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 specificity for branched chain acyl-CoA, determining the biosynthesis of branched-chain of fatty acids instead of straight-chain. acetyl-CoA + H(+) + malonyl-[ACP] = 3-oxobutanoyl-[ACP] + CO2 + CoA H(+) + malonyl-[ACP] + propanoyl-CoA = 3-oxopentanoyl-[ACP] + CO2 + CoA butanoyl-CoA + H(+) + malonyl-[ACP] = 3-oxohexanoyl-[ACP] + CO2 + CoA H(+) + malonyl-[ACP] + pentanoyl-CoA = 3-oxoheptanoyl-[ACP] + CO2 + CoA H(+) + hexanoyl-CoA + malonyl-[ACP] = 3-oxooctanoyl-[ACP] + CO2 + CoA H(+) + heptanoyl-CoA + malonyl-[ACP] = 3-oxononanoyl-[ACP] + CO2 + CoA 2-methylpropanoyl-CoA + H(+) + malonyl-[ACP] = 4-methyl-3-oxopentanoyl-[ACP] + CO2 + CoA 3-methylbutanoyl-CoA + H(+) + malonyl-[ACP] = 5-methyl-3-oxohexanoyl-[ACP] + CO2 + CoA (2S)-2-methylbutanoyl-CoA + H(+) + malonyl-[ACP] = (4S)-4-methyl-3-oxohexanoyl-[ACP] + CO2 + CoA Lipid metabolism; fatty acid biosynthesis. Homodimer. Down-regulated by FapR. The last Arg residue of the ACP-binding site is essential for the weak association between ACP/AcpP and FabH. Belongs to the thiolase-like superfamily. FabH family.