Correlating chromatin states with silencing phenotypes in Saccharomyces cerevisiae
The structure of chromatin is indicative of its expression status. This work explores the structure of repressive and non-repressive subtelomeric regions at native telomeres and the effect of both cis- and trans-acting factors on these regions.;The region adjacent to the subtelomeric core X element exhibited a heterochromatic MNase digestion pattern, indicative of phased nucleosomes, at the repressive telomere and a euchromatic structure at the non-repressive telomere. SIR2, SIR3 and SIR4, but not SIR1 were all required for formation of the heterochromatic structure in addition to the gene repression. However, deletion mutants of several histone modifiers (SET1, DOT1, BRE1, SAS2 and BDF1) disrupted silencing of the URA3 marker without affecting the heterochromatic structure. Deletion of yKU80, known to be essential for TPE, moderately disrupted the heterochromatic features. Therefore, formation of a heterochromatic structure is required but insufficient for silencing. Mutations in the ACS and Abf1p binding element in core X, known to decrease TPE, also caused a moderate disruption of the heterochromatic features. Therefore, core X is proposed to be required for the establishment but not maintenance of TPE.;A loop model of the telomere structure involving a telomere-core X interaction, stabilized by factors bound to both loci (ORC, Abf1p, Rap1p, Sir's and yKu), was previously proposed to explain the discontinuous nature of silencing close to telomeres. In this study, yKu80p is found to associate with core X elements at repressive and non-repressive telomeres, in addition to its known association with the telomere repeats. Disruption of the ORC and Abf1p binding sites in core X was insufficient to disrupt yKu binding. Therefore, the loop is proposed to be present at all telomeres, not just those with regions of repression around core X.