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. Optimum pH is 8.2-8.4. 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+). All the mutants (Ref.3 and PubMed:12083921) retained substantial pentalenene synthase activity accompanied by production of varying proportions of abortive cyclization products such as germacrene A, protoilludene and beta-caryophyllene. This is a true derailment or diversion of the normal cyclization reaction, and not simply the consequence of trapping of a normally cryptic, cationic intermediate. Belongs to the terpene synthase family.