DNA breakage, repair and lethality after 125I decay in rec+ and recA strains of Escherichia coli

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 -1960C, 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 induced one DSB, i.e. proportional to (DSB) = 1.0. For the recA strain, proportional to (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 370C before the extraction of DNA, demonstrated significant repair of 125I-induced DSBs by rec+ cells but none by recA cells. For small numbers of decays, there was approximately a 1 : 1 correlation, for either strain, between lethal decays and post-incubation residual DSBs. Comparison with data for larger numbers of decays indicated that a typical rec+ cell can repair no more than three to four DSBs per completed genome (2.5 x 109 daltons). (author)