Involved in the type II fatty acid elongation cycle. Catalyzes the elongation of a wide range of acyl-ACP by the addition of two carbons from malonyl-ACP to an acyl acceptor. Can also use unsaturated fatty acids. Catalyzes a key reaction in unsaturated fatty acid (UFA) synthesis, the elongation of the cis-3-decenoyl-ACP produced by FabA. a fatty acyl-[ACP] + H(+) + malonyl-[ACP] = a 3-oxoacyl-[ACP] + CO2 + holo-[ACP] (3Z)-decenoyl-[ACP] + H(+) + malonyl-[ACP] = 3-oxo-(5Z)-dodecenoyl-[ACP] + CO2 + holo-[ACP] Lipid metabolism; fatty acid biosynthesis. Homodimer. Belongs to the thiolase-like superfamily. Beta-ketoacyl-ACP synthases family.