DNA Breakage, Repair and Lethality after125I Decay inrec+andrecA Strains ofEscherichia Coli

Abstract

125I 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), a thymidine analogue. Radioactive bacteria were stored at -196.degree. 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 1 DSB, i.e., .alpha.(DSB) = 1.0. For the recA strain, .alpha.(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.degree. 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 3-4 DSBs per completed genome (2.5 .times. 109 daltons).