Abstract

As cells approach S phase, many changes occur to create an environment conducive for DNA synthesis and commitment to cell division. The transcription rate of many genes encoding enzymes involved in DNA synthesis, including the dihydrofolate reductase (dhfr) gene, increases at the G1/S boundary of the cell cycle. Although a number of transcription factors interact to finely tune the levels of dhfr RNA produced, two families of transcription factors, Sp1 and E2F, play central roles in modulating dhfr levels. A region containing several Sp1‐binding sites is required for both regulated and basal transcription levels. In contrast, the E2F‐binding sites near the transcription start site are required only for regulated transcription. A model is presented for the regulation of the dhfr gene which may also pertain to other cell cycle‐associated genes.