ABSTRACTHere we demonstrated that the inhibition of electron flux through the respiratory electron transport chain (ETC) by either the disruption of the gene for the major terminal oxidase (aa3cytochromecoxidase) or treatment with KCN resulted in the induction ofaldencoding alanine dehydrogenase inMycobacterium smegmatis. A decrease in functionality of the ETC shifts the redox state of the NADH/NAD+pool toward a more reduced state, which in turn leads to an increase in cellular levels of alanine by Ald catalyzing the conversion of pyruvate to alanine with the concomitant oxidation of NADH to NAD+. The induction ofaldexpression under respiration-inhibitory conditions inM. smegmatisis mediated by the alanine-responsive AldR transcriptional regulator. The growth defect ofM. smegmatisby respiration inhibition was exacerbated by inactivation of thealdgene, suggesting that Ald is beneficial toM. smegmatisin its adaptation and survival under respiration-inhibitory conditions by maintaining NADH/NAD+homeostasis. The low susceptibility ofM. smegmatistobcc1complex inhibitors appears to be, at least in part, attributable to the high expression level of thebdquinol oxidase inM. smegmatiswhen thebcc1-aa3branch of the ETC is inactivated.IMPORTANCEWe demonstrated that the functionality of the respiratory electron transport chain is inversely related to the expression level of thealdgene encoding alanine dehydrogenase inMycobacterium smegmatis. Furthermore, the importance of Ald in NADH/NAD+homeostasis during the adaptation ofM. smegmatisto severe respiration-inhibitory conditions was demonstrated in this study. On the basis of these results, we propose that combinatory regimens including both an Ald-specific inhibitor and respiration-inhibitory antitubercular drugs such as Q203 and bedaquiline are likely to enable a more efficient therapy for tuberculosis.
Terminal branching of the respiratory electron transport chain in Neisseria meningitidis.
