Role of trans-activating proteins in the generation of active chromatin at the PHO5 promoter in S. cerevisiae

Abstract

Induction of the PHOS gene in Saccharomyces cerevisiae by phosphate starvation was previously shown to be accompanied by the removal of four positioned nucleosomes from the promoter. We have now investigated the role of two trans-activating proteins, encoded by PH02 and PHO4, which bind to the PHOS promoter. Both proteins are absolutely required for the chromatin transition to occur as shown by analysis of null mutants of the two genes. Transformation of these mutant strains with plasmids containing the respective genes restores the wild type chromatin response. Increasing the gene dosage of PH02 and of PH04 makes it possible to differentiate functionally between the two proteins. From overexpressing PH04 in a wild type and also in a pho2 null mutant strain and complementary experiments with PHO2, it is concluded that the PH04 protein is the primary trigger for the chromatin transition, consistent with one of its two binding sites being located between positioned nucleosomes in repressed chromatin and thereby accessible. PH02, the binding site of which is located within a nucleosome under conditions of PHOS repression, contributes to the chromatin transition either by destabilizing histone-DNA interactions or by undergoing interactions with PHO4.