The Staphylococcal accessory regulator (sar) represses transcription of the Staphylococcus aureus collagen adhesin gene (cna) in an agr-independent manner

Modulation of fibronectin adhesins and other virulence factors in a teicoplanin-resistant derivative of methicillin-resistant Staphylococcus aureus

The impact of glycopeptide resistance on the molecular regulation of Staphylococcus aureus virulence and attachment to host tissues is poorly documented. We compared stable teicoplanin-resistant methicillin-resistant S. aureus (MRSA) strain 14-4 with its teicoplanin-susceptible MRSA parent, strain MRGR3, which exhibits a high degree of virulence in a rat model of chronic foreign body MRSA infection. The levels of fibronectin-mediated adhesion and surface display of fibronectin-binding proteins were higher in teicoplanin-resistant strain 14-4 than in its teicoplanin-susceptible parent or a teicoplanin-susceptible revertant (strain 14-4rev) that spontaneously emerged during tissue cage infection. Quantitative reverse transcription-PCR (qRT-PCR) showed four- and twofold higher steady-state levels of fnbA and fnbB transcripts, respectively, in strain 14-4 than in its teicoplanin-susceptible counterparts. Analysis of global regulatory activities by qRT-PCR revealed a strong reduction in the steady-state levels of RNAIII and RNAII in the teicoplanin-resistant strain compared to in its teicoplanin-susceptible counterparts. In contrast, sarA mRNA levels were more than fivefold higher in strain 14-4 than in MRGR3 and 14-4rev. Furthermore, the alternative transcription factor sigma B had a higher level of functional activity in the teicoplanin-resistant strain than in its teicoplanin-susceptible counterparts, as evidenced by significant increases in both the sigma B-dependent asp23 mRNA levels and the sarA P3 promoter-derived transcript levels, as assayed by qRT-PCR and Northern blotting, respectively. These data provide further evidence that the emergence of glycopeptide resistance is linked by still poorly understood molecular pathways with significant pleiotropic changes in the expression and regulation of some major virulence genes. These molecular and phenotypic changes may have a profound impact on the bacterial adhesion and colonization properties of such multiresistant organisms.

Regulation of virulence determinants inStaphylococcus aureus: complexity and applications

The virulence of Staphylococcus aureus is essentially determined by cell wall associated proteins and secreted toxins that are regulated and expressed according to growth phases and/or growth conditions. Gene expression is regulated by specific and sensitive mechanisms, most of which act at the transcriptional level. Regulatory factors constitute numerous complex networks, driving specific interactions with target gene promoters. These factors are largely regulated by two-component regulatory systems, such as the agr, saeRS, srrAB, arlSR and lytRS systems. These systems are sensitive to environmental signals and consist of a sensor histidine kinase and a response regulator protein. DNA-binding proteins, such as SarA and the recently identified SarA homologues (SarR, Rot, SarS, SarT, SarU), also regulate virulence factor expression. These homologues might be intermediates in the regulatory networks. The multiple pathways generated by these factors allow the bacterium to adapt to environmental conditions rapidly and specifically, and to develop infection. Precise knowledge of these regulatory mechanisms and how they control virulence factor expression would open up new perspectives for antimicrobial chemotherapy using key inhibitors of these systems.

Characterization of Staphylococcus aureus SarA binding sites

The staphylococcal accessory regulator locus (sarA) encodes a DNA-binding protein (SarA) that modulates expression of over 100 genes. Whether this occurs via a direct interaction between SarA and cis elements associated with its target genes is unclear, partly because the definitive characteristics of a SarA binding site have not been identified. In this work, electrophoretic mobility shift assays (EMSAs) were used to identify a SarA binding site(s) upstream of the SarA-regulated gene cna. The results suggest the existence of multiple high-affinity binding sites within the cna promoter region. Using a SELEX (systematic evolution of ligands by exponential enrichment) procedure and purified, recombinant SarA, we also selected DNA targets that contain a high-affinity SarA binding site from a random pool of DNA fragments. These fragments were subsequently cloned and sequenced. Randomly chosen clones were also examined by EMSA. These DNA fragments bound SarA with affinities comparable to those of recognized SarA-regulated genes, including cna, fnbA, and sspA. The composition of SarA-selected DNAs was AT rich, which is consistent with the nucleotide composition of the Staphylococcus aureus genome. Alignment of selected DNAs revealed a 7-bp consensus (ATTTTAT) that was present with no more than one mismatch in 46 of 56 sequenced clones. By using the same criteria, consensus binding sites were also identified upstream of the S. aureus genes spa, fnbA, sspA, agr, hla, and cna. With the exception of cna, which has not been previously examined, this 7-bp motif was within the putative SarA binding site previously associated with each gene.

Autoinduction and signal transduction in the regulation of staphylococcal virulence

The accessory genes of Staphylococcus aureus, including those involved in pathogenesis, are controlled by a complex regulatory network that includes at least four two-component systems, one of which, agr, is a quorum sensor, an alternative sigma factor and a large set of transcription factors, including at least two of the superantigen genes, tst and seb. These regulatory genes are hypothesized to act in a time- and population density-dependent manner to integrate signals received from the external environment with the internal metabolic machinery of the cell, in order to achieve the production of particular subsets of accessory/virulence factors at the time and in quantities that are appropriate to the needs of the organism at any given location. From the standpoint of pathogenesis, the regulatory agenda is presumably tuned to particular sites in the host organism. To address this hypothesis, it will be necessary to understand in considerable detail the regulatory interactions among the organism's numerous controlling systems. This review is an attempt to integrate a large body of data into the beginnings of a model that will hopefully help to guide research towards a full-scale test.

Caenorhabditis elegans as a model host for Staphylococcus aureus pathogenesis

Staphylococcus aureus, an important pathogen of humans and other warm-blooded animals, is also capable of killing the nematode Caenorhabditis elegans. Here, we show that C. elegans organisms that are fed S. aureus die over the course of several days in a process that is correlated with the accumulation of bacteria within the nematode digestive tract. Several S. aureus virulence determinants known or speculated to be important in mammalian pathogenesis, including the quorum-sensing global virulence regulatory system agr and the global virulence regulator sarA, the alternative sigma factor sigma(B), alpha-hemolysin, and V8 serine protease, are required for full pathogenicity in nematodes. In addition, several defined C. elegans mutants were examined for susceptibility to S. aureus infection. Enhanced susceptibility to S. aureus killing was observed with loss-of-function mutations in the C. elegans genes esp-2/sek-1 and esp-8/nsy-1, which encode components of a conserved p38 MAP kinase signaling pathway involved in nematode defense against multiple pathogens. These results suggest that key aspects of S. aureus pathogenesis have been conserved, irrespective of the host, and that specific C. elegans host factors can alter susceptibility to this gram-positive human pathogen.

Clinical isolates of Enterococcus faecium exhibit strain-specific collagen binding mediated by Acm, a new member of the MSCRAMM family

A collagen-binding adhesin of Enterococcus faecium, Acm, was identified. Acm shows 62% similarity to the Staphylococcus aureus collagen adhesin Cna over the entire protein and is more similar to Cna (60% and 75% similarity with Cna A and B domains respectively) than to the Enterococcus faecalis collagen-binding adhesin, Ace, which shares homology with Acm only in the A domain. Despite the detection of acm in 32 out of 32 E. faecium isolates, only 11 of these (all clinical isolates, including four vancomycin-resistant endocarditis isolates and seven other isolates) exhibited binding to collagen type I (CI). Although acm from three CI-binding vancomycin-resistant E. faecium clinical isolates showed 100% identity, analysis of acm genes and their promoter regions from six non-CI-binding strains identified deletions or mutations that introduced stop codons and/or IS elements within the gene or the promoter region in five out of six strains, suggesting that the presence of an intact functional acm gene is necessary for binding of E. faecium strains to CI. Recombinant Acm A domain showed specific and concentration-dependent binding to collagen, and this protein competed with E. faecium binding to immobilized CI. Consistent with the adherence phenotype and sequence data, probing with Acm-specific IgGs purified from anti-recombinant Acm A polyclonal rabbit serum confirmed the surface expression of Acm in three out of three collagen-binding clinical isolates of E. faecium tested, but in none of the strains with a non-functional pseudo acm gene. Introduction of a functional acm gene into two non-CI-binding natural acm mutant strains conferred a CI-binding phenotype, further confirming that native Acm is sufficient for the binding of E. faecium to CI. These results demonstrate that acm, which encodes a potential virulence factor, is functional only in certain infection-derived clinical isolates of E. faecium, and suggest that Acm is the primary adhesin responsible for the ability of E. faecium to bind collagen.

Role of sarA in the pathogenesis of Staphylococcus aureus musculoskeletal infection

We recently demonstrated that mutation of sarA in clinical isolates of Staphylococcus aureus results in a phenotype that is distinct by comparison to sarA mutants generated in the laboratory strain RN6390 (J. S. Blevins, K. E. Beenken, M. O. Elasri, B. K. Hurlburt, and M. S. Smeltzer, Infect. Immun. 70:470-480, 2002). This raises the possibility that studies demonstrating that RN6390 sarA mutants are attenuated do not accurately reflect the role of sarA in the pathogenesis of staphylococcal disease. To test this hypothesis, we used a murine model of musculoskeletal infection to assess the virulence of sarA and agr mutants generated in a clinical isolate of S. aureus (UAMS-1). By using this model, we confirmed that mutation of sarA and/or agr results in a reduced capacity to cause both septic arthritis and osteomyelitis.

sigmaB modulates virulence determinant expression and stress resistance: characterization of a functional rsbU strain derived from Staphylococcus aureus 8325-4

The accessory sigma factor sigmaB controls a general stress response that is thought to be important for Staphylococcus aureus survival and may contribute to virulence. The strain of choice for genetic studies, 8325-4, carries a small deletion in rsbU, which encodes a positive regulator of sigmaB activity. Consequently, to enable the role of sigmaB in virulence to be addressed, we constructed an rsbU(+) derivative, SH1000, using a method that does not leave behind an antibiotic resistance marker. The phenotypic properties of SH1000 (8325-4 rsbU(+)) were characterized and compared to those of 8325-4, the rsbU mutant, parent strain. A recognition site for sigmaB was located in the promoter region of katA, the gene encoding the sole catalase of S. aureus, by primer extension analysis. However, catalase expression and activity were similar in SH1000 (8325-4 rsbU(+)), suggesting that this promoter may have a minor role in catalase expression under normal conditions. Restoration of sigmaB activity in SH1000 (8325-4 rsbU(+)) resulted in a marked decrease in the levels of the exoproteins SspA and Hla, and this is likely to be mediated by reduced expression of agr in this strain. By using Western blotting and a sarA-lacZ reporter assay, the levels of SarA were found to be similar in strains 8325-4 and SH1000 (8325-4 rsbU(+)) and sigB mutant derivatives of these strains. This finding contrasts with previous reports that suggested that SarA expression levels are altered when they are measured transcriptionally. Inactivation of sarA in each of these strains resulted in an expected decrease in agr expression; however, the relative level of agr in SH1000 (8325-4 rsbU(+)) remained less than the relative levels in 8325-4 and the sigB mutant derivatives. We suggest that SarA is not likely to be the effector in the overall sigmaB-mediated effect on agr expression.

Variation in extracellular protease production among clinical isolates of Staphylococcus aureus due to different levels of expression of the protease repressor sarA

Staphylococcus aureus produces four major extracellular proteases: staphylococcal serine protease (V8 protease; SspA), cysteine protease (SspB), metalloprotease (aureolysin; Aur), and staphopain (Scp). Several in vitro studies have suggested that these enzymes are important virulence factors. Here we analyzed the protease production of 92 S. aureus strains from infected human soft tissue. Twenty-one strains produced variable zones of proteolysis on casein agar plates, while the remaining 71 strains appeared to be protease negative. The major protease genes were present in all protease-positive (n = 5) and protease-negative (n = 12) strains analyzed. Northern blotting showed that transcription of the protease genes was suppressed due to increased sigma factor B (SigB)-dependent expression of the protease repressor SarA. Other SigB-dependent traits such as pigmentation and expression of asp 23 were also increased in protease-negative compared to protease-positive strains. Inactivation of sarA in three protease-negative strains resulted in increased transcription of all protease genes and increased protease production, while overexpression of sarA in a strain producing protease at high levels repressed protease production. Our results suggest that the protease genes are conserved among clinical S. aureus strains and that the level of SigB-dependent expression of the protease repressor sarA determines the level of protease production in each strain.

Strain-dependent differences in the regulatory roles of sarA and agr in Staphylococcus aureus

The accessory gene regulator (agr) and the staphylococcal accessory regulator (sar) are central regulatory elements that control the production of Staphylococcus aureus virulence factors. To date, the functions of these loci have been defined almost exclusively using RN6390, which is representative of the laboratory strain 8325-4. However, RN6390 was recently shown to have a mutation in rsbU that results in a phenotype resembling that of a sigB mutant (I. Kullik et al., J. Bacteriol. 180:4814-4820, 1998). For that reason, it remains unclear whether the regulatory events defined in RN6390 are representative of the events that take place in clinical isolates of S. aureus. To address this issue, we generated mutations in the sarA and agr loci of three laboratory strains (RN6390, Newman, and S6C) and four clinical isolates (UAMS-1, UAMS-601, DB, and SC-1). Mutation of sarA in the cna-positive strains UAMS-1 and UAMS-601 resulted in an increased capacity to bind collagen, while mutation of agr had little impact. Northern blot analysis confirmed that the increase in collagen binding was due to increased cna transcription. Without exception, mutation of sarA resulted in increased production of proteases and a decreased capacity to bind fibronectin. Mutation of agr had the opposite effect. Although mutation of sarA resulted in a slight reduction in fnbA transcription, changes in the ability to bind fibronectin appeared to be more directly correlated with changes in protease activity. Lipase production was reduced in both sarA and agr mutants. While mutation of sarA in RN6390 resulted in reduced hemolytic activity, it had the opposite effect in all other strains. There appeared to be reduced levels of the sarC transcript in RN6390, but there was no difference in the overall pattern of sar transcription or the production of SarA. Although mutation of sarA resulted in decreased RNAIII transcription, this effect was not evident under all growth conditions. Taken together, these results suggest that studies defining the regulatory roles of sarA and agr by using RN6390 are not always representative of the events that occur in clinical isolates of S. aureus.

Regulation of Staphylococcus aureus capsular polysaccharide expression by agr and sarA

This study addresses the regulation of Staphylococcus aureus type 8 capsular polysaccharide (CP8) expression by the global regulators agr and sarA. We analyzed CP8 production, cap8-specific mRNA synthesis, and blaZ reporter gene activities of the transcriptional and translational fusions in strain Becker and its agr, sarA, and agr-sarA isogenic mutants during different phases of bacterial growth. In the wild-type strain, cap8 mRNA was undetectable until the mid-logarithmic phase of growth, whereas CP8 production was undetectable until 2 h later, at the onset of stationary phase. The delay most likely reflects the time needed for completing CP8 synthesis resulting from translation of cap8 mRNA. The agr mutation caused drastic reductions in CP8 production and cap8 gene transcription, suggesting that agr is a major positive regulator of CP8 expression. The results of gene fusion studies indicated that regulation by agr is exerted at the transcriptional level. In contrast, the sarA mutation caused only a slight reduction in cap8 mRNA synthesis and reporter gene activities. By comparing CP8 production and cap8 transcription, we observed that sarA affected CP8 production both trancriptionally and posttranslationally. We showed that agr was a major activator for cap gene expression not only in type 8 strain Becker but also in strains representing the four agr groups.

Influence of a functional sigB operon on the global regulators sar and agr in Staphylococcus aureus

The growth phase-dependent activity profile of the alternate transcription factor sigma(B) and its effects on the expression of sar and agr were examined in three different Staphylococcus aureus strains by Northern blot analyses and by the use of reporter gene fusion experiments. Significant sigma(B) activity was detectable only in the clinical isolates MSSA1112 and Newman, carrying the wild-type rsbU allele, but not in the NCTC8325 derivative BB255, which is defective in rsbU. sigma(B) activity peaked in the late exponential phase and diminished towards the stationary phase when bacteria were grown in Luria-Bertani medium. Transcriptional analysis and a sarP1-sarP2-sarP3 (sarP1-P2-P3)-driven firefly luciferase (luc+) reporter gene fusion demonstrated a strong sigma(B) activity- and growth phase-dependent increase in sar expression that was totally absent in either rsbU or Delta rsbUVWsigB mutants. In contrast, expression of the agr locus, as measured by RNAIII levels and by an hldp::luc+ fusion, was found to be higher in the absence of sigma(B) activity, such as in rsbU or Delta rsbUVWsigB mutants, than in wild-type strains. Overexpression of sigma(B) in BB255 derivatives resulted in a clear increase in sarP1-P2-P3::luc+ expression as well as a strong decrease in hldp::luc+ expression. The data presented here suggest that sigma(B) increases sar expression while simultaneously reducing the RNAIII level in a growth phase-dependent manner.

Loss of clumping factor B fibrinogen binding activity by Staphylococcus aureus involves cessation of transcription, shedding and cleavage by metalloprotease

The fibrinogen-binding protein clumping factor B (ClfB) of Staphylococcus aureus is present on the surface of cells from the early exponential phase of growth in greater amounts than on cells from late exponential phase and is barely detectable on cells from stationary phase. Expression of a clfB-lacZ fusion indicated that transcription stopped before the end of exponential phase. Mutations in the global regulators agr and sar had no effect on clfB transcription. The loss of ClfB protein from cells in stationary phase was due to expression ending before cells stopped growing, combined with shedding of some of the protein into the growth medium and dilution of those molecules remaining on the cell surface during the two to three cell division events leading to stationary phase. Two forms of the protein occurred on the cell surface, the smaller of which was generated by loss of a domain from the N terminus. The proportion of the smaller form increased as the cultures grew. The metalloprotease aureolysin was shown to be responsible for cleavage of ClfB. Cleavage was inhibited by EDTA and o-phenanthroline and did not occur in an aureolysin-deficient mutant. Purified aureolysin promoted cleavage of cell surface-located ClfB as well as the recombinant A domain of ClfB. Cleavage was detected at two sites, one located between residues Ser(197) and Leu(198) and the other between Ala(199) and Val(200). The truncated form of ClfB did not bind fibrinogen.

The two-component system ArlS-ArlR is a regulator of virulence gene expression in Staphylococcus aureus

Staphylococcus aureus is a major human pathogen that produces many virulence factors in a temporally regulated manner controlled by at least two global virulence regulatory loci (agr and sarA). We identified previously a two-component system, ArlS-ArlR, that modifies the activity of extracellular serine protease and may be involved in virulence regulation. Here, we show that mutations in either arlR or arlS increase the production of secreted proteins [alpha-toxin (Hla), beta-haemolysin, lipase, coagulase, serine protease (Ssp)] and especially protein A (Spa). Furthermore, the pattern of proteins secreted by both mutants was strikingly different from that of the wild-type strain. Transcriptional fusions showed that expression of hla, ssp and spa was higher in both mutants than in the wild-type strain, indicating that the arl operon decreases the production of virulence factors by downregulating the transcription of their genes. The arl mutation did not change spa expression in an agrA mutant or in a sarA mutant, suggesting that both the sarA and the agr loci are required for the action of arl on spa. Northern blot analyses indicated that the arl mutation increased the synthesis of both RNA II and RNA III, but decreased sarA transcription. Finally, arl was not autoregulated, but its expression was stimulated by agr and sarA. These results suggest that the Arl system interacts with both agr and sarA regulatory loci to modulate the virulence regulation network.

Impact of the regulatory loci agr, sarA and sae of Staphylococcus aureus on the induction of alpha-toxin during device-related infection resolved by direct quantitative transcript analysis

The cytotoxic alpha-toxin (encoded by hla) of Staphylococcus aureus is regulated by three loci, agr, sarA and sae, in vitro. Here, we assess the regulation of hla in a guinea pig model of device-related infection by quantifying RNAIII (the effector molecule of agr) and hla directly in exudates accumulating in infected devices without subculturing of the bacteria. LightCycler reverse transcription-polymerase chain reaction (RT-PCR) was used to quantify the transcripts. Strains RN6390 and Newman expressed considerably smaller amounts of RNAIII in the guinea pig than during in vitro growth. The residual RNAIII expression decreased during the course of infection and was negatively correlated with bacterial densities. As with RNAIII, the highest hla expression was detected in both strains early in infection. Even in strain Newman, a weak hla producer in vitro, a pronounced expression of hla was observed during infection. Likewise, four S. aureus isolates from cystic fibrosis (CF) patients expressed Q1hla despite an inactive agr during device-related infection as in the CF lung. Mutation of agr and sarA in strain Newman and RN6390 had no consequence for hla expression in vivo. In contrast, the mutation in sae resulted in severe downregulation of hla in vitro as well as in vivo. In conclusion, S. aureus seems to be provided with regulatory circuits different from those characterized in vitro to ensure alpha-toxin synthesis during infections.

Identification and characterization of a second superoxide dismutase gene (sodM) from Staphylococcus aureus

A gene encoding superoxide dismutase (SOD), sodM, from S. aureus was cloned and characterized. The deduced amino acid sequence specifies a 187-amino-acid protein with 75% identity to the S. aureus SodA protein. Amino acid sequence comparisons with known SODs and relative insensitivity to hydrogen peroxide and potassium cyanide indicate that SodM most likely uses manganese (Mn) as a cofactor. The sodM gene expressed from a plasmid rescued an Escherichia coli double mutant (sodA sodB) under conditions that are otherwise lethal. SOD activity gels of S. aureus RN6390 whole-cell lysates revealed three closely migrating bands of activity. The two upper bands were absent in a sodM mutant, while the two lower bands were absent in a sodA mutant. Thus, the middle band of activity most likely represents a SodM-SodA hybrid protein. All three bands of activity increased as highly aerated cultures entered the late exponential phase of growth, SodM more so than SodA. Viability of the sodA and sodM sodA mutants but not the sodM mutant was drastically reduced under oxidative stress conditions generated by methyl viologen (MV) added during the early exponential phase of growth. However, only the viability of the sodM sodA mutant was reduced when MV was added during the late exponential and stationary phases of growth. These data indicate that while SodA may be the major SOD activity in S. aureus throughout all stages of growth, SodM, under oxidative stress, becomes a major source of activity during the late exponential and stationary phases of growth such that viability and growth of an S. aureus sodA mutant are maintained.

Quorum sensing and the regulation of virulence gene expression in pathogenic bacteria

For many pathogens, the outcome of the interaction between host and bacterium is strongly affected by the bacterial population size. Coupling the production of virulence factors with cell population density ensures that the mammalian host lacks sufficient time to mount an effective defence against consolidated attack. Such a strategy depends on the ability of an individual bacterial cell to sense other members of the same species and in response, differentially express specific sets of genes. Such cell-cell communication is called "quorum sensing" and involves the direct or indirect activation of a response regulator by a small diffusible signal molecule. A number of chemically distinct quorum-sensing signal molecules have been described including the N-acyl-L-homoserine lactones (AHLs) in Gram-negative bacteria and post-translationally modified peptides in Gram-positive bacteria. For example, the human pathogens Pseudomonas aeruginosa and Staphylococcus aureus employ AHLs and peptides, respectively, to control the expression of multiple virulence genes in concert with cell population density. Apart from their role in signal transduction, certain quorum-sensing signal molecules, notably N-(3-oxododecanoyl)homoserine lactone, possess intrinsic pharmacological and immunomodulatory activities such that they may function as virulence determinants per se. While quorum-sensing signal molecules have been detected in tissues in experimental animal model and human infections, the mutation of genes involved in either quorum-sensing signal generation or signal transduction frequently results in the attenuation of virulence. Thus, interference with quorum sensing represents a promising strategy for the therapeutic or prophylactic control of infection.

Subinhibitory clindamycin differentially inhibits transcription of exoprotein genes in Staphylococcus aureus

It has long been known that certain antibiotics, at subinhibitory concentrations, differentially inhibit the synthesis of alpha-hemolysin and other staphylococcal virulence factors. In this report, we show that subinhibitory clindamycin (SBCL) eliminates production of nearly all exoproteins by Staphylococcus aureus but has virtually no effect on cytoplasmic proteins. The effect was abolished by a gene conferring resistance to macrolides-lincosamides-streptogramin B, showing that differential inhibition of protein synthesis is responsible; remarkably, however, subinhibitory clindamycin blocked production of several of the individual exoprotein genes, including spa (encoding protein A), hla (encoding alpha-hemolysin), and spr (encoding serine protease), at the level of transcription, suggesting that the primary effect must be differential inhibition of the synthesis of one or more regulatory proteins. In contrast to earlier reports, however, we found that subinhibitory clindamycin stimulates synthesis of coagulase and fibronectin binding protein B, also at the level of transcription. agr and sar expression was minimally affected by subinhibitory clindamycin. These effects varied from strain to strain and do not seem to be responsible for the effects of subinhibitory clindamycin on the overall exoprotein pattern.

Regulation of virulence determinants in Staphylococcus aureus

The pathogenicity of Staphylococcus aureus depends on the combined action of more than 40 different extracellular toxins, enzymes and cell surface proteins. A global regulator agr controls the production of many of these virulence factors by a regulating RNA molecule, RNAIII. Most of the virulence genes regulated by RNAIII are also regulated by SarA and a family of homologous proteins. The Sar proteins appear to repress transcription of individual virulence genes or sets of genes. As some Sar proteins also repress one or more sar homologous genes an increased production of a single Sar protein can result in decreased expression of some virulence genes, and an increased expression of others. Results are presented suggesting that RNAIII might function as an antirepressor, binding one or more of the Sar proteins.

Sigma(B) activity depends on RsbU in Staphylococcus aureus

Derivatives of the widely used laboratory strain Staphylococcus aureus NCTC8325, which are natural rsbU mutants, were shown to be unable to produce RsbU, a positive regulator of the alternative sigma factor sigma(B). The lack of RsbU prevented the heat-dependent production of sigma(B)-controlled transcripts and resulted in reduced H2O2 and UV tolerance, enhanced alpha-hemolysin activity, and the inability to produce the alkaline shock protein Asp23. After 48 h of growth, rsbU mutant strains failed to accumulate staphyloxanthin, the major stationary-phase carotenoid. Transcription of Asp23 was found to be exclusively controlled by sigma(B), making it an excellent target for the study of sigma(B) activity in S. aureus. Reporter gene experiments, using the firefly luciferase gene (luc+) fused to the sigma(B)-dependent promoter(s) of asp23, revealed that sigma(B) is almost inactive in 8325 derivatives. cis complementation of the 8325 derivative BB255 with the wild-type rsbU gene from strain COL produced the rsbU(+) derivative GP268, a strain possessing a sigma(B) activity profile comparable to that of the rsbU(+) wild-type strain Newman. In GP268, the heat inducibility of sigma(B)-dependent genes, Asp23 production, alpha-hemolysin activity, pigmentation, and susceptibility to H2O2 were restored to the levels observed in strain Newman, clearly demonstrating that RsbU is needed for activation of sigma(B) in S. aureus.

Crystal structures of SarA, a pleiotropic regulator of virulence genes in S. aureus

Staphylococcus aureus is a major human pathogen, the potency of which can be attributed to the regulated expression of an impressive array of virulence determinants. A key pleiotropic transcriptional regulator of these virulence factors is SarA, which is encoded by the sar (staphylococcal accessory regulator) locus. SarA was characterized initially as an activator of a second virulence regulatory locus, agr, through its interaction with a series of heptad repeats (AGTTAAG) within the agr promoter. Subsequent DNA-binding studies have revealed that SarA binds readily to multiple AT-rich sequences of variable lengths. Here we describe the crystal structure of SarA and a SarA-DNA complex at resolutions of 2.50 A and 2.95 A, respectively. SarA has a fold consisting of a four-helix core region and 'inducible regions' comprising a beta-hairpin and a carboxy-terminal loop. On binding DNA, the inducible regions undergo marked conformational changes, becoming part of extended and distorted alpha-helices, which encase the DNA. SarA recognizes an AT-rich site in which the DNA is highly overwound and adopts a D-DNA-like conformation by indirect readout. These structures thus provide insight into SarA-mediated transcription regulation.

SarA represses agr operon expression in a purified in vitro Staphylococcus aureus transcription system

Mutation and genetic complementation studies suggested that two chromosomal loci, agr and sar, are involved in the upregulation of several exotoxin genes and the downregulation of a number of surface protein genes in a growth phase-dependent manner in Staphylococcus aureus. We purified recombinant T7-tagged SarA from Escherichia coli and determined its effect on transcription from several S. aureus promoters by using purified RNA polymerase reconstituted with either sigma(A) or sigma(B) from S. aureus. Of the seven sigma(A)-dependent promoters that we tested, SarA repressed transcription from agrP2, agrP3, cna, sarP1, and sea promoters and did not affect sec and znt promoters. Furthermore, SarA had no effect on transcription from the sigma(B)-dependent sarP3 promoter. In vitro experimental data presented in this report suggest that SarA expression is autoregulated.

Analysis of genetic elements controlling Staphylococcus aureus lrgAB expression: potential role of DNA topology in SarA regulation

Penicillin-induced killing and murein hydrolase activity in Staphylococcus aureus are dependent on a variety of regulatory elements, including the LytSR two-component regulatory system and the virulence factor regulators Agr and Sar. The LytSR effects on these processes can be explained, in part, by the recent finding that a LytSR-regulated operon, designated lrgAB, affects murein hydrolase activity and penicillin tolerance. To examine the regulation of lrgAB expression in greater detail, we performed Northern blot and promoter fusion analyses. Both methods revealed that Agr and Sar, like LytSR, positively regulate lrgAB expression. A mutation in the agr locus reduced lrgAB expression approximately sixfold, while the sar mutation reduced lrgAB expression to undetectable levels. cis-acting regulatory elements involved in lrgAB expression were identified by fusing various fragments of the lrgAB promoter region to the xylE reporter gene and integrating these constructs into the chromosome. Catechol 2,3-dioxygenase assays identified DNA sequences, including an inverted repeat and intrinsic bend sites, that contribute to maximal lrgAB expression. Confirmation of the importance of the inverted repeat was achieved by demonstrating that multiple copies of the inverted repeat reduced lrgAB promoter activity, presumably by titrating out a positive regulatory factor. The results of this study demonstrate that lrgAB expression responds to a variety of positive regulatory factors and suggest that specific DNA topology requirements are important for optimal expression.

Identification and characterization of SarH1, a new global regulator of virulence gene expression in Staphylococcus aureus

The global regulators agr (accessory gene regulator) and sarA (staphylococcal accessory regulator) have been reported to be both activators and repressors of virulence gene expression in Staphylococcus aureus. How the effector of the agr system, RNAIII, interacts with target gene promoters is unknown. SarA, on the other hand, is a DNA-binding protein, which binds to conserved DNA motifs immediately upstream of both positively and negatively regulated promoters. Here, we searched for additional regulators that could explain the differential effects of RNAIII and SarA. Four differently regulated genes (hla, alpha-toxin; hld, RNAIII; spa, protein A; ssp, serine protease) were analysed for binding of potential regulatory proteins to the corresponding promoter DNA fragments, linked to magnetic beads. One protein (29 kDa), with affinity for all four promoters, showed a high degree of similarity to SarA and was named SarH1 (Sar homologue 1). Expression of sarH1 was strongly repressed by sarA and agr. Analysis of hla, hld, ssp and spa mRNAs in sarH1, sarA and agr mutants, and in sarA/sarH1 and agr/sarH1 double mutants, revealed that sarH1 has a strong repressive effect on hla and an activating effect on spa transcription. SDS-PAGE analysis of secreted proteins from the different mutants showed that the production of several other exoproteins was affected by sarH1. In conclusion, we show that both the agr-dependent suppression of protein A production and the sarA-dependent stimulation of alpha-toxin production is mediated via a new regulator, SarH1, which belongs to a family of Sar homologues.

Agr-independent regulation of fibronectin-binding protein(s) by the regulatory locus sar in Staphylococcus aureus

Fibronectin-binding proteins (FnBPs) are thought to be important for the attachment of Staphylococcus aureus during infection. The regulation of the genes fnbA and fnbB by the global regulatory loci sar and agr was examined using site-specific regulatory mutants of S. aureus strain Newman. The results from binding assays using both aqueous and solid-phase fibronectin as well as ligand blotting with biotinylated fibronectin showed that the expression of FnBPA is enhanced in the agr mutant but inhibited in the sar mutant and the sar-agr double mutant. The same regulatory pattern was observed in Northern blot analysis using fnbA-specific probes. The introduction of sar on a multicopy plasmid increased the already enhanced fnbA transcription of the agr mutant. FnBPB was not detectable by ligand blotting and the fnbB promoter activity in promoter fusion assays was not affected by either sar or agr. The sequence encompassing ORF3 located upstream of sarA was found to be essential for the activation of fnbA transcription. We hypothesize that this sequence may modulate SarA expression and/or activity on the post-transcriptional level. Gel shift assays demonstrated that SarA binds to the fnbA promoter fragments, probably as a dimer. DNase I footprinting assays with SarA revealed a protected area of 102 bp upstream of fnbA.

Characterization of the SarA virulence gene regulator of Staphylococcus aureus

Staphylococcus aureus is a potent human pathogen that expresses a large number of virulence factors in a temporally regulated fashion. Two pleiotropically acting regulatory loci were identified in previous mutational studies. The agr locus comprises two operons that express a quorum-sensing system from the P2 promoter and a regulatory RNA molecule from the P3 promoter. The sar locus encodes a DNA-binding protein that activates the expression of both agr operons. We have cloned the sarA gene, expressed SarA in Escherichia coli and purified the recombinant protein to apparent homogeneity. The purified protein was found to be dimeric in the presence and absence of DNA and to consist mostly of alpha-helices. DNase I footprinting of SarA on the putative regulatory region cis to the agr promoters revealed three high-affinity binding sites composed of two half-sites each. Quantitative electrophoretic mobility shift assays (EMSAs) were used to derive equilibrium binding constants (KD) for the interaction of SarA with these binding sites. An unusual ladder banding pattern was observed in EMSA with a large DNA fragment including all three binding sites. Our data indicate that SarA regulation of the agr operons involves binding to multiple half-sites and may involve other sites located downstream of the promoters.