Catabolite Repression and Induction of the Mg(2+)-Citrate Transporter CitM of Bacillus subtilis

In Bacillus subtilis the citM gene encodes the Mg(2+)-citrate transporter. A target site for carbon catabolite repression (cre site) is located upstream of citM. Fusions of the citM promoter region, including the cre sequence, to the β-galactosidase reporter gene were constructed and integrated into the amyE site of B. subtilis to study catabolic effects on citM expression. In parallel with β-galactosidase activity, the uptake of Ni(2+)-citrate in whole cells was measured to correlate citM promoter activity with the enzymatic activity of the CitM protein. In minimal media, CitM was only expressed when citrate was present. The presence of glucose in the medium completely repressed citM expression; repression was also observed in media containing glycerol, inositol, or succinate-glutamate. Studies with B. subtilis mutants defective in the catabolite repression components HPr, Crh, and CcpA showed that the repression exerted by all these medium components was mediated via the carbon catabolite repression system. During growth on inositol and succinate, the presence of glutamate strongly potentiated the repression of citM expression by glucose. A reasonable correlation between citM promoter activity and CitM transport activity was observed in this study, indicating that the Mg(2+)-citrate uptake activity of B. subtilis is mainly regulated at the transcriptional level.