Histone H3 Lysine 36 Methylation Antagonizes Silencing in Saccharomyces cerevisiae Independently of the Rpd3S Histone Deacetylase Complex

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Abstract
In yeast, methylation of histone H3 on lysine 36 (H3-K36) is catalyzed by the NSD1 leukemia oncoprotein homolog Set2. The histone deacetylase complex Rpd3S is recruited to chromatin via binding of the chromodomain protein Eaf3 to methylated H3-K36 to prevent erroneous transcription initiation. Here we identify a distinct function for H3-K36 methylation. We used random mutagenesis of histones H3 and H4 followed by a reporter-based screen to identify residues necessary to prevent the ectopic spread of silencing from the silent mating-type locus HMRa into flanking euchromatin. Mutations in H3-K36 or deletion of SET2 caused ectopic silencing of a heterochromatin-adjacent reporter. Transcriptional profiling revealed that telomere-proximal genes are enriched for those that display decreased expression in a set2Δ strain. Deletion of SIR4 rescued the expression defect of 26 of 37 telomere-proximal genes with reduced expression in set2Δ cells, implying that H3-K36 methylation prevents the spread of telomeric silencing. Indeed, Sir3 spreads from heterochromatin into neighboring euchromatin in set2Δ cells. Furthermore, genetic experiments demonstrated that cells lacking the Rpd3S-specific subunits Eaf3 or Rco1 did not display the anti-silencing phenotype of mutations in SET2 or H3-K36. Thus, antagonism of silencing is independent of the only known effector of this conserved histone modification.