Mechanisms of Transcriptional Activation and Repression can Both Involve TFIID

Regulation of transcription involves the activities of activators and repressors. Recent experiments have provided evidence that the function of both types of regulators can involve interactions with one or more component of the basal transcription machinery. A principal target appears to be TFIID, which consists of the TATA binding protein (TBP) and associated factors (TAFs). Here we describe experiments that provide added support for the idea that interactions affecting TFIID can play important roles in both activation and repression. We show, using transfection assays in Drosophila Schneider cells, that recruitment of TBP to a promoter as a GAL4-TBP fusion protein can provide a substantial activation of transcription. The conserved core of TBP is necessary and sufficient for this effect, which was observed with both TATA-containing and TATA-lacking basal promoters. These findings extend experiments performed in yeast, and strengthen the idea that recruitment of TBP (TFIID) can be an important mechanism of activation. We also provide further support for the idea that TBP can be a target for a transcriptional repressor, the Drosophila Even-skipped protein (Eve). We present evidence that the homeodomain, which is necessary for binding TBP in vitro, can also be required for repression in vivo, independent of its role in DNA binding. On the other hand, deletion of the alanine/proline-rich region that is essential for repression in vivo and TBP binding in vitro does not significantly affect DNA binding by the purified protein. These results strengthen the view that TBP, either directly or indirectly as a component of TFIID, can be a target of both activators and repressors.