Relation of Ultraviolet Light Mutagenesis to a Radiation-Damage Inducible System in Escherichia coli

Abstract

Revertants produced by UV light followed a square-law dependence on fluence up to about 0.5 J/m2, 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 the estimation of the fraction of cells in a population that is inducible after exposure to a particular UV fluence. Comparison with 2 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 3 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. UV-produced mutations in the presence or absence of an ionizing radiation predose appear to be due to C .fwdarw. T changes in the bacterial genome (C and T denote cytosine and thymine, respectively).