Induction by gamma irradiation of double-strand breaks of Escherichia coli chromosomes and their role in cell lethality

Viscoelastometric measurements of DNA from γ-irradiated bacteria were used to identify the induction of double-strand breaks (DSBs) in the chromosome of Escherichia coli. It is shown by means of inhibitors of repair endonucleases and different repair mutants that most DSBs in DNA of E. coli, γ-irradiated in buffer, arise from enzymatic incision of primary γ-damages; therefore, previous conclusions regarding DSB repair must be reconsidered. Based on these results, much of the repairable damage is single-strand breaks, and this damage can initiate formation of gaps and ultimately, when repair is insufficient, generation of enzymatically caused DSBs. After extensive repair, the first residual DSB in the E. coli chromosome is generated at approx.160 Gray (Gy), which corresponds to the D37 dose. We propose that DSBs induced directly by γ-irradiation are not repaired in wild-type strains. In a recently isolated γ-resistant strain, E. coli Gam/sup r/444, the dose required for observation of DSB after postirradiation incubation is 1000 Gy, which corresponds to the D37 of this strain. The resistance is proposed to be due to an ability to repair genuine DSBs