. We found that Cd-Hfq substitutes for most but not all of the tested functions of the Ec-Hfq protein. We also investigated the role of the C-terminal part of the Hfq proteins. We found that the C-terminal part of both Ec-Hfq and Cd-Hfq is not essential but contributes to some functions of both the E. coli and C. difficile chaperons.

Evolution of Escherichia coli to 42 °C and subsequent genetic engineering reveals adaptive mechanisms and novel mutations

Adaptive laboratory evolution (ALE) has emerged as a valuable method by which to investigate microbial adaptation to a desired environment. Here, we performed ALE to 42 °C of ten parallel populations of Escherichia coli K-12 MG1655 grown in glucose minimal media. Tightly controlled experimental conditions allowed selection based on exponential-phase growth rate, yielding strains that uniformly converged toward a similar phenotype along distinct genetic paths. Adapted strains possessed as few as 6 and as many as 55 mutations, and of the 144 genes that mutated in total, 14 arose independently across two or more strains. This mutational recurrence pointed to the key genetic targets underlying the evolved fitness increase. Genome engineering was used to introduce the novel ALE-acquired alleles in random combinations into the ancestral strain, and competition between these engineered strains reaffirmed the impact of the key mutations on the growth rate at 42 °C. Interestingly, most of the identified key gene targets differed significantly from those found in similar temperature adaptation studies, highlighting the sensitivity of genetic evolution to experimental conditions and ancestral genotype. Additionally, transcriptomic analysis of the ancestral and evolved strains revealed a general trend for restoration of the global expression state back toward preheat stressed levels. This restorative effect was previously documented following evolution to metabolic perturbations, and thus may represent a general feature of ALE experiments. The widespread evolved expression shifts were enabled by a comparatively scant number of regulatory mutations, providing a net fitness benefit but causing suboptimal expression levels for certain genes, such as those governing flagellar formation, which then became targets for additional ameliorating mutations. Overall, the results of this study provide insight into the adaptation process and yield lessons important for the future implementation of ALE as a tool for scientific research and engineering.

Efficient search, mapping, and optimization of multi-protein genetic systems in diverse bacteria

Developing predictive models of multi-protein genetic systems to understand and optimize their behavior remains a combinatorial challenge, particularly when measurement throughput is limited. We developed a computational approach to build predictive models and identify optimal sequences and expression levels, while circumventing combinatorial explosion. Maximally informative genetic system variants were first designed by the RBS Library Calculator, an algorithm to design sequences for efficiently searching a multi-protein expression space across a > 10,000-fold range with tailored search parameters and well-predicted translation rates. We validated the algorithm's predictions by characterizing 646 genetic system variants, encoded in plasmids and genomes, expressed in six gram-positive and gram-negative bacterial hosts. We then combined the search algorithm with system-level kinetic modeling, requiring the construction and characterization of 73 variants to build a sequence-expression-activity map (SEAMAP) for a biosynthesis pathway. Using model predictions, we designed and characterized 47 additional pathway variants to navigate its activity space, find optimal expression regions with desired activity response curves, and relieve rate-limiting steps in metabolism. Creating sequence-expression-activity maps accelerates the optimization of many protein systems and allows previous measurements to quantitatively inform future designs.

The Crc and Hfq proteins of Pseudomonas putida cooperate in catabolite repression and formation of ribonucleic acid complexes with specific target motifs

The Crc protein is a global regulator that has a key role in catabolite repression and optimization of metabolism in Pseudomonads. Crc inhibits gene expression post-transcriptionally, preventing translation of mRNAs bearing an AAnAAnAA motif [the catabolite activity (CA) motif] close to the translation start site. Although Crc was initially believed to bind RNA by itself, this idea was recently challenged by results suggesting that a protein co-purifying with Crc, presumably the Hfq protein, could account for the detected RNA-binding activity. Hfq is an abundant protein that has a central role in post-transcriptional gene regulation. Herein, we show that the Pseudomonas putida Hfq protein can recognize the CA motifs of RNAs through its distal face and that Crc facilitates formation of a more stable complex at these targets. Crc was unable to bind RNA in the absence of Hfq. However, pull-down assays showed that Crc and Hfq can form a co-complex with RNA containing a CA motif in vitro. Inactivation of the hfq or the crc gene impaired catabolite repression to a similar extent. We propose that Crc and Hfq cooperate in catabolite repression, probably through forming a stable co-complex with RNAs containing CA motifs to result in inhibition of translation initiation.

RNA binding by Hfq and ring-forming (L)Sm proteins: a trade-off between optimal sequence readout and RNA backbone conformation

The eukaryotic Sm and the Sm-like (LSm) proteins form a large family that includes LSm proteins in archaea and the Hfq proteins in bacteria. Commonly referred to as the (L)Sm protein family, the various members play important roles in RNA processing, decay, and riboregulation. Particularly interesting from a structural point of view is their ability to assemble into doughnut-shaped rings, which allows them to bind preferentially the uridine-rich 3′-end of RNA oligonucleotides. With an emphasis on Hfq, this review compares the RNA-binding properties of the various (L)Sm rings that were recently co-crystallized with RNA substrates, and it discusses how these properties relate to physiological function.

Rapid degradation of Hfq-free RyhB in Yersinia pestis by PNPase independent of putative ribonucleolytic complexes

The RNA chaperone Hfq in bacteria stabilizes sRNAs by protecting them from the attack of ribonucleases. Upon release from Hfq, sRNAs are preferably degraded by PNPase. PNPase usually forms multienzyme ribonucleolytic complexes with endoribonuclease E and/or RNA helicase RhlB to facilitate the degradation of the structured RNA. However, whether PNPase activity on Hfq-free sRNAs is associated with the assembly of RNase E or RhlB has yet to be determined. Here we examined the roles of the main endoribonucleases, exoribonucleases, and ancillary RNA-modifying enzymes in the degradation of Y. pestis RyhB in the absence of Hfq. Expectedly, the transcript levels of both RyhB1 and RyhB2 increase only after inactivating PNPase, which confirms the importance of PNPase in sRNA degradation. By contrast, the signal of RyhB becomes barely perceptible after inactivating of RNase III, which may be explained by the increase in PNPase levels resulting from the exemption of pnp mRNA from RNase III processing. No significant changes are observed in RyhB stability after deletion of either the PNPase-binding domain of RNase E or rhlB. Therefore, PNPase acts as a major enzyme of RyhB degradation independent of PNPase-containing RNase E and RhlB assembly in the absence of Hfq.

Genome-wide profiling of Hfq-binding RNAs uncovers extensive post-transcriptional rewiring of major stress response and symbiotic regulons in Sinorhizobium meliloti

The RNA chaperone Hfq is a global post-transcriptional regulator in bacteria. Here, we used RNAseq to analyze RNA populations from the legume symbiont Sinorhizobium meliloti that were co-immunoprecipitated (CoIP-RNA) with a FLAG-tagged Hfq in five growth/stress conditions. Hfq-bound transcripts (1315) were largely identified in stressed bacteria and derived from small RNAs (sRNAs), both trans-encoded (6.4%) and antisense (asRNAs; 6.3%), and mRNAs (86%). Pull-down with Hfq recovered a small proportion of annotated S. meliloti sRNAs (14% of trans-sRNAs and 2% of asRNAs) suggesting a discrete impact of this protein in sRNA pathways. Nonetheless, Hfq selectively stabilized CoIP-enriched sRNAs, anticipating that these interactions are functionally significant. Transcription of 26 Hfq-bound sRNAs was predicted to occur from promoters recognized by the major stress σ factors σ(E2) or σ(H1/2). Recovery rates of sRNAs in each of the CoIP-RNA libraries suggest a large impact of Hfq-assisted riboregulation in S. meliloti osmoadaptation. Hfq directly targeted 18% of the predicted S. meliloti mRNAs, which encode functionally diverse proteins involved in transport and metabolism, σ(E2)-dependent stress responses, quorum sensing, flagella biosynthesis, ribosome, and membrane assembly or symbiotic nitrogen fixation. Canonical targeting of the 5' regions of two of the ABC transporter mRNAs by the homologous Hfq-binding AbcR1 and AbcR2 sRNAs leading to inhibition of protein synthesis was confirmed in vivo. We therefore provide a comprehensive resource for the systems-level deciphering of hitherto unexplored S. meliloti stress and symbiotic post-transcriptional regulons and the identification of Hfq-dependent sRNA-mRNA regulatory pairs.

Impact of Hfq on global gene expression and intracellular survival in Brucella melitensis

Brucella melitensis is a facultative intracellular bacterium that replicates within macrophages. The ability of brucellae to survive and multiply in the hostile environment of host macrophages is essential to its virulence. The RNA-binding protein Hfq is a global regulator that is involved in stress resistance and pathogenicity. Here we demonstrate that Hfq is essential for stress adaptation and intracellular survival in B. melitensis. A B. melitensis hfq deletion mutant exhibits reduced survival under environmental stresses and is attenuated in cultured macrophages and mice. Microarray-based transcriptome analyses revealed that 359 genes involved in numerous cellular processes were dysregulated in the hfq mutant. From these same samples the proteins were also prepared for proteomic analysis to directly identify Hfq-regulated proteins. Fifty-five proteins with significantly affected expression were identified in the hfq mutant. Our results demonstrate that Hfq regulates many genes and/or proteins involved in metabolism, virulence, and stress responses, including those potentially involved in the adaptation of Brucella to the oxidative, acid, heat stress, and antibacterial peptides encountered within the host. The dysregulation of such genes and/or proteins could contribute to the attenuated hfq mutant phenotype. These findings highlight the involvement of Hfq as a key regulator of Brucella gene expression and facilitate our understanding of the role of Hfq in environmental stress adaptation and intracellular survival of B. melitensis.

Hfq binding changes the structure of Escherichia coli small noncoding RNAs OxyS and RprA, which are involved in the riboregulation of rpoS

OxyS and RprA are two small noncoding RNAs (sRNAs) that modulate the expression of rpoS, encoding an alternative sigma factor that activates transcription of multiple Escherichia coli stress-response genes. While RprA activates rpoS for translation, OxyS down-regulates the transcript. Crucially, the RNA binding protein Hfq is required for both sRNAs to function, although the specific role played by Hfq remains unclear. We have investigated RprA and OxyS interactions with Hfq using biochemical and biophysical approaches. In particular, we have obtained the molecular envelopes of the Hfq-sRNA complexes using small-angle scattering methods, which reveal key molecular details. These data indicate that Hfq does not substantially change shape upon complex formation, whereas the sRNAs do. We link the impact of Hfq binding, and the sRNA structural changes induced, to transcript stability with respect to RNase E degradation. In light of these findings, we discuss the role of Hfq in the opposing regulatory functions played by RprA and OxyS in rpoS regulation.

Modification of the RpoS network with a synthetic small RNA

Translation of the sigma factor RpoS is activated by DsrA, RprA and ArcA, three small non-coding sRNAs (sRNA) that expose the ribosome-binding site (RBS) by opening up an inhibitory loop. In the RpoS network, no sRNAs have been found to pair with the RBS, a most common sRNA target site in bacteria. Here, we generate Ribo-0, an artificial sRNA, which represses rpoS translation by pairing with the RBS. Ribo-0 bypasses the RNA chaperon Hfq but requires the RBS to be loosely blocked. Ribo-0 interacts with DsrA and reshapes the RpoS network. Specifically, in the intact RpoS network, DsrA activates rpoS translation by freeing up the RBS. In the modified RpoS network where Ribo-0 is introduced, the DsrA-caused RBS exposure facilitates Ribo-0 binding, thereby strengthening Ribo-0 inhibition. In other words, Ribo-0 changes DsrA from an activator to an accomplice for repressing rpoS translation. This work presents an artificial mechanism of rpoS regulation, reveals mutual effects of native and synthetic players and demonstrates genetic context-dependency of their functions.

The Vibrio parahaemolyticus small RNA RyhB promotes production of the siderophore vibrioferrin by stabilizing the polycistronic mRNA

High-affinity iron acquisition in Vibrio parahaemolyticus is mediated by the cognate siderophore vibrioferrin. We have previously reported that the vibrioferrin biosynthesis operon (pvsOp) is regulated at the transcriptional level by the iron-responsive repressor Fur (T. Tanabe, T. Funahashi, H. Nakao, S. Miyoshi, S. Shinoda, and S. Yamamoto, J. Bacteriol. 185:6938-6949, 2003). In this study, we identified the Fur-regulated small RNA RyhB and the RNA chaperone Hfq protein as additional regulatory proteins of vibrioferrin biosynthesis. We found that vibrioferrin production was greatly impaired in both the ryhB and hfq deletion mutants, and a TargetRNA search (http://snowwhite.wellesley.edu/targetRNA/index2.html) revealed that the 5'-untranslated region of pvsOp mRNA (pvsOp 5'-UTR) contains a potential base-pairing region required for the formation of the RyhB-pvsOp 5'-UTR duplex. An electrophoresis mobility shift assay indicated that RyhB can directly bind to the pvsOp 5'-UTR with the aid of Hfq. Rifampin chase experiments indicated that the half-life of pvsOp mRNA in the ryhB and hfq mutants was approximately 3-fold shorter than that in the parental strain, suggesting that both RyhB and Hfq are engaged in the stabilization of pvsOp mRNA. Chrome azurol S assays followed by electrophoresis mobility shift assays and rifampin chase experiments carried out for mutant strains indicated that base pairing between RyhB and the pvsOp 5'-UTR results in an increase in the stability of pvsOp mRNA, thereby leading to the promotion of vibrioferrin production. It is unprecedented that RyhB confers increased stability on a polycistronic mRNA involved in siderophore biosynthesis as a direct target.

Hfq restructures RNA-IN and RNA-OUT and facilitates antisense pairing in the Tn10/IS10 system

Hfq functions in post-transcriptional gene regulation in a wide range of bacteria, usually by promoting base-pairing of mRNAs and trans-encoded sRNAs that share partial sequence complementarity. It is less clear if Hfq is required for pairing of cis-encoded RNAs (i.e., antisense RNAs) with their target mRNAs. In the current work, we have characterized the interactions between Escherichia coli Hfq and the components of the Tn10/IS10 antisense system, RNA-IN and RNA-OUT. We show that Hfq interacts with RNA-OUT through its proximal RNA-binding surface, as is typical for Hfq and trans-encoded sRNAs. In contrast, RNA-IN binds both proximal and distal RNA-binding surfaces in Hfq with a higher affinity for the latter, as is typical for mRNA interactions in canonical sRNA-mRNA pairs. Importantly, an amino acid substitution in Hfq that interferes with RNA binding to the proximal site negatively impacts RNA-IN:OUT pairing in vitro and suppresses the ability of Hfq to negatively regulate IS10 transposition in vivo. We also show that Hfq binding to RNA-IN and RNA-OUT alters secondary structure elements in both of these RNAs and speculate that this could be important in how Hfq facilitates RNA-IN:OUT pairing. Based on the results presented here, we suggest that Hfq could be involved in regulating RNA pairing in other antisense systems, including systems encoded by other transposable elements.

Structure and RNA-binding properties of the bacterial LSm protein Hfq

Over the past years, small non-coding RNAs (sRNAs) emerged as important modulators of gene expression in bacteria. Guided by partial sequence complementarity, these sRNAs interact with target mRNAs and eventually affect transcript stability and translation. The physiological function of sRNAs depends on the protein Hfq, which binds sRNAs in the cell and promotes the interaction with their mRNA targets. This important physiological function of Hfq as a central hub of sRNA-mediated regulation made it one of the most intensely studied proteins in bacteria. Recently, a new model for sRNA binding by Hfq has been proposed that involves the direct recognition of the sRNA 3' end and interactions of the sRNA body with the lateral RNA-binding surface of Hfq. This review summarizes the current understanding of the RNA binding properties of Hfq and its (s)RNA complexes. Moreover, the implications of the new binding model for sRNA-mediated regulation are discussed.

Cycling of RNAs on Hfq

The RNA chaperone Hfq is a key player in small RNA (sRNA)-mediated regulation of target mRNAs in many bacteria. The absence of this protein causes pleiotropic phenotypes such as impaired stress regulation and, occasionally, loss of virulence. Hfq promotes rapid sRNA-target mRNA base pairing to allow for fast, adaptive responses. For this to happen, sRNAs and/or mRNAs must be bound by Hfq. However, when the intra- or extracellular environment changes, so does the intracellular RNA pool, and this, in turn, requires a correspondingly rapid change in the pool of Hfq-bound RNAs. Biochemical studies have suggested tight binding of Hfq to many RNAs, indicating very slow dissociation rates. In contrast, the changing pool of binding-competent RNAs must compete for access to this helper protein in a minute time frame (known response time for regulation). How rapid exchange of RNAs on Hfq in vivo can be reconciled with biochemically stable and very slowly dissociating Hfq-RNA complexes is the topic of this review. Several recent reports suggest that the time scale discrepancy can be resolved by an “active cycling” model: rapid exchange of RNAs on Hfq is not limited by slow intrinsic dissociation rates, but is driven by the concentration of free RNA. Thus, transient binding of competitor RNA to Hfq-RNA complexes increases cycling rates and solves the strong binding/high turnover paradox.