A nucleosome precludes binding of the transcription factor Pho4 in vivo to a critical target site in the PHO5 promoter.

Activation of the Saccharomyces cerevisiae PHO5 gene by phosphate starvation is accompanied by the disappearance of two pairs of positioned nucleosomes that flank a short hypersensitive region in the promoter. The transcription factor Pho4 is the key regulator of this transition. By in vitro footprinting it was previously shown that there is a low affinity site (UASp1) which is contained in the short hypersensitive region in the inactive promoter, and a high affinity site (UASp2) which is located in the adjacent nucleosome. To investigate the interplay between nucleosomes and Pho4, we have performed in vivo footprinting experiments with dimethylsulfate. Pho4 was found to bind to both sites in the active promoter. In contrast, it binds to neither site in the repressed promoter. Lack of binding under repressing conditions is largely due to the low affinity of Pho4 for its binding sites under these conditions. Despite the increased affinity of Pho4 for its target sites under activating conditions, binding to UASp2 is prevented by the presence of the nucleosome and can only occur after prior disruption of this nucleosome in a process that requires UASp1. Protection of the PHO5 UASp2 by the nucleosome is not absolute, however, since overexpression of Pho4 can disrupt this nucleosome even when UASp1 is deleted. Also under these conditions, with only UASp2 present, all four nucleosomes at the PHO5 promoter are disrupted, whereas no chromatin change at all is observed when both UAS elements are destroyed.

The acid phosphatase genes PHO10 and PHO11 in S. cerevisiae are located at the telomeres of chromosomes VIII and I.

Of the three regulated acid phosphatase genes in S. cerevisiae (PHO5, PHO10 and PHO11) two have previously been cloned (PHO5 and PHO11). We have now identified PHO10 and show by restriction mapping that it is highly homologous to PHO11. This homology includes not only the coding sequence but also a stretch of about 2 kb upstream and 2.2 kb downstream of the genes. Analysis of strains in which either gene had been disrupted shows that the two genes are located at the telomeres of two different chromosomes. PHO10 3.6 kb from the end of a chromosome I. This makes PHO11 the gene closest to the end of a chromosome that has been physically mapped so far in S. cerevisiae. The organization of the two genes varies strongly from strain to strain consistent with a high incidence of telomere rearrangement. In one of twenty transformants examined a conversion event could be directly demonstrated that resulted in a chromosome VIII which had acquired a copy of the telomere from chromosome I.