Involved in the production of the isoprenoid antibiotic terpentecin. Converts geranylgeranyl diphosphate (GGDP) into terpentedienol diphosphate (TDP) by a protonation-initiated cyclization. (2E,6E,10E)-geranylgeranyl diphosphate = terpentedienyl diphosphate Optimum pH is 6.8. Optimum temperature is 25-30 degrees Celsius. Antibiotic biosynthesis. Monomer. Mutants do not produce terpentecin. Belongs to the terpene synthase family.