Abstract
Isoprenoids are the most diverse and abundant group of natural products. In plants, farnesyl diphosphate (FPP) and geranylgeranyl diphosphate (GGPP) are precursors to many isoprenoids having essential functions. Terpenoids and sterols are derived from FPP, whereas gibberellins, carotenoids, casbenes, taxenes, and others originate from GGPP. The corresponding synthases (FPP synthase [FPPS] and GGPP synthase [GGPPS]) catalyze, respectively, the addition of two and three isopentenyl diphosphate molecules to dimethylallyl diphosphate. Maize (Zea mays L. cv B73) endosperm cDNAs encoding isoprenoid synthases were isolated by functional complementation of Escherichia coli cells carrying a bacterial gene cluster encoding all pathway enzymes needed for carotenoid biosynthesis, except for GGPPS. This approach indicated that the maize gene products were functional GGPPS enzymes. Yet, the predicted enzyme sequences revealed FPPS motifs and homology with FPPS enzymes. In vitro assays demonstrated that indeed these maize enzymes produced both FPP and GGPP and that the N-terminal sequence affected the ratio of FPP to GGPP. Their functionality in E. coli demonstrated that these maize enzymes can be coupled with a metabolon to provide isoprenoid substrates for pathway use, and suggests that enzyme bifunctionality can be harnessed. The maize cDNAs are encoded by a small gene family whose transcripts are prevalent in endosperm beginning mid development. These maize cDNAs will be valuable tools for assessing the critical structural properties determining prenyl transferase specificity and in metabolic engineering of isoprenoid pathways, especially in cereal crops.