Involved in some intermediate steps for the synthesis of the antibiotic polyketide bacillaene which is involved in secondary metabolism. It decarboxylates selectively the malonyl group attached on the acyl-carrier-protein AcpK (Mal-AcpK). H(+) + malonyl-[ACP] = acetyl-[ACP] + CO2 Antibiotic biosynthesis; bacillaene biosynthesis. Belongs to the thiolase-like superfamily. Beta-ketoacyl-ACP synthases family.