Repression of virulence genes by phosphorylation-dependent oligomerizationof CsrR at target promoters in S-pyogenes

csrRS encodes a two-component regulatory system that represses the transcri ption of a number of virulence factors in Streptococcus pyogenes, including the hyaluronic acid capsule and pyrogenic exotoxin B, CsrRS-regulated viru lence factors have diverse functions during pathogenesis and are differenti ally expressed throughout growth. This suggests that multiple signals induc e CsrRS-mediated gene regulation, or that regulated genes respond different ly to CsrR, or both. As a first step in dissecting the csrRS signal transdu ction pathway, we determined the mechanism by which CsrR mediates the repre ssion of its target promoters. We found that phosphorylated CsrR binds dire ctly to all but one of the promoters of its regulated genes, with different affinities. Phosphorylation of CsrR enhances both oligomerization and DNA binding. We defined the binding site of CsrR at each of the regulated promo ters using DNase I and hydroxyl radical footprinting. Based on these result s, we propose a model for differential regulation by CsrRS.