INDUCTION OF ESCHERICHIA-COLI OH(8)GUA ENDONUCLEASE BY OXIDATIVE STRESS - ITS SIGNIFICANCE IN AEROBIC LIFE

The induction of 8-hydroxyguanine (oh(8)Gua) endonuclease, a DNA repai r enzyme for an oxidatively modified guanine, oh(8)Gua was studied in various growth conditions in Escherichia coli (AB1157). Anaerobically grown E. coli were found to have a very low activity of this enzyme wh ile aerobically grown cells showed activity about 20 times that of the anaerobic level. Under the same condition, superoxide dismutase (SOD) showed about 6-fold increase in activity. A shift in growth condition s from anaerobic to aerobic resulted in rapid induction of this enzyme , and this induction was blocked (but not completely) by chloramphenic ol. It is indicated that molecular oxygen is an effective stimulator t o the induction of this enzyme and its induction depends partly on pro tein synthesis. Superoxide-producing compounds such as paraquat and me nadione also increased the activity of endonuclease as well as SOD, bu t H2O2 showed no effect, Thus, superoxides are also implied as a stimu lator. In contrast, hyperoxia induced only SOD not the endonuclease. T his induction of the endonuclease by hyperoxia was only observed in a SOD-deficient strain (QC774). The aerobic activity of the endonuclease in QC774 was the same as that of wild types (AB1157, GC4468). It is i mplied that the responsiveness of oh(8)Gua endonuclease to superoxides is less sensitive than that of SOD. The endonuclease was also induced by a temperature shift from 30 to 43 degrees C and treatment with nal idixic acid. Among the stimuli used, molecular oxygen seems to be most effective for its induction. The inducible nature of this enzyme will serve as an important mechanism for the protection of oxidative DNA d amage in the aerobic environment.