The CsrR/CsrS two-component system of group A Streptococcus responds to environmental Mg ²⁺

Abstract

Group A streptococci control expression of key virulence determinants via the two-component sensor͞regulator system CsrR͞CsrS. The membrane-bound sensor CsrS is thought to respond to previously unknown environmental signal(s) by controlling phosphorylation of its cognate regulator component CsrR. Phosphorylation of CsrR increases its affinity for binding to the promoter regions of Csr-regulated genes to repress transcription. Here we show that environmental Mg 2؉ concentration is a potent and specific stimulus for CsrR͞CsrS-mediated regulation. We studied the effect of divalent cations on expression of the Csr-regulated hyaluronic acid capsule genes (hasABC) by measuring chloramphenicol acetyltransferase (CAT) activity in a reporter strain of group A Streptococcus carrying a has operon promoter-cat fusion. Addition of Mg 2؉ , but not of Ca 2؉ , Mn 2؉ , or Zn 2؉ , repressed capsule gene expression by up to 80% in a dose-dependent fashion. The decrease in capsule gene transcription was associated with a marked reduction in cell-associated capsular polysaccharide. RNA hybridization analysis demonstrated reduced expression of the Csrregulated hasABC operon, streptokinase (ska), and streptolysin S (sagA) during growth in the presence of 15 mM Mg 2؉ for the wild-type strain 003CAT but not for an isogenic csrS mutant. We propose that Mg 2؉ binds to CsrS to induce phosphorylation of CsrR and subsequent repression of virulence gene expression. The low concentration of Mg 2؉ in extracellular body fluids predicts that the CsrR͞CsrS system is maintained in the inactive state during infection, thereby allowing maximal expression of critical virulence determinants in the human host.

Analysis of the Transcriptome of Group A Streptococcus in Mouse Soft Tissue Infection

“…Based on in vitro studies, high [Mg 2ϩ ] concentration has been proposed as an environmental stimulus responsible for up-regulation of CovR/S expression and therefore repression of the expression of CovR-regulated genes. 102 In vivo microenvironment analysis of magnesium concentrations throughout time, during the course of soft tissue infections may provide insight into the effectors regulating CovR/S expression.…”

Modulation ofcovRExpression inStreptococcus mutansUA159

“…In the absence of CovS, it is possible that Mg 2ϩ activates one of the previously identified TCS of S. mutans (5), which subsequently leads to the transcription of covR or the activation of CovR in S. mutans. However, the increase in the expression of covR under in vitro conditions was observed using concentrations of Mg 2ϩ that are significantly higher than physiological conditions; the concentration of Mg 2ϩ in saliva is 0.22 mM, while the concentration of Mg 2ϩ in blood is approximately 1 mM (22,47). As such, the physiological relevance of Mg 2ϩ for the expression of covR remains to be studied.…”

“…The increase in expression at pH 7.2 is not surprising, given that human saliva can be slightly alkaline. Gryllos et al (21,22) reported that the presence of Mg 2ϩ in the growth medium stimulated the activity of CovR/S, specifically acting as a stimulant for CovS. In the absence of CovS, it is possible that Mg 2ϩ activates one of the previously identified TCS of S. mutans (5), which subsequently leads to the transcription of covR or the activation of CovR in S. mutans.…”

“…Expression of covR is also influenced by the nutritional conditions of the extracellular environment; Steiner and Malke (49) reported that amino acid starvation led to the up-regulation of covR transcription by an as-yet-unidentified mechanism. Studies conducted by Gryllos et al (21,22) demonstrated that extracellular Mg 2ϩ was an effective stimulant of CovR/S, leading to the discovery of a subset of genes under the transcriptional control of CovR that had not previously been identified. Chaussee et al (10) reported that Rgg, a transcriptional regulator, positively regulates the expression of the covR/S system.…”

Effects of bittern water on cariogenic bacteria and saliva secretion

“…Although this system is common in gram-negative bacteria, a recent study has identified the same system in Group A Streptococcus (11), where it acts to repress the target genes thereby reducing its virulence. Intriguingly, the environmental Mg 2+ ion concentration has been shown to be a potent and specific stimulus for this system (11).…”