Roles of Alanine Dehydrogenase and Induction of Its Gene in Mycobacterium smegmatis under Respiration-Inhibitory Conditions

Here 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 (aa 3 cytochromec oxidase) or treatment with KCN resulted in the induction ofald encoding 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 ofald expression under respiration-inhibitory conditions inM. smegmatis is mediated by the alanine-responsive AldR transcriptional regulator. The growth defect ofM. smegmatis by respiration inhibition was exacerbated by inactivation of theald gene, suggesting that Ald is beneficial toM. smegmatis in its adaptation and survival under respiration-inhibitory conditions by maintaining NADH/NAD+ homeostasis. The low susceptibility ofM. smegmatis tobcc 1 complex inhibitors appears to be, at least in part, attributable to the high expression level of thebd quinol oxidase inM. smegmatis when thebcc 1 -aa 3 branch of the ETC is inactivated.IMPORTANCE We demonstrated that the functionality of the respiratory electron transport chain is inversely related to the expression level of theald gene encoding alanine dehydrogenase inMycobacterium smegmatis . Furthermore, the importance of Ald in NADH/NAD+ homeostasis during the adaptation ofM. smegmatis to 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.