Sesquiterpene synthase involved in the biosynthesis of volatile compounds (PubMed:21818683). Mediates the conversion of (2E,6E)-farnesyl diphosphate (FPP) into germacrene A (PubMed:21818683). (2E,6E)-farnesyl diphosphate = diphosphate + germacrene A Binds 3 Mg(2+) or Mn(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. Tpsa subfamily.