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 efficiently catalyze the conversion of palmitoleoyl-ACP (cis-hexadec-9-enoyl-ACP) to cis-vaccenoyl-ACP (cis-octadec-11-enoyl-ACP), an essential step in the thermal regulation of fatty acid composition. a fatty acyl-[ACP] + H(+) + malonyl-[ACP] = a 3-oxoacyl-[ACP] + CO2 + holo-[ACP] (9Z)-hexadecenoyl-[ACP] + H(+) + malonyl-[ACP] = 3-oxo-(11Z)-octadecenoyl-[ACP] + CO2 + holo-[ACP] Lipid metabolism; fatty acid biosynthesis. Homodimer. More than twofold more abundant in the large cell variant (LCV) stage than in the small cell variant (SCV) stage (at protein level). LCVs are more metabolically active than SCVs. Belongs to the thiolase-like superfamily. Beta-ketoacyl-ACP synthases family.