THE TRANSACTIVATION DOMAIN OF PHO4 IS REQUIRED FOR NUCLEOSOME DISRUPTION AT THE PHO5 PROMOTER

The chromatin structure of the PHO5 promoter is disrupted when the pro moter is derepressed by phosphate starvation. The transactivator, Pho4 , is primarily responsible for this change. We have used deletion muta tions of Pho4 in order to determine which protein domains are involved in nucleosome dissolution. Our results show that the DNA binding doma in by itself is not sufficient to trigger chromatin disruption, even w hen overexpressed. In vivo footprinting reveals that Pho4 derivatives lacking the N-terminal activation domain can bind to UAS(p)1, which re sides in a constitutively nucleosome-free region, but not to UAS(p)2, which lies within a nucleosome in the repressed PHO5 promoter. The aci dic activation domain of Pho4 appears to be involved in nucleosome dis ruption. Substitution of the native transactivation domain of Pho4 wit h that from VP16 results in substantial chromatin disruption. In every case, the ability of the Pho4 mutants to activate transcription corre lates with their ability to disrupt nucleosome structure in the PHO5 p romoter. Therefore, we conclude that the Pho4 activation domain has at least two roles: (i) to trigger disruption of nucleosome structure ov er the promoter, thereby facilitating the binding of transcription fac tors, and (ii) to interact with the transcriptional apparatus at the p roximal promoter.