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. Involved in the initial extension of the mycolate chain and forms monounsaturated fatty acids that averaged 40 carbons in length. 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] Lipid metabolism; mycolic acid biosynthesis. Belongs to the thiolase-like superfamily. Beta-ketoacyl-ACP synthases family.