Initiation of DNA Synthesis in a System of Synchronized L‐Cells: Rhythmicity of Thymidine Kinase Activity

In mechanically synchronized L‐cells, the activity of thymidine kinase and dCMP deaminase shows a distinct periodicity during the mitotic cycle. The rise in enzyme activities occurs at about the onset of DNA synthesis, 6 hours after mitosis, and reaches a maximum 16 hours after mitosis. The maximum rate of thymidine incorporation into DNA is found at 10 hours. The increase of enzyme activity is 6 fold for thymidine kinase and 5 fold for dCMP deaminase. A rapid fall of both enzyme activities is observed between 18 and 22 hours, coinciding with the second synchronous mitosis. The increase in enzyme activities is probably due to de novo synthesis, as judged by the effect of actinomycin D and puromycin. It is suggested that during the presynthetic phase of the cell cycle, messenger RNAs are synthesized for these enzymes. The enzymes in turn are required for the following DNA synthesis.