REDUCTION OF RADIATION CYTOTOXICITY BY HUMAN APURINIC ENDONUCLEASE IN A RADIATION-SENSITIVE ESCHERICHIA-COLI MUTANT

Ionizing radiation produces a variety of DNA damage through active oxygen species such as the superoxide radical (O2/·-), the hydroxyl radical (OH·), and hydrogen peroxide (H2O2). The removal of alkylation-induced apurinic (AP) sites and 3'-blocking deoxyribose fragments by exonuclease III (xth) and endonuclease IV (nfo) has been well demonstrated in E. coli. Very little information on the repair of radiation-induced DNA damage by human apurinic endonuclease is available. We examined the biological roles of the human AP endonuclease in the repair of radiation-induced DNA damage. An expression vector was constructed with human APE cDNA and transformed into radiation-sensitive E. coli mutants (xth- and nfo-). The radiation cytotoxicity was assayed by cell survival curves. Expression of human AP endonuclease in E. coli confirmed that AP endonuclease could complement exonuclease III functionally to diminish radiation cytotoxicity. In contrast, AP endonuclease was not able to increase resistance to H2O2, owing to a poor 3'-termini repair. We also tested whether AP endonuclease is a limiting factor for radiation cytotoxicity by using a plasmid nicking assay. Cell extracts from mutant cells with or without AP endonuclease expression were added to irradiated supercoiled plasmid DNA. The inability to convert supercoiled plasmid DNA to relaxed or linear forms suggested that there were large accumulations of AP sites in the mutant cell extracts. The AP endonuclease activities estimated from the plasmid nicking assays are 20- fold lower in the cell extracts of AP endonuclease-deficient mutant than in AP endonuclease-expressing cells. Therefore, AP endonuclease is a limiting step of base excision repair for the radiation-sensitive E. coli mutant, BW528. Our results conclude that AP endonuclease is responsible for the removal of AP sites from γ-radiation-induced base damage in E. coli.