engleski

Chromatin Remodeling Activities at the Yeast PHO84 Promoter

The yeast PHO84 promoter, which is coregulated with the PHO5 and PHO8 promoters in response to phosphate availability, is the strongest promoter of the PHO family, containing five binding sites for the specific activator Pho4. Two of these binding sites are located in a short hypersensitive region under repressive conditions. Upon induction, at least one nucleosome upstream and one downstream from the this region are remodeled and histone evicted, thereby exposing two additional Pho4 binding sites. The rate of histone eviction is strongly delayed in snf2 or gcn5 mutants. Without Snf2 chromatin remodeling is only partial, resulting in displacement of the downstream but not of the upstream nucleosome. Therefore, remodeling of these two nucleosomes requires different chromatin remodeling activities. In contrast to Snf2, Gcn5 is not required for complete remodeling of both nucleosomes upon full induction, either in the presence or absence of Snf2, and the same is true for Ino80. Gcn5 and Ino80 affects only the rate of remodeling, similar as we previously found for the PHO5 promoter. However, with respect to the requirement for Snf2, the PHO84 promoter chromatin structure is like a hybrid between the PHO5 and PHO8 promoters: the upstream nucleosome is strictly dependent on Snf2, like the PHO8 promoter and remodeling of the downstream nucleosome is only delayed without Snf2, like remodeling of the PHO5 promoter. This can maybe be expalined by different inherent nucleosome stabilities as assayed by in vitro reconstitution. We also examined a possible role of the histone variant H2A.Z in regulation of chromatin remodeling at PHO promoters. Interestingly, the absence of H2A.Z has only a slight effect on the PHO5 promoter, but causes a rather strong delay in the induction of the PHO84 promoter. The two promoters also respond differently to the absence of Snf1 kinase, which is involved in histone H3 phosphorilation. Under repressive conditions the absence of Snf1 has no effect on PHO5 activity, but at the PHO84 promoter there is significant remodeling of the downstream nucleosome. Consequently, we observed elevated activity under otherwise repressive conditions. This points to different meshanisms of chromatin remodeling at the two coregulated promoters.

yeast PHO84 promoter; chromatin remodeling

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