3'-BLOCKING DAMAGE OF DNA AS A MUTAGENIC LESION CAUSED BY HYDROGEN-PEROXIDE IN ESCHERICHIA-COLI

Ionizing radiation and hydrogen peroxide (H2O2) produce many types of oxidative DNA damage such as strand breaks, apurinic/apyrimidinic (AP) sites, base modifications and 3'-blocking damage such as 3'-phosphogl ycolated and S'-phosphorylated termini. AP sites and 3'-blocking damag e are repairable by exonuclease III and endonuclease IV in Escherichia coli. XthA-nfo double mutants of E. coli, which are deficient in exon uclease III and endonuclease IV, were highly sensitive to lethal and m utagenic effects of H2O2, compared with the wild-type strains. The pNT 180 and pNT186 plasmids containing wild-type nfo and mutant nfo-186 ge ne, respectively, were introduced into the xthA-nfo mutant. The nfo-18 6 gene product, Nfo186, retained normal AP endonuclease activity but c ould not remove 3'-blocking damage from DNA. The pNT180 corrected the sensitivity of the xthA-nfo mutant to lethal and mutagenic effects of H2O2. On the other hand, the pNT186 did not have any complementation e ffects. From these results it was concluded that 3'-blocking damage ra ther than an AP site is the primary lesion responsible for both lethal and mutagenic effects of H2O2.