The global transcriptional regulators, SSN6 and TUP1, play distinct roles in the establishment of a repressive chromatin structure.

Abstract

Repression of a-cell specific gene expression in yeast ~ cells requires MATch2 and MCM1, as well as two global repressors, SSN6 and TUP1. Previous studies demonstrated that nucleosomes positioned adjacent to the a2/MCM1 operator in c~ cells directly contribute to repression. To investigate the possibility that SSN6 and TUP1 provide a link between MAT~2/MCM1 and neighboring histones, nucleosome locations were examined in ssn6 and t u p l ot cells. In both cases, nucleosome positions downstream of the operator were disrupted, and the severity of the disruption correlated with the degree of derepression. Nevertheless, the observed changes in chromatin structure were not dependent on transcription. Our data strongly indicate that SSN6 and TUP1 directly organize repressive regions of chromatin.

A nucleosome positioned by α2/Mcm1 prevents Hap1 activator binding in vivo

“…Previous studies revealed that the α2/Mcm1 repressor, in conjunction with the Ssn6-Tup1 corepressor and the Isw2 complexes establishes an array of positioned nucleosomes to form a repressive chromatin structure [15,17,20,21,[28][29][30]32]. Our results imply that the positioned nucleosomes in the repressive chromatin function to limit the accessibility of at least a certain class of activators in vivo, therefore supporting the idea that nucleosome positioning plays a vital role in repression by α2/Mcm1.…”

“…The hypersensitive site was detectable in a cells, in which nucleosomes were not positioned, but it was strongly diminished in α cells, indicating that Hap1 binding is severely limited at the dyad in the positioned nucleosome IV. Similar results for Hap1 binding were obtained for TALS-UAS1b (lanes [19][20][21][22][23][24] and -UAS1c (lanes [28][29][30][31][32][33], in which the UAS1 site was located in the peripheral region of the positioned nucleosome IV; that is, the hypersensitive site is present in a cells, but absent in α cells. It should be noted that the location of UAS1 in TALSUASc is shifted to 5 bp exterior to the nucleosome edge, as compared to that in TALS-UAS1b.…”

“…The Isw2 chromatin remodeling complex positions nucleosomes in Tup1-regulated genes, such as a-cell-specific genes [15,[28][29][30][31]. Here, we analyzed the chromatin structure of TALS-UAS1a in isogenic isw2 mutant strains (Fig.…”

“…In a cells, nucleosomes were not positioned in these TALS minichromosomes. We also analyzed the chromatin structure of TRP1ARS1-UAS1 (lanes [25][26][27][28][29][30]. The HSR B region, where the UAS1 was inserted, was hypersensitive to MNase digestion in chromatin, as compared to naked DNA samples.…”

“…The Tup1-Ssn6 corepressor is required for repression and nucleosome positioning in a-cellspecific genes [26,27], and a tup1 mutation causes the loss of nucleosome positioning in TALS minichromosomes [28]. The Isw2 chromatin remodeling complex positions nucleosomes in Tup1-regulated genes, such as a-cell-specific genes [15,[28][29][30][31].…”