Heat shock factor-4 (HSF-4a) represses basal transcription through interaction with TFIIF

The heat shock transcription factors (HSFs) regulate the expression of heat shock proteins (hsps), which are critical for normal cellular proliferatio n and differentiation. One of the HSFs, HSF-4, contains two alternative spl ice variants, one of which possesses transcriptional repressor properties i n vivo, This repressor isoform inhibits basal transcription of hsps 27 and 90 in tissue culture cells. The molecular mechanisms of HSF-4a isoform-medi ated transcriptional repression is unknown, Here, we present evidence that HSF-4a inhibits basal transcription in vivo when it is artificially targete d to basal promoters via the DNA-binding domain of the yeast transcription factor, GAL4. By using a highly purified, reconstituted in vitro transcript ion system, we show that HSF-4a represses basal transcription at an early s tep during preinitiation complex assembly, as pre-assembled preinitiation c omplexes are refractory to the inhibitory effect on transcription. This rep ression occurs by the HSF-4a isoform, but not by the HSF-4b isoform, which we show is capable of activating transcription from a heat shock element-dr iven promoter in vitro, The repression of basal transcription by HSF-4a occ urs through interaction with the basal transcription factor TFIIF, TFIIF in teracts with a segment of HSF-4a that is required for the trimerization of HSF-4a, and deletion of this segment no longer inhibits basal transcription . These studies suggest that HSF-4a inhibits basal transcription both in vi vo and in vitro. Furthermore, this is the first report identifying an inter action between a transcriptional repressor with the basal transcription fac tor TFIIF.