Involved in some intermediate steps for the synthesis of the antibiotic polyketide bacillaene which is involved in secondary metabolism. It catalyzes the aldol condensation between the acetyl group attached to the acyl-carrier-protein AcpK (Ac-AcpK) and a beta-ketothioester polyketide intermediate linked to one of the consecutive thiolation domains of PksL. 3-oxobutanoyl-[ACP] + acetyl-[ACP] + H2O = (3S)-hydroxy-3-methylglutaryl-[ACP] + H(+) + holo-[ACP] Antibiotic biosynthesis; bacillaene biosynthesis. Belongs to the thiolase-like superfamily. HMG-CoA synthase family.