Sesquiterpene synthase involved in the biosynthesis of alpha-isocomene as the major product and detectable amounts of beta-caryophyllene, beta-isocomene, silphinene and modeph-2-ene. Produces exclusively the (-)-(E)-beta caryophyllene enantiomer. (2E,6E)-farnesyl diphosphate = (-)-alpha-isocomene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Highly expressed in roots, lower levels in stems and leaves and detected in disk florets, but not in ray florets. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family. Tpsa subfamily.