A Crh-specific function in carbon catabolite repression in Bacillus subtilis

Carbon catabolite repression in Bacillus subtilis is mediated by phosphorylation of the phosphoenolpyruvate:carbohydrate phosphotransferase system intermediate HPr at a serine residue catalyzed by HPr kinase. The orthologous protein Crh functions in a similar way, but, unlike HPr, it is not functional in carbohydrate uptake. A specific function for Crh is not known. The role of HPr and Crh in repressing the citM gene encoding the Mg(2+)-citrate transporter was investigated during growth of B. subtilis on different carbon sources. In glucose minimal medium, full repression was supported by both HPr and Crh. Strains deficient in Crh or the regulatory function of HPr revealed the same repression as the wild-type strain. In contrast, in a medium containing succinate and glutamate, repression was specifically mediated via Crh. Repression was relieved in the Crh-deficient strain, but still present in the HPr mutant strain. The data are the first demonstration of a Crh-specific function in B. subtilis and suggest a role for Crh in regulation of expression during growth on substrates other than carbohydrates.