Sesquiterpene synthase involved in germacrene A biosynthesis (PubMed:19580670). Germacrene A is a precursor of several sesquiterpene lactones (PubMed:19580670). (2E,6E)-farnesyl diphosphate = (+)-(R)-germacrene A + diphosphate Binds 3 Mg(2+) ions per subunit. Optimum pH is 7.7. Secondary metabolite biosynthesis; terpenoid biosynthesis. Monomer. Mainly expressed in sunflower trichomes. 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.