Part of the mycobacterial fatty acid elongation system FAS-II, which is involved in mycolic acid biosynthesis. Catalyzes the elongation of long chain acyl-ACP substrates by the addition of two carbons from malonyl-ACP to an acyl acceptor (PubMed:11600501, PubMed:12023885, PubMed:12464486, PubMed:16873379, PubMed:22017312, PubMed:24108128). Involved in the initial extension of the mycolate chain and forms monounsaturated fatty acids that averaged 40 carbons in length (PubMed:12464486). an ultra-long-chain mono-unsaturated fatty acyl-[ACP] + H(+) + malonyl-[ACP] = a 3-oxo-ultra-long-chain mono-unsaturated fatty acyl-[ACP] + CO2 + holo-[ACP] H(+) + hexadecanoyl-[ACP] + malonyl-[ACP] = 3-oxooctadecanoyl-[ACP] + CO2 + holo-[ACP] Phosphorylation decreases the condensation activity. kcat is 5.3 min(-1) with hexadecanoyl-[ACP] as substrate. kcat is 4.5 min(-1) with eicosanoyl-[ACP] as substrate. kcat is 4.8 min(-1) with malonyl-[ACP] as substrate (PubMed:11600501). kcat is 21 min(-1) with malonyl-CoA as substrate. kcat is 28 min(-1) with malonyl-AcpM as substrate. kcat is 5 min(-1) with hexadecanoyl-CoA as substrate (PubMed:22017312). Lipid metabolism; mycolic acid biosynthesis. Homodimer. Phe-404 residue probably plays a key role in the activation of the enzyme at the beginning of the catalytic cycle. A conformational change of Phe-404, possibly triggered by the substrate, is central for the activation because it switches KasA to the sufficiently reactive zwitterionic state. Phosphorylated in vitro by several Ser/Thr protein kinases (STPKs). Highly phosphorylated in vivo on threonines. Can be dephosphorylated by the Ser/Thr phosphatase PstP. Identified as a drug target (PubMed:10747933, PubMed:12023885, PubMed:22076471, PubMed:27581223, PubMed:32196311, PubMed:32197094). Inhibited by isoniazid (INH), thiolactomycin (TLM) and related analogs (PubMed:10747933, PubMed:12464486, PubMed:19604480, PubMed:24108128). Highly sensitive to cerulenin (PubMed:12023885). Is the biological target of GSK3011724A, an indazole sulfonamide (PubMed:27581223). Inhibited by the indazole JSF-3285, which is a promising preclinical candidate for tuberculosis (PubMed:32197094). Belongs to the thiolase-like superfamily. Beta-ketoacyl-ACP synthases family.