The Nature of the Damage to Escherichia Coli DNA Induced by γ-irradiation

Summary Quantitative studies of the number of γ-induced single-strand breaks (SSBs) and enzyme-labile sites (ELSs) were performed using the model of Col E1 plasmids, which undergo transition from the covalently closed form (CCF) into the open circular form (OCF) during γ-irradiation of the plasmid-bearing strain E. coli JC 411. By adding 0·5 M EDTA the repair endonucleases of the cell, which effect the transition of ELSs into SSBs during and after γ-irradiation, were totally inhibited. It was found that less than 15 per cent of the number of γ-induced lesions are primarily induced SSBs. About the same yield of SSBs is found if Col E1 plasmids are irradiated in vitro in broth, i.e. in conditions of direct radiation damage. The conclusions are that (1) the contribution of the direct radiation effect in the cell is greater than that of the indirect effect; (2) the main type of γ-induced lesions are the ELSs (most of which-more than 75 per cent-are alkalistable; (3) the enzymatic incision of γ-induced ELSs into SSBs is effected very quickly, mainly during irradiation; and (4) 0·5 M EDTA is a universal inhibitor of repair processes in the cell, including the action of N-glycosidases and endonucleases.