DNA strand breakage induced by X irradiation, by X irradiation and alkali treatment, or by X irradiation and subsequent incubation with Escherichia coli X-ray endonuclease was measured by the conversion of PM2 Type I DNA to Type II. The hydroxyl radical was found to mediate the formation of alkali-labile lesions and to a lesser extent simple strand breaks. All alkali-labile lesions were found also to be enzyme-sensitive sites. In addition, when the effective concentration of hydroxyl radicals was reduced by radical scavenging, reducing species appeared to produce enzyme sensitive sites which were not alkali labile. Tryptone-glucose-yeast extract medium, which was used in an attempt to simulate cellular conditions, protected against strand breakage, alkali-labile lesions, and non-alkali-labile enzyme-sensitive damages. Last, the E. coli X-ray endonuclease recognized osmium tetroxide-induced DNA damages which are identical to the radiolysis products of thymine produced by ionizing radiation.

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