Cooperative and competitive protein interactions at the Hsp70 promoter

Drosophila heat shock factor (HSF) binds to specific sequence elements of heat shock genes and can activate their transcription 200-fold. Though HSF has an acidic activation domain, the mechanistic details of heat shock gene activation remain undefined. Here we report that HSF interacts directly with the general transcription factor TBP (TATA-box binding protein), and these two factors bind cooperatively to heat shock promoters. A third factor that binds heat shock promoters, GAGA factor, also interacts with HSF and further stabilizes HSF binding to heat shock elements (HSEs). The interaction of HSF and TBP is explored in some detail here and is shown to be mediated by residues in both the amino- and carboxyl terminal portions of HSF. This HSF/TBP interaction can be specifically disrupted by competition with the potent acidic transcriptional activator VP16. We further show that the acidic domain of the largest subunit of Drosophila RNA polymerase II (Pol II) associates with TBP in vitro and is specifically displaced from TBP upon addition of HSF. The region of TBP that mediates both HSF and Pol II acidic domain binding maps to the conserved carboxyl terminal repeats and depends on at least one of the TBP residues known to be contacted by VP16 and to be critical for transcription activation. We discuss these findings in the context of a model in which HSF triggers hsp70 transcription by freeing the hsp70 promoter-paused Pol II from the constraints on elongation caused by the affinity of Pol II for general transcription factors.