Bifunctional sesterterpene synthase; part of the gene cluster that mediates the biosynthesis of the pentacyclic sesterterpene quiannulatic acid (PubMed:27447198). The first step of the pathway is performed by the sesterterpene synthase (QS) that possesses both prenyl transferase and terpene cyclase activity, converting isopentenyl diphosphate and dimethylallyl diphosphate into geranylfarnesyl diphosphate (GFPP) and further converting GFPP into quiannulatene via an unprecedented cyclization mode which involves three rounds of hydride shifts and two successive C-C bond migrations to construct the 5-6-5-5-5 fused ring (PubMed:27447198). The cytochrome P450 monooxygenase Qnn-P450 then oxidizes quiannulatene at C-19 in 3 successive reactions to afford quiannulatic acid (PubMed:27447198). (2E,6E)-farnesyl diphosphate + isopentenyl diphosphate = (2E,6E,10E)-geranylgeranyl diphosphate + diphosphate (2E,6E,10E,14E)-geranylfarnesyl diphosphate = diphosphate + quiannulatene Binds 4 Mg(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. In the N-terminal section; belongs to the terpene synthase family. In the C-terminal section; belongs to the FPP/GGPP synthase family.