Required for maintaining the appropriate mycolic acid composition and permeability of the envelope on its exposure to acidic pH. Upon expression in E.coli functions weakly as a triacylglycerol synthase, making triacylglycerol (TG) from diolein and long-chain fatty acyl-CoA. Has no wax synthase activity. a 1,2-diacyl-sn-glycerol + an acyl-CoA = a triacyl-sn-glycerol + CoA (9Z)-octadecenoyl-CoA + di-(9Z)-octadecenoylglycerol = 1,2,3-tri-(9Z-octadecenoyl)-glycerol + CoA Glycerolipid metabolism; triacylglycerol biosynthesis. Expression is controlled by VirS. Induced at acidic pH and in macrophages, and in response to low levels of nitric oxide (NO). Inactivation of the mymA operon causes altered cell wall structure, reduced contents and altered composition of mycolic acids along with the accumulation of saturated C24 and C26 fatty acids, and enhanced susceptibility to antibiotics, detergents and acidic pH. Also impairs ability to survive in macrophages. Belongs to the long-chain O-acyltransferase family.