The possibility that the radiation-induced radicals responsible for cell killing and for DNA strand breakage are different was studied by determining their relative reactivities towards oxygen under physiological conditions. The yield of x-ray-induced DNA single-strand breaks was determined by measuring the breaks in covalently closed circular λ DNA molecules superinfecting lysogenic strains of Escherichia coli K12. In the presence of oxygen the frequency of DNA breakage is four- to fivefold higher than under nitrogen anoxia. The concentration of oxygen that increases DNA breakage to half the maximal level, k, is 0.5 μM and is similar in repair proficient host cells and in cells mutated in the uvrA and/or recA genes. Intermediate levels of DNA strand breakage seen at low concentrations of oxygen are dependent on the concentration of cellular sulfhydryl compounds. The effectiveness of oxygen under comparable conditions in enhancing the radiation-induced killing of the non-lysogenic bacterial strains was determined. In cell killing k is 8 μM for repair-proficient cells as well as for cells mutated in the uvrA and/or recA genes. The 16-fold higher reactivity of oxygen in DNA breakage than in cell killing strongly suggests that radiation-induced reactions of oxygen leading to increased lethality differ from those promoting DNA single-strand breaks.

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