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.Escherichia coli, a member of the gram-negative and facultative anaerobic group of bacteria, usually obtains energy for growth through respiration under aerobic and anaerobic conditions, in addition to energy obtained from glycolysis (8, 10). In the respiratory chain of E. coli, two types of quinones, ubiquinone 8 (UQ-8) and menaquinone 8 (MK-8), are essential components (3,5,8,10). UQ-8 is necessary for the transfer of electrons from NADH to succinate in the electron transfer system that has molecular oxygen as the final electron acceptor. MK-8 functions for the transfer of electrons from formate in the anaerobic electron transfer system which uses nitrate as the final electron acceptor. While E. coli has both UQ-8 and MK-8, some microorganisms contain only one type of quinone; e.g., Bacillus species produce only MK and Acetobacter species generally produces only UQ (3, 5). Why does E. coli synthesize two kinds of quinones when other bacteria can subsist on only one? To address this question, mutants defective in the synthesis of UQ, MK, or both have been isolated (19). ubi and men mutants are respiration defective under aerobic and anaerobic conditions, respectively (10,19,20). A strain defective in both ubi and men has been constructed and found to grow very slowly (19). However, it appears likely that the ubi mutation was leaky in this strain, as a small amount of UQ could still be detected (19). We have taken a different approach by isolating a mutant with a deletion of the ispB gene, which encodes octaprenyl diphosphate synthase (1). This enzyme is responsible for the synthesis of the side chain of both UQ and MK, and strains defective in this enzyme should not be able to synthesize active forms of UQ or MK (1). However, we found that it was impossible to obtain an ispB deletion mutant unless the ispB gene or its homolog was supplied on a plasmid. Thus, we suggest that the ispB gene is essential for the normal growth of E. coli.Construction of an ispB-disrupted mutant. To investigate the function of the ispB gene, a plasmid (pTC2) used to disrupt this gene was constructed by inserting the chloramphenicol acetyltransferase (cat) gene into the ispB gene (Fig. 1). We attempted to obtain chloramphenicol-resistant strains by transforming strain FS1576 (recD) (15) wit...