HMG-CoA synthase; part of the gene cluster that mediates the biosynthesis of 1233A, a natural compound known as an inhibitor of HMG-CoA synthase in the mevalonate pathway and with antibacterial and antifungal activities (PubMed:32139880). This enzyme condenses acetyl-CoA with acetoacetyl-CoA to form HMG-CoA, which is the substrate for HMG-CoA reductase (By similarity). As part of the 1233A biosynthesis cluster, is involved in conferring self-resistance to 1233A (PubMed:32139880). acetoacetyl-CoA + acetyl-CoA + H2O = (3S)-hydroxy-3-methylglutaryl-CoA + CoA + H(+) Expression is increased upon exposure to hygromycin B. Does not affect the ability to produce 1233A and 1233B but leads to increased sensitivity to 1233A. Belongs to the thiolase-like superfamily. HMG-CoA synthase family.