Formation and Repair of DNA Double-Strand Breaks in Superinfecting Phage λ after Ionizing Irradiation of Escherichia coli Host Cells

Cells of Escherichia coli containing superinfecting phage λ DNA molecules were irradiated with 4-MeV electrons and lysed at neutral pH; the lysates were sedimented in neutral sucrose gradients to separate λ DNA molecules containing no strand breaks, one or more single-strand breaks, or a double-strand break. About 40 single-strand breaks were induced for every double-strand break in both the presence and absence of oxygen. The single-strand breaks were rapidly repaired after irradiation in nitrogen anoxia and subsequent incubation in aerobic growth medium, but no net reduction in the number of double-strand breaks was observed. This is similar to observations after oxic irradiation (E. Boye and R. E. Krisch, Int. J. Radiat. Biol., in press). No quadratic dose dependence of double-strand break induction was found up to 500 krad in oxygen and up to 1800 krad in nitrogen anoxia. Also, no difference in double-strand breakage was observed when equal doses in the range of 60-80 krad were delivered either (i) acutely, within 1 sec, or (ii) intermittently, with time to repair most single-strand breaks before onset of the next radiation pulse. It is concluded that the proposed mechanism whereby two independently induced single-strand breaks pair to form a double-strand break is not significant in the biological dose range.