Catabolite repression and induction of the Mg2+-citrate transporter CitM of Bacillus subtilis

In Bacillus subtilis the citM gene encodes the Mg2+-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 beta-galactosidase reporter gene were constructed and integrated int o the amyE site of B. subtilis to study catabolic effects on citM expressio n. In parallel with beta-galactosidase activity, the uptake of Ni2+-citrate in whole cells was measured to correlate citM promoter activity with the e nzymatic activity of the CitM protein. In minimal media, CitM was only expr essed when citrate was present. The presence of glucose in the medium compl etely repressed citM expression; repression was also observed in media cont aining glycerol, inositol, or succinate-glutamate. Studies with B. subtilis mutants defective in the catabolite repression components HPr, Crh, and Cc pA showed that the repression exerted by all these medium components was me diated via the carbon catabolite repression system. During growth on inosit ol and succinate, the presence of glutamate strongly potentiated the repres sion of citM expression by glucose. A reasonable correlation between citM p romoter activity and CitM transport activity was observed in this study, in dicating that the Mg2+-citrate uptake activity of B. subtilis is mainly reg ulated at the transcriptional level.