Dose-dependency of heterochromatin domains reveals subtelomeric structuration in budding yeast

The eukaryotic genome is divided into chromosomal domains of distinct gene activities. Transcriptionally silent chromatin is found in subtelomeric regions leading to telomeric position effect (TPE) in yeast, fly and man. Silent chromatin generally initiates at defined loci and tends to propagate from those sites by self-recruitment mechanisms implying the requirement for processes preventing ectopic spreading of silencing. Barrier elements that can block the spread of silent chromatin have been documented, but their relative efficiency is not known. Here we explore the dose-dependency of silencing factors for the extent of TPE in budding yeast. We characterized genome wide the impact of overexpressing the silencing factors Sir2 and Sir3 on the spreading of Sir3 and its impact on coding and non-coding transcription. We thus reveal that extension of silent domains can reach saturation. Analysis of published data sets enabled to uncover that the extension of Sir3 bound domains stops at zones corresponding to transitions of specific histone marks including H3K79 methylation that is deposited by the conserved enzyme Dot1. Importantly, DOT1 is essential for viability when Sir3 is in excess indicating that this transition actively blocks Sir3 spreading. Our work uncovers previously uncharacterized discrete chromosomal domains associated with specific chromatin features and demonstrates that TPE is efficiently restricted to subtelomeres by the preexisting chromatin landscape.真核基因组被划分为具有不同基因活性的染色体结构域。转录沉默染色质存在于亚端粒区域，可在酵母、果蝇与人类中引发端粒位置效应（telomeric position effect, TPE）。沉默染色质通常在特定基因座起始，并倾向于通过自我招募机制从这些位点扩散，这意味着需要存在调控过程以阻止沉默信号的异位扩散。已有研究报道了可阻断沉默染色质扩散的屏障元件，但其相对效率尚未明确。本研究以酿酒酵母为模型，探究沉默因子的剂量依赖性对TPE范围的影响。我们在全基因组范围内分析并表征了过表达沉默因子Sir2与Sir3对Sir3扩散的影响，以及其对编码转录与非编码转录的调控效应，由此发现沉默结构域的延伸可达到饱和状态。通过对已发表数据集的分析，我们揭示Sir3结合结构域的延伸会终止于对应特定组蛋白修饰转变的区域，其中包括由保守酶Dot1催化沉积的H3K79甲基化修饰。尤为重要的是，当Sir3过表达时，DOT1基因对细胞存活至关重要，这表明该组蛋白修饰转变可主动阻断Sir3的扩散。本研究揭示了此前未被表征的、与特定染色质特征相关的离散染色体结构域，并证实TPE可通过预先存在的染色质环境被有效限制在亚端粒区域。