Abstract
(5-T) uridine is a widely used radioactive precursor for the analysis of RNA metabolism. It has been proposed as a specific marker for RNA formation, cf. [ 1 ] because its label is lost during conversion from U to dT. As this may be valid for very short incubation times, it certainly introduces serious errors in studies exceeding l/ 10 of the cell doubling time. As shown in this paper, the radioactivity very soon enters DNA via [“HI UTP+[3H]CTP+[3H] dCTP conversion, thus labeling DNA exclusively in C bases. The U to C conversion was analysed in HeLa suspension cultures labeled with (5-T) uridine at different concentrations. C bases in RNA became rapidly labeled, though with some delay to U bases. The time needed to reach equilibrium depended on the concentration of the exogenous uridine indicating profound differences of UTP and CTP pools under these conditions. Base analysis of DNA showed a time-dependent increase in radioactivity which was exclusively located in the cytosine residues.
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