Involved in the biosynthesis of 12- and 14-membered ring macrolactone antibiotics such as methymycin, neomethymycin, narbomycin and pikromycin (PubMed:9770448, PubMed:10421766). Responsible for removing mis-formed acyl moieties (aberrant decarboxylation) that are bound to the PKS and could block it (PubMed:10421766, PubMed:12368286). Catalyzes the cleavage of methylmalonyl-[acp] (PubMed:12368286). It exhibits some acyl-group specificity, and catalyzes the cleavage of propionyl and butyryl derivatives faster than acetyl malonyl or methylmalonyl derivatives (PubMed:12368286). Antibiotic biosynthesis. Cells lacking this gene produce less than 5% of methymycin, neomethymycin and pikromycin. Belongs to the thioesterase family.