Geranylgeranyl pyrophosphate synthase; part of the gene cluster that mediates the biosynthesis of aphidicolin, a specific inhibitor of eukaryotic DNA synthesis and DNA polymerase alpha (PubMed:14745177, PubMed:21897020). The geranylgeranyl pyrophosphate synthase GGS is required for supplying a sufficient amount of geranylgeranyl diphosphate (GGDP), the general precursor of diterpenes (PubMed:21897020). The diterpene synthase ACS then catalyzes the conversion of geranylgeranyl diphosphate to aphidicolan-16-beta-ol via the intermediate syn-copalyldiphosphate (syn-CDP) (PubMed:11457369, PubMed:21897020). In addition to aphidicolan-16-beta-ol, the enzyme produces also low levels of amphidicol-15-ene and amphidicol-16-ene (PubMed:11457369). The cytochrome P450 monooxygenase P450-2 then catalyzes the two-step hydroxylation from aphidicolan-16-beta-ol to 3-deoxyaphidicolin via a 17,3-deoxyaphidicolin intermediate (PubMed:21897020). Finally, the cytochrome P450 monooxygenase P450-1 converts 3-deoxyaphidicolin to aphidicolin (PubMed:21897020). dimethylallyl diphosphate + isopentenyl diphosphate = (2E)-geranyl diphosphate + diphosphate (2E)-geranyl diphosphate + isopentenyl diphosphate = (2E,6E)-farnesyl diphosphate + diphosphate (2E,6E)-farnesyl diphosphate + isopentenyl diphosphate = (2E,6E,10E)-geranylgeranyl diphosphate + diphosphate Binds 3 Mg(2+) ions per subunit. Mycotoxin biosynthesis. Belongs to the FPP/GGPP synthase family.