Required for efficient production of poly(beta-hydroxybutyrate-co-beta-hydroxyvalerate) (PHBV). Catalyzes the condensation of acetyl-CoA and propionyl-CoA to form beta-ketovaleryl-CoA, and the condensation of two acetyl-CoA molecules to form acetoacetyl-CoA. acetyl-CoA + an acyl-CoA = a 3-oxoacyl-CoA + CoA 2 acetyl-CoA = acetoacetyl-CoA + CoA Mutants are impaired in their ability to produce PHBV when grown on propionate. Belongs to the thiolase-like superfamily. Thiolase family.