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In budding yeast, transcriptional silencing, which is important to regulate gene expression and maintain genome integrity, requires silent information regulator (Sir) proteins. In addition, Rtt106, a histone chaperone involved in nucleosome assembly, functions in transcriptional silencing. However, how transcriptional silencing is regulated during mitotic cell division is not well understood. We show that cells lacking Dia2, a component of the SCF Dia2  E3 ubiquitin ligase involved in DNA replication, display defects in silencing at the telomere and  HMR  locus and that the F-box and C-terminal regions of Dia2, two regions important for Dia2's ubiquitylation activity, are required for proper transcriptional silencing at these loci. In addition, we show that Sir proteins are mislocalized in  dia2 Δ mutant cells. Mutations in Dia2 and Rtt106 result in a synergistic loss of silencing at the  HMR  locus and significant elevation of Sir4 proteins at the  HMR  locus, suggesting that silencing defects in  dia2 Δ mutant cells are due, at least in part, to the altered levels of Sir4 at silent chromatin. Supporting this idea, we show that SCF Dia2  ubiquitylates Sir4  in vitro  and  in vivo . Furthermore, Sir4 binding to silent chromatin is dynamically regulated during the cell cycle, and this regulation is lost in  dia2 Δ mutant cells. These results demonstrate that the SCF Dia2  complex is involved in transcriptional silencing, ubiquitylates Sir4, and regulates transcriptional silencing during the cell cycle.

The SCFDia2 Ubiquitin E3 Ligase Ubiquitylates Sir4 and Functions in Transcriptional Silencing