Methylation of histone H3K9 is a hallmark of epigenetic silencing in eukaryotes. Nucleosome modifications often rely on positive feedback where enzymes are recruited by modified nucleosomes. A combination of local and global feedbacks has been proposed to account for some dynamic properties of heterochromatin, but the range at which the global feedbacks operate and the exact mode of heterochromatin propagation are not known. We investigated these questions in fission yeast. Guided by mathematical modeling, we incrementally increased the size of the mating-type region and profiled heterochromatin establishment over time. We observed exponential decays in the proportion of cells with active reporters, with rates that decreased with domain size. Establishment periods varied from a few generations in wild type to >200 generations in the longest region examined, and highly correlated silencing of two reporters located outside the nucleation center was observed. On a chromatin level, this indicates that individual regions are silenced in sudden bursts. Mathematical modeling accounts for these bursts if heterochromatic nucleosomes facilitate a deacetylation or methylation reaction at long range, in a distance-independent manner. A likely effector of three-dimensional interactions is the evolutionarily conserved Swi6^HP1 H3K9me reader, indicating the bursting behavior might be a general mode of heterochromatin propagation.

组蛋白 H3K9 的甲基化是真核生物表观遗传沉默的标志。核小体修饰通常依赖于正反馈，其中酶被修饰的核小体募集。已经提出了局部和全局反馈的组合来解释异染色质的一些动态特性，但全局反馈操作的范围和异染色质传播的确切模式尚不清楚。我们在裂殖酵母中研究了这些问题。在数学建模的指导下，随着时间的推移，我们逐渐增加了交配型区域的大小和异染色质的建立。我们观察到具有活性报告基因的细胞比例呈指数衰减，且速率随域大小而降低。建立时期从野生型的几代到> 检查最长区域的 200 代，观察到位于成核中心外的两个报告基因高度相关的沉默。在染色质水平上，这表明个别区域在突然爆发中沉默。如果异染色质核小体以与距离无关的方式促进远距离的去乙酰化或甲基化反应，则数学模型可以解释这些爆发。三维相互作用的一个可能效应器是进化上保守的 Swi6 以与距离无关的方式。三维相互作用的一个可能效应器是进化上保守的 Swi6 以与距离无关的方式。三维相互作用的一个可能效应器是进化上保守的 Swi6^HP1 H3K9me 阅读器，表明爆发行为可能是异染色质传播的一般模式。