The mechanistic basis for chromatin invasion and remodeling by the yeast pioneer transcription factor Rap1
Pioneer transcription factors (pTFs) bind to target sites within compact chromatin initiating chromatin remodeling and controlling the recruitment of downstream factors. The mechanisms by which pTFs overcome the chromatin barrier are not well understood. Here we reveal, using single-molecule fluorescence approaches, how the yeast transcription factor Rap1 invades and remodels chromatin. Using a reconstituted chromatin system replicating yeast promoter architecture we demonstrate that Rap1 can bind nucleosomal DNA within a chromatin fiber, but with shortened dwell times compared to naked DNA. Moreover, we show that Rap1 binding opens chromatin fiber structure by inhibiting nucleosome-nucleosome contacts. Finally, we reveal that Rap1 collaborates with the chromatin remodeler RSC to destabilize promoter nucleosomes, paving the way to form long-lived bound states on now exposed DNA. Together, our results provide a mechanistic view of how Rap1 gains access and opens chromatin, thereby establishing an active promoter architecture and controlling gene expression.