Amine termini of histones H3 and H4 are required for a1-α2 repression in yeast

The Saccharomyces cerevisiae α2 repressor controls two classes of cell- type-specific genes in yeast through association with different partners. α2-Mcm1 complexes repress a cell-specific gene expression in haploid α cells and diploid a/α cells, while al-α2 complexes repress haploid-specific genes in diploid cells. In both cases, repression is mediated through Ssn6- Tup1 corepressor complexes that are recruited via direct interactions with α2. We have previously shown that nucleosomes are positioned adjacent to the α2-Mcm1 operator under conditions of repression and that Tup1 interacts directly with histones H3 and H4. Here, we examine the role of chromatin in a1-α2 repression to determine if chromatin is a general feature of repression by Ssn6-Tup1. We find that mutations in the amine terminus of histone H4 cause a 4, to 11-fold derepression of a reporter gent under a1- α2 control, while truncation of the H3 amino terminus has a more modest (3- fold or less) effect. Strikingly, combination of the H3 truncation with an H4 mutation causes a 40-fold decrease in repression, clearly indicating a central role for these histones in al-α2-mediated repression. However, in contrast to the ordered positioning of nucleosomes adjacent to the α2-Mcm1 operator, nucleosomes are not positioned adjacent to the a1-α2 operator in diploid cells. Our data indicate that chromatin is important to Ssn6-Tup1- mediated repression but that the degrees of chromatin organization directed by these proteins differ at different promoters.