Yeast α2 repressor positions nucleosomes in TRP1/ARS1 chromatin

The yeast α2 repressor suppresses expression of a-mating-type-specific genes in haploid α and diploid a/α cell types. We inserted the α2-binding site into the multicopy TRP1/ARS1 yeast plasmid and examined the effects of α2 on the chromatin structure of the derivative plasmids in α cells, a cells and a/α cells. Whereas no effect on nucleosome position was observed in a cells, nucleosomes were precisely and stably positioned over sequences flanking the α2 operator in α and a/α cells. In addition, when the α2 operator was located upstream of the TRP1 gene, an extended array of positioned nucleosomes was formed in α cells and a/α cells, with formation of a nucleosome not present in a cells, and TRP1 mRNA production was substantially reduced. These data indicate that α2 causes a positioning of nucleosomes over sequences proximal to its operator in TRP1/ARS1 chromatin and suggest that changes in chromatin structure may be related to α2 repression of cell-type-specific genes.