The Nature of the Damage toEscherichia ColiDNA Induced by γ-irradiation

Abstract

Quantitative studies of the number of .gamma.-induced single-strand breaks (SSB) and enzyme-labile sites (ELS) 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 M EDTA the repair endonucleases of the cell, which effect the transition of ELS into SSB during and after .gamma.-irradiation, were totally inhibited. Less than 15% of the number of .gamma.-induced lesions are primarily induced SSB. About the same yield of SBB is found if Col E1 plasmids are irradiated in vitro in broth, i.e., in conditions of direct radiation damage. The contribution of the direct radiation effect in the cell is greater than that of the indirect effect. The main type of .gamma.-induced lesions are the ELS (most of which-more than 75%-are alkali-stable). The enzymatic incision of .gamma.-induced ELS into SSB is effected very quickly, mainly during irradiation; 0.5 M EDTA is a universal inhibitor of repair processes in the cell, including the action of N-glycosidases and endonucleases.