Hydroxymethylglutaryl-CoA synthase; part of the first module of ergosterol biosynthesis pathway that includes the early steps of the pathway, conserved across all eukaryotes, and which results in the formation of mevalonate from acetyl-coenzyme A (acetyl-CoA) (By similarity). Erg13A and erg13B condense acetyl-CoA with acetoacetyl-CoA to form hydroxymethylglutaryl-CoA (HMG-CoA) (By similarity). The first module starts with the action of the cytosolic acetyl-CoA acetyltransferase erg10B that catalyzes the formation of acetoacetyl-CoA. The hydroxymethylglutaryl-CoA synthases erg13A and erg13B then condense acetyl-CoA with acetoacetyl-CoA to form HMG-CoA. The rate-limiting step of the early module is the reduction to mevalonate by the 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductases hmg1 and hmg2. Mevalonate is also a precursor for the extracellular siderophore triacetylfusarinine C (TAFC) (PubMed:16110826, PubMed:22106303) (Probable). acetoacetyl-CoA + acetyl-CoA + H2O = (3S)-hydroxy-3-methylglutaryl-CoA + CoA + H(+) Metabolic intermediate biosynthesis; (R)-mevalonate biosynthesis; (R)-mevalonate from acetyl-CoA: step 2/3. Belongs to the thiolase-like superfamily. HMG-CoA synthase family.