Heterochromatin spreading at yeast telomeres occurs in M phase

Heterochromatin regulation of gene expression exhibits epigenetic inheritance, in which some feature of the structure is retained and can reseed formation in new cells. To understand the cell-cycle events that influence heterochromatin assembly and maintenance in budding yeast, we have conducted two types of experiments. First we have examined the kinetics of heterochromatin spreading at telomeres. We have constructed a strain in which the efficient silencing of a telomere-linked URA3 gene depends on the inducible expression of the Sir3 silencing factor. Prior studies determined that S-phase passage was required for the establishment of silencing at the HM loci in yeast. We find that establishment of silencing in our strain occurs at a point coincident with mitosis and does not require S-phase passage. In addition, we find that passage through mitosis is sufficient to establish silencing at the HML locus in a strain bearing a conditional allele of SIR3. Finally, we have also assessed the stability of yeast heterochromatin in the absence of the cis-acting elements required for its establishment. We show that silencing is stable through S phase in the absence of silencers and therefore possesses the ability to self-propagate through DNA replication. However, silencing is lost in the absence of silencers during progression through M phase. These experiments point to crucial events in mitosis influencing the assembly and persistence of heterochromatin.