Fusicoccins are a class of diterpene glucosides produced by the plant-pathogenic fungus Phomopsis amygdali. As modulators of 14-3-3 proteins, fusicoccin A functions as a potent activator of plasma memblane H^+-ATPase in plants. From the mycelia, a cDNA encoding fusicocca-2,10(14)-diene (a tricyclic hydrocarbon precursor for fusicoccins) synthase (PaFS) was isolated. This enzyme is chimera protein, which possesses not only cyclase activity but also prenyltransferase (geranylgeranyl diphosphate synthase) activity. The functional analysis of truncated mutants and site-directed mutagenesis demonstrated that PaFS consists of two domains, a terpene cyclase domain at the N terminus and a prenyltransferase domain at the C terminus. Our data base survey suggested that such chimeric terpene synthase also occur in other fungal species. According to these results, we can propose the unusual diterpene synthase family, specific to fungi.