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 (PubMed:9013860, PubMed:8910376, PubMed:19679654, PubMed:22017312). Can also use unsaturated fatty acids (PubMed:3076377, PubMed:8910376, PubMed:19679654). Catalyzes a key reaction in unsaturated fatty acid (UFA) synthesis, the elongation of the cis-3-decenoyl-ACP produced by FabA (PubMed:19679654). Can use acyl chains from C-6 to C-14 (PubMed:9013860, PubMed:8910376, PubMed:19679654, PubMed:22017312). Has an absolute requirement for an ACP substrate as the acyl donor, and no activity is detected when both substrates are based on CoA (PubMed:22017312). 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] H(+) + hexanoyl-[ACP] + malonyl-[ACP] = 3-oxooctanoyl-[ACP] + CO2 + holo-[ACP] H(+) + malonyl-[ACP] + octanoyl-[ACP] = 3-oxodecanoyl-[ACP] + CO2 + holo-[ACP] decanoyl-[ACP] + H(+) + malonyl-[ACP] = 3-oxododecanoyl-[ACP] + CO2 + holo-[ACP] dodecanoyl-[ACP] + H(+) + malonyl-[ACP] = 3-oxotetradecanoyl-[ACP] + CO2 + holo-[ACP] H(+) + malonyl-[ACP] + tetradecanoyl-[ACP] = 3-oxohexadecanoyl-[ACP] + CO2 + holo-[ACP] (7Z)-tetradecenoyl-[ACP] + H(+) + malonyl-[ACP] = 3-oxo-(9Z)-hexadecenoyl-[ACP] + CO2 + holo-[ACP] kcat is 6.6 min(-1) with malonyl-[ACP] as substrate in the presence of dodecanoyl-[ACP]. kcat is 3.4 min(-1) with dodecanoyl-[ACP] in the presence of malonyl-[ACP]. Lipid metabolism; fatty acid biosynthesis. Homodimer. Mainly activated by FadR, but minor repression is also conferred by FabR (PubMed:11859088, PubMed:21276098). Belongs to the thiolase-like superfamily. Beta-ketoacyl-ACP synthases family.