Sesquiterpene synthase producing exclusively alpha-farnesene. Associated with the production of sesquiterpenes responsible for the aroma of the fruit. (2E,6E)-farnesyl diphosphate = (3E,6E)-alpha-farnesene + diphosphate Binds 3 Mg(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Expressed in the rind tissues of ripe fruits. Expressed during ripening. 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.