Sesquiterpene synthase involved in the biosynthesis of bisabolene. (2E,6E)-farnesyl diphosphate = (S)-beta-bisabolene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Expressed only in young rhizomes. Not detected in leaves, roots and mature rhizomes. 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.