Bifunctional terpene synthase; part of the gene cluster that mediates the biosynthesis of betaestacins (Ref.3). The bifunctional terpene synthase btcA converts isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) into the sesterterpene betaestacin I (Ref.3). The C-terminal prenyltransferase (PT) domain of btcA catalyzes formation of GFPP, whereas the N-terminal terpene cyclase (TC) domain catalyzes the cyclization of GFPP into betaestacin I (Ref.3). The cytochrome P450 monooxygenase btcB oxidizes the C25 methyl group of betaestacin I to yield the carboxylic acid betaestacin IV via the alcohol betaestacin III (Ref.3). The cytochrome P450 monooxygenase btcC further catalyzes the multistep oxidation of betaestacin IV to produce several compounds, including betaestacins Va, Vb, Vc and VI (Ref.3). (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 Secondary metabolite biosynthesis; terpenoid biosynthesis. Hexamer. The conserved DDXXD motifs as well as the NSE/DTE motif are important for the catalytic activity, presumably through binding to Mg(2+). Does not affect growth and pigmentation on plateculture, nor conidial formation. In the N-terminal section; belongs to the terpene synthase family. In the C-terminal section; belongs to the FPP/GGPP synthase family.