Bifunctional sesterterpene synthase that converts isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) into ophiobolin F (PubMed:32725018). The C-terminal prenyltransferase (PT) domain of AcldOS converts isopentenyl diphosphate and dimethylallyl diphosphate into geranylfarnesyl diphosphate (GFPP), whereas the N-terminal terpene cyclase (TC) domain catalyzes the cyclization of GFPP to ophiobolin F (PubMed:32725018). (2E,6E)-farnesyl diphosphate + isopentenyl diphosphate = (2E,6E,10E)-geranylgeranyl diphosphate + diphosphate (2E,6E,10E)-geranylgeranyl diphosphate + isopentenyl diphosphate = (2E,6E,10E,14E)-geranylfarnesyl diphosphate + diphosphate (2E,6E,10E,14E)-geranylfarnesyl diphosphate + H2O = diphosphate + ophiobolin F Binds 4 Mg(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. The conserved DDXXD motifs as well as the NSE/DTE motif are important for the catalytic activity, presumably through binding to Mg(2+). In the N-terminal section; belongs to the terpene synthase family. In the C-terminal section; belongs to the FPP/GGPP synthase family.