SERUM-RESPONSIVE EXPRESSION FROM THE MURINE THYMIDINE KINASE PROMOTERIS SPECIFICALLY DISRUPTED IN A TRANSFORMED-CELL LINE

Thymidine kinase (TK) gene expression is controlled in normal cells at both the transcriptional and posttranscriptional levels. Together, th ese regulatory systems mediate the 20-50-fold induction of TK mRNA obs erved as cells traverse the G(1)-S boundary of the cell cycle. Previou sly, we have reported that a ''Yi'' protein complex was observed to bi nd the mouse TK promoter in a cell cycle-dependent manner in nontransf ormed cells (Q-P. Dou, J. L. Fridovich-Keil, and A. B. Pardee, Proc. N atl. Acad. Sci. USA, 88: 1157-1161, 1991) and bound constitutively in transformed cells (D. W. Bradley, Q-P. Dou, J. L. Fridovich-Keil, and A. B. Pardee, Proc. Natl. Acad. Sci. USA, 87: 9310-9314, 1990). Noneth eless, TK mRNA levels in these cells continue to exhibit a marked S-sp ecific induction (>10 fold), raising the question: what is the status of TK promoter-mediated, as opposed to posttranscriptional, gene regul ation in these transformed cells? To address this question, we have us ed cell synchrony experiments involving both transformed and nontransf ormed cells stably transfected with a TK promoter-beta-globin reporter gene construct. We have found that, in marked contrast to the tight r egulation of reporter gene expression observed in nontransformed cells (J. L. Fridovich-Keil, J. M. Gudas, Q-P. Dou, I. Bouvard, and A. B. P ardee, Cell Growth and Differ., 2: 67-76, 1991), reporter gene express ion in the transformed cells is constitutive and, therefore, closely p arallels the presence of Yi DNA-binding activity. These data are fully consistent with other recently published observations concerning diff erential controls of TK transcriptional and posttranscriptional regula tion (J. M. Cudas, J. L. Fridovich-Keil, and A. B. Pardee, Cell Growth and Regul., 4: 421-430, 1993) and support the hypothesis that, in tra nsformed cells, endogenous TK is regulated predominantly at the posttr anscriptional level.