Studies on Mutations Induced by Nonlethal Dosages of Ultraviolet Light

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Abstract
Studies are reported on the dosages of UV light that induce mutations in the streptomycin resistance locus in Escherichia coli B/r without appreciable lethality. The nonlethal system is unique in that 2 types of mutational events can be demonstrated, 1) mutants expressed on minimal media, and 2) additional mutants expressed on enriched media. At higher levels of irradiation only the former can be recovered. A very few minutes on the expression medium following irradiation will determine which mutational pattern will be followed. Modification of the expression medium by inhibitors supports the view that both direct and indirect radiation effects are involved and that at higher doses one of these effects is masked.