Catalyzes the condensation reaction of fatty acid synthesis by the addition to an acyl acceptor of two carbons from malonyl-ACP. Catalyzes the first condensation reaction which initiates fatty acid synthesis and may therefore play a role in governing the total rate of fatty acid production. Possesses both acetoacetyl-ACP synthase and acetyl transacylase activities. Has some substrate preference for acetyl-CoA. Its substrate specificity determines the biosynthesis of straight-chain of fatty acids. acetyl-CoA + H(+) + malonyl-[ACP] = 3-oxobutanoyl-[ACP] + CO2 + CoA Lipid metabolism; fatty acid biosynthesis. Homodimer. The last Arg residue of the ACP-binding site is essential for the weak association between ACP/AcpP and FabH. Inhibited by the SB418011 antibiotic. Not inhibited by cerulenin, and weakly inhibited by thiolactomycin. Belongs to the thiolase-like superfamily. FabH family.