Geranylgeranyl pyrophosphate synthase; part of the gene cluster that mediates the biosynthesis of the indole diterpenes penitrems. The geranylgeranyl diphosphate (GGPP) synthase ptmG catalyzes the first step in penitrem biosynthesis via conversion of farnesyl pyrophosphate and isopentyl pyrophosphate into geranylgeranyl pyrophosphate (GGPP). Condensation of indole-3-glycerol phosphate with GGPP by the prenyl transferase ptmC then forms 3-geranylgeranylindole (3-GGI). Epoxidation by the FAD-dependent monooxygenase ptmM leads to a epoxidized-GGI that is substrate of the terpene cyclase ptmB for cyclization to yield paspaline. Paspaline is subsequently converted to 13-desoxypaxilline by the cytochrome P450 monooxygenase ptmP, the latter being then converted to paxilline by the cytochrome P450 monooxygenase ptmQ. Paxilline is converted to beta-paxitriol via C-10 ketoreduction by the short-chain dehydrogenase ptmH which can be monoprenylated at the C-20 by the indole diterpene prenyltransferase ptmD. A two-step elimination (acetylation and elimination) process performed by the O-acetyltransferase ptmV and ptmI leads to the production of the prenylated form of penijanthine. The FAD-linked oxidoreductase ptmO then converts the prenylated form of penijanthine into PC-M5 which is in turn transformed into PC-M4 by the aromatic dimethylallyltransferase ptmE. Five sequential oxidative transformations performed by the cytochrome P450 monooxygenases ptmK, ptmU, ptmL, ptmN and ptmJ yield the various penitrem compounds. PtmK, ptmU and ptmM are involved in the formation of the key bicyclic ring of penitrem C via the formation of the intermediates secopenitrem D and penitrem D. PtmL catalyzes the epoxidation of penitrem D and C to yield penitrem B and F, respectively. PtmJ catalyzes the last benzylic hydroxylation to convert penitrem B to prenitrem E and penitrem F to penitrem A