Department of Computational and Systems Biology, John Innes Centre, Norwich Research Park, Norwich NR4 7UH, UK.
Nature. 2011 Jul 24;476(7358):105-8. doi: 10.1038/nature10241.
The conserved Polycomb repressive complex 2 (PRC2) generates trimethylation of histone 3 lysine 27 (H3K27me3), a modification associated with stable epigenetic silencing. Much is known about PRC2-induced silencing but key questions remain concerning its nucleation and stability. Vernalization, the perception and memory of winter in plants, is a classic epigenetic process that, in Arabidopsis, involves PRC2-based silencing of the floral repressor FLC. The slow dynamics of vernalization, taking place over weeks in the cold, generate a level of stable silencing of FLC in the subsequent warm that depends quantitatively on the length of the prior cold. These features make vernalization an ideal experimental system to investigate both the maintenance of epigenetic states and the switching between them. Here, using mathematical modelling, chromatin immunoprecipitation and an FLC:GUS reporter assay, we show that the quantitative nature of vernalization is generated by H3K27me3-mediated FLC silencing in the warm in a subpopulation of cells whose number depends on the length of the prior cold. During the cold, H3K27me3 levels progressively increase at a tightly localized nucleation region within FLC. At the end of the cold, numerical simulations predict that such a nucleation region is capable of switching the bistable epigenetic state of an individual locus, with the probability of overall FLC coverage by silencing H3K27me3 marks depending on the length of cold exposure. Thus, the model predicts a bistable pattern of FLC gene expression in individual cells, a prediction we verify using the FLC:GUS reporter system. Our proposed switching mechanism, involving the local nucleation of an opposing histone modification, is likely to be widely relevant in epigenetic reprogramming.
保守的多梳抑制复合物 2（PRC2）生成组蛋白 3 赖氨酸 27 三甲基化（H3K27me3），这种修饰与稳定的表观遗传沉默有关。人们对 PRC2 诱导的沉默了解很多，但关于其核形成和稳定性仍存在关键问题。植物中的春化作用是一种经典的表观遗传过程，即植物感知和记忆冬季，在拟南芥中，它涉及基于 PRC2 的 floral 抑制剂 FLC 的沉默。春化作用的缓慢动力学在寒冷中需要数周的时间，因此在随后的温暖中，FLC 的稳定沉默程度在很大程度上取决于先前寒冷的持续时间。这些特征使得春化作用成为研究表观遗传状态的维持和它们之间的转换的理想实验系统。在这里，我们使用数学建模、染色质免疫沉淀和 FLC:GUS 报告基因检测，表明春化作用的定量性质是由 H3K27me3 介导的在温暖条件下 FLC 的沉默引起的，这种沉默发生在依赖于先前寒冷持续时间的细胞亚群中。在寒冷期间，H3K27me3 水平在 FLC 内的一个紧密定位的核形成区域逐渐增加。在寒冷结束时，数值模拟预测这样的核形成区域能够切换单个基因座的双稳态表观遗传状态，通过 H3K27me3 标记沉默覆盖整个 FLC 的概率取决于寒冷暴露的持续时间。因此，该模型预测了单个细胞中 FLC 基因表达的双稳态模式，我们使用 FLC:GUS 报告基因系统验证了这一预测。我们提出的切换机制涉及到相反的组蛋白修饰的局部核形成，这可能在表观遗传重编程中具有广泛的相关性。
