Involved in sesquiterpene lactone biosynthesis. Produces exclusively (+)-germacrene A. (2E,6E)-farnesyl diphosphate = (+)-(R)-germacrene A + diphosphate Binds 3 Mg(2+) ions per subunit. Optimum pH is 6.8. Secondary metabolite biosynthesis; terpenoid biosynthesis. Expressed in roots, in head core tissue and leaves, in green and etiolated seedlings, and at a much lower level in green leaves. 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.