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CcpA-Independent Regulation of Expression of the Mg²⁺-Citrate Transporter Gene citM by Arginine Metabolism in Bacillus subtilis

Molecular Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, 9751 NN Haren, The Netherlands,¹ Instituto de Biología Molecular y Celular de Rosaria (IBR-CONICET) and Departamento de Microbiología, Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, 2000 Rosario, Argentina²

Received 3 September 2002/ Accepted 2 November 2002

Transcriptional regulation of the Mg²⁺-citrate transporter, CitM, the main citrate uptake system of Bacillus subtilis, was studied during growth in rich medium. Citrate in the growth medium was required for induction under all growth conditions. In Luria-Bertani medium containing citrate, citM expression was completely repressed during the exponential growth phase, marginally expressed in the transition phase, and highly expressed in the stationary growth phase. The repression was relieved when the cells were grown in spent Luria-Bertani medium. The addition of a mixture of 18 amino acids restored repression. L-Arginine in the mixture appeared to be solely responsible for the repression, and ornithine appeared to be an equally potent repressor of citM expression. Studies of mutant strains deficient in RocR and SigL, proteins required for the expression of the enzymes of the arginase pathway, confirmed that uptake into the cell and, most likely, conversion of arginine to ornithine were required for repression. Arginine-mediated repression was independent of a functional CcpA, the global regulator protein in carbon catabolite repression (CCR). Nevertheless, CCR-mediated repression was the major mechanism controlling the expression during exponential growth, while the newly described, CcpA-independent arginine-mediated repression was specifically apparent during the transition phase of growth.

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