Redundant function of cmaA2 and mmaA2 in Mycobacterium tuberculosis cis cyclopropanation of oxygenated mycolates.
The Mycobacterium tuberculosis cell envelope contains a wide variety of lipids and glycolipids, including mycolic acids, long-chain branched fatty acids that are decorated by cyclopropane rings. Genetic analysis of the mycolate methyltransferase family has been a powerful approach to assign functions to each of these enzymes but has failed to reveal the origin of cis cyclopropanation of the oxygenated mycolates. Here we examine potential redundancy between mycolic acid methyltransferases by generating and analyzing M. tuberculosis strains lacking mmaA2 and cmaA2, mmaA2 and cmaA1, or mmaA1 alone. M. tuberculosis lacking both cmaA2 and mmaA2 cannot cis cyclopropanate methoxymycolates or ketomycolates, phenotypes not shared by the mmaA2 and cmaA2 single mutants. In contrast, a combined loss of cmaA1 and mmaA2 had no effect on mycolic acid modification compared to results with a loss of mmaA2 alone. Deletion of mmaA1 from M. tuberculosis abolishes trans cyclopropanation without accumulation of trans-unsaturated oxygenated mycolates, placing MmaA1 in the biosynthetic pathway for trans-cyclopropanated oxygenated mycolates before CmaA2. These results define new functions for the mycolic acid methyltransferases of M. tuberculosis and indicate a substantial redundancy of function for MmaA2 and CmaA2, the latter of which can function as both a cis and trans cyclopropane synthase for the oxygenated mycolates.