ABSTRACT
Exonuclease III (Exo III) and endonuclease IV (Endo IV) play a critical role in the base excision repair (BER) of Escherichia coli. Both are endowed with AP endonucleolytic activity, cleaving the 5' phosphodiester bond adjacent to spontaneous or induced abasic sites in DNA. Although mutants defective in Exo III (xthA) are usually hypersensitive to oxidative agents such as hydrogen peroxide, near-UV-light and X-rays, mutants defective in Endo IV (nfo) are not as sensitive as the xthA strain. To further investigate the roles of these AP endonucleases in DNA repair, we evaluated the sensitivity and mutagenesis of xthA and nfo strains after UVB and compared with UVC light. Our results revealed that xthA but not nfo strain was hypersensitive to UVB. The use of Fe(+2) ion chelator (dipyridyl), prior to irradiation, completely protected the xthA mutant against UVB lethal lesions, suggesting the generation of toxic oxidative lesions mediated by transition metal reactions. The nfo strain displayed increased UVB-induced mutagenesis, which was significantly suppressed by pre-treatment with dipyridyl. Although xthA strain did not display increased mutagenesis after UVC and UVB treatments, this phenotype was not related to xthA mutation, but rather to an unknown secondary mutation specifying an antimutator phenotype. After UVB irradiation, the base substitution spectra of nfo strain revealed a bias towards AT-->GC transitions and GC-->CG transversions, which were also suppressed by previous treatment with the iron chelator. Overall, on the basis of the differential sensitivities and mutational spectra displayed after UVC and UVB treatments, we propose a role for Endo IV and Exo III to counteract DNA damage induced by the oxidative counterpart of UVB in E.coli.
