The bidirectionally transcribed dihydrofolate reductase and rep-3a promoters are growth regulated by distinct mechanisms.

The mouse dihydrofolate reductase (dhfr) gene possesses a bidirectional promoter that produces functional transcripts in the opposite direction. These opposite strand transcripts encode the Rep-3 gene product, a protein that has homology to DNA mismatch repair enzymes. The core of the bidirectional promoter consists of four consensus binding sites for the transcription factor Sp1. These binding sites have been shown to be important for basal transcription from both the rep-3a and dhfr promoters. Extensive characterization of the dhfr promoter has shown that growth-dependent regulation requires the E2F binding sites that flank the transcription initiation site. Here we show that endogenous rep-3a mRNA and the rep-3a promoter are growth regulated, in a manner very similar to the regulation of the dhfr mRNA and promoter region. However, we find that the E2F sites required for dhfr regulation are dispensible for regulation of the rep-3a promoter. Instead, we have shown that the rep-3a initiation region is critical for the G1S phase-specific activation of this promoter. Gel mobility shift experiments indicate that a member of the Sp1 family of transcription factors binds to the rep-3a initiation region, suggesting that this family of transcription factors may play a role in cell growth control.