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

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Abstract
Cells of E. coli containing superinfecting phage .gamma. DNA molecules were irradiated with 4 MeV electrons and lysed at neutral pH; the lysates were sedimented in neutral sucrose gradients to separate .gamma. DNA molecules containing no strand breaks, 1 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 O2. The single-strand breaks were rapidly repaired after irradiation in N2 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. No quadratic dose dependence of double-strand break induction was found up to 500 krad in O2 and up to 1800 krad in N2 anoxia. Also, no difference in double-strand breakage was observed when equal doses in the range of 60-80 krad were delivered either acutely, within 1 s, or intermittently, with time to repair most single-strand breaks before onset of the next radiation pulse. The proposed mechanism whereby 2 independently induced single-strand breaks pair to form a double-strand break is not significant in the biological dose range.