Ionizing radiation and hydrogen peroxide (H2O2) produce many types of oxidative DNA damage sucn as strand breaks, apurinic/apyrimidinic (AP) sites, base modifications and 3′-blocking damage such as 3′-phosphoglycolated and 3′-phosphorylated termini. AP sites and 3′-blocking damage are repairable by exonuclease III and endonuclease IV in Escherichia coli. XthA-nfo double mutants of E. coli, which are deficient in exonuclease III and endonuclease IV. were highly sensitive to lethal and mutagenic effects ot H2O2, compared with the wild-type strains. The pNT180 and pNT186 plasmids containing wild-type nfo and mutant nfa-186 gene, respectively, were introduced into the xthA-nfo mutant. The nfo-186 gene product. Nfo186. retained normal AP endonuclease activity but could 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 effects. From these results it was concluded that 3′-blocking damage rather than an AP site is the primary lesion responsible for both lethal and mutagenic effects of H2O2.
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