Inhibition of nucleolar and chromatin RNA synthesis in Hela cells by excess thymidine

Summary Inhibition of RNA synthesis in Hela cells, evoked by high doses of thymidine, was followed. The author used labelling with ³H-uridine and quantitative autoradiography in two groups of experiments: first, of 4 mM thymidine was being followed for the period of 1, 6, 12 and 24 hours; second, the effect of 0.33, 1, 3, 9 and 27 mM of thymidine for the period of 21 hours. A significant decrease of both the nucleolar and extranucleolar chromatin RNA synthesis was detected as a due to the exposure to 4 mM thymidine for the period of 1–6 hours or to the exposure to 1 mM thymidine for the period of 21 hours. A further deepening of the inhibition of the extranucleolar RNA synthesis appears by protracting the period of action or by increasing the concentration of thymidine in the medium.The author is of the opinion that the inhibition of the nucleolar RNA synthesis is very likely a result of the blockade of the DNA reduplication and of the cell synchronization. The inhibition of the extranucleolar RNA synthesis is on the other hand caused directly, without the intermediate of DNA.