Survival, mutation and capacity to repair single-strand DNA breaks after gamma irradiation in different Exr? strains of Escherichia coli

Summary Strains of Escherichia coli K-12 and B/r made by transduction of the exrA allele from a B_s-2 derivative have been compared with Exr(W)^– strains derived from B_s-1 and B_s-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.