Involved in the synthesis of beta-D-mannosyl phosphomycoketide (MPM), an antigenic mycobacterial polyketide (PubMed:15611286, PubMed:18613748). Binds a fatty acyl-CoA as a starter unit, and extends it by five rounds of alternative additions of malonyl-CoA and methylmalonyl-CoA extender units. Depending on the starter unit, the enzyme forms mycoketide-CoAs of different lengths (PubMed:15611286, PubMed:18613748). Shows preference for small-/medium-chain starter fatty acyl substrates (PubMed:18613748). Uses a hybrid modularly iterative mechanism, by forming a supramolecular assembly to perform repetitive cycles of iterations (PubMed:18613748). 5 (S)-methylmalonyl-CoA + a medium-chain fatty acyl-CoA + 32 H(+) + 5 malonyl-CoA + 22 NADPH = a mycoketide-CoA + 10 CO2 + 10 CoA + 11 H2O + 22 NADP(+) Lipid metabolism; fatty acid metabolism. Forms a large supramolecular assembly mediated through specific interactions between the N- and C-terminus linkers. Contains 12 catalytic domains constituting two modules. Both modules contain a complete set of catalytic and auxiliary domains. Deletion of the gene completely abolishes the ability of the strain to activate CD1c-restricted T cells (PubMed:15611286). Growth of the mutant is attenuated in mouse alveolar macrophage cell line, and the virulence of the mutant in vivo is highly attenuated in a murine model (PubMed:12819062). Sirakova et al. showed that disruption of the gene causes a drastic decrease in the synthesis of dimycocerosyl phthiocerol (DIM), but Matsunaga et al. demonstrated later that DIM is produced by the disruption mutant (PubMed:12819062, PubMed:15611286). The alkane moiety distinguishes these mycobacterial lipid antigens from mammalian mannosyl beta-1-phosphodolichol (MPD) and is necessary for activation of CD1c-restricted T cells.