The Paf1 Complex Represses ARG1 Transcription in Saccharomyces cerevisiae by Promoting Histone Modifications

The conserved multifunctional Paf1 complex is important for the proper transcription of numerous genes, and yet the exact mechanisms by which it controls gene expression remain unclear. While previous studies indicate that the Paf1 complex is a positive regulator of transcription, the repression of many genes also requires the Paf1 complex. In this study we usedARG1 as a model gene to study transcriptional repression by the Paf1 complex inSaccharomyces cerevisiae . We found that several members of the Paf1 complex contribute toARG1 repression and that the complex localizes to theARG1 promoter and coding region in repressing conditions, which is consistent with a direct repressive function. Furthermore, Paf1 complex-dependent histone modifications are enriched at theARG1 locus in repressing conditions, and histone H3 lysine 4 methylation contributes toARG1 repression. Consistent with previous reports, histone H2B monoubiquitylation, the mark upstream of histone H3 lysine 4 methylation, is also important forARG1 repression. To begin to identify the mechanistic basis for Paf1 complex-mediated repression ofARG1 , we focused on the Rtf1 subunit of the complex. Through an analysis ofRTF1 mutations that abrogate known Rtf1 activities, we found that Rtf1 mediatesARG1 repression primarily by facilitating histone modifications. Other members of the Paf1 complex, such as Paf1, appear to repressARG1 through additional mechanisms. Together, our results suggest that Rtf1-dependent histone H2B ubiquitylation and H3 K4 methylation repressARG1 expression and that histone modifications normally associated with active transcription can occur at repressed loci and contribute to transcriptional repression.