Amino termini of histones H3 and H4 are required for a1-alpha 2 repression in yeast

The Saccharomyces cerevisiae alpha 2 repressor controls two classes of cell-type-specific genes in yeast through association with different partners, alpha 2-Mcm1 complexes repress a cell-specific gene expression in haploid a cells and diploid a/alpha cells, while a1-alpha 2 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 alpha 2. We have previously shown that nucleosomes are positioned adjacent to the alpha 2-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-alpha 2 repression to determine if chromatin is a general feature of repression by Ssn6-Tup1. We find that mutations in the amino terminus of histone H4 cause a 4- to II-fold derepression of a reporter gene under a1-alpha 2 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 a1-alpha 2-mediated repression, However, in contrast to the ordered positioning of nucleosomes adjacent to the alpha 2-Mcm1 operator, nucleosomes are not positioned adjacent to the a1-alpha 2 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.