In Corynebacterium glutamicum, cyclic adenosine monophosphate (cAMP) serves as an effector of the global transcriptional regulator GlxR. Synthesis of cAMP is catalyzed by the membrane-bound adenylate cyclase CyaB. In this study, we investigated theconsequences of decreased intracellular cAMP levels in a 1cyaB mutant. While no growth defect of the 1cyaB strain was observed on glucose, fructose, sucrose, or gluconate alone, the addition of acetate to these growth media resulted in a severe growth inhibition, which could be reversed by plasmid-based cyaB expression or by supplementation of the medium with cAMP. The effect was concentration- and pHdependent, suggesting a link to the uncoupling activity of acetate. In agreement, the 1cyaB mutant had an increased sensitivity to the protonophore carbonyl cyanidem-chlorophenyl hydrazone (CCCP). The increased uncoupler sensitivity correlated with a lowered membrane potential of acetate-grown 1cyaB cells compared to wild-type cells. A reduced membrane potential affects major cellular processes, such as ATPsynthesis by F1FO-ATP synthase and numerous transport processes. The impaired membrane potential of the 1cyaB mutant could be due to a decreased expression of the cytochrome bc1-aa3 supercomplex, which is the major contributor of proton-motive force in C. glutamicum. Expression of the supercomplex genes was previously reported to be activated by GlxR-cAMP. A suppressor mutant of the 1cyaB strain with improved growth on acetate was isolated, which carried a single mutation in the genome leading to an Ala131Thr exchange in GlxR. Introduction of this point mutation into the original 1cyaB mutant restored the growth defect on acetate. This supported the importance of GlxR for the phenotype of the 1cyaB mutant and, more generally, of the cAMP-GlxR system for the control of energy metabolism in C. glutamicum.
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