Remodeling of Mycobacterium tuberculosis lipids regulates prpCD during acid growth arrest

Mycobacterium tuberculosis (Mtb) establishes a state of non-replicating persistence when it is cultured at acidic pH with glycerol as a sole carbon source. Growth can be restored by spontaneous mutations in the ppe51 gene or supplementation with pyruvate, supporting that acid growth arrests is a genetically controlled, adaptive process and not simply a physiological limitation associated with acidic pH. Transcriptional profiling identified the methylcitrate synthase and methylcitrate dehydratase genes ( prpC and prpD , respectively) as being selectively induced during acid growth arrest. prpCD along with isocitrate lyase ( icl ) enable Mtb to detoxify propionyl-CoA through the methylcitrate cycle. The goal of this study was to examine mechanisms underlying the regulation of prpCD during acid growth arrest. Induction of prpCD during acid growth arrest was reduced when the medium was supplemented with vitamin B12 (which enables an alternative propionate detoxification pathway) and enhanced in an icl mutant (which is required for the propionate detoxification), suggesting that Mtb is responding to elevated levels of propionyl-CoA during acidic growth arrest. We hypothesized that an endogenous source of propionyl-CoA generated during metabolism of methyl-branched lipids may be regulating prpCD . Using Mtb radiolabeled with 14C-propionate or 14C-acetate, it was observed that lipids are remodeled during acid growth arrest, with triacylglycerol being catabolized and sulfolipid and trehalose dimycolate being synthesized. Blocking TAG lipolysis using the lipase inhibitor tetrahydrolipstatin, resulted in enhanced prpC induction during acid growth arrest, suggesting that lipid remodeling may function, in part, to detoxify propionate. Notably, prpC was not induced during acid growth arrest when using lactate instead of glycerol. We propose that metabolism of glycerol at acidic pH may result in the accumulation of propionyl-CoA and that lipid remodeling may function as a detoxification mechanism.