The etiological agent of Lyme disease, Borrelia burgdorferi, appears to contain only a few small RNA molecules

Potential Regulatory Role in Mammalian Host Adaptation for a Small Intergenic Region of Lp17 in the Lyme Disease Spirochete

The bacterial agent of Lyme disease, Borrelia burgdorferi, relies on an intricate gene regulatory network to transit between the disparate Ixodes tick vector and mammalian host environments. We recently reported that a B. burgdorferi mutant lacking a transcriptionally active intergenic region of lp17 displayed attenuated murine tissue colonization and pathogenesis due to altered expression of multiple antigens. In this study, a more detailed characterization of the putative regulatory factor encoded by the intergenic region was pursued. In cis complemented strains featuring mutations aimed at eliminating potential protein translation were capable of full tissue colonization, suggesting that the functional product encoded by the intergenic region is not a protein as previously predicted. In trans complementation of the intergenic region resulted in elevated transcription of the sequence compared to wild type and was found to completely abolish infectivity in both immunocompetent "and immunodeficient mice. Quantitative analysis of transcription of the intergenic region by wild-type B. burgdorferi showed it to be highly induced during murine infection relative to in vitro culture. Lastly, targeted deletion of this intergenic region resulted in significant changes to the transcriptome, including genes with potential roles in transmission and host adaptation. The findings reported herein strongly suggest that this segment of lp17 serves a potentially critical role in the regulation of genes required for adaptation and persistence of the pathogen in a mammalian host.

The intergenic small non-coding RNA ittA is required for optimal infectivity and tissue tropism in Borrelia burgdorferi

Post-transcriptional regulation via small regulatory RNAs (sRNAs) has been implicated in diverse regulatory processes in bacteria, including virulence. One class of sRNAs, termed trans-acting sRNAs, can affect the stability and/or the translational efficiency of regulated transcripts. In this study, we utilized a collaborative approach that employed data from infection with the Borrelia burgdorferi Tn library, coupled with Tn-seq, together with borrelial sRNA and total RNA transcriptomes, to identify an intergenic trans-acting sRNA, which we designate here as ittA for infectivity-associated and tissue-tropic sRNA locus A. The genetic inactivation of ittA resulted in a significant attenuation in infectivity, with decreased spirochetal load in ear, heart, skin and joint tissues. In addition, the ittA mutant did not disseminate to peripheral skin sites or heart tissue, suggesting a role for ittA in regulating a tissue-tropic response. RNA-Seq analysis determined that 19 transcripts were differentially expressed in the ittA mutant relative to its genetic parent, including vraA, bba66, ospD and oms28 (bba74). Subsequent proteomic analyses also showed a significant decrease of OspD and Oms28 (BBA74) proteins. To our knowledge this is the first documented intergenic sRNA that alters the infectivity potential of B. burgdorferi.

Transcriptional profiling of Rickettsia prowazekii coding and non-coding transcripts during in vitro host-pathogen and vector-pathogen interactions

Natural pathogen transmission of Rickettsia prowazekii, the etiologic agent of epidemic typhus, to humans is associated with arthropods, including human body lice, ticks, and ectoparasites of eastern flying squirrel. Recently, we have documented the presence of small RNAs in Rickettsia species and expression of R. prowazekii sRNAs during infection of cultured human microvascular endothelial cells (HMECs), which represent the primary target cells during human infections. Bacterial noncoding transcripts are now well established as critical post-transcriptional regulators of virulence and adaptation mechanisms in varying host environments. Despite their importance, little is known about the expression profile and regulatory activities of R. prowazekii sRNAs (Rp_sRs) in different host cells encountered as part of the natural life-cycle. To investigate the sRNA expression profile of R. prowazekii during infection of arthropod host cells, we employed an approach combining in vitro infection, bioinformatics, RNA sequencing, and PCR-based quantitation. Global analysis of R. prowazekii transcriptome by strand-specific RNA sequencing enabled us to identify 67 cis-acting (antisense) and 26 trans-acting (intergenic) Rp_sRs expressed during the infection of Amblyomma americanum (AAE2) cells. Comparative evaluation of expression during R. prowazekii infection of HMECs and AAE2 cells by quantitative RT-PCR demonstrated significantly higher expression of four selected Rp_sRs in tick AAE2 cells. Examination of the coding transcriptome revealed differential up-regulation of >150 rickettsial genes in either HMECs or AAE2 cells and yielded evidence for host cell-dependent utilization of alternative transcription start sites by 18 rickettsial genes. Our results thus suggest noticeable differences in the expression of both Rp_sRs as well as the coding transcriptome and the exploitation of multiple transcription initiation sites for select genes during the infection of human endothelium and tick vector cells as the host and yield new insights into rickettsial virulence and transmission mechanisms.

RNA-Seq of Borrelia burgdorferi in Multiple Phases of Growth Reveals Insights into the Dynamics of Gene Expression, Transcriptome Architecture, and Noncoding RNAs

Borrelia burgdorferi, the agent of Lyme disease, differentially expresses numerous genes and proteins as it cycles between mammalian hosts and tick vectors. Insights on regulatory mechanisms have been provided by earlier studies that examined B. burgdorferi gene expression patterns during cultivation. However, prior studies examined bacteria at only a single time point of cultivation, providing only a snapshot of what is likely a dynamic transcriptional program driving B. burgdorferi adaptations to changes during culture growth phases. To address that concern, we performed RNA sequencing (RNA-Seq) analysis of B. burgdorferi cultures at early-exponential, mid-exponential, and early-stationary phases of growth. We found that expression of nearly 18% of annotated B. burgdorferi genes changed significantly during culture maturation. Moreover, genome-wide mapping of the B. burgdorferi transcriptome in different growth phases enabled insight on transcript boundaries, operon structures, and identified numerous putative non-coding RNAs. These RNA-Seq data are discussed and presented as a resource for the community of researchers seeking to better understand B. burgdorferi biology and pathogenesis.

Insights into the biology of Borrelia burgdorferi gained through the application of molecular genetics

Borrelia burgdorferi, the vector-borne bacterium that causes Lyme disease, was first identified in 1982. It is known that much of the pathology associated with Lyme borreliosis is due to the spirochete's ability to infect, colonize, disseminate, and survive within the vertebrate host. Early studies aimed at defining the biological contributions of individual genes during infection and transmission were hindered by the lack of adequate tools and techniques for molecular genetic analysis of the spirochete. The development of genetic manipulation techniques, paired with elucidation and annotation of the B. burgdorferi genome sequence, has led to major advancements in our understanding of the virulence factors and the molecular events associated with Lyme disease. Since the dawn of this genetic era of Lyme research, genes required for vector or host adaptation have garnered significant attention and highlighted the central role that these components play in the enzootic cycle of this pathogen. This chapter covers the progress made in the Borrelia field since the application of mutagenesis techniques and how they have allowed researchers to begin ascribing roles to individual genes. Understanding the complex process of adaptation and survival as the spirochete cycles between the tick vector and vertebrate host will lead to the development of more effective diagnostic tools as well as identification of novel therapeutic and vaccine targets. In this chapter, the Borrelia genes are presented in the context of their general biological roles in global gene regulation, motility, cell processes, immune evasion, and colonization/dissemination.

Dealing with oxidative stress and iron starvation in microorganisms: an overview

Iron starvation and oxidative stress are 2 hurdles that bacteria must overcome to establish an infection. Pathogenic bacteria have developed many strategies to efficiently infect a broad range of hosts, including humans. The best characterized systems make use of regulatory proteins to sense the environment and adapt accordingly. For example, iron-sulfur clusters are critical for sensing the level and redox state of intracellular iron. The regulatory small RNA (sRNA) RyhB has recently been shown to play a central role in adaptation to iron starvation, while the sRNA OxyS coordinates cellular response to oxidative stress. These regulatory sRNAs are well conserved in many bacteria and have been shown to be essential for establishing a successful infection. An overview of the different strategies used by bacteria to cope with iron starvation and oxidative stress is presented here.

CsrA modulates levels of lipoproteins and key regulators of gene expression critical for pathogenic mechanisms of Borrelia burgdorferi

Carbon storage regulator A (CsrA) is an RNA binding protein that has been characterized in many bacterial species to play a central regulatory role by modulating several metabolic processes. We recently showed that a homolog of CsrA in Borrelia burgdorferi (CsrA(Bb), BB0184) was upregulated in response to propagation of B. burgdorferi under mammalian host-specific conditions. In order to further delineate the role of CsrA(Bb), we generated a deletion mutant designated ES10 in a linear plasmid 25-negative isolate of B. burgdorferi strain B31 (ML23). The deletion mutant was screened by PCR and Southern blot hybridization, and a lack of synthesis of CsrA(Bb) in ES10 was confirmed by immunoblot analysis. Analysis of ES10 propagated at pH 6.8/37°C revealed a significant reduction in the levels of OspC, DbpA, BBK32, and BBA64 compared to those for the parental wild-type strain propagated under these conditions, while there were no significant changes in the levels of either OspA or P66. Moreover, the levels of two regulatory proteins, RpoS and BosR, were also found to be lower in ES10 than in the control strain. Quantitative real-time reverse transcription-PCR analysis of total RNA extracted from the parental strain and csrA(Bb) mutant revealed significant differences in gene expression consistent with the changes at the protein level. Neither the csrA(Bb) mutant nor the trans-complemented strain was capable of infection following intradermal needle inoculation in C3H/HeN mice at either 10³ or 10⁵ spirochetes per mouse. The further characterization of molecular basis of regulation mediated by CsrA(Bb) will provide significant insights into the pathophysiology of B. burgdorferi.

Evidence that two ATP-dependent (Lon) proteases in Borrelia burgdorferi serve different functions

The canonical ATP-dependent protease Lon participates in an assortment of biological processes in bacteria, including the catalysis of damaged or senescent proteins and short-lived regulatory proteins. Borrelia spirochetes are unusual in that they code for two putative ATP-dependent Lon homologs, Lon-1 and Lon-2. Borrelia burgdorferi, the etiologic agent of Lyme disease, is transmitted through the blood feeding of Ixodes ticks. Previous work in our laboratory reported that B. burgdorferi lon-1 is upregulated transcriptionally by exposure to blood in vitro, while lon-2 is not. Because blood induction of Lon-1 may be of importance in the regulation of virulence factors critical for spirochete transmission, the clarification of functional roles for these two proteases in B. burgdorferi was the object of this study. On the chromosome, lon-2 is immediately downstream of ATP-dependent proteases clpP and clpX, an arrangement identical to that of lon of Escherichia coli. Phylogenetic analysis revealed that Lon-1 and Lon-2 cluster separately due to differences in the NH₂-terminal substrate binding domains that may reflect differences in substrate specificity. Recombinant Lon-1 manifested properties of an ATP-dependent chaperone-protease in vitro but did not complement an E. coli Lon mutant, while Lon-2 corrected two characteristic Lon-mutant phenotypes. We conclude that B. burgdorferi Lons -1 and -2 have distinct functional roles. Lon-2 functions in a manner consistent with canonical Lon, engaged in cellular homeostasis. Lon-1, by virtue of its blood induction, and as a unique feature of the Borreliae, may be important in host adaptation from the arthropod to a warm-blooded host.

Lyme disease, caused by the spirochete Borrelia burgdorferi, is the most prevalent arthropod-borne disease in North America. In nature, the bacterium oscillates between its tick vector host (Ixodes spp.) and small rodents (Peromyscus spp.). B. burgdorferi is able to persist in these two very different host environments by modulating the expression of surface lipoproteins proteins, or other proteins, in response to host factors or environmental cues such as temperature and pH. Our interest in this process led to the identification of a homolog of the E. coli ATP-dependent lon protease (lon-1) in B. burgdorferi that was upregulated in response to blood. The prototypical Lon of E. coli is a conserved protease important for the destruction of abnormal and short-lived proteins. B. burgdorferi is unusual in that it also codes for a second lon homolog, lon-2, that was not upregulated in response to blood. In this study, we sought to clarify the roles for Lon-1 and Lon-2 in B. burgdorferi. We present evidence that Lon-1 is an ATP- and Mg²⁺-dependent protease but does not function in a manner consistent with a prototypical Lon. Lon-2, however, functionally complemented Lon in E. coli. Thus, Lon-1 and Lon-2 appear to have distinct roles in B. burgdorferi; Lon-1 by virtue of its blood induction may be important in host adaptation, while Lon-2 is the functional homolog of E. coli Lon.

Stem loop sequences specific to transposable element IS605 are found linked to lipoprotein genes in Borrelia plasmids

Background

Plasmids of Borrelia species are dynamic structures that contain a large number of repetitive genes, gene fragments, and gene fusions. In addition, the transposable element IS605/200 family, as well as degenerate forms of this IS element, are prevalent. In Helicobacter pylori, flanking regions of the IS605 transposase gene contain sequences that fold into identical small stem loops. These function in transposition at the single-stranded DNA level.

Methodology/Principal Findings

In work reported here, bioinformatics techniques were used to scan Borrelia plasmid genomes for IS605 transposable element specific stem loop sequences. Two variant stem loop motifs are found in the left and right flanking regions of the transposase gene. Both motifs appear to have dispersed in plasmid genomes and are found “free-standing” and phylogenetically conserved without the associated IS605 transposase gene or the adjacent flanking sequence. Importantly, IS605 specific stem loop sequences are also found at the 3′ ends of lipoprotein genes (PFam12 and PFam60), however the left and right sequences appear to develop their own evolutionary patterns. The lipoprotein gene-linked left stem loop sequences maintain the IS605 stem loop motif in orthologs but only at the RNA level. These show mutations whereby variants fold into phylogenetically conserved RNA-type stem loops that contain the wobble non-Watson-Crick G-U base-pairing. The right flanking sequence is associated with the family lipoprotein-1 genes. A comparison of homologs shows that the IS605 stem loop motif rapidly dissipates, but a more elaborate secondary structure appears to develop in its place.

Conclusions/Significance

Stem loop sequences specific to the transposable element IS605 are present in plasmid regions devoid of a transposase gene and significantly, are found linked to lipoprotein genes in Borrelia plasmids. These sequences are evolutionarily conserved and/or structurally developed in an RNA format. The findings show that IS605 stem loop sequences are multifaceted and are selectively conserved during evolution when the transposable element dissipates.

A genome-wide analysis of small regulatory RNAs in the human pathogen group A Streptococcus

The coordinated regulation of gene expression is essential for pathogens to infect and cause disease. A recently appreciated mechanism of regulation is that afforded by small regulatory RNA (sRNA) molecules. Here, we set out to assess the prevalence of sRNAs in the human bacterial pathogen group A Streptococcus (GAS). Genome-wide identification of candidate GAS sRNAs was performed through a tiling Affymetrix microarray approach and identified 40 candidate sRNAs within the M1T1 GAS strain MGAS2221. Together with a previous bioinformatic approach this brings the number of novel candidate sRNAs in GAS to 75, a number that approximates the number of GAS transcription factors. Transcripts were confirmed by Northern blot analysis for 16 of 32 candidate sRNAs tested, and the abundance of several of these sRNAs were shown to be temporally regulated. Six sRNAs were selected for further study and the promoter, transcriptional start site, and Rho-independent terminator identified for each. Significant variation was observed between the six sRNAs with respect to their stability during growth, and with respect to their inter- and/or intra-serotype-specific levels of abundance. To start to assess the contribution of sRNAs to gene regulation in M1T1 GAS we deleted the previously described sRNA PEL from four clinical isolates. Data from genome-wide expression microarray, quantitative RT-PCR, and Western blot analyses are consistent with PEL having no regulatory function in M1T1 GAS. The finding that candidate sRNA molecules are prevalent throughout the GAS genome provides significant impetus to the study of this fundamental gene-regulatory mechanism in an important human pathogen.

Overexpression of CsrA (BB0184) alters the morphology and antigen profiles of Borrelia burgdorferi

Borrelia burgdorferi, the agent of Lyme disease, alters its gene expression in response to highly disparate environmental signals encountered in its hosts. Among the relatively few regulators of adaptive gene expression present in the borrelial genome is an open reading frame (ORF), BB0184, annotated as CsrA (carbon storage regulator A). CsrA, in several bacterial species, has been characterized as a small RNA binding protein that functions as a global regulator affecting mRNA stability or levels of translation of multiple ORFs. Consistent with known functions of CsrA, overexpression of CsrA from B. burgdorferi (CsrABb) in Escherichia coli resulted in reduced accumulation of glycogen. We determined that csrABb is part of the flgK motility operon and that the synthesis of CsrABb was increased when B. burgdorferi was propagated under fed-tick conditions. Overexpression of CsrABb in B. burgdorferi strain B31 (ML23, lp25-negative clonal isolate) resulted in a clone, designated ES25, which exhibited alterations in colony morphology and a significant reduction in the levels of FlaB. Several lipoproteins previously characterized as playing a role in infectivity were also altered in ES25. Real-time reverse transcription-PCR analysis of RNA revealed significant differences in the transcriptional levels of ospC in ES25, while there were no such differences in the levels of other transcripts, suggesting posttranscriptional regulation of expression of these latter genes. These observations indicate that CsrABb plays a role in the regulation of expression of pathophysiological determinants of B. burgdorferi, and further characterization of CsrABb will help in better understanding of the regulators of gene expression in B. burgdorferi.

On the facultative requirement of the bacterial RNA chaperone, Hfq

The pleiotropic post-transcriptional regulator Hfq is an RNA chaperone that facilitates pairing interactions between small regulatory RNAs (sRNAs) and their mRNA targets in several bacteria. However, this classical pattern, derived from the Escherichia coli model, is not applicable to the whole bacterial kingdom. In this article we discuss the facultative requirement for Hfq for sRNA-mRNA duplex formation among bacteria and the specific features of the Hfq protein and RNA duplexes that might account for the dispensability or requirement of the chaperone. Apparent links between the need for Hfq, the GC content of bacterial genomes and the free energy of experimentally validated sRNA-mRNA pairing interactions are presented.

Intergenic regions of Borrelia plasmids contain phylogenetically conserved RNA secondary structure motifs

BACKGROUND

Borrelia species are unusual in that they contain a large number of linear and circular plasmids. Many of these plasmids have long intergenic regions. These regions have many fragmented genes, repeated sequences and appear to be in a state of flux, but they may serve as reservoirs for evolutionary change and/or maintain stable motifs such as small RNA genes.

RESULTS

In an in silico study, intergenic regions of Borrelia plasmids were scanned for phylogenetically conserved stem loop structures that may represent functional units at the RNA level. Five repeat sequences were found that could fold into stable RNA-type stem loop structures, three of which are closely linked to protein genes, one of which is a member of the Borrelia lipoprotein_1 super family genes and another is the complement regulator-acquiring surface protein_1 (CRASP-1) family. Modeled secondary structures of repeat sequences display numerous base-pair compensatory changes in stem regions, including C-G-->A-U transversions when orthologous sequences are compared. Base-pair compensatory changes constitute strong evidence for phylogenetic conservation of secondary structure.

CONCLUSION

Intergenic regions of Borrelia species carry evolutionarily stable RNA secondary structure motifs. Of major interest is that some motifs are associated with protein genes that show large sequence variability. The cell may conserve these RNA motifs whereas allow a large flux in amino acid sequence, possibly to create new virulence factors but with associated RNA motifs intact.

Small RNA gene identification and mRNA target predictions in bacteria

MOTIVATION

Bacterial small ribonucleic acids (sRNAs) that are not ribosomal and transfer or messenger RNAs were initially identified in the sixties, whereas their molecular functions are still under active investigation today. It is now widely accepted that most play central roles in gene expression regulation in response to environmental changes. Interestingly, some are also implicated in bacterial virulence. Functional studies revealed that a large subset of these sRNAs act by an antisense mechanism thanks to pairing interactions with dedicated mRNA targets, usually around their translation start sites, to modulate gene expression at the posttranscriptional level. Some sRNAs modulate protein activity or mimic the structure of other macromolecules. In the last few years, in silico methods have been developed to detect more bacterial sRNAs. Among these, computational analyses of the bacterial genomes by comparative genomics have predicted the existence of a plethora of sRNAs, some that were confirmed to be expressed in vivo. The prediction accuracy of these computational tools is highly variable and can be perfectible. Here we review the computational studies that have contributed to detecting the sRNA gene and mRNA targets in bacteria and the methods for their experimental testing. In addition, the remaining challenges are discussed.

Small non-coding RNAs in Caulobacter crescentus

Small non-coding RNAs (sRNAs) are active in many bacterial cell functions, including regulation of the cell's response to environmental challenges. We describe the identification of 27 novel Caulobacter crescentus sRNAs by analysis of RNA expression levels assayed using a tiled Caulobacter microarray and a protocol optimized for detection of sRNAs. The principal analysis method involved identification of sets of adjacent probes with unusually high correlation between the individual intergenic probes within the set, suggesting presence of a sRNA. Among the validated sRNAs, two are candidate transposase gene antisense RNAs. The expression of 10 of the sRNAs is regulated by either entry into stationary phase, carbon starvation, or rich versus minimal media. The expression of four of the novel sRNAs changes as the cell cycle progresses. One of these shares a promoter motif with several genes expressed at the swarmer-to-stalked cell transition; while another appears to be controlled by the CtrA global transcriptional regulator. The probe correlation analysis approach reported here is of general use for large-scale sRNA identification for any sequenced microbial genome.

Temperature-induced regulation of RpoS by a small RNA in Borrelia burgdorferi

The alternative sigma factor RpoS (sigma38 or sigmaS) plays a central role in the reciprocal regulation of the virulence-associated major outer surface proteins OspC and OspA in Borrelia burgdorferi, the Lyme disease spirochete. Temperature is one of the key environmental signals controlling RpoS, but the molecular mechanism by which the signal is transduced remains unknown. Herein, we identify and describe a small non-coding RNA, DsrABb, that regulates the temperature-induced increase in RpoS. A novel 5' end of the rpoS mRNA was identified and DsrABb has the potential to extensively base-pair with the upstream region of this rpoS transcript. We demonstrate that B. burgdorferi strains lacking DsrABb do not upregulate RpoS and OspC in response to an increase in temperature, but do regulate RpoS and OspC in response to changes in pH and cell density. Analyses of the rpoS and ospC steady-state mRNA levels in the dsrABb mutant indicate that DsrABb regulates RpoS post-transcriptionally. The 5' and 3' ends of DsrABb were mapped, demonstrating that at least four species exist with sizes ranging from 213 to 352 nucleotides. We hypothesize that DsrABb binds to the upstream region of the rpoS mRNA and stimulates translation by releasing the Shine-Dalgarno sequence and start site from a stable secondary structure. Therefore, we postulate that DsrABb is a molecular thermometer regulating RpoS in Borrelia burgdorferi.

Computational approaches for the discovery of bacterial small RNAs

Recent work has uncovered a growing number of bacterial small RNAs (sRNAs), some of which have been shown to regulate critical cellular processes. Computational approaches, in combination with experiments, have played an important role in the discovery of these sRNAs. In this article, we first give an overview of different computational approaches for genome-wide prediction of sRNAs. These approaches have led to the discovery of several novel sRNAs, however the regulatory roles are not yet known for a majority of these sRNAs. By contrast, several recent studies have highlighted the inverse problem where the functional role of the sRNA is already known and the challenge is to identify its genomic location. The focus of this article is on computational tools and strategies for identifying these specific sRNAs which function as key components of known regulatory pathways.

A cyanobacterial non-coding RNA, Yfr1, is required for growth under multiple stress conditions

Small, regulatory, non-coding RNA (ncRNA) is involved in various cell functions in both prokaryotes and eukaryotes. However, information on ncRNA in cyanobacteria is still scarce. We studied ncRNA genes by computational screening to compare the intergenic regions of the Synechococcus elongatus PCC 6301 genome with the genomes of three freshwater cyanobacteria. We identified an ncRNA gene in S. elongatus, which has been previously described as yfr1 in marine cyanobacteria. The S. elongatus yfr1 gene is 65 nucleotides long and is positioned between guaB and trxA. We found a high conservation of the yfr1 gene in most cyanobacterial lineages. A yfr1-deficient mutant showed reduced growth under various stress conditions, e.g. oxidative stress and high salt stress conditions, and showed unusual accumulation of sbtA mRNA. A gel shift assay demonstrated interaction of the Yfr1 RNA with sbtA mRNA in vitro. This suggests that the sbtA transcript is a target RNA for the Yfr1 RNA.

Proteins that interact with bacterial small RNA regulators

Small regulatory RNAs have been identified in a wide range of organisms, where they modify mRNA stability, translation or protein function. Small RNA regulators (sRNAs) either pair with mRNA targets or modify protein activities. Here we discuss current knowledge of the various proteins that interact with RNA regulators and review the physiologic implications of sRNA-protein complexes in DNA, RNA and protein metabolism, as well as in RNA and protein quality control in prokaryotes. Proteins that interact with the sRNAs can possess catalytic activity, induce conformational changes of the sRNA, or be sequestered by the sRNA to prevent the action of the protein.

No detectable effect of RNA-binding protein Hfq absence in Staphylococcus aureus

BACKGROUND

The RNA-binding protein Hfq is involved in stress and virulence of several pathogens, probably due to its role as mediator in small RNA (sRNA)-mRNA interactions. In this study, we investigate the function of Hfq in the Gram-positive pathogen Staphylococcus aureus, by constructing hfq null mutant derivatives.

RESULTS

We report that unexpectedly, in S. aureus, Hfq does not seem to play a crucial role in stress response, RNAIII or spa mRNA quantity and exoprotein expression, as tested in three virulent genetic backgrounds. Moreover, a global analysis of the RN6390 hfq mutant, which tests approximately 2000 phenotypes, supports our results concerning the non-implication of Hfq in stress response, and shows that Hfq is also not involved in resistance to several chemical agents and antibiotics and does not seem to be implicated in metabolic pathways.

CONCLUSION

Our data suggest that although sRNA-mRNA interactions in S. aureus are decisive for gene expression regulation, they do not require the RNA-chaperone protein Hfq. These interactions possibly require an RNA-chaperone protein other than Hfq, which remains to be found.

Borrelia burgdorferi ftsZ plays a role in cell division

ftsZ is essential for cell division in many microorganisms. In Escherichia coli and Bacillus subtilis, FtsZ plays a role in ring formation at the leading edge of the cell division septum. An ftsZ homologue is present in the Borrelia burgdorferi genome (ftsZ(Bbu)). Its gene product (FtsZ(Bbu)) is strongly homologous to other bacterial FtsZ proteins, but its function has not been established. Because loss-of-function mutants of ftsZ(Bbu) might be lethal, the tetR/tetO system was adapted for regulated control of this gene in B. burgdorferi. Sixty-two nucleotides of an ftsZ(Bbu) antisense DNA sequence under the control of a tetracycline-responsive modified hybrid borrelial promoter were cloned into pKFSS1. This construct was electroporated into a B. burgdorferi host strain carrying a chromosomally located tetR under the control of the B. burgdorferi flaB promoter. After induction by anhydrotetracycline, expression of antisense ftsZ RNA resulted in generation of filamentous B. burgdorferi that were unable to divide and grew more slowly than uninduced cells. To determine whether FtsZ(Bbu) could interfere with the function of E. coli FtsZ, ftsZ(Bbu) was amplified from chromosomal DNA and placed under the control of the tetracycline-regulated hybrid promoter. After introduction of the construct into E. coli and induction with anhydrotetracycline, overexpression of ftsZ(Bbu) generated a filamentous phenotype. This suggested interference of ftsZ(Bbu) with E. coli FtsZ function and confirmed the role of ftsZ(Bbu) in cell division. This is the first report of the generation of a B. burgdorferi conditional lethal mutant equivalent by tetracycline-controlled expression of antisense RNA.

Genes for small, noncoding RNAs under sporulation control in Bacillus subtilis

The process of sporulation in the bacterium Bacillus subtilis is known to involve the programmed activation of several hundred protein-coding genes. Here we report the discovery of previously unrecognized genes under sporulation control that specify small, non-protein-coding RNAs (sRNAs). Genes for sRNAs were identified by transcriptional profiling with a microarray bearing probes for intergenic regions in the genome and by use of a comparative genomics algorithm that predicts regions of conserved RNA secondary structure. The gene for one such sRNA, SurA, which is located in the region between yndK and yndL, was induced at the start of development under the indirect control of the master regulator for entry into sporulation, Spo0A. The gene for a second sRNA, SurC, located in the region between dnaJ and dnaK, was switched on at a late stage of sporulation by the RNA polymerase sigma factor sigmaK, which directs gene transcription in the mother cell compartment of the developing sporangium. Finally, a third intergenic region, that between polC and ylxS, which specified several sRNAs, including two transcripts produced under the control of the forespore-specific sigma factor sigmaG and a third transcript generated by sigmaK, was identified. Our results indicate that the full repertoire of sporulation-specific gene expression involves the activation of multiple genes for small, noncoding RNAs.

How to find small non-coding RNAs in bacteria

Small non-coding RNAs (sRNAs) have attracted considerable attention as an emerging class of gene expression regulators. In bacteria, a few regulatory RNA molecules have long been known, but the extent of their role in the cell was not fully appreciated until the recent discovery of hundreds of potential sRNA genes in the bacteriumEscherichia coli. Orthologs of theseE. colisRNA genes, as well as unrelated sRNAs, were also found in other bacteria. Here we review the disparate experimental approaches used over the years to identify sRNA molecules and their genes in prokaryotes. These include genome-wide searches based on the biocomputational prediction of non-coding RNA genes, global detection of non-coding transcripts using microarrays, and shotgun cloning of small RNAs (RNomics). Other sRNAs were found by either co-purification with RNA-binding proteins, such as Hfq or CsrA/RsmA, or classical cloning of abundant small RNAs after size fractionation in polyacrylamide gels. In addition, bacterial genetics offers powerful tools that aid in the search for sRNAs that may play a critical role in the regulatory circuit of interest, for example, the response to stress or the adaptation to a change in nutrient availability. Many of the techniques discussed here have also been successfully applied to the discovery of eukaryotic and archaeal sRNAs.