Hydrophobic surface exposure-mediated CIRCE repression by the Ca2+-sensor, HrcA, in Streptococcus pneumoniae

Abstract Heat shock-induced expression of the CIRCE ( c ontrolling i nverted r epeat of c haperone e xpression) regulon in Streptococcus pneumoniae was examined in the presence of Ca 2+ . In the presence of the Ca 2+ , thermo-resistance and expression of the CIRCE regulon of S. pneumoniae were significantly repressed. In addition, hydrophobicity of the HrcA was increased in the presence of the metal ion suggesting that Ca 2+ induces conformational changes such as exposure of the hydrophobic surfaces of HrcA, which facilitate binding to GroEL. This in turn enhances access to CIRCE and leads to repression of the dnaK and groE operons.