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

Quantitative studies of the number of gamma-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 gamma-irradiation of the plasmid-bearing strain E. coli JC 411. By adding 0.5 MEDTA the repair endonucleases of the cell, which effect the transition of ELSs into SSBs during and after gamma-irradiation, were totally inhibited. It was found thless than 15 per cent of the number of gamma-induced lesions are primarily induced SSBs. About the saditions 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 gamma-induced lesions are the ELSs (most of which--more than 75 per cent--are alkali-stable; (3) the enzymatic incision of gamma-induced ELSs into SSBs is effected very quickly, mainly during irradiation; and (4) 0.5 MEDTA is a universal inhibitor of repair processes in cell, including the action of N-glycosidases and endonucleases.