Abstract
The Escherichia coli ispB gene encoding octaprenyl diphosphate synthase is responsible for the synthesis of the side chain of isoprenoid quinones. We tried to construct an E. coli ispB-disrupted mutant but could not isolate the chromosomal ispB disrupted mutant unless the ispB gene or its homolog was supplied on a plasmid. The chromosomal ispB disruptants that harbored plasmids carrying the ispB homologs from Haemophilus influenzae and Synechocystis sp. strain PCC6803 produced mainly ubiquinone 7 and ubiquinone 9, respectively. Our results indicate that the function of the ispB gene is essential for normal growth and that this function can be substituted for by homologs of the ispB gene from other organisms that produce distinct forms of ubiquinone.