SUMMARY: Curves for the absolute yield of revertants as a function of ultraviolet (uv) radiation dose were obtained after post-irradiation incubation in saline and after exposure to reactivating light. The curves exhibited plateau regions and final slopes which were equal to the slopes of the survival curves for the auxotrophic Escherichia coli parent. These features may be due to a saturation of the mutagenic effect at low doses. This saturation would be the result of (a) heterogeneity in the unirradiated population with respect to the amount of protein and RNA synthesis which can occur before DNA replicates in the irradiated organisms, combined with (b) limitation of the observable yield by the selection method. The observable yield would be derived from the fraction of the population most resistant to the lethal effect of radiation.
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