Synthesis-mediated release of a small RNA inhibitor of RNA polymerase

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.

Structural and mechanistic characterization of 6S RNA from the hyperthermophilic bacterium Aquifex aeolicus

Bacterial 6S RNAs competitively inhibit binding of RNA polymerase (RNAP) holoenzymes to DNA promoters, thereby globally regulating transcription. RNAP uses 6S RNA itself as a template to synthesize short transcripts, termed pRNAs (product RNAs). Longer pRNAs (approx. ≥ 10 nt) rearrange the 6S RNA structure and thereby disrupt the 6S RNA:RNAP complex, which enables the enzyme to resume transcription at DNA promoters. We studied 6S RNA of the hyperthermophilic bacterium Aquifex aeolicus, representing the thermodynamically most stable 6S RNA known so far. Applying structure probing and NMR, we show that the RNA adopts the canonical rod-shaped 6S RNA architecture with little structure formation in the central bulge (CB) even at moderate temperatures (≤37 °C). 6S RNA:pRNA complex formation triggers an internal structure rearrangement of 6S RNA, i.e. formation of a so-called central bulge collapse (CBC) helix. The persistence of several characteristic NMR imino proton resonances upon pRNA annealing demonstrates that defined helical segments on both sides of the CB are retained in the pRNA-bound state, thus representing a basic framework of the RNA's architecture. RNA-seq analyses revealed pRNA synthesis from 6S RNA in A. aeolicus, identifying 9 to ∼17-mers as the major length species. A. aeolicus 6S RNA can also serve as a template for in vitro pRNA synthesis by RNAP from the mesophile Bacillus subtilis. Binding of a synthetic pRNA to A. aeolicus 6S RNA blocks formation of 6S RNA:RNAP complexes. Our findings indicate that A. aeolicus 6S RNA function in its hyperthermophilic host is mechanistically identical to that of other bacterial 6S RNAs. The use of artificial pRNA variants, designed to disrupt helix P2 from the 3'-CB instead of the 5'-CB but preventing formation of the CBC helix, indicated that the mechanism of pRNA-induced RNAP release has been evolutionarily optimized for transcriptional pRNA initiation in the 5'-CB.

Sibling rivalry: related bacterial small RNAs and their redundant and non-redundant roles

Small RNA molecules (sRNAs) are now recognized as key regulators controlling bacterial gene expression, as sRNAs provide a quick and efficient means of positively or negatively altering the expression of specific genes. To date, numerous sRNAs have been identified and characterized in a myriad of bacterial species, but more recently, a theme in bacterial sRNAs has emerged: the presence of more than one highly related sRNAs produced by a given bacterium, here termed sibling sRNAs. Sibling sRNAs are those that are highly similar at the nucleotide level, and while it might be expected that sibling sRNAs exert identical regulatory functions on the expression of target genes based on their high degree of relatedness, emerging evidence is demonstrating that this is not always the case. Indeed, there are several examples of bacterial sibling sRNAs with non-redundant regulatory functions, but there are also instances of apparent regulatory redundancy between sibling sRNAs. This review provides a comprehensive overview of the current knowledge of bacterial sibling sRNAs, and also discusses important questions about the significance and evolutionary implications of this emerging class of regulators.

RNA mango aptamer-fluorophore: a bright, high-affinity complex for RNA labeling and tracking

Because RNA lacks strong intrinsic fluorescence, it has proven challenging to track RNA molecules in real time. To address this problem and to allow the purification of fluorescently tagged RNA complexes, we have selected a high affinity RNA aptamer called RNA Mango. This aptamer binds a series of thiazole orange (fluorophore) derivatives with nanomolar affinity, while increasing fluorophore fluorescence by up to 1,100-fold. Visualization of RNA Mango by single-molecule fluorescence microscopy, together with injection and imaging of RNA Mango/fluorophore complex in C. elegans gonads demonstrates the potential for live-cell RNA imaging with this system. By inserting RNA Mango into a stem loop of the bacterial 6S RNA and biotinylating the fluorophore, we demonstrate that the aptamer can be used to simultaneously fluorescently label and purify biologically important RNAs. The high affinity and fluorescent properties of RNA Mango are therefore expected to simplify the study of RNA complexes.

Differential RNA-seq: the approach behind and the biological insight gained

RNA-sequencing has revolutionized the quantitative and qualitative analysis of transcriptomes in both prokaryotes and eukaryotes. It provides a generic approach for gene expression profiling, annotation of transcript boundaries and operons, as well as identifying novel transcripts including small noncoding RNA molecules and antisense RNAs. We recently developed a differential RNA-seq (dRNA-seq) method which in addition to the above, yields information as to whether a given RNA is a primary or processed transcript. Originally applied to describe the primary transcriptome of the gastric pathogen Helicobacter pylori, dRNA-seq has since provided global maps of transcriptional start sites in diverse species, informed new biology in the CRISPR-Cas9 system, advanced to a tool for comparative transcriptomics, and inspired simultaneous RNA-seq of pathogen and host.

Riboregulation in plant-associated α-proteobacteria

The symbiotic α-rhizobia Sinorhizobium meliloti, Bradyrhizobium japonicum, Rhizobium etli and the related plant pathogen Agrobacterium tumefaciens are important model organisms for studying plant-microbe interactions. These metabolically versatile soil bacteria are characterized by complex lifestyles and large genomes. Here we summarize the recent knowledge on their small non-coding RNAs (sRNAs) including conservation, function, and interaction of the sRNAs with the RNA chaperone Hfq. In each of these organisms, an inventory of hundreds of cis- and trans-encoded sRNAs with regulatory potential was uncovered by high-throughput approaches and used for the construction of 39 sRNA family models. Genome-wide analyses of hfq mutants and co-immunoprecipitation with tagged Hfq revealed a major impact of the RNA chaperone on the physiology of plant-associated α-proteobacteria including symbiosis and virulence. Highly conserved members of the SmelC411 family are the AbcR sRNAs, which predominantly regulate ABC transport systems. AbcR1 of A. tumefaciens controls the uptake of the plant-generated signaling molecule GABA and is a central regulator of nutrient uptake systems. It has similar functions in S. meliloti and the human pathogen Brucella abortus. As RNA degradation is an important process in RNA-based gene regulation, a short overview on ribonucleases in plant-associated α-proteobacteria concludes this review.

Genomewide comparison and novel ncRNAs of Aquificales

Background

The Aquificales are a diverse group of thermophilic bacteria that thrive in terrestrial and marine hydrothermal environments. They can be divided into the families Aquificaceae, Desulfurobacteriaceae and Hydrogenothermaceae. Although eleven fully sequenced and assembled genomes are available, only little is known about this taxonomic order in terms of RNA metabolism.

Results

In this work, we compare the available genomes, extend their protein annotation, identify regulatory sequences, annotate non-coding RNAs (ncRNAs) of known function, predict novel ncRNA candidates, show idiosyncrasies of the genetic decoding machinery, present two different types of transfer-messenger RNAs and variations of the CRISPR systems. Furthermore, we performed a phylogenetic analysis of the Aquificales based on entire genome sequences, and extended this by a classification among all bacteria using 16S rRNA sequences and a set of orthologous proteins.

Combining several in silico features (e.g. conserved and stable secondary structures, GC-content, comparison based on multiple genome alignments) with an in vivo dRNA-seq transcriptome analysis of Aquifex aeolicus, we predict roughly 100 novel ncRNA candidates in this bacterium.

Conclusions

We have here re-analyzed the Aquificales, a group of bacteria thriving in extreme environments, sharing the feature of a small, compact genome with a reduced number of protein and ncRNA genes. We present several classical ncRNAs and riboswitch candidates. By combining in silico analysis with dRNA-seq data of A. aeolicus we predict nearly 100 novel ncRNA candidates.

In vitro characterization of 6S RNA release-defective mutants uncovers features of pRNA-dependent release from RNA polymerase in E. coli

6S RNA is a noncoding RNA that inhibits bacterial transcription by sequestering RNA polymerase holoenzyme (Eσ(70)) in low-nutrient conditions. This transcriptional block can be relieved by the synthesis of a short product RNA (pRNA) using the 6S RNA as a template. Here, we selected a range of 6S RNA release-defective mutants from a high diversity in vitro pool. Studying the release-defective variant R9-33 uncovered complex interactions between three regions of the 6S RNA. As expected, mutating the transcriptional start site (TSS) slowed and partially inhibited release. Surprisingly, additional mutations near the TSS were found that rescued this effect. Likewise, three mutations in the top strand of the large open bubble (LOB) could considerably slow release but were rescued by the addition of upstream mutations found between a highly conserved "-35" motif and the LOB. Combining the three top strand LOB mutations with mutations near the TSS, however, was particularly effective at preventing release, and this effect could be further enhanced by inclusion of the upstream mutations. Overexpressing R9-33 and a series of milder release-defective mutants in Escherichia coli resulted in a delayed entry into exponential phase together with a decrease in cell survival that correlated well with the severity of the in vitro phenotypes. The complex crosstalk observed between distinct regions of the 6S RNA supports a scrunching type model of 6S RNA release, where at least three regions of the 6S RNA must interact with Eσ(70) in a cooperative manner so as to ensure effective pRNA-dependent release.

RNAs nonspecifically inhibit RNA polymerase II by preventing binding to the DNA template

Many RNAs are known to act as regulators of transcription in eukaryotes, including certain small RNAs that directly inhibit RNA polymerases both in prokaryotes and eukaryotes. We have examined the potential for a variety of RNAs to directly inhibit transcription by yeast RNA polymerase II (Pol II) and find that unstructured RNAs are potent inhibitors of purified yeast Pol II. Inhibition by RNA is achieved by blocking binding of the DNA template and requires binding of the RNA to Pol II prior to open complex formation. RNA is not able to displace a DNA template that is already stably bound to Pol II, nor can RNA inhibit elongating Pol II. Unstructured RNAs are more potent inhibitors than highly structured RNAs and can also block specific transcription initiation in the presence of basal transcription factors. Crosslinking studies with ultraviolet light show that unstructured RNA is most closely associated with the two large subunits of Pol II that comprise the template binding cleft, but the RNA has contacts in a basic residue channel behind the back wall of the active site. These results are distinct from previous observations of specific inhibition by small, structured RNAs in that they demonstrate a sensitivity of the holoenzyme to inhibition by unstructured RNA products that bind to a surface outside the DNA cleft. These results are discussed in terms of the need to prevent inhibition by RNAs, either though sequestration of nascent RNA or preemptive interaction of Pol II with the DNA template.

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.

Global regulation of transcription by a small RNA: a quantitative view

Small RNAs are integral regulators of bacterial gene expression, the majority of which act posttranscriptionally by basepairing with target mRNAs, altering translation or mRNA stability. 6S RNA, however, is a small RNA that is a transcriptional regulator, acting by binding directly to σ(70)-RNA polymerase (σ(70)-RNAP) and preventing its binding to gene promoters. At the transition from exponential to stationary phase, 6S RNA accumulates and globally downregulates the transcription of hundreds of genes. At the transition from stationary to exponential phase (outgrowth), 6S RNA is released from σ(70)-RNAP, resulting in a fast increase in free σ(70)-RNAP and transcription of many genes. The transition from stationary to exponential phase is sharp, and is thus accessible for experimental study. However, the transition from exponential to stationary phase is gradual and complicated by changes in other factors, making it more difficult to isolate 6S RNA effects experimentally at this transition. Here, we use mathematical modeling and simulation to study the dynamics of 6S RNA-dependent regulation, focusing on transitions in growth mediated by altered nutrient availability. We first show that our model reproduces the sharp increase in σ(70)-RNAP at outgrowth, as well as the behavior of two experimentally tested mutants, thus justifying its use for characterizing the less accessible dynamics of the transition from exponential to stationary phase. We characterize the dynamics of the two transitions for Escherichia coli wild-type, as well as for mutants with various 6S RNA-RNAP affinities, demonstrating that the 6S RNA regulation mechanism is generally robust to a wide range of such mutations, although the level of regulation at single promoters and their resulting expression fold change will be altered with changes in affinity. Our results provide insight into the potential advantage of transcription regulation by 6S RNA, as it enables storage and efficient release of σ(70)-RNAP during transitions in nutrient availability, which is likely to give a competitive advantage to cells encountering diverse environmental conditions.

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.

Parallel evolution of genome structure and transcriptional landscape in the Epsilonproteobacteria

Background

Gene reshuffling, point mutations and horizontal gene transfer contribute to bacterial genome variation, but require the genome to rewire its transcriptional circuitry to ensure that inserted, mutated or reshuffled genes are transcribed at appropriate levels. The genomes of Epsilonproteobacteria display very low synteny, due to high levels of reshuffling and reorganisation of gene order, but still share a significant number of gene orthologs allowing comparison. Here we present the primary transcriptome of the pathogenic Epsilonproteobacterium Campylobacter jejuni, and have used this for comparative and predictive transcriptomics in the Epsilonproteobacteria.

Results

Differential RNA-sequencing using 454 sequencing technology was used to determine the primary transcriptome of C. jejuni NCTC 11168, which consists of 992 transcription start sites (TSS), which included 29 putative non-coding and stable RNAs, 266 intragenic (internal) TSS, and 206 antisense TSS. Several previously unknown features were identified in the C. jejuni transcriptional landscape, like leaderless mRNAs and potential leader peptides upstream of amino acid biosynthesis genes. A cross-species comparison of the primary transcriptomes of C. jejuni and the related Epsilonproteobacterium Helicobacter pylori highlighted a lack of conservation of operon organisation, position of intragenic and antisense promoters or leaderless mRNAs. Predictive comparisons using 40 other Epsilonproteobacterial genomes suggests that this lack of conservation of transcriptional features is common to all Epsilonproteobacterial genomes, and is associated with the absence of genome synteny in this subdivision of the Proteobacteria.

Conclusions

Both the genomes and transcriptomes of Epsilonproteobacteria are highly variable, both at the genome level by combining and division of multicistronic operons, but also on the gene level by generation or deletion of promoter sequences and 5′ untranslated regions. Regulatory features may have evolved after these species split from a common ancestor, with transcriptome rewiring compensating for changes introduced by genomic reshuffling and horizontal gene transfer.

Regulation of bacterial metabolism by small RNAs using diverse mechanisms

Bacteria live in many dynamic environments with alternating cycles of feast or famine that have resulted in the evolution of mechanisms to quickly alter their metabolic capabilities. Such alterations often involve complex regulatory networks that modulate expression of genes involved in nutrient uptake and metabolism. A great number of protein regulators of metabolism have been characterized in depth. However, our ever-increasing understanding of the roles played by RNA regulators has revealed far greater complexity to regulation of metabolism in bacteria. Here, we review the mechanisms and functions of selected bacterial RNA regulators and discuss their importance in modulating nutrient uptake as well as primary and secondary metabolic pathways.

Single-strand promoter traps for bacterial RNA polymerase

Besides canonical double-strand DNA promoters, multisubunit RNAPs (RNA polymerases) recognize a number of specific single-strand DNA and RNA templates, resulting in synthesis of various types of RNA transcripts. The general recognition principles and the mechanisms of transcription initiation on these templates are not fully understood. To investigate further the molecular mechanisms underlying the transcription of single-strand templates by bacterial RNAP, we selected high-affinity single-strand DNA aptamers that are specifically bound by RNAP holoenzyme, and characterized a novel class of aptamer-based transcription templates. The aptamer templates have a hairpin structure that mimics the upstream part of the open promoter bubble with accordingly placed specific promoter elements. The affinity of the RNAP holoenzyme to such DNA structures probably underlies its promoter-melting activity. Depending on the template structure, the aptamer templates can direct synthesis of productive RNA transcripts or effectively trap RNAP in the process of abortive synthesis, involving DNA scrunching, and competitively inhibit promoter recognition. The aptamer templates provide a novel tool for structure-function studies of transcription initiation by bacterial RNAP and its inhibition.

Initiating nucleotide identity determines efficiency of RNA synthesis from 6S RNA templates in Bacillus subtilis but not Escherichia coli

The 6S RNA is a non-coding small RNA that binds within the active site of housekeeping forms of RNA polymerases (e.g. Eσ⁷⁰ in Escherichia coli, Eσ^A in Bacillus subtilis) and regulates transcription. Efficient release of RNA polymerase from 6S RNA regulation during outgrowth from stationary phase is dependent on use of 6S RNA as a template to generate a product RNA (pRNA). Interestingly, B. subtilis has two 6S RNAs, 6S-1 and 6S-2, but only 6S-1 RNA appears to be used efficiently as a template for pRNA synthesis during outgrowth. Here, we demonstrate that the identity of the initiating nucleotide is particularly important for the B. subtilis RNA polymerase to use RNA templates. Specifically, initiation with guanosine triphosphate (GTP) is required for efficient pRNA synthesis, providing mechanistic insight into why 6S-2 RNA does not support robust pRNA synthesis as it initiates with adenosine triphosphate (ATP). Intriguingly, E. coli RNA polymerase does not have a strong preference for initiating nucleotide identity. These observations highlight an important difference in biochemical properties of B. subtilis and E. coli RNA polymerases, specifically in their ability to use RNA templates efficiently, which also may reflect the differences in GTP and ATP metabolism in these two organisms.

6S-1 RNA function leads to a delay in sporulation in Bacillus subtilis

We have discovered that 6S-1 RNA (encoded by bsrA) is important for appropriate timing of sporulation in Bacillus subtilis in that cells lacking 6S-1 RNA sporulate earlier than wild-type cells. The time to generate a mature spore once the decision to sporulate has been made is unaffected by 6S-1 RNA, and, therefore, we propose that it is the timing of onset of sporulation that is altered. Interestingly, the presence of cells lacking 6S-1 RNA in coculture leads to all cell types exhibiting an early-sporulation phenotype. We propose that cells lacking 6S-1 RNA modify their environment in a manner that promotes early sporulation. In support of this model, resuspension of wild-type cells in conditioned medium from ΔbsrA cultures also resulted in early sporulation. Use of Escherichia coli growth as a reporter of the nutritional status of conditioned media suggested that B. subtilis cells lacking 6S-1 RNA reduce the nutrient content of their environment earlier than wild-type cells. Several pathways known to impact the timing of sporulation, such as the skf- and sdp-dependent cannibalism pathways, were eliminated as potential targets of 6S-1 RNA-mediated changes, suggesting that 6S-1 RNA activity defines a novel mechanism for altering the timing of onset of sporulation. In addition, 6S-2 RNA does not influence the timing of sporulation, providing further evidence of the independent influences of these two related RNAs on cell physiology.

RNA polymerase II acts as an RNA-dependent RNA polymerase to extend and destabilize a non-coding RNA

RNA polymerase II (Pol II) is a well-characterized DNA-dependent RNA polymerase, which has also been reported to have RNA-dependent RNA polymerase (RdRP) activity. Natural cellular RNA substrates of mammalian Pol II, however, have not been identified and the cellular function of the Pol II RdRP activity is unknown. We found that Pol II can use a non-coding RNA, B2 RNA, as both a substrate and a template for its RdRP activity. Pol II extends B2 RNA by 18 nt on its 3'-end in an internally templated reaction. The RNA product resulting from extension of B2 RNA by the Pol II RdRP can be removed from Pol II by a factor present in nuclear extracts. Treatment of cells with α-amanitin or actinomycin D revealed that extension of B2 RNA by Pol II destabilizes the RNA. Our studies provide compelling evidence that mammalian Pol II acts as an RdRP to control the stability of a cellular RNA by extending its 3'-end.

Identification of small RNAs in Mycobacterium smegmatis using heterologous Hfq

Gene regulation by small RNAs (sRNAs) has been extensively studied in various bacteria. However, the presence and roles of sRNAs in mycobacteria remain largely unclear. Immunoprecipitation of RNA chaperone Hfq to enrich for sRNAs is one of the effective methods to isolate sRNAs. However, the lack of an identified mycobacterial hfq restricts the feasibility of this approach. We developed a novel method that takes advantage of the conserved inherent sRNAs-binding capability of heterologous Hfq from Escherichia coli to enrich sRNAs from Mycobacterium smegmatis, a model organism for studying Mycobacterium tuberculosis. We validated 12 trans-encoded and 12 cis-encoded novel sRNAs in M. smegmatis. Many of these sRNAs are differentially expressed at exponential phase compared with stationary phase, suggesting that sRNAs are involved in the growth of mycobacteria. Intriguingly, five of the cis-encoded novel sRNAs target known transposases. Phylogenetic conservation analysis shows that these sRNAs are pathogenicity dependent. We believe that our findings will serve as an important reference for future analysis of sRNAs regulation in mycobacteria and will contribute significantly to the development of sRNAs prediction programs. Moreover, this novel method of using heterologous Hfq for sRNAs enrichment can be of general use for the discovery of bacterial sRNAs in which no endogenous Hfq is identified.

Basic and applied uses of genome-scale metabolic network reconstructions of Escherichia coli

The genome-scale model (GEM) of metabolism in the bacterium Escherichia coli K-12 has been in development for over a decade and is now in wide use. GEM-enabled studies of E. coli have been primarily focused on six applications: (1) metabolic engineering, (2) model-driven discovery, (3) prediction of cellular phenotypes, (4) analysis of biological network properties, (5) studies of evolutionary processes, and (6) models of interspecies interactions. In this review, we provide an overview of these applications along with a critical assessment of their successes and limitations, and a perspective on likely future developments in the field. Taken together, the studies performed over the past decade have established a genome-scale mechanistic understanding of genotype-phenotype relationships in E. coli metabolism that forms the basis for similar efforts for other microbial species. Future challenges include the expansion of GEMs by integrating additional cellular processes beyond metabolism, the identification of key constraints based on emerging data types, and the development of computational methods able to handle such large-scale network models with sufficient accuracy.

E. coli 6S RNA release from RNA polymerase requires σ70 ejection by scrunching and is orchestrated by a conserved RNA hairpin

The 6S RNA in Escherichia coli suppresses housekeeping transcription by binding to RNA polymerase holoenzyme (core polymerase + σ⁷⁰) under low nutrient conditions and rescues σ⁷⁰-dependent transcription in high nutrient conditions by the synthesis of a short product RNA (pRNA) using itself as a template. Here we characterize a kinetic intermediate that arises during 6S RNA release. This state, consisting of 6S RNA and core polymerase, is related to the formation of a top-strand "release" hairpin that is conserved across the γ-proteobacteria. Deliberately slowing the intrinsic 6S RNA release rate by nucleotide feeding experiments reveals that σ⁷⁰ ejection occurs abruptly once a pRNA length of 9 nucleotides (nt) is reached. After σ⁷⁰ ejection, an additional 4 nt of pRNA synthesis is required before the 6S:pRNA complex is finally released from core polymerase. Changing the E. coli 6S RNA sequence to preclude formation of the release hairpin dramatically slows the speed of 6S RNA release but, surprisingly, does not alter the abruptness of σ⁷⁰ ejection. Rather, the pRNA size required to trigger σ⁷⁰ release increases from 9 nt to 14 nt. That a precise pRNA length is required to trigger σ⁷⁰ release either with or without a hairpin implicates an intrinsic "scrunching"-type release mechanism. We speculate that the release hairpin serves two primary functions in the γ-proteobacteria: First, its formation strips single-stranded "-10" 6S RNA interactions away from σ⁷⁰. Second, the formation of the hairpin accumulates RNA into a region of the polymerase complex previously associated with DNA scrunching, further destabilizing the 6S:pRNA:polymerase complex.

Functional characterization of the RNA chaperone Hfq in the opportunistic human pathogen Stenotrophomonas maltophilia

Hfq is an RNA-binding protein known to regulate a variety of cellular processes by interacting with small RNAs (sRNAs) and mRNAs in prokaryotes. Stenotrophomonas maltophilia is an important opportunistic pathogen affecting primarily hospitalized and immunocompromised hosts. We constructed an hfq deletion mutant (Δhfq) of S. maltophilia and compared the behaviors of wild-type and Δhfq S. maltophilia cells in a variety of assays. This revealed that S. maltophilia Hfq plays a role in biofilm formation and cell motility, as well as susceptibility to antimicrobial agents. Moreover, Hfq is crucial for adhesion to bronchial epithelial cells and is required for the replication of S. maltophilia in macrophages. Differential RNA sequencing analysis (dRNA-seq) of RNA isolated from S. maltophilia wild-type and Δhfq strains showed that Hfq regulates the expression of genes encoding flagellar and fimbrial components, transmembrane proteins, and enzymes involved in different metabolic pathways. Moreover, we analyzed the expression of several sRNAs identified by dRNA-seq in wild-type and Δhfq S. maltophilia cells grown in different conditions on Northern blots. The accumulation of two sRNAs was strongly reduced in the absence of Hfq. Furthermore, based on our dRNA-seq analysis we provide a genome-wide map of transcriptional start sites in S. maltophilia.

Identification by cDNA cloning of abundant sRNAs in a human-avirulent Yersinia pestis strain grown under five different growth conditions

AIMS

sRNA regulation is supposedly involved in the stress response of a pathogen during infection. Yersinia pestis, the etiologic agent of plague, must encounter temperature and microenvironment changes, given its lifestyle. Here, we used the cDNA cloning approach to discover full-length sRNA candidates that are highly expressed in Y. pestis under five different growth conditions.

MATERIALS & METHODS

The cDNA cloning approach was improved by combining the traditional cDNA library construction with the prevalent rapid amplification of cDNA ends and RNA size selection techniques.

RESULTS

In total, 43 RNA species, including six previously annotated sRNAs, were identified. Of these, 25 sRNAs were encoded on the antisense strand of the annotated genes. Interestingly, two of these sRNAs were found on the complementary strand of noncoding RNAs. In addition, eight novel sRNAs encoded in the intergenic regions were also revealed. Ten sRNA candidates chosen for the northern blot analysis were successfully detected. Analysis of the expression patterns of 29 candidate sRNAs showed that 24 sRNAs are highly abundant in Y. pestis upon entry into the stationary growth phase.

CONCLUSION

Our preliminary attempt at screening the novel sRNA candidates will lay the foundation for understanding the roles of sRNAs in Y. pestis physiology and pathogenesis.

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.

Transcriptome complexity and riboregulation in the human pathogen Helicobacter pylori

The Gram-negative Epsilonproteobacterium Helicobacter pylori is considered as one of the major human pathogens and many studies have focused on its virulence mechanisms as well as genomic diversity. In contrast, only very little is known about post-transcriptional regulation and small regulatory RNAs (sRNAs) in this spiral-shaped microaerophilic bacterium. Considering the absence of the common RNA chaperone Hfq, which is a key-player in post-transcriptional regulation in enterobacteria, H. pylori was even regarded as an organism without riboregulation. However, analysis of the H. pylori primary transcriptome using RNA-seq revealed a very complex transcriptional output from its small genome. Furthermore, the identification of a wealth of sRNAs as well as massive antisense transcription indicates that H. pylori uses riboregulation for its gene expression control. The ongoing functional characterization of sRNAs along with the identification of associated RNA binding proteins will help to understand their potential roles in Helicobacter virulence and stress response. Moreover, research on riboregulation in H. pylori will provide new insights into its virulence mechanisms and will also help to shed light on post-transcriptional regulation in other Epsilonproteobacteria, including widespread and emerging pathogens such as Campylobacter.

A pRNA-induced structural rearrangement triggers 6S-1 RNA release from RNA polymerase in Bacillus subtilis

Bacillus subtilis 6S-1 RNA binds to the housekeeping RNA polymerase (σ(A)-RNAP) and directs transcription of short 'product' RNAs (pRNAs). Here, we demonstrate that once newly synthesized pRNAs form a sufficiently stable duplex with 6S-1 RNA, a structural rearrangement is induced in cis, which involves base-pairing between sequences in the 5'-portion of the central bulge and nucleotides that become available as a result of pRNA invasion. The rearrangement decreases 6S-1 RNA affinity for σ(A)-RNAP. Among the pRNA length variants synthesized by σ(A)-RNAP (up to ∼14 nt), only the longer ones, such as 12-14-mers, form a duplex with 6S-1 RNA that is sufficiently long-lived to induce the rearrangement. Yet, an LNA (locked nucleic acid) 8-mer can induce the same rearrangement due to conferring increased duplex stability. We propose that an interplay of rate constants for polymerization (k(pol)), for pRNA:6S-1 RNA hybrid duplex dissociation (k(off)) and for the rearrangement (k(conf)) determines whether pRNAs dissociate or rearrange 6S-1 structure to trigger 6S-1 RNA release from σ(A)-RNAP. A bioinformatic screen suggests that essentially all bacterial 6S RNAs have the potential to undergo a pRNA-induced structural rearrangement.

Structure of noncoding RNA is a determinant of function of RNA binding proteins in transcriptional regulation

The majority of the noncoding regions of mammalian genomes have been found to be transcribed to generate noncoding RNAs (ncRNAs), resulting in intense interest in their biological roles. During the past decade, numerous ncRNAs and aptamers have been identified as regulators of transcription. 6S RNA, first described as a ncRNA in E. coli, mimics an open promoter structure, which has a large bulge with two hairpin/stalk structures that regulate transcription through interactions with RNA polymerase. B2 RNA, which has stem-loops and unstructured single-stranded regions, represses transcription of mRNA in response to various stresses, including heat shock in mouse cells. The interaction of TLS (translocated in liposarcoma) with CBP/p300 was induced by ncRNAs that bind to TLS, and this in turn results in inhibition of CBP/p300 histone acetyltransferase (HAT) activity in human cells. Transcription regulator EWS (Ewing's sarcoma), which is highly related to TLS, and TLS specifically bind to G-quadruplex structures in vitro. The carboxy terminus containing the Arg-Gly-Gly (RGG) repeat domains in these proteins are necessary for cis-repression of transcription activation and HAT activity by the N-terminal glutamine-rich domain. Especially, the RGG domain in the carboxy terminus of EWS is important for the G-quadruplex specific binding. Together, these data suggest that functions of EWS and TLS are modulated by specific structures of ncRNAs.

Native gel electrophoresis to study the binding and release of RNA polymerase by 6S RNA

RNA-protein interactions are critical in diverse aspects of gene expression and often serve to mediate regulatory events. Many procedures are available to gain information about RNA-protein interactions. They span from initial identification of an interaction, such as through co-immunoprecipitation studies, to highly detailed atomic resolution definition of the interaction gained from crystallographic and NMR studies. One of the most versatile techniques uses native gel electrophoresis to study RNA-protein complexes, which is often called band shift, gel retardation, or electrophoretic mobility shift assays. Gel shift assays have been used to study a plethora of RNA-protein interactions in all organisms, but here we will use the 6S RNA:RNA polymerase interaction from Escherichia coli as an example to direct discussion of questions that can be addressed, including the ability to follow the dynamics of complexes over time.

Lag phase is a distinct growth phase that prepares bacteria for exponential growth and involves transient metal accumulation

Lag phase represents the earliest and most poorly understood stage of the bacterial growth cycle. We developed a reproducible experimental system and conducted functional genomic and physiological analyses of a 2-h lag phase in Salmonella enterica serovar Typhimurium. Adaptation began within 4 min of inoculation into fresh LB medium with the transient expression of genes involved in phosphate uptake. The main lag-phase transcriptional program initiated at 20 min with the upregulation of 945 genes encoding processes such as transcription, translation, iron-sulfur protein assembly, nucleotide metabolism, LPS biosynthesis, and aerobic respiration. ChIP-chip revealed that RNA polymerase was not "poised" upstream of the bacterial genes that are rapidly induced at the beginning of lag phase, suggesting a mechanism that involves de novo partitioning of RNA polymerase to transcribe 522 bacterial genes within 4 min of leaving stationary phase. We used inductively coupled plasma mass spectrometry (ICP-MS) to discover that iron, calcium, and manganese are accumulated by S. Typhimurium during lag phase, while levels of cobalt, nickel, and sodium showed distinct growth-phase-specific patterns. The high concentration of iron during lag phase was associated with transient sensitivity to oxidative stress. The study of lag phase promises to identify the physiological and regulatory processes responsible for adaptation to new environments.

Bacterial small RNA regulators: versatile roles and rapidly evolving variations

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

RNA mimicry, a decoy for regulatory proteins

Small non-coding RNA molecules (sRNA) are key regulators participating in complex networks, which adapt metabolism in response to environmental changes. In this issue of Molecular Microbiology, and in a related paper in Proc. Natl. Acad. Sci. USA, Moreno et al. (2011) and Sonnleitner et al. (2009) report on novel sRNAs, which act as decoys to inhibit the activity of the master post-transcriptional regulatory protein Crc. Crc is a key protein involved in carbon catabolite repression that optimizes metabolism improving the adaptation of the bacteria to their diverse habitats. Crc is a novel RNA-binding protein that regulates translation of multiple target mRNAs. Two regulatory sRNAs in Pseudomonas putida mimic the natural mRNA targets of Crc and counteract the action of Crc by sequestrating the protein when catabolite repression is absent. Crc trapping by a sRNA is a mechanism reminiscent to the regulation of the repressor of secondary metabolites (RsmA) in Pseudomonas, and highlights the suitability of RNA-dependent regulation to rapidly adjust cell growth in response to environmental changes.

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.

The transcriptional landscape of Chlamydia pneumoniae

Background

Gene function analysis of the obligate intracellular bacterium Chlamydia pneumoniae is hampered by the facts that this organism is inaccessible to genetic manipulations and not cultivable outside the host. The genomes of several strains have been sequenced; however, very little information is available on the gene structure and transcriptome of C. pneumoniae.

Results

Using a differential RNA-sequencing approach with specific enrichment of primary transcripts, we defined the transcriptome of purified elementary bodies and reticulate bodies of C. pneumoniae strain CWL-029; 565 transcriptional start sites of annotated genes and novel transcripts were mapped. Analysis of adjacent genes for co-transcription revealed 246 polycistronic transcripts. In total, a distinct transcription start site or an affiliation to an operon could be assigned to 862 out of 1,074 annotated protein coding genes. Semi-quantitative analysis of mapped cDNA reads revealed significant differences for 288 genes in the RNA levels of genes isolated from elementary bodies and reticulate bodies. We have identified and in part confirmed 75 novel putative non-coding RNAs. The detailed map of transcription start sites at single nucleotide resolution allowed for the first time a comprehensive and saturating analysis of promoter consensus sequences in Chlamydia.

Conclusions

The precise transcriptional landscape as a complement to the genome sequence will provide new insights into the organization, control and function of genes. Novel non-coding RNAs and identified common promoter motifs will help to understand gene regulation of this important human pathogen.

Bacterial transcriptomics: what is beyond the RNA horiz-ome?

Over the past 3 years, bacterial transcriptomics has undergone a massive revolution. Increased sequencing capacity and novel tools have made it possible to explore the bacterial transcriptome to an unprecedented depth, which has revealed that the transcriptome is more complex and dynamic than expected. Alternative transcripts within operons challenge the classic operon definition, and many small RNAs involved in the regulation of transcription, translation and pathogenesis have been discovered. Furthermore, mRNAs may localize to specific areas in the cell, and the spatial organization and dynamics of the chromosome have been shown to be important for transcription. Epigenetic modifications of DNA also affect transcription, and RNA processing affects translation. Therefore, transcription in bacteria resembles that in eukaryotes in terms of complexity more closely than was previously thought. Here we will discuss the contribution of 'omics' approaches to these discoveries as well as the possible impact that they are expected to have in the future.

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.

Evolution of multisubunit RNA polymerases in the three domains of life

RNA polymerases (RNAPs) carry out transcription in all living organisms. All multisubunit RNAPs are derived from a common ancestor, a fact that becomes apparent from their amino acid sequence, subunit composition, structure, function and molecular mechanisms. Despite the similarity of these complexes, the organisms that depend on them are extremely diverse, ranging from microorganisms to humans. Recent findings about the molecular and functional architecture of RNAPs has given us intriguing insights into their evolution and how their activities are harnessed by homologous and analogous basal factors during the transcription cycle. We provide an overview of the evolutionary conservation of and differences between the multisubunit polymerases in the three domains of life, and introduce the 'elongation first' hypothesis for the evolution of transcriptional regulation.

An experimentally anchored map of transcriptional start sites in the model cyanobacterium Synechocystis sp. PCC6803

There has been an increasing interest in cyanobacteria because these photosynthetic organisms convert solar energy into biomass and because of their potential for the production of biofuels. However, the exploitation of cyanobacteria for bioengineering requires knowledge of their transcriptional organization. Using differential RNA sequencing, we have established a genome-wide map of 3,527 transcriptional start sites (TSS) of the model organism Synechocystis sp. PCC6803. One-third of all TSS were located upstream of an annotated gene; another third were on the reverse complementary strand of 866 genes, suggesting massive antisense transcription. Orphan TSS located in intergenic regions led us to predict 314 noncoding RNAs (ncRNAs). Complementary microarray-based RNA profiling verified a high number of noncoding transcripts and identified strong ncRNA regulations. Thus, ∼64% of all TSS give rise to antisense or ncRNAs in a genome that is to 87% protein coding. Our data enhance the information on promoters by a factor of 40, suggest the existence of additional small peptide-encoding mRNAs, and provide corrected 5' annotations for many genes of this cyanobacterium. The global TSS map will facilitate the use of Synechocystis sp. PCC6803 as a model organism for further research on photosynthesis and energy research.

Rho-dependent termination of ssrS (6S RNA) transcription in Escherichia coli: implication for 3' processing of 6S RNA and expression of downstream ygfA (putative 5-formyl-tetrahydrofolate cyclo-ligase)

It is well known that 6S RNA, a global regulatory noncoding RNA that modulates gene expression in response to the cellular stresses in Escherichia coli, is generated by processing from primary ssrS (6S RNA) transcripts derived from two different promoters. The 5' processing of 6S RNA from primary transcripts has been well studied; however, it remains unclear how the 3'-end of this RNA is generated although previous studies have suggested that exoribonucleolytic trimming is necessary for 3' processing. Here, we describe several Rho-dependent termination sites located ∼90 bases downstream of the mature 3'-end of 6S RNA. Our data suggest that the 3'-end of 6S RNA is generated via exoribonucleolytic trimming, rather than endoribonucleolytic cleavage, following the transcription termination events. The termination sites identified in this study are within the open reading frame of the downstream ygfA (putative 5-formyl-tetrahydrofolate cyclo-ligase) gene, a part of the highly conserved bacterial operon ssrS-ygfA, which is up-regulated during the biofilm formation. Our findings reveal that ygfA expression, which also aids the formation of multidrug-tolerant persister cells, could be regulated by Rho-dependent termination activity in the cell.

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.

6S RNA-dependent inhibition of RNA polymerase is released by RNA-dependent synthesis of small de novo products

6S RNA from Escherichia coli is known to bind to RNA polymerase, preventing interaction with many promoters during stationary growth. The resulting repression is released under conditions of nutritional upshift, when the growth situation improves. 6S RNA, which binds to the active site of RNA polymerase, has the particularly interesting feature to act as a template, causing the transcription of defined de novo RNAs (dnRNA) that are complementary to a specific sequence region of the 6S RNA. We analyzed the conditions of dnRNA synthesis and determined their effect on the 6S RNA-mediated inhibition of RNA polymerase in vitro and in vivo. Upon nutritional upshift the RNA polymerase/6S RNA complex induces the rapid synthesis of dnRNAs, which form stable hybrids with the 6S RNA template. The resulting structural change destabilizes the inactivated RNA polymerase complex, causing sigma subunit release. Both dnRNA and 6S RNA are rapidly degraded after complex disintegration. Experiments using the transcriptional inhibitor rifampicin demonstrate that active transcription is required for the disintegration of the RNA polymerase/6S RNA complex. Our results support the conclusion that 6S RNA not only inhibits transcription during stationary growth but also enables cells to resume rapid growth after starvation and help to escape from stationary phase.

Northern blot detection of endogenous small RNAs (approximately14 nt) in bacterial total RNA extracts

Here we describe a northern blot procedure that allows the detection of endogenous RNAs as small as approximately 14 nt in total RNA extracts from bacteria. RNAs that small and as part of total bacterial RNA extracts usually escape detection by northern blotting. The approach combines LNA probes 5'-digoxigenin-endlabeled for non-radioactive probe detection with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide-mediated chemical crosslinking of RNAs to nylon membranes, and necessitates the use of native PAGE either with the TBE or MOPS buffer system.

Dampening DNA binding: a common mechanism of transcriptional repression for both ncRNAs and protein domains

With eukaryotic non-coding RNAs (ncRNAs) now established as critical regulators of cellular transcription, the true diversity with which they can elicit biological effects is beginning to be appreciated. Two ncRNAs, mouse B2 RNA and human Alu RNA, have been found to repress mRNA transcription in response to heat shock. They do so by binding directly to RNA polymerase II, assembling into complexes on promoter DNA, and disrupting contacts between the polymerase and the DNA. Such a mechanism of repression had not previously been observed for a eukaryotic ncRNA; however, there are examples of eukaryotic protein domains that repress transcription by blocking essential protein-DNA interactions. Comparing the mechanism of transcriptional repression utilized by these protein domains to that used by B2 and Alu RNAs raises intriguing questions regarding transcriptional control, and how B2 and Alu RNAs might themselves be regulated.

Binding and release of the 6S transcriptional control RNA

6S RNA is an important noncoding RNA that regulates eubacterial transcription. In Escherichia coli this RNA binds to the sigma(70) RNA polymerase holoenzyme and is released by the synthesis of a short product RNA. In order to determine how binding and release are controlled by the 6S RNA sequence, we used in vitro selection to screen a high diversity library containing approximately 4 x 10(12) sequences for functional 6S RNA variants. Residues critical for binding were found to be located in a "-35" region upstream of the 6S RNA transcription bubble mimic structure. Mutating these phylogenetically conserved residues invariably led to decreases in binding and removing them abolished binding, implicating these nucleotides in a biologically important interaction with the Esigma(70) complex. Interestingly, mutation of phylogenetically conserved "-10" residues that were also upstream of the site of pRNA synthesis was found to influence 6S RNA release rates in addition to modulating -35 binding. These results indicate how 6S RNA -35 binding to sigma(70) RNA polymerase holoenzyme can regulate expression from "strong" and "weak" -35 DNA promoters and suggest that 6S RNA release rates have been fine tuned over evolutionary time so as to correctly regulate cellular levels of transcription.