Lanzuolo Chiara, Roure Virginie, Dekker Job, Bantignies Frédéric, Orlando Valerio
Dulbecco Telethon Institute at IGB CNR, Epigenetics and Genome Reprogramming, Via Pietro Castellino 111, 80131 Naples, Italy.
Nat Cell Biol. 2007 Oct;9(10):1167-74. doi: 10.1038/ncb1637. Epub 2007 Sep 9.
In Drosophila, the function of the Polycomb group genes (PcGs) and their target sequences (Polycomb response elements (PREs)) is to convey mitotic heritability of transcription programmes--in particular, gene silencing. As part of the mechanisms involved, PREs are thought to mediate this transcriptional memory function by building up higher-order structures in the nucleus. To address this question, we analysed in vivo the three-dimensional structure of the homeotic locus bithorax complex (BX-C) by combining chromosome conformation capture (3C) with fluorescent in situ hybridization (FISH) and FISH immunostaining (FISH-I) analysis. We found that, in the repressed state, all major elements that have been shown to bind PcG proteins, including PREs and core promoters, interact at a distance, giving rise to a topologically complex structure. We show that this structure is important for epigenetic silencing of the BX-C, as we find that major changes in higher-order structures must occur to stably maintain alternative transcription states, whereas histone modification and reduced levels of PcG proteins determine an epigenetic switch that is only partially heritable.
在果蝇中，多梳蛋白家族基因（PcGs）及其靶序列（多梳反应元件（PREs））的功能是传递转录程序的有丝分裂遗传性——特别是基因沉默。作为相关机制的一部分，PREs被认为通过在细胞核中构建高阶结构来介导这种转录记忆功能。为了解决这个问题，我们通过将染色体构象捕获（3C）与荧光原位杂交（FISH）和FISH免疫染色（FISH-I）分析相结合，在体内分析了同源异型基因座双胸复合体（BX-C）的三维结构。我们发现，在抑制状态下，所有已被证明能结合PcG蛋白的主要元件，包括PREs和核心启动子，在一定距离处相互作用，形成一种拓扑复杂的结构。我们表明这种结构对于BX-C的表观遗传沉默很重要，因为我们发现高阶结构的重大变化必须发生才能稳定维持交替的转录状态，而组蛋白修饰和PcG蛋白水平的降低决定了一种仅部分可遗传的表观遗传开关。
