Catalyzes the cyclization of farnesyl diphosphate (FPP) to the tricyclic sesquiterpene pentalenene, which is the hydrocarbon precursor of the pentalenolactone family of antibiotics produced by a variety of Streptomyces species. (2E,6E)-farnesyl diphosphate = diphosphate + pentalenene Binds 3 Mg(2+) ions per subunit. Sesquiterpene biosynthesis; pentalenene biosynthesis; pentalenene from farnesyl diphosphate: step 1/1. Antibiotic biosynthesis; pentalenolactone biosynthesis. Monomer. 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.