Murine Dihydrofolate Reductase Transcripts Through the Cell Cycle

Abstract

The murine dihydrofolate reductase gene codes for mRNAs that differ in the length of their 3′ untranslated region as well as in the length of their 5′ leader sequence. In addition, the dihydrofolate reductase promoter functions bidirectionally, producing a series of RNAs from the opposite strand than the dihydrofolate reductase mRNAs. We have examined the production of these RNAs and their heterogeneous 5′ and 3′ termini as mouse 3T6 cells progress through a physiologically continuous cell cycle. We found that all of the transcripts traverse the cell cycle in a similar manner, increasing at the G1/S boundary without significantly changing their ratios relative to one another. We conclude that cell-cycle regulation of dihydrofolate reductase is achieved without recruiting new transcription initiation sites and without a change in polyadenylation sites. It appears that the mechanism responsible for the transcriptional cell-cycle regulation of the dihydrofolate reductase gene is manifested only by transiently increasing the efficiency of transcription at the dihydrofolate reductase promoter.