In most eukaryotes, constitutive heterochromatin, defined by histone H3 lysine 9 methylation (H3K9me), is enriched on repetitive DNA, such as pericentromeric repeats and transposons. Furthermore, repetitive transgenes also induce heterochromatin formation in diverse model organisms. However, the mechanisms that promote heterochromatin formation at repetitive DNA elements are still not clear. Here, using fission yeast, we show that tandemly repeated mRNA genes promote RNA interference (RNAi)-mediated heterochromatin formation in cooperation with an antisilencing factor, Epe1. Although the presence of tandemly repeated genes itself does not cause heterochromatin formation, once complementary small RNAs are artificially supplied in trans, the RNAi machinery assembled on the repeated genes starts producing cognate small RNAs in cis to autonomously maintain heterochromatin at these sites. This “repeat-induced RNAi” depends on the copy number of repeated genes and Epe1, which is known to remove H3K9me and derepress the transcription of genes underlying heterochromatin. Analogous to repeated genes, the DNA sequence underlying constitutive heterochromatin encodes widespread transcription start sites (TSSs), from which Epe1 activates ncRNA transcription to promote RNAi-mediated heterochromatin formation. Our results suggest that when repetitive transcription units underlie heterochromatin, Epe1 generates sufficient transcripts for the activation of RNAi without disruption of heterochromatin.

中文翻译：

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串联重复基因通过裂殖酵母中的抗沉默因子 Epe1 促进 RNAi 介导的异染色质形成

在大多数真核生物中，由组蛋白 H3 赖氨酸 9 甲基化 (H3K9me) 定义的组成型异染色质富含重复 DNA，例如着丝粒周围重复序列和转座子。此外，重复转基因还会在不同的模型生物体中诱导异染色质形成。然而，促进重复 DNA 元件异染色质形成的机制仍不清楚。在这里，我们使用裂殖酵母，表明串联重复的 mRNA 基因与抗沉默因子 Epe1 合作促进 RNA 干扰 (RNAi) 介导的异染色质形成。虽然串联重复基因的存在本身并不会导致异染色质形成，但一旦反式人工提供互补的小 RNA，组装在重复基因上的 RNAi 机制开始产生顺式同源小 RNA，以在这些位点自主维持异染色质。这种“重复诱导的 RNAi”取决于重复基因和 Epe1 的拷贝数，众所周知，Epe1 可以去除 H3K9me 并去抑制异染色质下基因的转录。类似于重复基因，组成型异染色质的 DNA 序列编码广泛的转录起始位点 (TSS)，Epe1 从该位点激活 ncRNA 转录以促进 RNAi 介导的异染色质形成。我们的结果表明，当重复转录单位构成异染色质的基础时，Epe1 会生成足够的转录物来激活 RNAi，而不会破坏异染色质。