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Time Scale for Rejoining of Bacteriophage Deoxyribonucleic Acid Molecules in Superinfected pol⁺ and polA1 Strains of Escherichia coli After Exposure to 4 MeV Electrons

Department of Medical Physics, The Norwegian Radium Hospital, Montebello; Institute of General Genetics, The University of Oslo and Department of Biophysics, Norsk Hydro's Institute for Cancer Research, Montebello, Oslo 3, Norway

ABSTRACT

The time scale for rejoining of radiation-induced deoxyribonucleic acid (DNA) single-strand breaks was measured in the presence and absence of oxygen. The involvement of DNA polymerase I in this repair process was studied. Formation and rejoining of DNA strand breaks were measured in DNA infecting lysogenic pol⁺ and polA1 strains of Escherichia coli irradiated by 4 MeV electrons under identical conditions. Irradiation and transfer to alkaline detergent could be completed in less than 180 ms. The initial yields of DNA strand breaks were identical in pol⁺ and polA1 host cells and four- to fivefold higher in the presence of oxygen than in nitrogen anoxia. Evidence for the existence of a very fast repair process, independent of DNA polymerase I, was not found, since no rejoining of radiation-induced DNA strand breaks was observed during incubation from 45 ms to 3 s. In pol⁺ host cells most of the strand breaks produced in the presence of oxygen were rejoined within the first 30 to 40 s of incubation, whereas no rejoining could be detected within the same period of time in anoxic cells. Since no rejoining of broken DNA molecules was observed in polA1 host cells, it is concluded that the synthetase activity of DNA polymerase I is involved in the rejoining of DNA breaks induced by radiation in the presence of oxygen.