Abstract
In chronic infection, Mycobacterium tuberculosis bacilli are thought to enter a metabolic program that provides sufficient energy for maintenance of the protonmotive force, but is insufficient to meet the demands of cellular growth. We sought to understand this metabolic downshift genetically by targeting succinate dehydrogenase, the enzyme which couples the growth processes controlled by the TCA cycle with the energy production resulting from the electron transport chain. M. tuberculosis contains two operons which are predicted to encode succinate dehydrogenase enzymes (sdh-1 and sdh-2); we found that deletion of Sdh1 contributes to an inability to survive long term stationary phase. Stable isotope labeling and mass spectrometry revealed that Sdh1 functions as a succinate dehydrogenase during aerobic growth, and that Sdh2 is dispensable for this catalysis, but partially overlapping activities ensure that the loss of one enzyme can incompletely compensate for loss of the other. Deletion of Sdh1 disturbs the rate of respiration via the mycobacterial electron transport chain, resulting in an increased proportion of reduced electron carrier (menaquinol) which leads to increased oxygen consumption. The loss of respiratory control leads to an inability to recover from stationary phase. We propose a model in which succinate dehydrogenase is a governor of cellular respiration in the adaptation to low oxygen environments.