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. Th ough HSF has an acidic activation domain, the mechanistic details of h eat shock gene activation remain undefined. Here we report that HSF in teracts directly with the general transcription factor TBP (TATA-box b inding protein), and these two factors bind cooperatively to heat shoc k promoters. A third factor that binds heat shock promoters, GAGA fact or, 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 a mino- and carboxyl terminal portions of HSF. This HSF/TBP interaction can be specifically disrupted by competition with the potent acidic tr anscriptional activator VP16. We further show that the acidic domain o f the largest subunit of Drosophila RNA polymerase II (Pol II) associa tes with TBP in vitro and is specifically displaced from TBP upon addi tion of HSF. The region of TBP that mediates both HSF and Pol II acidi c domain binding maps to the conserved carboxyl terminal repeats and d epends on at least one of the TBP residues known to be contacted by VP 16 and to be critical for transcription activation. We discuss these f indings in the context of a model in which HSF triggers hsp70 transcri ption by freeing the hsp70 promoter-paused Pol II from the constraints on elongation caused by the affinity of Pol II for general transcript ion factors.