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. Possesses a clear preference for long-chain acyl-CoA substrates rather than acyl-ACP primers, and shows only weak activity with acetyl-CoA. Its substrate specificity determines the biosynthesis of mycolic acid fatty acid chain, which is characteristic of mycobacterial cell wall. acetyl-CoA + H(+) + malonyl-[ACP] = 3-oxobutanoyl-[ACP] + CO2 + CoA H(+) + hexanoyl-CoA + malonyl-[ACP] = 3-oxooctanoyl-[ACP] + CO2 + CoA H(+) + malonyl-[ACP] + octanoyl-CoA = 3-oxodecanoyl-[ACP] + CO2 + CoA decanoyl-CoA + H(+) + malonyl-[ACP] = 3-oxododecanoyl-[ACP] + CO2 + CoA dodecanoyl-CoA + H(+) + malonyl-[ACP] = 3-oxotetradecanoyl-[ACP] + CO2 + CoA H(+) + malonyl-[ACP] + tetradecanoyl-CoA = 3-oxohexadecanoyl-[ACP] + CO2 + CoA H(+) + hexadecanoyl-CoA + malonyl-[ACP] = 3-oxooctadecanoyl-[ACP] + CO2 + CoA Sensitive to thiolactomycin and resistant to cerulenin in vitro. Lipid metabolism; fatty acid biosynthesis. Lipid metabolism; mycolic acid biosynthesis. Homodimer. The last Arg residue of the ACP-binding site is essential for the weak association between ACP/AcpP and FabH. Was identified as a high-confidence drug target. Belongs to the thiolase-like superfamily. FabH family.