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:12464486, PubMed:16873379). Involved in extension of the mycolate chains to full lengths and produces longer chain multiunsaturated hydrocarbons averaging 54 carbons in length (PubMed:12464486). Essential for resistance to macrophage antimicrobial activity (PubMed:12950920). an ultra-long-chain di-unsaturated fatty acyl-[ACP] + H(+) + malonyl-[ACP] = a 3-oxo-ultra-long-chain di-unsaturated fatty acyl-[ACP] + CO2 + holo-[ACP] H(+) + hexadecanoyl-[ACP] + malonyl-[ACP] = 3-oxooctadecanoyl-[ACP] + CO2 + holo-[ACP] Phosphorylation decreases the condensing activity of KasB and leads to the production of shorter mycolic acids, which is associated to dramatic phenotype changes, such as loss of acid-fastness, increase of cell wall permeability, severe attenuation in infected mice and defect in macrophage colonization. kcat is 1.6 min(-1) with hexadecanoyl-[ACP] as substrate. kcat is 0.6 min(-1) with eicosanoyl-[ACP] as substrate. kcat is 1.4 min(-1) with malonyl-[ACP] as substrate. Lipid metabolism; mycolic acid biosynthesis. Homodimer. Phosphorylated on Thr-313 and Thr-315 (PubMed:24809459). Phosphorylated in vitro by several Ser/Thr protein kinases (STPKs) (PubMed:16873379). Deletion of the gene results in loss of acid-fast staining, which is a simple and rapid diagnostic test for detecting M.tuberculosis (PubMed:17360388, PubMed:22516756). The region between the inner and outer membranes of the mutant, which is composed mainly of cell wall anchored mycolic acids, shows a significant decrease in electron density as compared to the wild type. It suggests that altered mycolic acids patterns in the mutant may have affected the packing of the lipid rich layer of the M.tuberculosis cell envelope, resulting in a reduced electron density of this layer and loss of acid-fastness in light microscopical observation (PubMed:22516756). Mutants synthesize mycolic acids that are 2-6 carbons shorter than wild type and show a significant reduction in the abundance of keto-mycolates (PubMed:12950920, PubMed:17360388). Deletion of the gene affects mycolic acid trans-cyclopropanation, alters the colony morphology and abolishes classic serpentine growth (PubMed:17360388). Mutants exhibit strikingly altered cell wall permeability, leading to a marked increase in susceptibility to lipophilic antibiotics and the host antimicrobial molecules defensin and lysozyme (PubMed:12950920). Deletion mutant can persist in infected immunocompetent mice for up to 600 days without causing disease or mortality (PubMed:17360388). Identified as a drug target (PubMed:10747933, PubMed:12950920). Inhibited by isoniazid (INH), thiolactomycin (TLM) and related analogs (PubMed:10747933, PubMed:12464486). Belongs to the thiolase-like superfamily. Beta-ketoacyl-ACP synthases family. Extended N-terminus.