Complete auxotrophy for unsaturated fatty acids requires deletion of two sets of genes in Mycobacterium smegmatis.

The synthesis of unsaturated fatty acids in Mycobacterium smegmatis is poorly characterized. Bioinformatic analysis revealed four putative fatty acid desaturases in its genome, one of which, MSMEG_1886, is highly homologous to desA3, the only palmitoyl/stearoyl desaturase present in the Mycobacterium tuberculosis genome. A MSMEG_1886 deletion mutant was partially auxotrophic for oleic acid and viable at 37°C and 25°C, although with a long lag phase in liquid medium. Fatty acid analysis suggested that MSMEG_1886 is a palmitoyl/stearoyl desaturase, as the synthesis of palmitoleic acid was abrogated, while oleic acid contents dropped by half in the mutant. Deletion of the operon MSMEG_1741-1743 (highly homologous to a Pseudomonas aeruginosa acyl-CoA desaturase) had little effect on growth of the parental strain; however the double mutant MSMEG_1886-MSMEG_1741-1743 strictly required oleic acid for growth. The ΔMSMEG_1886-ΔMSMEG_1741 double mutant was able to grow (poorly but better than the ΔMSMEG_1886 single mutant) in solid and liquid media devoid of oleic acid, suggesting a repressor role for ΔMSMEG_1741. Fatty acid analysis of the described mutants suggested that MSMEG_1742-43 desaturates C18:0 and C24:0 fatty acids. Thus, although the M. smegmatis desA3 homologue is the major player in unsaturated fatty acid synthesis, a second set of genes is also involved.