The Bacillus subtilis rocG gene, encoding catabolic glutamate dehydrogenase, was found to be subject to direct CcpA-dependent glucose repression. The effect of CcpA required the presence of both the HPr and Crh proteins. The primary CcpA binding site was identified by mutational analysis and DNase I footprinting. In the absence of inducers of the Roc pathway, rocG was still expressed at a low level due to readthrough transcription. CcpA-dependent repression of rocG readthrough transcription proved to contribute to the slow growth rate of B. subtilis cells in glucose-glutamate medium. Increased readthrough expression of rocG was shown to be partially responsible for the growth defect of ccpA strains in glucose-ammonium medium.Interconversions of ␣-ketoglutarate and glutamate are major links between carbon and nitrogen metabolism. These reactions are catalyzed by several enzymes: glutamate synthase [␣-ketoglutarate ϩ glutamine ϩ NAD(P)H 3 2ϫ glutamate ϩ NAD ϩ (P)], glutamate dehydrogenase (GlutDH) [␣-ketoglutarate ϩ NH 3 ϩ NAD(P)H^glutamate ϩ NAD ϩ (P)], and glutamate-dependent aminotransferases [glutamate ϩ keto acid (or aldehyde)^␣-ketoglutarate ϩ amino acid].In Bacillus subtilis, two genes code for GlutDH proteins. The rocG gene product is the major catabolic GlutDH, while gudB encodes an intrinsically inactive GlutDH (7). Spontaneous gain-of-function mutations in gudB generate a second catabolic GlutDH, GudB1 (7). No anabolic glutamate dehydrogenase is present in B. subtilis, and all de novo synthesis of glutamate is catalyzed by glutamate synthase (3).The B. subtilis rocG gene is a member of the RocR regulon (5). RocR also controls the rocABC and rocDEF operons, whose products catalyze degradation of arginine to glutamate (9,16,17). All three roc transcription units have SigL-dependent promoters, require RocR and AhrC proteins for expression, and are induced by arginine, ornithine, or proline. The RocG-catalyzed reaction (glutamate ϩ NAD ϩ 3 ␣-ketoglutarate ϩ NH 3 ϩ NADH) can be viewed as the final step in the utilization of arginine, ornithine, and proline, providing rapidly metabolizable carbon-or nitrogen-containing compounds for biosynthesis.Many B. subtilis genes involved in utilization of alternative carbon sources are subject to carbon catabolite repression mediated by the CcpA protein (8,11,25,34). We show here that rocG expression is also subject to CcpA-dependent carbon catabolite repression. In the course of this work, we discovered that rocG is transcribed not only from its own SigL-dependent promoter but also by readthrough from an upstream gene. In fact, the known growth defect of ccpA mutants in glucoseammonium medium (13,30,33) can be partially relieved by blocking readthrough transcription of rocG.
MATERIALS AND METHODSBacterial strains and culture media. The B. subtilis strains used in this study are listed in Table 1 or described in the table footnotes. The media and growth conditions for B. subtilis and Escherichia coli strains were described previously (7). TSS minimal medium was suppleme...