Prenyl transferase; part of the gene cluster that mediates the biosynthesis of the indole diterpenes janthitremanes such as shearinine K or shearinine A (PubMed:26213965). The geranylgeranyl diphosphate (GGPP) synthase janG catalyzes the first step in janthitremane biosynthesis via conversion of farnesyl pyrophosphate and isopentyl pyrophosphate into geranylgeranyl pyrophosphate (GGPP) (PubMed:26213965). Condensation of indole-3-glycerol phosphate with GGPP by the prenyl transferase janC then forms 3-geranylgeranylindole (3-GGI) (PubMed:26213965). Epoxidation by the FAD-dependent monooxygenase janM leads to a epoxidized-GGI that is substrate of the terpene cyclase janB for cyclization to yield paspaline (PubMed:26213965). Paspaline is subsequently converted to 13-desoxypaspaline by the cytochrome P450 monooxygenase janP, via beta-PC-M6 in a series of alpha-face oxidations (Probable). The cytochrome P450 monooxygenase janQ is proposed to carry out sequential beta-face oxidation steps at C-7 and C-13 of 13-desoxypaspaline to form paspalicine and paspalinine respectively (Probable). The indole diterpene prenyltransferase janD may then convert paspalinine into shearinine K which is substrate of janO and/or additional enzymes for oxidation and cyclization to generate shearinine A (Probable). Secondary metabolite biosynthesis. Belongs to the FPP/GGPP synthase family.