Regulation of the PHO5 gene. Shown are diagrams of PHO5 when repressed (top) and induced (bottom). In the repressed state (high phosphate), four nucleosomes (−4 to −1, shown in light blue) span the PHO5 regulatory region, including nucleosome −2, which blocks one of the Pho4-bindings sites (red line). Several factors, shown on the left and described in the text, are required for PHO5 induction, which results in loss of nucleosomes over the PHO5 regulatory region, the binding of Pho4, and activation of transcription. The dotted lines around nucleosomes −4 and −1 indicate a more variable degree of loss. Upon addition of phosphate, the nucleosomes reassemble in the PHO5 regulatory region in an Spt6-dependent fashion, resulting in transcriptional repression.

Chromatin structure over growth genes and stress genes. (A) Growth genes generally have a nucleosome-free region (red) that does not contains a TATA box, flanked by a −1 (turquoise) and +1 (blue) nucleosome. The +1 nucleosome contains Htz1 and the −1 nucleosome sometimes does. The downstream nucleosomes (yellow) are less well positioned. (B) Stress genes generally contain a TATA box, and the −1 and +1 nucleosome positions are less well defined, as indicated in the diagram.