Polycomb group (PcG) proteins are an important group of transcriptional repressors that act by modifying chromatin. PcG target genes are covered by the repressive chromatin mark H3K27me3. Polycomb repressive complex 2 (PRC2) is a multiprotein complex that is responsible for generating H3K27me3. In Drosophila, PRC2 is recruited by Polycomb Response Elements (PREs) and then tri-methylates flanking nucleosomes, spreading the H3K27me3 mark over large regions of the genome, the "Polycomb domains". What defines the boundary of a Polycomb domain? There is experimental evidence that insulators, PolII, and active transcription can all form the boundaries of Polycomb domains. Here we divide the boundaries of larval Polycomb domains into six different categories. In one category, genes are transcribed toward the Polycomb domain, where active transcription is thought to stop the spreading of H3K27me3. In agreement with this, we show that introducing a transcriptional terminator into such a transcription unit causes an extension of the Polycomb domain. Additional data suggest that active transcription of a boundary gene may restrict the range of enhancer activity of a Polycomb-regulated gene.

中文翻译：

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定义果蝇多梳结构域的边界。

多梳族 (PcG) 蛋白是一组重要的转录抑制蛋白，通过修饰染色质发挥作用。PcG 靶基因被抑制性染色质标记 H3K27me3 覆盖。Polycomb 抑制复合物 2 (PRC2) 是一种多蛋白复合物，负责生成 H3K27me3。在果蝇中，PRC2 被多梳响应元件 (PRE) 招募，然后对核小体侧翼进行三甲基化，将 H3K27me3 标记传播到基因组的较大区域（“多梳结构域”）。什么定义了 Polycomb 域的边界？有实验证据表明绝缘子、PolII 和主动转录都可以形成 Polycomb 结构域的边界。在这里，我们将幼虫 Polycomb 域的边界分为六个不同的类别。在一类中，基因向 Polycomb 结构域转录，其中活跃的转录被认为可以阻止 H3K27me3 的传播。与此一致的是，我们表明将转录终止子引入到这样的转录单元中会导致 Polycomb 结构域的延伸。其他数据表明，边界基因的活跃转录可能会限制 Polycomb 调节基因的增强子活性范围。