Control of thymidine kinase mRNA during the cell cycle

To investigate the mechanism which controls the onset of DNA synthesis, we examined the regulation of thymidine kinase (TK) and its mRNA in the cell cycle. TK activity provides a useful marker for the onset of the S phase in mammalian cells. The present analysis of regulation of TK mRNA in BALB/c 3T3 cells showed that (i) the increase in TK activity depended on the availability of TK mRNA, (ii) the level of TK mRNA between G0 and S increased more than 20-fold, (iii) the rate of run-on TK transcription increased at most 2- to 4-fold between the G0 and S phases, (iv) the half-life of TK mRNA was greater than 8 to 12 h in the S and M phases and decreased as cells entered quiescence, (v) the TK mRNA increase was fully blocked by inhibition of protein synthesis by only 60%, (vi) this inhibition was completely effective for up to about 10 h following serum addition and progressively much less effective when the drugs were added later. These results suggest that the appearance of TK mRNA at the beginning of the S phase in serum-stimulated 3T3 cells is controlled not only by the rate of gene transcription but importantly also by the decreased rate of mRNA degradation. Similar mechanisms may be involved in regulation of the onset of DNA synthesis and the increase in TK mRNA since both are controlled in a manner consistent with a requirement for a labile protein.