POSTTRANSCRIPTIONAL CONTROL OF THYMIDINE KINASE MESSENGER-RNA ACCUMULATION IN CELLS RELEASED FROM G0-G1 PHASE BLOCKS

In this study, we have utilized thymidine kinase (TK) mRNA induction a s a model for investigating regulatory events at the G1-S boundary of the cell cycle. Using three independent methods for synchronizing dipl oid, nontumorigenic CHEF/18 cells, we found that the mechanism(s) unde rlying TK mRNA accumulation varied with the method of cell synchrony u sed. When cells were arrested by serum deprivation, both transcription al and posttranscriptional controls contributed to the observed accumu lation of TK mRNA at the G1-S boundary. When synchronized by isoleucin e deprivation, mature TK mRNA and TK pre-mRNAs increased significantly at the G1-S boundary of the cell cycle with no detectable change in t he rate of TK gene transcription. Following lovastatin treatment, whic h appears to arrest cells at a point very early in G1, posttranscripti onal mechanisms were solely responsible for the subsequent accumulatio n of TK mRNA observed upon mevalonate repletion. We confirmed that tra nscriptional mechanisms were involved in TK mRNA regulation only when cells progressed from G0 into S phase using reporter genes transcribed from the heterologous human TK promoter. Taken together, these result s indicate that posttranscriptional mechanism(s) are primarily respons ible for regulating the abundance of TK mRNA during the cell cycle in CHEF/18 cells and further suggest uncoupling of transcriptional and po sttranscriptional controls following different physiological condition s of cell cycle arrest.