Can 3H-uridine-induced sister chromatid exchange be used as a measure of transcriptional activity of chromosomes?

Abstract

In order to test the validity of using ³H-uridine-induced chromosome damage as a measure of the transcriptional activity of chromosomes, experiments were performed to examine the mechanism by which ³H-uridine induces sister chromatid exchanges. DON and B₁₄FAF₂₈ Chinese hamster cells exposed to ³H-uridine showed a dose-dependent increase in SCE frequency, whereas unlabelled uridine produced no increase. ³H-uridine labelling in the presence of increasing concentrations of unlabelled uridine eliminated this increase in a concentration dependent manner. Competition between ³H-uridine and other unlabelled pyrimidines (thymidine and cytidine) was equally as effective; however, unlabelled hypoxanthine had no effect on ³H-uridine-induced SCEs. This suggested that ³H-uridine caused SCEs after interconversion into deoxycytidine followed by incorporation into DNA. Significant incorporation of ³H into DNA was confirmed by direct comparisons of label in the DNA and RNA fractions, and about 90% of this label was found to be in the cytosine fraction of hydrolysates. We therefore conclude that ³H-uridine produces chromosome damage because of the incorporation of a tritiated conversion product into DNA, rather than by exposure of transcriptionally active DNA to ³H-uridine-labelled RNA. We suggest therefore, that the results of earlier experiments, in which ³H-uridine chromosome damage has been used to assay the transcriptional activity of chromosomes, may need to be reinterpreted.