The CsrR/CsrS two-component system of group A Streptococcus responds to environmental Mg2+.

Group A streptococci control expression of key virulence determinants via the two-component sensorregulator system CsrRCsrS. 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 CsrRCsrS-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 Csr-regulated 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 CsrRCsrS system is maintained in the inactive state during infection, thereby allowing maximal expression of critical virulence determinants in the human host.