Prenyltransferases catalyze the consecutive condensation of isopentenyl diphosphate with allylic diphosphates to produce prenyl diphosphates whose chain lengths are absolutely determined by each enzyme. To investigate the mechanism of the consecutive reaction and the determination of the ultimate chain length, a random mutational approach was planned. A geranylgeranyl-diphosphate synthase gene from Sulfolobus acidocaldarius was randomly mutagenized by NaNO2 treatment to construct a library of mutated geranylgeranyl-diphosphate synthase genes on a yeast expression vector. The library was screened for suppression of a pet phenotype of yeast C296-LH3, which is deficient in hexaprenyl-diphosphate synthase. Five mutants that could grow on a YEPG plate, which contained only glycerol as an energy source instead of glucose, were selected from approximately 1,400 mutants. All selected mutated enzymes catalyzed the formation of polyprenyl diphosphates with prenyl chains longer than geranylgeranyl diphosphate. Especially mutants 1, 3, and 5 showed the strongest elongation activity to produce large amounts of geranylfarnesyl diphosphate with a concomitant amount of hexaprenyl diphosphate. Sequence analysis revealed that each mutant contained a few amino acid substitutions and that the mutation of Phe-77, which is located on the fifth amino acid upstream from the first aspartate-rich consensus motif, is the most effective for elongating the ultimate product. Amino acid alignment of known prenyltransferases around this position and our previous observations on farnesyl-diphosphate synthase (Ohnuma, S.-i., Nakazawa, T., Hemmi, H., Hallberg, A.-M., Koyama, T., Ogura, K., and Nishino, T.(1996) J. Biol. Chem. 271, 10087-10095) clearly indicate that the amino acid at the position of all prenyltransferases must regulate the chain elongation.