sigmaB and the sigmaB-dependent arlRS and yabJ-spoVG loci affect capsule formation in Staphylococcus aureus

The alternative transcription factor sigma(B) of Staphylococcus aureus affects the transcription of the cap gene cluster, required for the synthesis of capsular polysaccharide (CP), although this operon is lacking an apparent sigma(B)-dependent promoter. Regulation of cap expression and CP production in S. aureus strain Newman was shown here to be influenced by sigma(B), the two-component signal transduction regulatory system ArlRS, and the yabJ-spoVG locus to different extents. Inactivation of arlR or deletion of the sigB operon strongly suppressed capA (CP synthesis enzyme A) transcription. Deletion of spoVG had a polar effect on yabJ-spoVG transcription and resulted in a two- to threefold decrease in capA transcription. Interestingly, immunofluorescence showed that CP production was strongly impaired in all three mutants, signaling that the yabJ-spoVG inactivation, despite its only partial effect on capA transcription, abolished capsule formation. trans-Complementation of the DeltaspoVG mutant with yabJ-spoVG under the control of its native promoter restored CP-5 production and capA expression to levels seen in the wild type. Northern analyses revealed a strong impact of sigma(B) on arlRS and yabJ-spoVG transcription. We hypothesize that ArlR and products of the yabJ-spoVG locus may serve as effectors that modulate sigma(B) control over sigma(B)-dependent genes lacking an apparent sigma(B) promoter.

The Legionella pneumophila response regulator LqsR promotes host cell interactions as an element of the virulence regulatory network controlled by RpoS and LetA

Legionella pneumophila is an opportunistic human pathogen that replicates within environmental amoebae including Acanthamoeba castellanii and Dictyostelium discoideum. The Icm/Dot type IV secretion system promotes phagocytosis and intracellular replication of L. pneumophila in an endoplasmic reticulum-derived 'Legionella-containing vacuole' (LCV). L. pneumophila adopts a biphasic life cycle consisting of a replicative growth phase and a transmissive (stationary) phase, the latter of which is characterized by the preferential expression of genes required for motility and virulence. A bioinformatic analysis of the L. pneumophila genome revealed a gene cluster homologous to the Vibrio cholerae cqsAS genes, encoding a putative quorum sensing autoinducer synthase (lqsA) and a sensor kinase (lqsS), which flank a novel response regulator (lqsR). We report here that an L. pneumophila lqsR deletion mutant grew in broth with the same rate as wild-type bacteria, but entered the replicative growth phase earlier. Overexpression of lqsR led to an elongated morphology of the bacteria. The lqsR mutant strain was found to be more salt-resistant and impaired for intracellular growth in A. castellanii, D. discoideum and macrophages, formation of the ER-derived LCV and toxicity. Moreover, L. pneumophila lacking LqsR, as well as strains lacking the stationary sigma factor RpoS or the two-component response regulator LetA, were phagocytosed less efficiently by A. castellanii, D. discoideum or macrophages. The expression of lqsR was dependent on RpoS and, to a lesser extent, also on LetA. DNA microarray experiments revealed that lqsR regulates the expression of genes involved in virulence, motility and cell division, consistent with a role for LqsR in the transition from the replicative to the transmissive (virulent) phase. Our findings indicate that LqsR is a novel pleiotropic regulator involved in RpoS- and LetA-controlled interactions of L. pneumophila with phagocytes.

Adaptation in bacterial flagellar and motility systems: from regulon members to 'foraging'-like behavior in E. coli

Bacterial flagellar motility and chemotaxis help cells to reach the most favorable environments and to successfully compete with other micro-organisms in response to external stimuli. Escherichia coli is a motile gram-negative bacterium, and the flagellar regulon in E. coli is controlled by a master regulator FlhDC as well as a second regulator, flagellum-specific sigma factor, σ^F. To define the physiological role of these two regulators, we carried out transcription profiling experiments to identify, on a genome-wide basis, genes under the control of these two regulators. In addition, the synchronized pattern of increasing CRP activity causing increasing FlhDC expression with decreasing carbon source quality, together with the apparent coupling of motility activity with the activation of motility and chemotaxis genes in poor quality carbon sources, highlights the importance of CRP activation in allowing E. coli to devote progressively more of its limited reserves to search out better conditions. In adaptation to a variety of carbon sources, the motile bacteria carry out tactical responses by increasing flagellar operation but restricting costly flagellar synthesis, indicating its capability of strategically using the precious energy in nutrient-poor environments for maximizing survival.

Signaling mechanisms for activation of extracytoplasmic function (ECF) sigma factors

A variety of mechanisms are used to signal extracytoplasmic conditions to the cytoplasm. These mechanisms activate extracytoplasmic function (ECF) sigma factors which recruit RNA-polymerase to specific genes in order to express appropriate proteins in response to the changing environment. The two best understood ECF signaling pathways regulate sigma(E)-mediated expression of periplasmic stress response genes in Escherichia coli and FecI-mediated expression of iron-citrate transport genes in E. coli. Homologues from other Gram-negative bacteria suggest that these two signaling mechanisms and variations on these mechanisms may be the general schemes by which ECF sigma factors are regulated in Gram-negative bacteria.

The phrA gene of Rhodobacter sphaeroides encodes a photolyase and is regulated by singlet oxygen and peroxide in a σ E-dependent manner

The genome of the facultatively photosynthetic bacterium Rhodobacter sphaeroides encodes three proteins of the photolyase/cryptochrome family. This paper shows that phrA (RSP2143) encodes a functional photolyase, which is an enzyme that repairs UV radiation-induced DNA damage in a blue light dependent manner. Expression of phrA is upregulated in response to light, with no photoreceptor or the photosynthetic electron transport being involved. The results reveal that singlet oxygen and hydrogen peroxide dependent signals are transmitted by the sigma(E) factor and the anti-sigma(E) factor ChrR affecting phrA expression, while superoxide anions do not stimulate phrA expression. Thus, the sigma(E) regulon is involved not only in the response to singlet oxygen but also in the hydrogen peroxide response.

Mycobacterium tuberculosis SigF regulates genes encoding cell wall-associated proteins and directly regulates the transcriptional regulatory gene phoY1

Mycobacterium tuberculosis SigF is homologous to stress response and sporulation sigma factors in many bacteria. Consistent with a possible role in mycobacterial survival under stress conditions, M. tuberculosis sigF is strongly induced within cultured human macrophages and upon nutrient starvation, and SigF has been implicated in M. tuberculosis entry into stationary phase. On the other hand, SigF appears to contribute to the immune pathology of tuberculosis (TB), and a sigF-deficient mutant has altered cell membrane properties. Using an M. tuberculosis sigF deletion mutant, we show here that sigF is not required for bacillary survival under nutrient starvation conditions and within activated murine macrophages or for extracellular persistence in an in vivo granuloma model of latent TB infection. Using a chemically inducible recombinant strain to conditionally overexpress sigF, we did not observe arrest or retardation of growth in exponentially growing cultures or reduced susceptibility to rifampin and isoniazid. Consistent with our hypothesis that SigF may mediate TB immunopathogenesis by altering cell membrane properties, we found that overexpression of sigF resulted in the differential regulation of many cell wall-associated proteins, including members of the MmpL, PE, and PPE families, several of which have been shown to influence host-pathogen interactions. The most highly upregulated gene by quantitative reverse transcription-PCR at all time points following sigF induction was Rv3301c (phoY1), which encodes a probable transcriptional regulatory protein homologous to PhoU proteins involved in regulation of phosphate uptake. Using in vitro transcription analysis, we show that SigF directly regulates phoY1, whose proposed promoter sequence is GGATTG-N(16)-GGGTAT.

Extracytoplasmic-stress-responsive pathways modulate type III secretion in Yersinia pseudotuberculosis

Three signal transduction pathways, the two-component systems CpxRA and BaeSR and the alternative sigma factor sigma(E), respond to extracytoplasmic stress that facilitates bacterial adaptation to changing environments. At least the CpxRA and sigma(E) pathways control the production of protein-folding and degradation factors that counter the effects of protein misfolding in the periplasm. This function also influences the biogenesis of multicomponent extracellular appendages that span the bacterial envelope, such as various forms of pili. Herein, we investigated whether any of these regulatory pathways in the enteropathogen Yersinia pseudotuberculosis affect the functionality of the Ysc-Yop type III secretion system. This is a multicomponent molecular syringe spanning the bacterial envelope used to inject effector proteins directly into eukaryotic cells. Disruption of individual components revealed that the Cpx and sigma(E) pathways are important for Y. pseudotuberculosis type III secretion of Yops (Yersinia outer proteins). In particular, a loss of CpxA, a sensor kinase, reduced levels of structural Ysc (Yersinia secretion) components in bacterial membranes, suggesting that these mutant bacteria are less able to assemble a functional secretion apparatus. Moreover, these bacteria were no longer capable of localizing Yops into the eukaryotic cell interior. In addition, a cpxA lcrQ double mutant engineered to overproduce and secrete Yops was still impaired in intoxicating cells. Thus, the Cpx pathway might mediate multiple influences on bacterium-target cell contact that modulate Yersinia type III secretion-dependent host cell cytotoxicity.

The extracytoplasmic function-type sigma factor SigM of Corynebacterium glutamicum ATCC 13032 is involved in transcription of disulfide stress-related genes

The gene for the extracytoplasmic function (ECF) sigma factor SigM was deleted from the chromosome of the gram-positive soil bacterium Corynebacterium glutamicum to elucidate the role of the SigM protein in the regulation of gene expression. Comparative DNA microarray hybridizations of the C. glutamicum wild type and sigM-deficient mutant C. glutamicum DN1 revealed 23 genes with enhanced expression in the sigM-proficient strain, encoding functions in the assembly of iron-sulfur clusters (suf operon), thioredoxin reductase (trxB), thioredoxins (trxC, trxB1), chaperones (groES, groEL, clpB), and proteins involved in the heat shock response (hspR, dnaJ, grpE). Deletion of the sigM gene rendered the C. glutamicum cells more sensitive to heat, cold, and the presence of the thiol oxidant diamide. Transcription of the sigM gene increased under different stress conditions, including heat shock, cold shock, and disulfide stress caused by diamide treatment, suggesting a regulatory role for SigM under thiol-oxidative stress conditions. Stress-responsive promoters were determined upstream of the suf operon and of the trxB, trxC, and trxB1 genes. The deduced SigM consensus promoter is characterized by the -35 hexamer gGGAAT and the -10 hexamer YGTTGR. Transcription of the sigM gene is apparently controlled by the ECF sigma factor SigH, since a sigH mutant was unable to enhance the expression of sigM and the SigM regulon under thiol-oxidative stress conditions. A typical SigH-responsive promoter was mapped upstream of the sigM gene. The ECF sigma factor SigM is apparently part of a regulatory cascade, and its transcription is controlled by SigH under conditions of thiol-oxidative stress.

Regulated proteolysis controls mucoid conversion in Pseudomonas aeruginosa

Overproduction of the exopolysaccharide alginate causes mucoid conversion in Pseudomonas aeruginosa and is a poor prognosticator in cystic fibrosis. The ECF sigma factor AlgU and its cognate anti-sigma factor MucA are two principal regulators of alginate production. Here, we report the identification of three positive regulators of alginate biosynthesis: PA4033 (designated mucE), PA3649 (designated mucP), and algW. MucE, a small protein (9.5 kDa), was identified as part of a global mariner transposon screen for new regulators of alginate production. A transposon located in its promoter caused the overexpression of MucE and mucoid conversion in P. aeruginosa strains PAO1 and PA14. Accumulation of MucE in the envelope resulted in increased AlgU activity and reduced MucA levels. Three critical amino acid residues at the C terminus of MucE (WVF) were required for mucoid conversion via two predicted proteases AlgW (DegS) and MucP (RseP/YaeL). Moreover, as in Escherichia coli, the PDZ domain of AlgW was required for signal transduction. These results suggest that AlgU is regulated similarly to E. coli sigma(E) except that the amino acid triad signals from MucE and other envelope proteins that activate AlgW are slightly different from those activating DegS.

Quorum sensing enhances the stress response in Vibrio cholerae

Vibrio cholerae lives in aquatic environments and causes cholera. Here, we show that quorum sensing enhances V. cholerae viability under certain stress conditions by upregulating the expression of RpoS, and this regulation acts through HapR, suggesting that a quorum-sensing-enhanced stress response plays a role in V. cholerae environmental survival.

SigM-responsive genes of Bacillus subtilis and their promoters

Promoters of nine Bacillus subtilis genes (bcrC, yacK, ydaH, yfnI, yjbD, ypbG, ypuA, yraA, and ysxA), all responsive to artificially induced increases in the stress-responsive extracytoplasmic function sigma factor, SigM, were mapped by rapid amplification of cDNA ends-PCR. The resulting promoter consensus suggests that overlapping control by SigX or SigW is common.

Identification of the sigmaB regulon of Bacillus cereus and conservation of sigmaB-regulated genes in low-GC-content gram-positive bacteria

The alternative sigma factor sigma(B) has an important role in the acquisition of stress resistance in many gram-positive bacteria, including the food-borne pathogen Bacillus cereus. Here, we describe the identification of the set of sigma(B)-regulated genes in B. cereus by DNA microarray analysis of the transcriptome upon a mild heat shock. Twenty-four genes could be identified as being sigma(B) dependent as witnessed by (i) significantly lower expression levels of these genes in mutants with a deletion of sigB and rsbY (which encode the alternative sigma factor sigma(B) and a crucial positive regulator of sigma(B) activity, respectively) than in the parental strain B. cereus ATCC 14579 and (ii) increased expression of these genes upon a heat shock. Newly identified sigma(B)-dependent genes in B. cereus include a histidine kinase and two genes that have predicted functions in spore germination. This study shows that the sigma(B) regulon of B. cereus is considerably smaller than that of other gram-positive bacteria. This appears to be in line with phylogenetic analyses where sigma(B) of the B. cereus group was placed close to the ancestral form of sigma(B) in gram-positive bacteria. The data described in this study and previous studies in which the complete sigma(B) regulon of the gram-positive bacteria Bacillus subtilis, Listeria monocytogenes, and Staphylococcus aureus were determined enabled a comparison of the sets of sigma(B)-regulated genes in the different gram-positive bacteria. This showed that only three genes (rsbV, rsbW, and sigB) are conserved in their sigma(B) dependency in all four bacteria, suggesting that the sigma(B) regulon of the different gram-positive bacteria has evolved to perform niche-specific functions.

SigmaE regulates and is regulated by a small RNA in Escherichia coli

RybB is a small, Hfq-binding noncoding RNA originally identified in a screen of conserved intergenic regions in Escherichia coli. Fusions of the rybB promoter to lacZ were used to screen plasmid genomic libraries and genomic transposon mutants for regulators of rybB expression. A number of plasmids, including some carrying rybB, negatively regulated the fusion. An insertion in the rep helicase and one upstream of dnaK decreased expression of the fusion. Multicopy suppressors of these insertions led to identification of two plasmids that stimulated the fusion. One contained the gene for the response regulator OmpR; the second contained mipA, encoding a murein hydrolase. The involvement of MipA and OmpR in cell surface synthesis suggested that the rybB promoter might be dependent on sigma(E). The sequence upstream of the +1 of rybB contains a consensus sigma(E) promoter. The activity of rybB-lacZ was increased in cells lacking the RseA anti-sigma factor and when sigma(E) was overproduced from a heterologous promoter. The activity of rybB-lacZ and the detection of RybB were totally abolished in an rpoE-null strain. In vitro, sigma(E) efficiently transcribes from this promoter. Both a rybB mutation and an hfq mutation significantly increased expression of both rybB-lacZ and rpoE-lacZ fusions, consistent with negative regulation of the sigma(E) response by RybB and other small RNAs. Based on the plasmid screens, NsrR, a repressor sensitive to nitric oxide, was also found to negatively regulate sigma(E)-dependent promoters in an RseA-independent fashion.

An extracytoplasmic function sigma factor acts as a general stress response regulator in Sinorhizobium meliloti

Sinorhizobium meliloti genes transcriptionally up-regulated after heat stress, as well as upon entry into stationary phase, were identified by microarray analyses. Sixty stress response genes were thus found to be up-regulated under both conditions. One of them, rpoE2 (smc01506), encodes a putative extracytoplasmic function (ECF) sigma factor. We showed that this sigma factor controls its own transcription and is activated by various stress conditions, including heat and salt, as well as entry into stationary phase after either carbon or nitrogen starvation. We also present evidence that the product of the gene cotranscribed with rpoE2 negatively regulates RpoE2 activity, and we therefore propose that it plays the function of anti-sigma factor. By combining transcriptomic, bioinformatic, and quantitative reverse transcription-PCR analyses, we identified 44 RpoE2-controlled genes and predicted the number of RpoE2 targets to be higher. Strikingly, more than one-third of the 60 stress response genes identified in this study are RpoE2 targets. Interestingly, two genes encoding proteins with known functions in stress responses, namely, katC and rpoH2, as well as a second ECF-encoding gene, rpoE5, were found to be RpoE2 regulated. Altogether, these data suggest that RpoE2 is a major global regulator of the general stress response in S. meliloti. Despite these observations, and although this sigma factor is well conserved among alphaproteobacteria, no in vitro nor in planta phenotypic difference from the wild-type strain could be detected for rpoE2 mutants. This therefore suggests that other important actors in the general stress response have still to be identified in S. meliloti.

Altered Proteome in Burkholderia pseudomallei rpoE Operon Knockout Mutant: Insights into Mechanisms of rpoE Operon in Stress Tolerance, Survival, and Virulence

We have previously shown that the alternative sigma factor sigmaE (RpoE), encoded by rpoE, is involved in stress tolerance and survival of Burkholderia pseudomallei. However, its molecular and pathogenic mechanisms remain unclear. In the present study, we applied gel-based, differential proteomics to compare the cellular proteome of an rpoE operon knockout mutant (RpoE Mut) to that of wild-type (K96243 WT) B. pseudomallei. Quantitative intensity analysis (n = 5 gels from 5 individual culture flasks in each group) revealed significantly differential expression of 52 proteins, which were subsequently identified by Q-TOF MS/MS. These included oxidative, osmotic, and other stress response proteins; chaperones; transcriptional/translational regulators; metabolic enzymes; proteins involved in cell wall synthesis, fatty synthesis, glycogen synthesis, and storage; exported proteins; secreted proteins; adhesion molecule; protease/peptidase; protease inhibitor; signaling proteins; and other miscellaneous proteins. The down-regulation of several stress response proteins, chaperones, transcriptional/translational regulators, and proteins involved in cell wall synthesis in RpoE Mut provided some new insights into the mechanisms of the rpoE operon for the stress tolerance and survival of B. pseudomallei. In addition, the proteomic data and in vivo study indicated that the rpoE operon is also involved in the virulence of B. pseudomallei. Our findings underscore the usefulness of proteomics for unraveling pathogenic mechanisms of diseases at the molecular level.

Physiological effects of Crl in Salmonella are modulated by sigmaS level and promoter specificity

The small regulatory protein Crl activates sigma(S) (RpoS), the stationary-phase and general stress response sigma factor. Crl has been reported to bind sigma(S) in vitro and to facilitate the formation of RNA polymerase holoenzyme. In Salmonella enterica serovar Typhimurium, Crl is required for the development of the rdar morphotype and transcription initiation of the sigma(S)-dependent genes csgD and adrA, involved in curli and cellulose production. Here, we examined the expression of other sigma(S)-dependent phenotypes and genes in a Deltacrl mutant of Salmonella. Gene fusion analyses and in vitro transcription assays indicate that the magnitude of Crl activation differs between promoters and is highly dependent on sigma(S) levels. We replaced the wild-type rpoS allele in S. enterica serovar Typhimurium strain ATCC 14028 with the rpoS(LT2) allele that shows reduced expression of sigma(S); the result was an increased Crl activation ratio and larger physiological effects of Crl on oxidative, thermal, and acid stress resistance levels during stationary phase. We also found that crl, rpoS, and crl rpoS strains grew better on succinate than did the wild type and expressed the succinate dehydrogenase sdhCDBA operon more strongly. The crl and rpoS(LT2) mutations also increased the competitive fitness of Salmonella in stationary phase. These results show that Crl contributes to negative regulation by sigma(S), a finding consistent with a role for Crl in sigma factor competition via the facilitation of sigma(S) binding to core RNA polymerase.

Probing the Importance of Selected Phylum-specific Amino Acids in σA of Bacteroides fragilis, a Primary σ Factor Naturally Devoid of an N-terminal Acidic Region 1.1

The sigmaA factor of Bacteroides fragilis is the prototype of a novel subgroup of primary sigma factors that are essential for growth and ensure the initiation of transcription of the housekeeping genes. This subgroup is confined to the phyla Bacteroidetes and Chlorobi. Its members carry a specific amino acid signature and are notably characterized by a short, basic N-terminal segment instead of the typical acidic region 1.1. Using in vitro mutagenesis, we investigated the importance of this basic segment and of several residues of the signature for the function of sigmaA. We have shown that the conserved residues Phe-61 and Lys-265, located in the core binding and DNA binding subregions 2.1 and 4.2, respectively, are critical for full function of the B. fragilis holoenzyme. With respect to the unusual subregion composition of sigmaA, we have shown that truncation of the basic N-terminal segment, or reversion of its charge, strongly affects the overall transcriptional activity of B. fragilis RNA polymerase in vitro. Our results indicate that the presence of the intact basic segment is required for the formation of RNA polymerase (RNAP)-promoter open complexes, the correct architecture of the transcription bubble, and efficient promoter clearance.

Group 3 sigma factors in the marine cyanobacterium Synechococcus sp. strain PCC 7002 are required for growth at low temperature

Three genes, sigF, sigG and sigH, encoding group 3 sigma factors have been cloned and characterized in the marine cyanobacterium Synechococcus sp. strain PCC 7002. The sigF gene product was similar to sigma factors involved in general stress response and sporulation in other organisms, and the sigG and sigH gene products were similar to extracytoplasmic function (ECF) sigma factors. The sigG and sigH genes were associated with the putative regulatory genes and the sizes of transcripts for sigG and sigH genes were large enough to be cotranscribed with the associated downstream genes. The sigG downstream gene was designated sapG (sigG-associated protein), and yeast two-hybrid analysis demonstrated that SigG and SapG interact when produced in yeast cells. Null mutants of these three group 3 sigma factor genes were created by interposon mutagenesis. The growth of the sigF mutant strain was much slower than the wild-type strain at 15 degrees C, although the growth rates at 22 degrees C and 38 degrees C were identical to those of the wild-type strain. The sigG mutant could not grow continuously at 22 degrees C, and no growth occurred at 15 degrees C. Since SigG and SapG interact in yeast cells and the sigG and sapG mutants showed a similar growth phenotype, SapG is likely to be a regulatory protein for SigG involved in the same pathway in transcriptional regulation in this cyanobacterium.

The sigmaS subunit of RNA polymerase as a signal integrator and network master regulator in the general stress response in Escherichia coli

The sigmaS (RpoS) subunit of RNA polymerase in Escherichia coli is a key master regulator which allows this bacterial model organism and important pathogen to adapt to and survive environmentally rough times. While hardly present in rapidly growing cells, sigmaS strongly accumulates in response to many different stress conditions, partly replaces the vegetative sigma subunit in RNA polymerase and thereby reprograms this enzyme to transcribe sigmaS-dependent genes (up to 10% of the E. coli genes). In this review, we summarize the extremely complex regulation of sigmaS itself and multiple signal input at the level of this master regulator, we describe the way in which sigmaS specifically recognizes "stress" promoters despite their similarity to vegetative promoters, and, while being far from comprehensive, we give a short overview of the far-reaching physiological impact of sigmaS. With sigmaS being a central and multiple signal integrator and master regulator of hundreds of genes organized in regulatory cascades and sub-networks or regulatory modules, this system also represents a key model system for analyzing complex cellular information processing and a starting point for understanding the complete regulatory network of an entire cell.

A rationale for repression and/or loss of motility by pathogenic Yersinia in the mammalian host

Pathogenic yersiniae either repress flagella expression under host conditions (Yersinia enterocolitica and Yersinia pseudotuberculosis) or have permanently lost this capability by mutation (Yersinia pestis). The block in flagella synthesis for the enteropathogenic Yersinia centers on fliA (sigmaF) repression. This repression ensures the downstream repression of flagellin structural genes which can be cross-recognized and secreted by virulence type III secretion systems. Y. pestis carries several flagellar mutations including a frame shift mutation in flhD, part of the flagellar master control operon. Repression of flagellins in the host environment may be critical because they are potent inducers of innate immunity. Artificial expression of flagellin in Y. enterocolitica completely attenuates virulence, supporting the hypothesis that motility is a liability in the mammalian host.

σBand SarA independently regulate polysaccharide intercellular adhesin production inStaphylococcus epidermidis

The production of polysaccharide intercellular adhesin (PIA) is an essential process in foreign body infections mediated by Staphylococcus epidermidis. Transcriptional regulation of the icaADBC operon, the genes responsible for production of enzymes that synthesize PIA, is multi-factorial and involves at least SarA and σB. Transcriptional and promoter fusion studies revealed that the decreased transcription of the icaADBC operon observed in a S. epidermidis 1457 sigB mutant is not mediated through a direct interaction of σB–RNA polymerase at the icaADBC promoter region but instead through the upregulation of IcaR, a known repressor of icaADBC transcription. Transcriptional analysis of a 1457 sigB–icaR double mutant confirmed that the decreased icaADBC transcript in 1457 sigB is IcaR dependent. Furthermore, primer extension studies suggest that the icaR promoter appears to be σAdependent, suggesting that σBindirectly controls icaR transcription through an unknown pathway. In addition, it was confirmed that the loss of SarA results in the loss of icaADBC transcription and PIA production in S. epidermidis. It was further demonstrated, through the over-production of SarA in 1457 sigB, that the loss of sarP1 promoter activity in 1457 sigB has little or no effect on the loss of PIA production in this mutant. Finally, it was demonstrated that PIA production could be restored in both 1457 sigB and 1457 sarA by complementing these mutants with a full-length icaADBC operon controlled by a cadmium-inducible noncognate promoter. It is concluded that σBand SarA operate independently of each other to regulate PIA production and biofilm development in S. epidermidis.Key words: Staphylococcus epidermidis, biofilm, σB, SarA, icaADBC.

Role of the alternative sigma factors σ E and σ S in survival of Salmonella enterica serovar Typhimurium during starvation, refrigeration and osmotic shock

The ability of Salmonella enterica serovar Typhimurium to survive environmental stress requires specific, coordinated, responses, which induce resistance to the stress condition. This study investigated the relative contribution of sigmaE and sigmaS, the sigma factors regulating extracytoplasmic and general stress response functions, respectively, to survival at low temperature and also in media of differing osmotic strength, conditions relevant to food preservation. To determine if low-temperature storage is a signal for sigmaE- and sigmaS-mediated survival, the ability of S. Typhimurium rpoE, rpoS and rpoE/rpoS mutants to survive in a saline starvation-survival model at a refrigeration temperature (4.5 degrees C) was examined. Under these conditions, the rpoE mutant was significantly (P<0.05) compromised compared to the parent and to an rpoS mutant. The double mutant in rpoE and rpoS displayed a cumulative defect in survival. In hyperosmotic environments (low aw) containing 6 % NaCl and at refrigeration temperature, both sigma factors were important for maximum survival but sigmaS played the dominant role. Analysis of the metabolic activity of starved populations at 4.5 and 37 degrees C revealed significantly (P<0.001) elevated electron-transport system activity in mutants in rpoE and rpoS, indicating a role for sigmaE- and sigmaS-regulated genes in maintaining energy homeostasis. Together these data demonstrate that sigmaE and sigmaS are important for survival of S. Typhimurium in conditions encountered during food processing and that the relative contribution of sigmaE and sigmaS is critically dependent on the precise nature of the stress.

The sigma factors of Mycobacterium tuberculosis

Mycobacterium tuberculosis is a remarkable pathogen capable of adapting and surviving in various harsh conditions. Correct gene expression regulation is essential for the success of this process. The reversible association of different sigma factors is a common mechanism for reprogramming bacterial RNA polymerase and modulating the transcription of numerous genes. Thirteen putative sigma factors are encoded in the M. tuberculosis genome, several being important for virulence. Here, we analyse the latest information available on mycobacterial sigma factors and discuss their roles in the physiology and virulence of M. tuberculosis.

The alternative sigma factor sigmaH is required for toxin gene expression by Bacillus anthracis

Expression of the structural genes for the anthrax toxin proteins is coordinately controlled by host-related signals, such as elevated CO(2), and the trans-acting positive regulator AtxA. In addition to these requirements, toxin gene expression is under growth phase regulation. The transition state regulator AbrB represses atxA expression to influence toxin synthesis. During the late exponential phase of growth, when AbrB levels begin to decrease, toxin synthesis increases. Here we report that toxin gene expression also requires the presence of sigH, a gene encoding the RNA polymerase sigma factor associated with development in Bacillus subtilis. In the well-studied B. subtilis system, sigma(H) is required for sporulation and other post-exponential-phase processes and is part of a feedback control pathway for abrB expression. Our data indicate that a Bacillus anthracis sigH-null mutant is asporogenous and toxin deficient. Yet the sigma factor is required for toxin gene expression in a manner that is independent of the pathway leading to post-exponential-phase gene expression. Sigma(H) positively controls atxA in an AbrB-independent manner. These findings, combined with previous observations, suggest that the steady-state level of atxA expression is critical for optimal toxin gene transcription. We propose a model whereby, under toxin-inducing growth conditions, control of toxin gene expression is fine-tuned by the independent effects of sigma(H) and AbrB on the expression of atxA.