Sesquiterpene synthase that catalyzes the formation of germacrene A. Can use farnesyl diphosphate as substrate, but not geranyl diphosphate or geranylgeranyl diphosphate. Beta-elemene, the initially measured product in the assay, is derived nonenzymatically from germacrene A. (2E,6E)-farnesyl diphosphate = (+)-(R)-germacrene A + diphosphate Secondary metabolite biosynthesis; terpenoid biosynthesis. Expressed in young leaves. Detected in trichomes and cones. 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.