Sesquiterpene synthase involved in germacrene A biosynthesis. Produces also dditional sesquiterpene products, including 4,5-di-epi-aristolochene, eremophilene, alpha-selinene. (2E,6E)-farnesyl diphosphate = (+)-(R)-germacrene A + diphosphate Binds 3 Mg(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. 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.