Search

The mechanisms, which cell has evolved, to modulate the chromatin structure and consequently gene expression were extensively studied at the yeast PHO5 and PHO8 genes. In order to get further insight into this issue, we started to study the yeast PHO84 promoter, which is coregulated with the PHO5 and PHO8 promoters by the phosphate availability and is the strongest PHO promoter. In the repressed state this promoter exhibits a short hypersensitive region flanked by two positioned nucleosomes and harbouring two high affinity Pho4 binding sites. Upon induction, the chromatin structure is remodeled into an extensive hypersensitive region that allows access to two additional UASp elements. This chromatin transition is strongly dependent on Snf2, Ino80 and Gcn5. Interestingly, the two nucleosomes show a markedly different degree of cofactor requirement: remodeling of the upstream is critically dependent on Snf2, while remodeling of the downstream is only stimulated by this cofactor. We also showed that remodeling of the upstream nucleosome is more dependent on Ino80, but this cofactor is, however, not essentially required for complete remodeling of the downstream nucleosome upon full induction, even in the absence of Snf2. Chromatin remodeling at the PHO84 promoter leads to the eviction of histones from the promoter, so we also examined a possible role of histone chaperone Asf1 for the activation of this promoter, and found that this protein has a stimulating but not essential role in PHO84 promoter induction, less pronounced than at the PHO5 promoter. Moreover, at the PHO84 promoter, but also at the PHO5 promoter, Asf1 likely functions through its essential action on the histone acetyltransferase Rtt109, which acetylates histone H3 at lysine 56.

The yeast PHO5 and PHO8 promoters were extensively studied by us and others as a model system for elucidation of promoter regulation by chromatin structure remodeling. We have previously shown that requirements for chromatin modifying and remodeling cofactors at these two promoters were rather different even though they are both activated by the same specific activator. In order to get better understanding of cofactor requirement mechanisms we included a third, the strongest PHO promoter, PHO84 in our study. Upon induction under phosphate starvation conditions, chromatin remodeling at the PHO84 promoter leads to the eviction of histones resulting in an extensive nucleosome-free region. The eviction of the two nucleosomes is strongly dependent on chromatin remodeling complexes SWI/SNF and Ino80 and histone acetyltransferase Gcn5. Interestingly, for the first time we found that the two nucleosomes at the same promoter showed markedly different degree of cofactors requirement ; remodeling of one is strictly dependent on SWI/SNF, while the other is completely remodeled in the absence of both the SWI/SNF and Ino80 complexes. The essential, most abundant chromatin remodeling complex, RSC, affects nucleosome remodeling at PHO84 and PHO5 promoters but is not critically required. The histone chaperone Asf1 also has stimulating role in chromatin remodeling at both PHO5 and PHO84 promoters, at least in part through its essential action on histone H3K56 acetyltransferase Rtt109.