Catalyzes the cyclization of farnesyl diphosphate (FPP) to the sesquiterpene germacrene A. (2E,6E)-farnesyl diphosphate = 5-epi-alpha-selinene + diphosphate Binds 3 Mg(2+) ions per subunit. kcat is 0.044 sec(-1) with FPP as substrate (at pH 8). 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.