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Subtelomeres are imperfect repeats adjacent to telomeres that are repressed by heterochromatin. Although essential for genome integrity, their repetitive nature has thwarted dissection of local heterochromatin assembly and maintenance mechanisms. Here, we engineeredSchizosaccharomyces pombestrains carrying fluorescent reporters at a single subtelomere. We find that subtelomeric heterochromatin is organized into discrete subdomains that nucleate at telomere-proximal and cryptic internal sites. Telomere-proximal regions depend on canonical shelterin or RNA interference nucleation pathways, while telomere-distal regions require nucleosome remodelers, histone chaperones, and boundary-associated factors. Using multi-generational live imaging and targeted perturbations, we show that subtelomeric subdomains display position-specific, clonally variable silencing across a spectrum of robust to fragile epigenetic states. This clonal variegation is also induced by naturally occurring subtelomeric structural variants. These findings demonstrate that subtelomeric heterochromatin maintenance is not uniform but rather governed by local chromatin context and architecture. 

Abstract Heterochromatic gene silencing relies on combinatorial control by specific histone modifications, the occurrence of transcription, and/or RNA degradation. Once nucleated, heterochromatin propagates within defined chromosomal regions and is maintained throughout cell divisions to warrant proper genome expression and integrity. In the fission yeast Schizosaccharomyces pombe, the Ccr4-Not complex partakes in gene silencing, but its relative contribution to distinct heterochromatin domains and its role in nucleation versus spreading have remained elusive. Here, we unveil major functions for Ccr4-Not in silencing and heterochromatin spreading at the mating type locus and subtelomeres. Mutations of the catalytic subunits Caf1 or Mot2, involved in RNA deadenylation and protein ubiquitinylation, respectively, result in impaired propagation of H3K9me3 and massive accumulation of nucleation-distal heterochromatic transcripts. Both silencing and spreading defects are suppressed upon disruption of the heterochromatin antagonizing factor Epe1. Overall, our results position the Ccr4-Not complex as a critical, dual regulator of heterochromatic gene silencing and spreading.