Survival, mutation and capacity to repair single-strand DNA breaks after gamma irradiation in different Exr− strains of Escherichia coli
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Strains of Escherichia coli K-12 and B/r made by transduction of the exrA allele from a Bs-2 derivative have been compared with Exr(W)− strains derived from Bs-1 and Bs-2 by mutation (E.M. Witkin, 1967). Both transduced exrA and Exr(W)− strains were almost unmutable by gamma radiation, but the former class were as sensitive to gamma radiation as recA strains and, like them, were unable to repair single-strand DNA breaks as detected by the McGrath-Williams technique. In contrast the Exr(W)−strains were as resistant to gammaradiation as Exr(W)+ strains derived from them and were equally efficient in repairing single-strand breaks. The existence of Exr(W)−strains suggests that the mutagenicity of single-strand breaks may depend entirely on the way in which they are repaired. The properties of the (Exr(W)−strains cannot be ascribed solely to the transducable exrA allele.
A large effect of diffuse daylight in lowering the molecular weight of DNA on alkaline sucrose gradients is described which, unless prevented, may lead to erroneous results in such experiments.
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