The requirements for COMPASS and Paf1 in transcriptional silencing and methylation of histone H3 in Saccharomyces cerevisiae.

The Set1-containing complex, COMPASS, methylates histone H3 on lysine 4 (K4) in Saccharomyces cerevisiae. Despite the preferential association of K4-trimethylated H3 with regions of the genome that are transcribed by RNA polymerase II, transcriptional silencing is one of the few cases in S. cerevisiae where histone-methylation defects have a clear effect on gene expression. To better understand the role of COMPASS in transcriptional silencing, we have determined which members of COMPASS are required for silencing at the ribosomal DNA locus (rDNA), a telomere, and the silent mating loci (HM) using Northern analyses. Our findings indicate that most members of COMPASS are required for silencing at the rDNA and telomere, while none are required for silencing of endogenous genes at the HM loci. To complement gene-expression analysis, quantitative Western blot experiments were performed to determine the members of COMPASS that are required for methylation of histone H3. While most are required for trimethylation, cells lacking certain COMPASS proteins maintain reduced levels of K4 mono- and dimethylated H3, suggesting that some COMPASS members have redundant function. Finally, we show Paf1 is required for silencing and K4-methylated H3 at the rDNA, suggesting a possible direct role for K4-methylated H3 in gene silencing.