Enzyme characteristics of trans-prenyl diphosphate synthase (Tk-IdsA) from Thermococcus kodakaraensis, which catalyzes the consecutive trans-condensation of isopentenyl diphosphate (C-5) units with allylic diphosphate, were examined. Product analysis revealed that Tk-IdsA is a bifunctional enzyme, farnesyl diphosphate (FPP, C-15)/geranylgeranyl diphosphate (GGPP, C-20) synthase, and mainly yields, both C-15 and C-20. The FPP/GGPP product ratio increases with the rise of the reaction temperature. The kinetic parameters obtained at 70 and 90 degreesC demonstrated that the rise of the temperature elevates the k(0) value for the C-10 allylic substrate to more than those for the C5 and Cry allylic substrates. These data suggest that Tk-IdsA contributes to adjust the membrane composition to the cell growth temperature by modulating its substrate and product specificities. Mutation study indicated that the aromatic side chain of Tyr-81 acts as a steric hindrance to terminate the chain elongation and defines the final product length. (C) 2004 Elsevier Inc. All rights reserved.