Bifunctional terpene synthase; part of the gene cluster that mediates the biosynthesis of betaestacins (PubMed:29185768, Ref.2). The bifunctional terpene synthase btcA converts isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) into the sesterterpene betaestacin I (PubMed:29185768, Ref.2). 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 (PubMed:29185768, Ref.2). The cytochrome P450 monooxygenase btcB is then responsible for the six-step oxidation of betaestacin I to yield betaestacin II (Ref.2). The roles of the cytochrome P450 monooxygenase btcC and the alpha-ketoglutarate-dependent dioxygenase btcD have not been identified yet (Probable). (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+). In the N-terminal section; belongs to the terpene synthase family. In the C-terminal section; belongs to the FPP/GGPP synthase family.