Sesquiterpene synthase that catalyzes the formation of alpha-muurolene, and at lower level (+)-(R)-germacrene A and gamma-muurolene. (2E,6E)-farnesyl diphosphate = alpha-muurolene + diphosphate (2E,6E)-farnesyl diphosphate = diphosphate + gamma-muurolene (2E,6E)-farnesyl diphosphate = (+)-(R)-germacrene A + diphosphate Binds 3 Mg(2+) ions per subunit. 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.