A Cis-Acting tRNA Gene Imposes the Cell Cycle Progression Requirement for Establishing Silencing at the HMR Locus in Yeast

Numerous studies have determined that the establishment of Sir protein-dependent transcriptional silencing in yeast requires progression through the cell cycle. In our study we examined the cell cycle requirement for the establishment of silencing at the HML and HMR loci using strains bearing conditional or inducible SIR3 alleles. Consistent with prior reports, we observed that establishing silencing at HMR required progression through the cell cycle. Unexpectedly, we found that the HML locus is far less dependent on cell cycle progression to establish silencing. Seeking cis-acting elements that could account for this difference, we found that deletion of a tRNA gene that serves as a chromatin boundary at HMR abolishes the cell cycle progression requirement at this locus, while insertion of sequences containing this tRNA gene adjacent to HML imposes dependence on cell cycle progression for the full establishment of silencing. Our results indicate that the cell cycle progression requirement is not a property intrinsic to the formation of heterochromatin in yeast, but is instead a cis-limited, locus-specific phenomenon. We show that inactivation of the Scc1 cohesin also abolishes the requirement for cell cycle progression and test models based on a possible link between the tRNA gene and cohesin association.