Figure 4. Pho4 binding in vivo to PHO promoters, and model of threshold-dynamic range decoupling.

a, Chromatin immunoprecipitation of Pho4 to PHO target genes and the H4 PHO5 promoter variant (see Fig. 1a) shows differential occupancy in intermediate P_i conditions at promoters with an exposed high- (H) versus low (L)-affinity site. Data points represent mean ± s.d. from at least three independent experiments. b, Schematic depicting a possible mechanism that decouples promoter induction threshold from dynamic range. In high P_i (top row), nucleosomes are fully assembled; in intermediate P_i (middle row), substantial Pho4 occupancy occurs only at promoters with exposed high-affinity sites, resulting in chromatin remodelling and transcription commensurate to the total Pho4 recruited; and in P_i starvation (bottom row), saturating Pho4 activity results in remodelling and maximum expression at all promoters. Nucleosome occupancy is indicated by the opacity of grey ovals, Pho4 by yellow ovals, and the amount of transcription by the thickness of the green arrows. See text for description.