Depletion of the non-coding regulatory 6S RNA in E. coli causes a surprising reduction in the expression of the translation machinery

Identification and Characterization of Non-protein Coding RNA Homologs in Serratia Marcescens by Comparative Transcriptomics

The Serratia marcescens is a Gram-negative bacterium from the Enterobacteriaceae family. Recently, S. marcescens have evolved to become a versatile and opportunistic pathogen. Furthermore, this bacterium is also a multi-drug resistant pathogen exhibiting Extended-Spectrum Beta-Lactamases (ESBL) activity. This bacterium is highly associated with infections in healthcare settings and even leads to death. Hence, an advanced approach based on non-protein coding RNA (npcRNA) of S. marcescens was considered in this study to understand its regulatory roles in virulence, pathogenesis, and the differential expression of these transcripts in various growth phases of the bacterium. BLASTn search of known npcRNAs from Salmonella typhi, Escherichia coli, and Yersinia pestis against S. marcescens was performed to discover putative conserved homologous transcripts. The novelty of these putative homologous npcRNAs was verified by screening through the Rfam web tool. The target mRNA for the homologs was predicted via the TargetRNA2 webtool to understand the possible regulatory roles of these transcripts. The npcRNA homologs, which were predicted to regulate virulence target mRNA were assessed for their expression profile at different growth stages via reverse transcription PCR and the band intensity was quantitatively analysed using the Image J tool. The known npcRNA ssrS, from S. typhi showed expression in S. marcescens during three growth stages (lag, log, and stationary). Expression was observed to be high during the lag phase followed by a similarly low-level expression during the log and no expression during stationary phase. This ssrS homolog was predicted to regulate mRNA that encodes for protein FliR, which is associated with virulence. This is a preliminary study that lay the foundation for further elucidation of more virulence-associated npcRNAs that are yet to be discovered from S. marcescens, which can be useful for diagnostics and therapeutic applications.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12088-023-01160-y.

Ms1 RNA Interacts With the RNA Polymerase Core in Streptomyces coelicolor and Was Identified in Majority of Actinobacteria Using a Linguistic Gene Synteny Search

Bacteria employ small non-coding RNAs (sRNAs) to regulate gene expression. Ms1 is an sRNA that binds to the RNA polymerase (RNAP) core and affects the intracellular level of this essential enzyme. Ms1 is structurally related to 6S RNA that binds to a different form of RNAP, the holoenzyme bearing the primary sigma factor. 6S RNAs are widespread in the bacterial kingdom except for the industrially and medicinally important Actinobacteria. While Ms1 RNA was identified in Mycobacterium, it is not clear whether Ms1 RNA is present also in other Actinobacteria species. Here, using a computational search based on secondary structure similarities combined with a linguistic gene synteny approach, we identified Ms1 RNA in Streptomyces. In S. coelicolor, Ms1 RNA overlaps with the previously annotated scr3559 sRNA with an unknown function. We experimentally confirmed that Ms1 RNA/scr3559 associates with the RNAP core without the primary sigma factor HrdB in vivo. Subsequently, we applied the computational approach to other Actinobacteria and identified Ms1 RNA candidates in 824 Actinobacteria species, revealing Ms1 RNA as a widespread class of RNAP binding sRNAs, and demonstrating the ability of our multifactorial computational approach to identify weakly conserved sRNAs in evolutionarily distant genomes.

6S RNA-Dependent Susceptibility to RNA Polymerase Inhibitors

Bacterial small RNAs (sRNAs) contribute to a variety of regulatory mechanisms that modulate a wide range of pathways, including metabolism, virulence, and antibiotic resistance. We investigated the involvement of sRNAs in rifampicin resistance in the opportunistic pathogen Staphylococcus aureus. Using a competition assay with an sRNA mutant library, we identified 6S RNA as being required for protection against low concentrations of rifampicin, an RNA polymerase (RNAP) inhibitor. This effect applied to rifabutin and fidaxomicin, two other RNAP-targeting antibiotics. 6S RNA is highly conserved in bacteria, and its absence in two other major pathogens, Salmonella enterica and Clostridioides difficile, also impaired susceptibility to RNAP inhibitors. In S. aureus, 6S RNA is produced from an autonomous gene and accumulates in stationary phase. In contrast to what was reported for Escherichia coli, S. aureus 6S RNA does not appear to play a critical role in the transition from exponential to stationary phase but affects σ^B-regulated expression in prolonged stationary phase. Nevertheless, its protective effect against rifampicin is independent of alternative sigma factor σ^B activity. Our results suggest that 6S RNA helps maintain RNAP-σ^A integrity in S. aureus, which could in turn help bacteria withstand low concentrations of RNAP inhibitors.

Involvement of E. coli 6S RNA in Oxidative Stress Response

6S RNA, a small non-coding RNA present in almost all bacteria, inhibits transcription via direct binding to RNA polymerase holoenzymes. The mechanism of 6S RNA action was investigated to a large extent in E. coli, however, lack of 6S RNA (ΔssrS) was demonstrated to be unfavorable but not essential for cell survival under various growth conditions. In the present study, we revealed, for the first time, a lethal phenotype of the ΔssrS strain in the presence of high concentrations of H₂O₂. This phenotype was rescued by complementation of the ssrS gene on a plasmid. We performed comparative qRT-PCR analyses on an enlarged set of mRNAs of genes associated with the oxidative stress response, allowing us to identify four genes known to be involved in this pathway (soxS, ahpC, sodA and tpx) that had decreased mRNA levels in the ΔssrS strain. Finally, we performed comparative proteomic analyses of the wild-type and ΔssrS strains, confirming that ΔssrS bacteria have reduced levels of the proteins AhpC and Tpx involved in H₂O₂ reduction. Our findings substantiate the crucial role of the riboregulator 6S RNA for bacterial coping with extreme stresses.

Structural and Functional Insight into the Mechanism of Bacillus subtilis 6S-1 RNA Release from RNA Polymerase

Here we investigated the refolding of Bacillus subtilis 6S-1 RNA and its release from σ^A-RNA polymerase (σ^A-RNAP) in vitro using truncated and mutated 6S-1 RNA variants. Truncated 6S-1 RNAs, only consisting of the central bubble (CB) flanked by two short helical arms, can still traverse the mechanistic 6S RNA cycle in vitro despite ~10-fold reduced σ^A-RNAP affinity. This indicates that the RNA’s extended helical arms including the ‘−35′-like region are not required for basic 6S-1 RNA functionality. The role of the ‘central bubble collapse helix’ (CBCH) in pRNA-induced refolding and release of 6S-1 RNA from σ^A-RNAP was studied by stabilizing mutations. This also revealed base identities in the 5’-part of the CB (5’-CB), upstream of the pRNA transcription start site (nt 40), that impact ground state binding of 6S-1 RNA to σ^A-RNAP. Stabilization of the CBCH by the C44/45 double mutation shifted the pRNA length pattern to shorter pRNAs and, combined with a weakened P2 helix, resulted in more effective release from RNAP. We conclude that formation of the CBCH supports pRNA-induced 6S-1 RNA refolding and release. Our mutational analysis also unveiled that formation of a second short hairpin in the 3′-CB is detrimental to 6S-1 RNA release. Furthermore, an LNA mimic of a pRNA as short as 6 nt, when annealed to 6S-1 RNA, retarded the RNA’s gel mobility and interfered with σ^A-RNAP binding. This effect incrementally increased with pLNA 7- and 8-mers, suggesting that restricted conformational flexibility introduced into the 5’-CB by base pairing with pRNAs prevents 6S-1 RNA from adopting an elongated shape. Accordingly, atomic force microscopy of free 6S-1 RNA versus 6S-1:pLNA 8- and 14-mer complexes revealed that 6S-1:pRNA hybrid structures, on average, adopt a more compact structure than 6S-1 RNA alone. Overall, our findings also illustrate that the wild-type 6S-1 RNA sequence and structure ensures an optimal balance of the different functional aspects involved in the mechanistic cycle of 6S-1 RNA.

6S-Like scr3559 RNA Affects Development and Antibiotic Production in Streptomyces coelicolor

Regulatory RNAs control a number of physiological processes in bacterial cells. Here we report on a 6S-like RNA transcript (scr3559) that affects both development and antibiotic production in Streptomyces coelicolor. Its expression is enhanced during the transition to stationary phase. Strains that over-expressed the scr3559 gene region exhibited a shortened exponential growth phase in comparison with a control strain; accelerated aerial mycelium formation and spore maturation; alongside an elevated production of actinorhodin and undecylprodigiosin. These observations were supported by LC-MS analyses of other produced metabolites, including: germicidins, desferrioxamines, and coelimycin. A subsequent microarray differential analysis revealed increased expression of genes associated with the described morphological and physiological changes. Structural and functional similarities between the scr3559 transcript and 6S RNA, and its possible employment in regulating secondary metabolite production are discussed.

Gene Regulation and Transcriptomics

Borrelia (Borreliella) burgdorferi, along with closely related species, is the etiologic agent of Lyme disease. The spirochete subsists in an enzootic cycle that encompasses acquisition from a vertebrate host to a tick vector and transmission from a tick vector to a vertebrate host. To adapt to its environment and persist in each phase of its enzootic cycle, B. burgdorferi wields three systems to regulate the expression of genes: the RpoN-RpoS alternative sigma factor cascade, the Hk1/Rrp1 two-component system and its product c-di-GMP, and the stringent response mediated by Rel_Bbu and DksA. These regulatory systems respond to enzootic phase-specific signals and are controlled or fine- tuned by transcription factors, including BosR and BadR, as well as small RNAs, including DsrABb and Bb6S RNA. In addition, several other DNA-binding and RNA-binding proteins have been identified, although their functions have not all been defined. Global changes in gene expression revealed by high-throughput transcriptomic studies have elucidated various regulons, albeit technical obstacles have mostly limited this experimental approach to cultivated spirochetes. Regardless, we know that the spirochete, which carries a relatively small genome, regulates the expression of a considerable number of genes required for the transitions between the tick vector and the vertebrate host as well as the adaptation to each.

Similarities and differences between 6S RNAs from Bradyrhizobium japonicum and Sinorhizobium meliloti

6S RNA, a conserved and abundant small non-coding RNA found in most bacteria, regulates gene expression by inhibiting RNA polymerase (RNAP) holoenzyme. 6S RNAs from α-proteobacteria have been studied poorly so far. Here, we present a first in-depth analysis of 6S RNAs from two α-proteobacteria species, Bradyrhizobium japonicum and Sinorhizobium meliloti. Although both belong to the order Rhizobiales and are typical nitrogen-fixing symbionts of legumes, their 6S RNA expression profiles were found to differ: B. japonicum 6S RNA accumulated in the stationary phase, thus being reminiscent of Escherichia coli 6S RNA, whereas S. meliloti 6S RNA level peaked at the transition to the stationary phase, similarly to Rhodobacter sphaeroides 6S RNA. We demonstrated in vitro that both RNAs have hallmarks of 6S RNAs: they bind to the σ⁷⁰-type RNAP holoenzyme and serve as templates for de novo transcription of so-called product RNAs (pRNAs) ranging in length from ∼13 to 24 nucleotides, with further evidence of the synthesis of even longer pRNAs. Likewise, stably bound pRNAs were found to rearrange the 6S RNA structure to induce its dissociation from RNAP. Compared with B. japonicum 6S RNA, considerable conformational heterogeneity was observed for S. meliloti 6S RNA and its complexes with pRNAs, even though the two 6S RNAs share ∼75% sequence identity. Overall, our findings suggest that the two rhizobial 6S RNAs have diverged with respect to their regulatory impact on gene expression throughout the bacterial life cycle.

Characterization of 6S RNA in the Lyme disease spirochete

6S RNA binds to RNA polymerase and regulates gene expression, contributing to bacterial adaptation to environmental stresses. In this study, we examined the role of 6S RNA in murine infectivity and tick persistence of the Lyme disease spirochete Borrelia (Borreliella) burgdorferi. B. burgdorferi 6S RNA (Bb6S RNA) binds to RNA polymerase, is expressed independent of growth phase or nutrient stress in culture, and is processed by RNase Y. We found that rny (bb0504), the gene encoding RNase Y, is essential for B. burgdorferi growth, while ssrS, the gene encoding 6S RNA, is not essential, indicating a broader role for RNase Y activity in the spirochete. Bb6S RNA regulates expression of the ospC and dbpA genes encoding outer surface protein C and decorin binding protein A, respectively, which are lipoproteins important for host infection. The highest levels of Bb6S RNA are found when the spirochete resides in unfed nymphs. ssrS mutants lacking Bb6S RNA were compromised for infectivity by needle inoculation, but injected mice seroconverted, indicating an ability to activate the adaptive immune response. ssrS mutants were successfully acquired by larval ticks and persisted through fed nymphs. Bb6S RNA is one of the first regulatory RNAs identified in B. burgdorferi that controls the expression of lipoproteins involved in host infectivity.

Mechanisms by Which Small RNAs Affect Bacterial Activity

The oral cavity contains a distinct habitat that supports diverse bacterial flora. Recent observations have provided additional evidence that sRNAs are key regulators of bacterial physiology and pathogenesis. These sRNAs have been divided into 5 functional groups: cis-encoded RNAs, trans-encoded RNAs, RNA regulators of protein activity, bacterial CRISPR (clustered regularly interspaced short palindromic repeat) RNAs, and a novel category of miRNA-size small RNAs (msRNAs). In this review, we discuss a critical group of key commensal and opportunistic oral pathogens. In general, supragingival bacterial sRNAs function synergistically to fine-tune the regulation of cellular processes and stress responses in adaptation to environmental changes. Particularly in the cariogenic bacteria Streptococcus mutans, both the antisense vicR RNA and msRNA1657 can impede the metabolism of bacterial exopolysaccharides, prevent biofilm formation, and suppress its cariogenicity. In Enterococcus faecalis, selected sRNAs control the expression of proteins involved in diverse cellular processes and stress responses. In subgingival plaques, sRNAs from periodontal pathogens can function as novel bacterial signaling molecules that mediate bacterial-human interactions in periodontal homeostasis. In Porphyromonas gingivalis, the expression profiles of putative sRNA101 and sRNA42 were found to respond to hemin availability after hemin starvation. Regarding Aggregatibacter actinomycetemcomitans (previously Actinobacillus actinomycetemcomitans), a major periodontal pathogen associated with aggressive periodontitis, the predicted sRNAs interact with several virulence genes, including those encoding leukotoxin and cytolethal distending toxin. Furthermore, in clinical isolates, these associated RNAs could be explored not only as potential biomarkers for oral disease monitoring but also as alternative types of regulators for drug design. Thus, this emerging subspecialty of bacterial regulatory RNAs could reshape our understanding of bacterial gene regulation from their key roles of endogenous regulatory RNAs to their activities in pathologic processes.

Triggering the stringent response: signals responsible for activating (p)ppGpp synthesis in bacteria

The stringent response is a conserved bacterial stress response mechanism that allows bacteria to respond to nutritional challenges. It is mediated by the alarmones pppGpp and ppGpp, nucleotides that are synthesized and hydrolyzed by members of the RSH superfamily. Whilst there are key differences in the binding targets for (p)ppGpp between Gram-negative and Gram-positive bacterial species, the transient accumulation of (p)ppGpp caused by nutritional stresses results in a global change in gene expression in all species. The RSH superfamily of enzymes is ubiquitous throughout the bacterial kingdom, and can be split into three main groups: the long-RSH enzymes; the small alarmone synthetases (SAS); and the small alarmone hydrolases (SAH). Despite the prevalence of these enzymes, there are important differences in the way in which they are regulated on a transcriptional and post-translational level. Here we provide an overview of the diverse regulatory mechanisms that are involved in governing this crucial signalling network. Understanding how the RSH superfamily members are regulated gives insights into the varied important biological roles for this signalling pathway across the bacteria.

Small RNAs Involved in Regulation of Nitrogen Metabolism

Global (metabolic) regulatory networks allow microorganisms to survive periods of nitrogen starvation or general nutrient stress. Uptake and utilization of various nitrogen sources are thus commonly tightly regulated in Prokarya (Bacteria and Archaea) in response to available nitrogen sources. Those well-studied regulations occur mainly at the transcriptional and posttranslational level. Surprisingly, and in contrast to their involvement in most other stress responses, small RNAs (sRNAs) involved in the response to environmental nitrogen fluctuations are only rarely reported. In addition to sRNAs indirectly affecting nitrogen metabolism, only recently it was demonstrated that three sRNAs were directly involved in regulation of nitrogen metabolism in response to changes in available nitrogen sources. All three trans-acting sRNAs are under direct transcriptional control of global nitrogen regulators and affect expression of components of nitrogen metabolism (glutamine synthetase, nitrogenase, and PII-like proteins) by either masking the ribosome binding site and thus inhibiting translation initiation or stabilizing the respective target mRNAs. Most likely, there are many more sRNAs and other types of noncoding RNAs, e.g., riboswitches, involved in the regulation of nitrogen metabolism in Prokarya that remain to be uncovered. The present review summarizes the current knowledge on sRNAs involved in nitrogen metabolism and their biological functions and targets.

Ms1 RNA increases the amount of RNA polymerase in Mycobacterium smegmatis

Ms1 is a sRNA recently found in mycobacteria and several other actinobacterial species. Ms1 interacts with the RNA polymerase (RNAP) core devoid of sigma factors, which differs from 6S RNA that binds to RNAP holoenzymes containing the primary sigma factor. Here we show that Ms1 is the most abundant non-rRNA transcript in stationary phase in Mycobacterium smegmatis. The accumulation of Ms1 stems from its high-level synthesis combined with decreased degradation. We identify the Ms1 promoter, P_Ms1 , and cis-acting elements important for its activity. Furthermore, we demonstrate that PNPase (an RNase) contributes to the differential accumulation of Ms1 during growth. Then, by comparing the transcriptomes of wt and ΔMs1 strains from stationary phase, we reveal that Ms1 affects the intracellular level of RNAP. The absence of Ms1 results in decreased levels of the mRNAs encoding β and β' subunits of RNAP, which is also reflected at the protein level. Thus, the ΔMs1 strain has a smaller pool of RNAPs available when the transcriptional demand increases. This contributes to the inability of the ΔMs1 strain to rapidly react to environmental changes during outgrowth from stationary phase.

Regulation of Global Transcription in Escherichia coli by Rsd and 6S RNA

In Escherichia coli, the sigma factor σ⁷⁰ directs RNA polymerase to transcribe growth-related genes, while σ³⁸ directs transcription of stress response genes during stationary phase. Two molecules hypothesized to regulate RNA polymerase are the protein Rsd, which binds to σ⁷⁰, and the non-coding 6S RNA which binds to the RNA polymerase-σ⁷⁰ holoenzyme. Despite multiple studies, the functions of Rsd and 6S RNA remain controversial. Here we use RNA-Seq in five phases of growth to elucidate their function on a genome-wide scale. We show that Rsd and 6S RNA facilitate σ³⁸ activity throughout bacterial growth, while 6S RNA also regulates widely different genes depending upon growth phase. We discover novel interactions between 6S RNA and Rsd and show widespread expression changes in a strain lacking both regulators. Finally, we present a mathematical model of transcription which highlights the crosstalk between Rsd and 6S RNA as a crucial factor in controlling sigma factor competition and global gene expression.

6S RNA, a Global Regulator of Transcription

6S RNA is a small RNA regulator of RNA polymerase (RNAP) that is present broadly throughout the bacterial kingdom. Initial functional studies in Escherichia coli revealed that 6S RNA forms a complex with RNAP resulting in regulation of transcription, and cells lacking 6S RNA have altered survival phenotypes. The last decade has focused on deepening the understanding of several aspects of 6S RNA activity, including (i) addressing questions of how broadly conserved 6S RNAs are in diverse organisms through continued identification and initial characterization of divergent 6S RNAs; (ii) the nature of the 6S RNA-RNAP interaction through examination of variant proteins and mutant RNAs, cross-linking approaches, and ultimately a cryo-electron microscopic structure; (iii) the physiological consequences of 6S RNA function through identification of the 6S RNA regulon and promoter features that determine 6S RNA sensitivity; and (iv) the mechanism and cellular impact of 6S RNA-directed synthesis of product RNAs (i.e., pRNA synthesis). Much has been learned about this unusual RNA, its mechanism of action, and how it is regulated; yet much still remains to be investigated, especially regarding potential differences in behavior of 6S RNAs in diverse bacteria.

c-di-GMP Regulates Various Phenotypes and Insecticidal Activity of Gram-Positive Bacillus thuringiensis

C-di-GMP has been well investigated to play significant roles in the physiology of many Gram-negative bacteria. However, its effect on Gram-positive bacteria is less known. In order to more understand the c-di-GMP functions in Gram-positive bacteria, we have carried out a detailed study on the c-di-GMP-metabolizing enzymes and their physiological functions in Bacillus thuringiensis, a Gram-positive entomopathogenic bacterium that has been applied as an insecticide successfully. We performed a systematic study on the ten putative c-di-GMP-synthesizing enzyme diguanylate cyclases (DGCs) and c-di-GMP-degrading enzyme phosphodiesterases (PDEs) in B. thuringiensis BMB171, and artificially elevated the intracellular c-di-GMP level in BMB171 by deleting one or more pde genes. We found increasing level of intracellular c-di-GMP exhibits similar activities as those in Gram-negative bacteria, including altered activities in cell motility, biofilm formation, and cell-cell aggregation. Unexpectedly, we additionally found a novel function exhibited by the increasing level of c-di-GMP to promote the insecticidal activity of this bacterium against Helicoverpa armigera. Through whole-genome transcriptome profile analyses, we found that 4.3% of the B. thuringiensis genes were differentially transcribed when c-di-GMP level was increased, and 77.3% of such gene products are involved in some regulatory pathways not reported in other bacteria to date. In summary, our study represents the first comprehensive report on the c-di-GMP-metabolizing enzymes, their effects on phenotypes, and the transcriptome mediated by c-di-GMP in an important Gram-positive bacterium.

6S RNA plays a role in recovery from nitrogen depletion in Synechocystis sp. PCC 6803

Background

The 6S RNA is a global transcriptional riboregulator, which is exceptionally widespread among most bacterial phyla. While its role is well-characterized in some heterotrophic bacteria, we subjected a cyanobacterial homolog to functional analysis, thereby extending the scope of 6S RNA action to the special challenges of photoautotrophic lifestyles.

Results

Physiological characterization of a 6S RNA deletion strain (ΔssaA) demonstrates a delay in the recovery from nitrogen starvation. Significantly decelerated phycobilisome reassembly and glycogen degradation are accompanied with reduced photosynthetic activity compared to the wild type. Transcriptome profiling further revealed that predominantly genes encoding photosystem components, ATP synthase, phycobilisomes and ribosomal proteins were negatively affected in ΔssaA. In vivo pull-down studies of the RNA polymerase complex indicated that the presence of 6S RNA promotes the recruitment of the cyanobacterial housekeeping σ factor SigA, concurrently supporting dissociation of group 2 σ factors during recovery from nitrogen starvation.

Conclusions

The combination of genetic, physiological and biochemical studies reveals the homologue of 6S RNA as an integral part of the cellular response of Synechocystis sp. PCC 6803 to changing nitrogen availability. According to these results, 6S RNA supports a rapid acclimation to changing nitrogen supply by accelerating the switch from group 2 σ factors SigB, SigC and SigE to SigA-dependent transcription. We therefore introduce the cyanobacterial 6S RNA as a novel candidate regulator of RNA polymerase sigma factor recruitment in Synechocystis sp. PCC 6803. Further studies on mechanistic features of the postulated interaction should shed additional light on the complexity of transcriptional regulation in cyanobacteria.

Electronic supplementary material

The online version of this article (10.1186/s12866-017-1137-9) contains supplementary material, which is available to authorized users.

6S RNA in Rhodobacter sphaeroides: 6S RNA and pRNA transcript levels peak in late exponential phase and gene deletion causes a high salt stress phenotype

The function of 6S RNA, a global regulator of transcription, was studied in the photosynthetic α-proteobacterium Rhodobacter sphaeroides. The cellular levels of R. sphaeroides 6S RNA peak toward the transition to stationary phase and strongly decrease during extended stationary phase. The synthesis of so-called product RNA transcripts (mainly 12-16-mers) on 6S RNA as template by RNA polymerase was found to be highest in late exponential phase. Product RNA ≥ 13-mers are expected to trigger the dissociation of 6S RNA:RNA polymerase complexes. A 6S RNA deletion in R. sphaeroides had no impact on growth under various metabolic and oxidative stress conditions (with the possible exception of tert-butyl hydroperoxide stress). However, the 6S RNA knockout resulted in a robust growth defect under high salt stress (0.25 M NaCl). Remarkably, the sspA gene encoding the putative salt stress-induced membrane protein SspA and located immediately downstream of the 6S RNA (ssrS) gene on the antisense strand was expressed at elevated levels in the ΔssrS strain when grown in the presence of 250 mM NaCl.

Overexpression of two stress-responsive, small, non-coding RNAs, 6S and tmRNA, imparts butanol tolerance in Clostridium acetobutylicum

While extensively studied in several model organisms, the role of small, non-coding RNAs in the stress response remains largely unexplored in Clostridium organisms. About 100 years after the first industrial Acetone-Butanol-Ethanol fermentation process, based on the Weizmann Clostridium acetobutylicum strain, strain tolerance to butanol remains a crucial factor limiting the economics of the process. Several studies have examined the response of this organism to metabolite stress, and several genes have been engaged to impart enhanced tolerance, but no sRNAs have yet been directly engaged in this task. We show that the two stress-responsive sRNAs, 6S and tmRNA, upon overexpression impart tolerance to butanol as assessed by viability assays under process-relevant conditions. 6S overexpression enhances cell densities as well as butanol titres. We discuss the likely mechanisms that these two sRNAs might engage in this tolerance phenotype. Our data support the continued exploration of sRNAs as a basis for engineering enhanced tolerance and enhanced solvent production, especially because sRNA-based strategies impose a minimal metabolic burden on the cells.

Maturation of 6S regulatory RNA to a highly elongated structure

As bacterial populations leave the exponential growth phase and enter the stationary phase, their patterns of gene expression undergo marked changes. A key effector of this change is 6S RNA, which is a highly conserved regulatory RNA that impedes the transcription of genes associated with exponential growth by forming an inactivating ternary complex with RNA polymerase and sigma factor σ(70) (σ(70)-RNAP). In Escherichia coli, the endoribonuclease RNase E generates 6S RNA by specific cleavage of a precursor that is nearly twice the size of the 58 kDa mature form. We have explored recognition of the precursor by RNase E, and observed that processing is inhibited under conditions of excess substrate. This finding supports a largely distributive mechanism, meaning that each round of catalysis results in enzyme dissociation and re-binding to the substrate. We show that the precursor molecule and the mature 6S share a structural core dominated by an A-type helix, indicating that processing is not accompanied by extensive refolding. Both precursor and mature forms of 6S have a highly stable secondary structure, adopt an elongated shape, and show the potential to form dimers under specific conditions; nonetheless, 6S has a high structural plasticity that probably enables it to be structurally adapted upon binding to its cognate protein partners. Analysis of the 6S-σ(70)-RNAP complex by native mass spectrometry reveals a stable association with a stoichiometry of 1:1:1. A theoretical 3D model of mature 6S is presented, which is consistent with the experimental data and supports a previously proposed structure with a small stem-loop inside the central bubble.

Dissemination of 6S RNA among bacteria

6S RNA is a highly abundant small non-coding RNA widely spread among diverse bacterial groups. By competing with DNA promoters for binding to RNA polymerase (RNAP), the RNA regulates transcription on a global scale. RNAP produces small product RNAs derived from 6S RNA as template, which rearranges the 6S RNA structure leading to dissociation of 6S RNA:RNAP complexes. Although 6S RNA has been experimentally analysed in detail for some species, such as Escherichia coli and Bacillus subtilis, and was computationally predicted in many diverse bacteria, a complete and up-to-date overview of the distribution among all bacteria is missing. In this study we searched with new methods for 6S RNA genes in all currently available bacterial genomes. We ended up with a set of 1,750 6S RNA genes, of which 1,367 are novel and bona fide, distributed among 1,610 bacteria, and had a few tentative candidates among the remaining 510 assembled bacterial genomes accessible. We were able to confirm two tentative candidates by Northern blot analysis. We extended 6S RNA genes of the Flavobacteriia significantly in length compared to the present Rfam entry. We describe multiple homologs of 6S RNAs (including split 6S RNA genes) and performed a detailed synteny analysis.

Phenotypic characterization and complementation analysis of Bacillus subtilis 6S RNA single and double deletion mutants

6S RNA, a global regulator of transcription in bacteria, binds to housekeeping RNA polymerase (RNAP) holoenzymes to competitively inhibit transcription from DNA promoters. Bacillus subtilis encodes two 6S RNA homologs whose differential functions are as yet unclear. We constructed derivative strains of B. subtilis PY79 lacking 6S-1 RNA (ΔbsrA), 6S-2 RNA (ΔbsrB) or both (ΔbsrAB) to study the physiological role of the two 6S RNAs. We observed two growth phenotypes of mutant strains: (i) accelerated decrease of optical density toward extended stationary phase and (ii) faster outgrowth from stationary phase under alkaline stress conditions (pH 9.8). The first phenotype was observed for bacteria lacking bsrA, and even more pronounced for ΔbsrAB bacteria, but not for those lacking bsrB. The magnitude of the second phenotype was relatively weak for ΔbsrB, moderate for ΔbsrA and again strongest for ΔbsrAB bacteria. Whereas ΔbsrAB bacteria complemented with bsrB or bsrA (strains ΔbsrAB + B and ΔbsrAB + A) mimicked the phenotypes of the ΔbsrA and ΔbsrB strains, respectively, complementation with the gene ssrS encoding Escherichia coli 6S RNA failed to cure the "low stationary optical density" phenotype of the double mutant, despite ssrS expression, in line with previous findings. Finally, proteomics (two-dimensional differential gel electrophoresis, 2D-DIGE) of B. subtilis 6S RNA deletion strains unveiled a set of proteins that were expressed at higher levels particularly during exponential growth and preferentially in mutant strains lacking 6S-2 RNA. Several of these proteins are involved in metabolism and stress responses.

NADPH-dependent reductive biotransformation with Escherichia coli and its pfkA deletion mutant: influence on global gene expression and role of oxygen supply

An Escherichia coli ΔpfkA mutant lacking the major phosphofructokinase possesses a partially cyclized pentose phosphate pathway leading to an increased NADPH per glucose ratio. This effect decreases the amount of glucose required for NADPH regeneration in reductive biotransformations, such as the conversion of methyl acetoacetate (MAA) to (R)-methyl 3-hydroxybutyrate (MHB) by an alcohol dehydrogenase from Lactobacillus brevis. Here, global transcriptional analyses were performed to study regulatory responses during reductive biotransformation. DNA microarray analysis revealed amongst other things increased expression of soxS, supporting previous results indicating that a high NADPH demand contributes to the activation of SoxR, the transcriptional activator of soxS. Furthermore, several target genes of the ArcAB two-component system showed a lower mRNA level in the reference strain than in the ΔpfkA mutant, pointing to an increased QH2 /Q ratio in the reference strain. This prompted us to analyze yields and productivities of MAA reduction to MHB under different oxygen regimes in a bioreactor. Under anaerobic conditions, the specific MHB production rates of both strains were comparable (7.4 ± 0.2 mmolMHB  h(-1)  gcdw (-1) ) and lower than under conditions of 15% dissolved oxygen, where those of the reference strain (12.8 mmol h(-1)  gcdw (-1) ) and of the ΔpfkA mutant (11.0 mmol h(-1)  gcdw (-1) ) were 73% and 49% higher. While the oxygen transfer rate (OTR) of the reference strain increased after the addition of MAA, presumably due to the oxidation of the acetate accumulated before MAA addition, the OTR of the ΔpfkA strain strongly decreased, indicating a very low respiration rate despite sufficient oxygen supply. The latter effect can likely be attributed to a restricted conversion of NADPH into NADH via the soluble transhydrogenase SthA, as the enzyme is outcompeted in the presence of MAA by the recombinant NADPH-dependent alcohol dehydrogenase. The differences in respiration rates can explain the suggested higher ArcAB activity in the reference strain.

Regulation of transcription by 6S RNAs: insights from the Escherichia coli and Bacillus subtilis model systems

Whereas, the majority of bacterial non-coding RNAs and functional RNA elements regulate post-transcriptional processes, either by interacting with other RNAs via base-pairing or through binding of small ligands (riboswitches), 6S RNAs affect transcription itself by binding to the housekeeping holoenzyme of RNA polymerase (RNAP). Remarkably, 6S RNAs serve as RNA templates for bacterial RNAP, giving rise to the de novo synthesis of short transcripts, termed pRNAs (product RNAs). Hence, 6S RNAs prompt the enzyme to act as an RNA-dependent RNA polymerase (RdRP). Synthesis of pRNAs exceeding a certain length limit (~13 nt) persistently rearrange the 6S RNA structure, which in turn, disrupts the 6S RNA:RNAP complex. This pRNA synthesis-mediated "reanimation" of sequestered RNAP molecules represents the conceivably fastest mechanism for resuming transcription in cells that enter a new exponential growth phase. The many different 6S RNAs found in a wide variety of bacteria do not share strong sequence homology but have in common a conserved rod-shaped structure with a large internal loop, termed the central bulge; this architecture mediates specific binding to the active site of RNAP. In this article, we summarize the overall state of knowledge as well as very recent findings on the structure, function, and physiological effects of 6S RNA examples from the two model organisms, Escherichia coli and Bacillus subtilis. Comparison of the presently known properties of 6S RNAs in the two organisms highlights common principles as well as diverse features.

Mechanistic comparison of Bacillus subtilis 6S-1 and 6S-2 RNAs--commonalities and differences

Bacterial 6S RNAs bind to the housekeeping RNA polymerase (σ(A)-RNAP in Bacillus subtilis) to regulate transcription in a growth phase-dependent manner. B. subtilis expresses two 6S RNAs, 6S-1 and 6S-2 RNA, with different expression profiles. We show in vitro that 6S-2 RNA shares hallmark features with 6S-1 RNA: Both (1) are able to serve as templates for pRNA transcription; (2) bind with comparable affinity to σ(A)-RNAP; (3) are able to specifically inhibit transcription from DNA promoters, and (4) can form stable 6S RNA:pRNA hybrid structures that (5) abolish binding to σ(A)-RNAP. However, pRNAs of equal length dissociate faster from 6S-2 than 6S-1 RNA, owing to the higher A,U-content of 6S-2 pRNAs. This could have two mechanistic implications: (1) Short 6S-2 pRNAs (<10 nt) dissociate faster instead of being elongated to longer pRNAs, which could make it more difficult for 6S-2 RNA-stalled RNAP molecules to escape from the sequestration; and (2) relative to 6S-1 RNA, 6S-2 pRNAs of equal length will dissociate more rapidly from 6S-2 RNA after RNAP release, which could affect pRNA turnover or the kinetics of 6S-2 RNA binding to a new RNAP molecule. As 6S-2 pRNAs have not yet been detected in vivo, we considered that cellular RNAP release from 6S-2 RNA might occur via 6S-1 RNA displacing 6S-2 RNA from the enzyme, either in the absence of pRNA transcription or upon synthesis of very short 6S-2 pRNAs (∼ 5-mers, which would escape detection by deep sequencing). However, binding competition experiments argued against these possibilities.

6S RNA: recent answers--future questions

6S RNA is a non-coding RNA, found in almost all phylogenetic branches of bacteria. Through its conserved secondary structure, resembling open DNA promoters, it binds to RNA polymerase and interferes with transcription at many promoters. That way, it functions as transcriptional regulator facilitating adaptation to stationary phase conditions. Strikingly, 6S RNA acts as template for the synthesis of small RNAs (pRNA), which trigger the disintegration of the inhibitory RNA polymerase-6S RNA complex releasing 6S RNA-dependent repression. The regulatory implications of 6S RNAs vary among different bacterial species depending on the lifestyle and specific growth conditions that they have to face. The influence of 6S RNA can be seen on many different processes including stationary growth, sporulation, light adaptation or intracellular growth of pathogenic bacteria. Recent structural and functional studies have yielded details of the interaction between E. coli 6S RNA and RNA polymerase. Genome-wide transcriptome analyses provided insight into the functional diversity of 6S RNAs. Moreover, the mechanism and physiological consequences of pRNA synthesis have been explored in several systems. A major function of 6S RNA as a guardian regulating the economic use of cellular resources under limiting conditions and stress emerges as a common perception from numerous recent studies.

Regulation of transcription from two ssrS promoters in 6S RNA biogenesis

ssrS-encoded 6S RNA is an abundant noncoding RNA that binds σ(70)-RNA polymerase and regulates expression at a subset of promoters in Escherichia coli. It is transcribed from two tandem promoters, ssrS P1 and ssrS P2. Regulation of transcription from two ssrS promoters in 6S RNA biogenesis was examined. Both P1 and P2 were growth phase-dependently regulated. Depletion of 6S RNA had no effect on growth-phase-dependent transcription from either promoter, whereas overexpression of 6S RNA increased P1 transcription and decreased P2 transcription, suggesting that transcription from P1 and P2 is subject to feedback activation and feedback inhibition, respectively. This feedback regulation disappeared in Δfis strains, supporting involvement of Fis in this process. The differential feedback regulation may provide a means for maintaining appropriate cellular concentrations of 6S RNA.

ncRNAs and thermoregulation: a view in prokaryotes and eukaryotes

During cellular stress response, a widespread inhibition of transcription and blockade of splicing and other post-transcriptional processing is detected, while certain specific genes are induced. In particular, free-living cells constantly monitor temperature. When the thermal condition changes, they activate a set of genes coding for proteins that participate in the response. Non-coding RNAs, ncRNAs, and conformational changes in specific regions of mRNAs seem also to be crucial regulators that enable the cell to adjust its physiology to environmental changes. They exert their effects following the same principles in all organisms and may affect all steps of gene expression. These ncRNAs and structural elements as related to thermal stress response in bacteria are reviewed. The resemblances to eukaryotic ncRNAs are highlighted.

6S RNA - an old issue became blue-green

6S RNA from Escherichia coli acts as a versatile transcriptional regulator by binding to the RNA polymerase and changing promoter selectivity. Although homologous 6S RNA structures exist in a wide range of bacteria, including cyanobacteria, our knowledge of 6S RNA function results almost exclusively from studies with E. coli. To test for potential structural and functional conservation, we selected four predicted cyanobacterial 6S RNAs (Synechocystis, Synechococcus, Prochlorococcus and Nostoc), which we compared with their E. coli counterpart. Temperature-gradient gel electrophoresis revealed similar thermodynamic transition profiles for all 6S RNAs, indicating basically similar secondary structures. Subtle differences in melting behaviour of the different RNAs point to minor structural variations possibly linked to differences in optimal growth temperature. Secondary structural analysis of three cyanobacterial 6S RNAs employing limited enzymic hydrolysis and in-line probing supported the predicted high degree of secondary structure conservation. Testing for functional homology we found that all cyanobacterial 6S RNAs were active in binding E. coli RNA polymerase and transcriptional inhibition, and had the ability to act as template for transcription of product RNAs (pRNAs). Deletion of the 6S RNA gene in Synechocystis did not significantly affect cell growth in liquid media but reduced fitness during growth on solid agar. While our study shows that basic 6S RNA functions are conserved in species as distantly related as E. coli and cyanobacteria, we also noted a subtle degree of divergence, which might reflect fundamental differences in transcriptional regulation and lifestyle, thus providing the first evidence for a possible physiological role in cyanobacteria.

Regulation of 6S RNA by pRNA synthesis is required for efficient recovery from stationary phase in E. coli and B. subtilis

6S RNAs function through interaction with housekeeping forms of RNA polymerase holoenzyme (Eσ⁷⁰ in Escherichia coli, Eσ^A in Bacillus subtilis). Escherichia coli 6S RNA accumulates to high levels during stationary phase, and has been shown to be released from Eσ⁷⁰ during exit from stationary phase by a process in which 6S RNA serves as a template for Eσ⁷⁰ to generate product RNAs (pRNAs). Here, we demonstrate that not only does pRNA synthesis occur, but it is an important mechanism for regulation of 6S RNA function that is required for cells to exit stationary phase efficiently in both E. coli and B. subtilis. Bacillus subtilis has two 6S RNAs, 6S-1 and 6S-2. Intriguingly, 6S-2 RNA does not direct pRNA synthesis under physiological conditions and its non-release from Eσ^A prevents efficient outgrowth in cells lacking 6S-1 RNA. The behavioral differences in the two B. subtilis RNAs clearly demonstrate that they act independently, revealing a higher than anticipated diversity in 6S RNA function globally. Overexpression of a pRNA-synthesis-defective 6S RNA in E. coli leads to decreased cell viability, suggesting pRNA synthesis-mediated regulation of 6S RNA function is important at other times of growth as well.

Regulation by small RNAs in bacteria: expanding frontiers

Research on the discovery and characterization of small, regulatory RNAs in bacteria has exploded in recent years. These sRNAs act by base pairing with target mRNAs with which they share limited or extended complementarity, or by modulating protein activity, in some cases by mimicking other nucleic acids. Mechanistic insights into how sRNAs bind mRNAs and proteins, how they compete with each other, and how they interface with ribonucleases are active areas of discovery. Current work also has begun to illuminate how sRNAs modulate expression of distinct regulons and key transcription factors, thus integrating sRNA activity into extensive regulatory networks. In addition, the application of RNA deep sequencing has led to reports of hundreds of additional sRNA candidates in a wide swath of bacterial species. Most importantly, recent studies have served to clarify the abundance of remaining questions about how, when, and why sRNA-mediated regulation is of such importance to bacterial lifestyles.

Genome-wide detection of novel regulatory RNAs in E. coli

The intergenic regions in bacterial genomes can contain regulatory leader sequences and small RNAs (sRNAs), which both serve to modulate gene expression. Computational analyses have predicted the presence of hundreds of these noncoding regulatory RNAs in Escherichia coli; however, only about 80 have been experimentally validated. By applying a deep-sequencing approach, we detected and quantified the vast majority of the previously validated regulatory elements and identified 10 new sRNAs and nine new regulatory leader sequences in the intergenic regions of E. coli. Half of the newly discovered sRNAs displayed enhanced stability in the presence of the RNA-binding protein Hfq, which is vital to the function of many of the known E. coli sRNAs. Whereas previous methods have often relied on phylogenetic conservation to identify regulatory leader sequences, only five of the newly discovered E. coli leader sequences were present in the genomes of other enteric species. For those newly identified regulatory elements having orthologs in Salmonella, evolutionary analyses showed that these regions encoded new noncoding elements rather than small, unannotated protein-coding transcripts. In addition to discovering new noncoding regulatory elements, we validated 53 sRNAs that were previously predicted but never detected and showed that the presence, within intergenic regions, of σ(70) promoters and sequences with compensatory mutations that maintain stable RNA secondary structures across related species is a good predictor of novel sRNAs.

The E. coli anti-sigma factor Rsd: studies on the specificity and regulation of its expression

Background

Among the seven different sigma factors in E. coli σ⁷⁰ has the highest concentration and affinity for the core RNA polymerase. The E. coli protein Rsd is regarded as an anti-sigma factor, inhibiting σ⁷⁰-dependent transcription at the onset of stationary growth. Although binding of Rsd to σ⁷⁰ has been shown and numerous structural studies on Rsd have been performed the detailed mechanism of action is still unknown.

Methodology/Principal Findings

We have performed studies to unravel the function and regulation of Rsd expression in vitro and in vivo. Cross-linking and affinity binding revealed that Rsd is able to interact with σ⁷⁰, with the core enzyme of RNA polymerase and is able to form dimers in solution. Unexpectedly, we find that Rsd does also interact with σ³⁸, the stationary phase-specific sigma factor. This interaction was further corroborated by gel retardation and footprinting studies with different promoter fragments and σ³⁸- or σ⁷⁰-containing RNA polymerase in presence of Rsd. Under competitive in vitro transcription conditions, in presence of both sigma factors, a selective inhibition of σ⁷⁰-dependent transcription was prevailing, however. Analysis of rsd expression revealed that the nucleoid-associated proteins H-NS and FIS, StpA and LRP bind to the regulatory region of the rsd promoters. Furthermore, the major promoter P2 was shown to be down-regulated in vivo by RpoS, the stationary phase-specific sigma factor and the transcription factor DksA, while induction of the stringent control enhanced rsd promoter activity. Most notably, the dam-dependent methylation of a cluster of GATC sites turned out to be important for efficient rsd transcription.

Conclusions/Significance

The results contribute to a better understanding of the intricate mechanism of Rsd-mediated sigma factor specificity changes during stationary phase.

Small Regulatory RNA and Legionella pneumophila

Legionella pneumophila is a gram-negative bacterial species that is ubiquitous in almost any aqueous environment. It is the agent of Legionnaires’ disease, an acute and often under-reported form of pneumonia. In mammals, L. pneumophila replicates inside macrophages within a modified vacuole. Many protein regulators have been identified that control virulence-related properties, including RpoS, LetA/LetS, and PmrA/PmrB. In the past few years, the importance of regulation of virulence factors by small regulatory RNA (sRNAs) has been increasingly appreciated. This is also the case in L. pneumophila where three sRNAs (RsmY, RsmZ, and 6S RNA) were recently shown to be important determinants of virulence regulation and 79 actively transcribed sRNAs were identified. In this review we describe current knowledge about sRNAs and their regulatory properties and how this relates to the known regulatory systems of L. pneumophila. We also provide a model for sRNA-mediated control of gene expression that serves as a framework for understanding the regulation of virulence-related properties of L. pneumophila.

Stress response regulators identified through genome-wide transcriptome analysis of the (p)ppGpp-dependent response in Rhizobium etli

BACKGROUND

The alarmone (p)ppGpp mediates a global reprogramming of gene expression upon nutrient limitation and other stresses to cope with these unfavorable conditions. Synthesis of (p)ppGpp is, in most bacteria, controlled by RelA/SpoT (Rsh) proteins. The role of (p)ppGpp has been characterized primarily in Escherichia coli and several Gram-positive bacteria. Here, we report the first in-depth analysis of the (p)ppGpp-regulon in an α-proteobacterium using a high-resolution tiling array to better understand the pleiotropic stress phenotype of a relA/rsh mutant.

RESULTS

We compared gene expression of the Rhizobium etli wild type and rsh (previously rel) mutant during exponential and stationary phase, identifying numerous (p)ppGpp targets, including small non-coding RNAs. The majority of the 834 (p)ppGpp-dependent genes were detected during stationary phase. Unexpectedly, 223 genes were expressed (p)ppGpp-dependently during early exponential phase, indicating the hitherto unrecognized importance of (p)ppGpp during active growth. Furthermore, we identified two (p)ppGpp-dependent key regulators for survival during heat and oxidative stress and one regulator putatively involved in metabolic adaptation, namely extracytoplasmic function sigma factor EcfG2/PF00052, transcription factor CH00371, and serine protein kinase PrkA.

CONCLUSIONS

The regulatory role of (p)ppGpp in R. etli stress adaptation is far-reaching in redirecting gene expression during all growth phases. Genome-wide transcriptome analysis of a strain deficient in a global regulator, and exhibiting a pleiotropic phenotype, enables the identification of more specific regulators that control genes associated with a subset of stress phenotypes. This work is an important step toward a full understanding of the regulatory network underlying stress responses in α-proteobacteria.

Regulatory RNA in bacterial pathogens

Bacteria constitute a large and diverse class of infectious agents, causing devastating diseases in humans, animals, and plants. Our understanding of gene expression control, which forms the basis for successful prevention and treatment strategies, has until recently neglected the many roles that regulatory RNAs might have in bacteria. In recent years, several such regulators have been found to facilitate host-microbe interactions and act as key switches between saprophytic and pathogenic lifestyles. This review covers the versatile regulatory RNA mechanisms employed by bacterial pathogens and highlights the dynamic interplay between riboregulation and virulence factor expression.

6S RNA regulation of relA alters ppGpp levels in early stationary phase

6S RNA is a small, non-coding RNA that interacts directly with σ⁷⁰-RNA polymerase and regulates transcription at many σ⁷⁰-dependent promoters. Here, we demonstrate that 6S RNA regulates transcription of relA, which encodes a ppGpp synthase. The 6S RNA-dependent regulation of relA expression results in increased ppGpp levels during early stationary phase in cells lacking 6S RNA. These changes in ppGpp levels, although modest, are sufficient to result in altered regulation of transcription from σ⁷⁰-dependent promoters sensitive to ppGpp, including those promoting expression of genes involved in amino acid biosynthesis and rRNA. These data place 6S RNA as another player in maintaining appropriate gene expression as cells transition into stationary phase. Independent of this ppGpp-mediated 6S RNA-dependent regulation, we also demonstrate that in later stationary phase, 6S RNA continues to downregulate transcription in general, and specifically at a subset of the amino acid promoters, but through a mechanism that is independent of ppGpp and which we hypothesize is through direct regulation. In addition, 6S RNA-dependent regulation of σ^S activity is not mediated through observed changes in ppGpp levels. We suggest a role for 6S RNA in modulating transcription of several global regulators directly, including relA, to downregulate expression of key pathways in response to changing environmental conditions.