Serum-responsive expression from the murine thymidine kinase promoter is 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, these regulatory systems mediate the 20-50-fold induction of TK mRNA observed as cells traverse the G1-S boundary of the cell cycle. Previously, we have reported that a "Yi" protein complex was observed to bind the mouse TK promoter in a cell cycle-dependent manner in nontransformed cells (Q-P. Dou, J. L. Fridovich-Keil, and A. B. Pardee, Proc. Natl. 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). Nonetheless, TK mRNA levels in these cells continue to exhibit a marked S-specific induction (> 10 fold), raising the question: what is the status of TK promoter-mediated, as opposed to posttranscriptional, gene regulation in these transformed cells? To address this question, we have used cell synchrony experiments involving both transformed and nontransformed cells stably transfected with a TK promoter-beta-globin reporter gene construct. We have found that, in marked contrast to the tight regulation of reporter gene expression observed in nontransformed cells (J. L. Fridovich-Keil, J. M. Gudas, Q-P. Dou, I. Bouvard, and A. B. Pardee, Cell Growth & Differ., 2: 67-76, 1991), reporter gene expression in the transformed cells is constitutive and, therefore, closely parallels the presence of Yi DNA-binding activity. These data are fully consistent with other recently published observations concerning differential controls of TK transcriptional and posttranscriptional regulation (J. M. Gudas, J. L. Fridovich-Keil, and A. B. Pardee, Cell Growth & Regul., 4: 421-430, 1993) and support the hypothesis that, in transformed cells, endogenous TK is regulated predominantly at the posttranscriptional level.