A nucleosome positioned by α2/Mcm1 prevents Hap1 activator binding in vivo

Nucleosome positioning has been proposed as a mechanism of transcriptional repression. Here, we examined whether nucleosome positioning affects activator binding in living yeast cells. We introduced the cognate Hap1 binding site (UAS1) at a location 24-43 bp, 29-48 bp, or 61-80 bp interior to the edge of a nucleosome positioned by α2/Mcm1 in yeast minichromosomes. Hap1 binding to the UAS1 was severely inhibited, not only at the pseudo-dyad but also in the peripheral region of the positioned nucleosome in α cells, while it was detectable in a cells, in which the nucleosomes were not positioned. Hap1 binding was restored in α cells with tup1 or isw2 mutations, which caused the loss of nucleosome positioning. These results support the mechanism in which α2/Mcm1-dependent nucleosome positioning has a regulatory function to limit the access of transcription factors.