Transcription initiation and temporal expression of thymidine kinase mRNA in Chinese hamster cells

The induction of thymidine kinase mRNA has proved to be a valuable model for understanding regulatory events at the G1/S boundary of the cell cycle (1, 2, 3). As an initial step toward characterizing the regulation of this gene in Chinese hamster cells, I have mapped the transcription start sites for TK mRNA in CHEF/18 cells. Two closely spaced sites of transcription initiation were detected downstream of a nonconsensus TATAA element in the promoter region. Using primer extension analyses, I demonstrated that the transcription initiation sites remained constant while the absolute levels of TK mRNA varied during the cell cycle in synchronized populations of Chinese hamster cells.