Dynamic Chromatin Remodeling At The Budding Yeast Pho5 Promoter

Promoter transactivation is often associated with the disruption or remodeling of nucleosomes in chromatin. The budding yeast PHO5 promoter, which drives expression of the major secreted acid phosphatase, is a proven model system for investigating gene regulatory mechanisms in the context of chromatin. Under conditions of high external phosphate concentration, PHO5 transcription is repressed through an upstream signaling cascade and an array of nucleosomes that are well positioned and hinder access to crucial regulatory elements. Much recent progress has been made in elucidating the molecular steps that lead to promoter activation upon phosphate starvation. Inhibition of an upstream repressive signaling cascade enables DNA binding by site-specific activator proteins, which, in turn, lead to the direct recruitment of two histone modifying and two ATP-dependent chromatin remodeling complexes. Using a novel single-molecule assay for chromatin remodeling in vivo, we have shown that the number of nucleosomes disrupted from cell-to-cell is highly variable. Moreover, nucleosomes adjacent to activator binding sites are disrupted earlier and at higher frequency than are those more distal. The results suggest that the localized recruitment of chromatin modifying and remodeling complexes leads to nucleosome disruption that spreads from sites of transactivator association with DNA and attenuates with distance.