Multiple pathways tether telomeres and silent chromatin at the nuclear periphery: functional implications for sir-mediated repression.

The positioning of chromosomal domains within interphase nuclei is thought to contribute to establishment and maintenance of epigenetic control. Using GFP-tagged chromosomal domains, LexA-fusion targeting and live microscopy, we investigated mechanisms through which chromatin can be anchored to the nuclear envelope (NE). We find that a subdomain of the silencing information regulator Sir4 (Sir4(PAD)) and yKu80 are sufficient to tether a chromatin region to the nuclear periphery, independently of their silencing function. Sir4(PAD) interacts with Esc1 (Establishes Silent Chromatin 1), a large acidic protein, localized at the nuclear periphery in the absence of Sir4 and yKu. Sir4 also binds to the periphery through yKu80, whose perinuclear ligand is unidentified. Both pathways are involved in the localization of natural telomeres. To show that silent chromatin can also mediate anchorage, we uncoupled the HMR silent mating-type locus from the chromosome using inducible site-specific recombination. Real-time cytological techniques reveal the position and dynamics of the excised locus. We show that the silent HMR ring associates with the NE in a SIR-dependent manner, while derepressed excised rings move without detectable constraint throughout the nucleoplasm. Dual anchoring pathways thus cooperate to generate high concentrations of SIR proteins in perinuclear foci, which in turn promote repression.