Enzymatic photoreactivation of Escherichia coli after ionizing irradiation: chemical evidence for the production of pyrimidine dimers

By comparing the results for a uvrA recA and a uvrA uvrB recA phr strain, we conclude that the photoreactivation (PR) observed after ionizing irradiation involves the same enzyme that is responsible for the PR of ultraviolet (uv) radiation-induced cyclobutadipyrimidines (Py<>Py). A comparison of the photoreactivable sectors in a uvrA recA strain after uv irradiation and 137Cs γ irradiation indicates that 10 krad of γ radiation produces about 0.07 Jm-2-equivalents of 254-nm radiation-induced photoreactivable damage. After 400 krad, an acid hydrolysate of the isolated DNA revealed the presence of thymine-containing Py<>Py, as evidenced by their chromatographic properties and their photochemical reversibility. The yield of thymine dimers after 400 krad of γ irradiation, as detected chromatographically, was in close agreement with the yield that was predicted from our PR studies. Thus, the PR observed after ionizing irradiation in strains that are blocked in excision repair and in postreplication repair is due to the production of trace amounts of Py<>Py