Formation of a Polycomb-Domain in the Absence of Strong Polycomb Response Elements

Polycomb group response elements (PREs) in Drosophila are DNA-elements that recruit Polycomb proteins (PcG) to chromatin and regulate gene expression. PREs are easily recognizable in the Drosophila genome as strong peaks of PcG-protein binding over discrete DNA fragments; many small but statistically significant PcG peaks are also observed in PcG domains. Surprisingly, in vivo deletion of the four characterized strong PREs from the PcG regulated invected-engrailed (inv-en) gene complex did not disrupt the formation of the H3K27me3 domain and did not affect inv-en expression in embryos or larvae suggesting the presence of redundant PcG recruitment mechanism. Further, the 3D-structure of the inv-en domain was only minimally altered by the deletion of the strong PREs. A reporter construct containing a 7.5kb en fragment that contains three weak peaks but no large PcG peaks forms an H3K27me3 domain and is PcG-regulated. Our data suggests a model for the recruitment of PcG-complexes to Drosophila genes via interactions with multiple, weak PREs spread throughout an H3K27me3 domain.

果蝇体内的多梳蛋白家族应答元件（Polycomb group response elements，PREs）是一类可招募多梳蛋白家族（Polycomb group proteins，PcG）至染色质并调控基因表达的DNA元件。在果蝇基因组中，PREs可被轻易识别为离散DNA片段上呈现强结合峰的多梳蛋白结合区域；此外在多梳蛋白结构域（PcG domains）中也存在大量虽强度较低但具有统计学显著性的多梳蛋白结合峰。令人意外的是，在体内对受PcG调控的invected-engrailed（inv-en）基因复合体中4个已鉴定的强PREs进行敲除，既未破坏组蛋白H3赖氨酸27三甲基化结构域（H3K27me3 domain）的形成，也未影响胚胎及幼虫阶段inv-en的表达，这表明存在冗余的PcG招募机制。进一步研究发现，强PREs的敲除仅对inv-en结构域的三维结构产生了极轻微的改变。一款包含7.5kb en片段的报告基因构建体，该片段含有3个强度较低的结合峰但无明显的多梳蛋白结合大峰，可形成H3K27me3结构域并受PcG调控。本研究数据支持如下模型：果蝇体内的PcG复合物可通过与分布于整个H3K27me3结构域的多个弱PREs相互作用，被招募至靶基因处。