A core nucleosome surface crucial for transcriptional silencing

Transcriptional silencing in yeast provides a genetically tractable system for analyzing the formation and maintenance of heterochromatin, a transcriptionally repressive chromatin structure found in all organisms. The nucleosome constitutes the central structure of chromatin and comprises two chains each of histones H2A, H2B, H3 and H4. The structure of the nucleosome consists of a central globular core surrounded by outwardly protruding amino-terminal histone tails. We show that a specific surface of the assembled nucleosome core is required for silencing in yeast. This surface is located at a H3/H4 histone-fold motif and contains amino-acid side chains located on the nucleosome disk surface and on an adjacent surface that interacts with DNA. The side chains, identified from mutants in which all three forms of silencing (rDNA, telomere and silent mating locus silencing) are eliminated, are centered around Lys79 of histone H3, a residue methylated by the yeast Dot1 protein. Moreover, mutations in the genes encoding H3 (HHT1 and HHT2) and H4 (HHF1 and HHF2) mapping to spatially adjacent amino-acid residues affected the three forms of silencing distinctly, suggesting that specific interactions mediate each form of silencing. Several of the mutations that we identified resemble those in a cluster of previously identified mutations affecting a distinct histone-fold motif elsewhere in the nucleosome core. These two clusters relieve distinct forms of transcriptional repression (silencing versus repression resulting from lack of Swi/Snf chromatin remodeling activity).