125I Decay in Microorganisms: A Prokaryotic Model System for Examining the Role of Unrepaired Double-Strand DNA Breaks in Cell Killing
Iodine-125, when incorporated into the DNA of microorganisms as iododeoxyuridine, has been demonstrated to induce double-strand DNA breaks (DSBs) with approximately 100% efficiency when it undergoes radioactive decay. In the biological dose range approximately 2/3 of these breaks are repaired in a wild type (rec +) strain of E. coli K12, but none are repaired in the corresponding recombination deficient (recA) strain. Each unrepaired DSB appears to produce a lethal event in either strain. Survival experiments imply that maximum repair of DSBs in rec + cells can occur in the presence of only a single genome per cell and therefore does not require a recombi-national event. However, experiments with 125I-labelled small intracellular closed circular DNA mulecules (super-infecting phage λ, MW=33×106d) demonstrate no repair of decay-induced DSBs, suggesting that the superstructure of the large cellular chromosome plays an important role in the repair of DSBs, even in the absence of recombination.
KeywordsEthyl Iodide recA Cell Biological Repair Nuclear Recoil Energy Unrepaired DSBs
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