A mutant endonuclease IV of Escherichia coli loses the ability to repair lethal DNA damage induced by hydrogen peroxide but not that induced by methyl methanesulfonate

A mutant allele of the Escherichia coli nfo gene encoding endonuclease IV, nfo-186, was cloned into plasmid pUC18. When introduced into an E. cofi xthA nfo mutant, the gene product of nfo-186 complemented the hypersensitivity of the mutant to methyl methanesulfonate (MMS) but not to hydrogen peroxide (H202) and bleomycin. These results suggest that the mutant endonuclease IV has normal activity for repairing DNA damages induced by MMS but not those induced by H202 and bleomycin. A missense mutation in the cloned nfo-186 gene, in which the wild-type glycine 149 was replaced by aspartic acid, was detected by DNA sequencing. The wild-type and mutant endonuclease IV were purified to near homogeneity, and their apurinic (AP) endonuclease and 3'-phosphatase activities were determined. No difference was observed in the AP endonuclease activities of the wild-type and mutant proteins. However, 3'-phosphatase activity was dramati-cally reduced in the mutant protein. From these results, it is concluded that the endonuclease IV186 protein is specifically deficient in the ability to remove 3'-terminus-blocking damage, which is required for DNA repair synthesis, and it is possible that the lethal DNA damage by H202 is 3'-blocking damage and not AP-site damage. Partially reduced oxygen species, such as superoxide anion radical, hydrogen peroxide (H202), and hydroxy