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首页> 外文期刊>PLoS Genetics >A unique enhancer boundary complex on the mouse ribosomal RNA genes persists after loss of Rrn3 or UBF and the inactivation of RNA polymerase I transcription
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A unique enhancer boundary complex on the mouse ribosomal RNA genes persists after loss of Rrn3 or UBF and the inactivation of RNA polymerase I transcription

机译：缺失Rrn3或UBF并失活RNA聚合酶I转录后，小鼠核糖体RNA基因上的独特增强子边界复合物仍然存在

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Transcription of the several hundred of mouse and human Ribosomal RNA (rRNA) genes accounts for the majority of RNA synthesis in the cell nucleus and is the determinant of cytoplasmic ribosome abundance, a key factor in regulating gene expression. The rRNA genes, referred to globally as the rDNA, are clustered as direct repeats at the Nucleolar Organiser Regions, NORs, of several chromosomes, and in many cells the active repeats are transcribed at near saturation levels. The rDNA is also a hotspot of recombination and chromosome breakage, and hence understanding its control has broad importance. Despite the need for a high level of rDNA transcription, typically only a fraction of the rDNA is transcriptionally active, and some NORs are permanently silenced by CpG methylation. Various chromatin-remodelling complexes have been implicated in counteracting silencing to maintain rDNA activity. However, the chromatin structure of the active rDNA fraction is still far from clear. Here we have combined a high-resolution ChIP-Seq protocol with conditional inactivation of key basal factors to better understand what determines active rDNA chromatin. The data resolve questions concerning the interdependence of the basal transcription factors, show that preinitiation complex formation is driven by the architectural factor UBF (UBTF) independently of transcription, and that RPI termination and release corresponds with the site of TTF1 binding. They further reveal the existence of an asymmetric Enhancer Boundary Complex formed by CTCF and Cohesin and flanked upstream by phased nucleosomes and downstream by an arrested RNA Polymerase I complex. We find that the Enhancer Boundary Complex is the only site of active histone modification in the 45kbp rDNA repeat. Strikingly, it not only delimits each functional rRNA gene, but also is stably maintained after gene inactivation and the re-establishment of surrounding repressive chromatin. Our data define a poised state of rDNA chromatin and place the Enhancer Boundary Complex as the likely entry point for chromatin remodelling complexes.
机译：数百个小鼠和人类核糖体RNA（rRNA）基因的转录占细胞核中RNA合成的大部分，并且是细胞质核糖体丰度的决定因素，而核糖体丰度是调节基因表达的关键因素。 rRNA基因，在全球范围内被称为rDNA，聚集在几个染色体的核仁组织区NORs上，作为直接重复序列，在许多细胞中，活性重复序列以接近饱和的水平转录。 rDNA也是重组和染色体断裂的热点，因此了解其控制具有广泛的重要性。尽管需要高水平的rDNA转录，但通常只有一小部分rDNA具有转录活性，并且某些NOR被CpG甲基化永久沉默。各种染色质重塑复合物已被认为可以抵消沉默以维持rDNA活性。但是，活性rDNA组分的染色质结构仍很不清楚。在这里，我们将高分辨率ChIP-Seq协议与关键基础因子的条件灭活相结合，以更好地了解决定活性rDNA染色质的因素。数据解决了有关基础转录因子相互依赖性的问题，表明预起始复合物的形成受建筑因子UBF（UBTF）的驱动，与转录无关，并且RPI的终止和释放与TTF1结合位点相对应。他们进一步揭示了不对称增强子边界复合物的存在，该复合物由CTCF和Cohesin形成，在上游侧为分相的核小体，在下游侧为被阻滞的RNA聚合酶I复合体。我们发现增强子边界复合物是45kbp rDNA重复序列中活性组蛋白修饰的唯一位点。引人注目的是，它不仅界定了每个功能性rRNA基因的界限，而且在基因失活和周围抑制性染色质的重建后也得以稳定维持。我们的数据定义了rDNA染色质的平衡状态，并将增强子边界复合物放置为染色质重塑复合物的可能入口点。

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