Catalyzes the cyclization of farnesyl diphosphate (FPP) to the tricyclic sesquiterpene pentalenene in the biosynthesis of neopentalenolactone antibiotic. (2E,6E)-farnesyl diphosphate = diphosphate + pentalenene Binds 3 Mg(2+) ions per subunit. kcat is 0.605 sec(-1). Antibiotic biosynthesis; neopentalenolactone biosynthesis. Monomer. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). S.avermitilis does not produce pentalenolactone itself in vivo but instead a group of new metabolites that are neopentalenolactone derivatives. Belongs to the terpene synthase family.