Chromatin opening and transactivator potentiation by RAP1 in Saccharomycescerevisiae

Transcriptional activators function in vivo via binding sites that may be p ackaged into chromatin. Here we show that whereas the transcriptional activ ator GAL4 is strongly able to perturb chromatin structure via a nucleosomal binding site in yeast, GCN4 does so poorly. Correspondingly, GCN4 requires assistance from an accessory protein, RAP1, for activation of the HIS4 pro moter, whereas GAIA does not. The requirement for RAP1 for GCN4-mediated HI S4 activation is dictated by the DNA-binding domain of GCN4 and not the act ivation domain, suggesting that RAP1 assists GCN4 in gaining access to its binding site. Consistent with this, overexpression of GCN4 partially allevi ates the requirement for RAP1, whereas HIS4 activation via a weak GAL4 bind ing site requires RAP1. RAP1 is extremely effective at interfering with pos itioning of a nucleosome containing its binding site, consistent with a rol e in opening chromatin at the HIS4 promoter. Furthermore, increasing the sp acing between binding sites for RAP1 and GCN4 by 5 or 10 bp does not impair HIS4 activation, indicating that cooperative protein-protein interactions are not involved in transcriptional facilitation by RAP1. We conclude that an important role of RAP1 is to assist activator binding by opening chromat in.