Heterochromatin spreading at yeast telomeres occurs in M-phase.
Kristen Martins-Taylor, Scott Holmes
MB&B, Wesleyan University, Lawn Ave., Middletown, CT, 06459, USA
Heterochromatin regulation of gene expression exhibits epigenetic inheritance, in which some feature of the structure is retained and can re-seed formation in new cells. To understand the cell-cycle events that influence heterochromatin assembly and maintenance in budding yeast we have examined the kinetics of heterochromatin spreading at telomeres. We have constructed a strain in which efficient silencing of telomere-linked URA3 gene depends on the controlled expression of an inducible SIR3 gene. Prior studies determined that S-phase passage was required for the establishment of silencing 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. Furthermore, we find that passage through mitosis is sufficient to establish silencing at the HML locus in a strain bearing a conditional allele of SIR3. These experiments point to crucial events in mitosis influencing the assembly and persistence of heterochromatin. To investigate these critical events in mitosis, we are examining the roles of the cohesin and condensin complexes through the use of temperature sensitive alleles. In addition, we are also examining the assembly of heterochromatin as a function of the cell cycle.
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