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. Can use a wide range of acyl-CoAs as the primer substrate in vitro, with a slight preference for short, medium-straight chain acyl-CoAs. Can also use branched-chain acyl-CoAs and acetyl-CoA