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 functio n of both types of regulators can involve interactions with one or mor e component of the basal transcription machinery. A principal target a ppears to be TFIID, which consists of the TATA binding protein (TBP) a nd associated factors (TAFs). Here we describe experiments that provid e added support for the idea that interactions affecting TFIID can pla y important roles in both activation and repression. We show, using tr ansfection assays in Drosophila Schneider cells, that recruitment of T BP to a promoter as a GAL4-TBP fusion protein can provide a substantia l activation of transcription. The conserved core of TBP is necessary and sufficient for this effect, which was observed with both TATA-cont aining and TATA-lacking basal promoters. These findings extend experim ents performed in yeast, and strengthen the idea that recruitment of T BP (TFIID) can be an important mechanism of activation. We also provid e further support for the idea that TBP can be a target for a transcri ptional repressor, the Drosophila Even-skipped protein (Eve). We prese nt evidence that the homeodomain, which is necessary for binding TBP i n vitro, can also be required for repression in vivo, independent of i ts role in DNA binding. On the other hand, deletion of the alanine/pro line-rich region that is essential for repression in vivo and TBP bind ing 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 activat ors and repressors.