Inhibition of uracil-DNA glycosylase increases SCEs in BrdU-treated and visible light-irradiated cells.

We have approached the study of the ability of different types of lesions produced by DNA-damaging agents to develop sister-chromatid exchanges (SCEs) by analyzing SCE levels observed in Allium cepa L cells with BrdU-substituted DNA and exposed to visible light (VL), an irradiation which produces uracil residues in DNA after debromination of bromouracil and enhances SCE levels but only above a certain dose. We have partially purified an uracil-DNA glycosylase activity from A. cepa L root meristem cells, which removes uracil from DNA, the first step in the excision repair of this lesion. This enzyme was inhibited in vitro by 6-amino-uracil and uracil but not by thymine. When cells exposed to VL, at a dose that did not produce per se an SCE increase, were immediately post-treated with these inhibitors of uracil-DNA glycosylase, a significant increase in SCE levels was obtained. Moreover, SCE levels in irradiated cells dropped to control level when a short holding time (less than 15 min) elapsed between exposure to VL and the beginning of post-treatment with the inhibitor. Thus, our results (1) showed that inhibitors of uracil-DNA glycosylase enhanced SCE levels in cells with unifilarly BrdU-substituted DNA exposed to visible light; (2) pointed to uracils and/or to some products of their repair as lesions responsible for SCE formation under our experimental conditions; and (3) indicated the existence of a very rapid repair of SCE-inducing lesions produced by visible light irradiation of cells with unifilarly BrdU-containing DNA.