DNA Breakage, Repair and Lethality after ¹²⁵I Decay in rec⁺ and recA Strains of Escherichia Coli

Summary

Iodine-125 decays by electron capture and is known to cause extensive molecular fragmentation via the Auger effect. ¹²⁵I was incorporated into the DNA of exponentially-growing E. coli K12 AB2487, a recA mutant, and E. coli K12 AB2497, the corresponding rec⁺ strain, as 5-iododeoxyuridine (IUdR), an analogue of thymidine. Radioactive bacteria were stored at −196°C, and samples were periodically assayed for loss of viability and for the induction of double-strand breaks (DSBs) in DNA. Each ¹²⁵I decay in the DNA of either strain induces one DSB, i.e. α(DSB) = 1·0. For the recA strain, α(lethal) = 0·9 and for the rec⁺ strain, 0·4. Assays for biological repair of DSBs, involving incubation of thawed samples in growth-medium at 37°C before the extraction of DNA, demonstrate significant repair of ¹²⁵I-induced DSBs by rec⁺ cells but none by recA cells. For small numbers of decays, there is approximately a 1 : 1 correlation, for either strain, between lethal decays and post-incubation residual DSBs. Comparison with data for larger numbers of decays indicates that a typical rec⁺ cell can repair no more than three to four DSBs per completed genome (2·5 × 10⁹ daltons).