Sesquiterpene synthase involved in germacrene A biosynthesis (PubMed:21620424). Germacrene A is a precursor of several sesquiterpene lactones (PubMed:21620424). (2E,6E)-farnesyl diphosphate = (+)-(R)-germacrene A + diphosphate Binds 3 Mg(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Monomer. Expressed in glandular trichomes of all aerial tissues, with highest levels in tissues accumulating parthenolide (e.g. flowers and, to some extent, leaves). During flowers development, mostly observed in ovaries during the biosynthetically most active stages (e.g. stages 2, 3 and 4, when petals are progressively opening). 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.