Revertants produced by ultraviolet light (uv) were shown to follow a square-law dependence on fluence up to about <tex-math>$0.5\ {\rm J}/{\rm m}^{2}$</tex-math>, after which the dependence became closer to linear. This behavior can be associated with an induction process, and a new way of expressing mutation data is presented that allows one to estimate the fraction of cells in a population that is inducible after exposure to a particular uv fluence. Comparison with two other kinds of radiation-induced behavior (induction of inhibition of post-irradiation DNA degradation and induced radioresistance) shows that the fluence dependence of induction is similar in all three cases. A predose with ionizing radiation followed by an incubation period and subsequent exposure to a graded set of uv fluences at 265 nm gave an increase in the number of revertants and a more linear dose dependence. The same result was obtained if the predose was a uv fluence at 265 nm and the graded set of doses was at 313 nm, a wavelength which by itself produces very few revertants. These changes in the mutagenic response were also consistent with the induction of a mutation-potentiating system by the predose. Ultraviolet-produced mutations in the presence or absence of an ionizing radiation predose appear to be due to C → T changes in the bacterial genome (C and T denote cytosine and thymine, respectively).

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