Bivalency in <i>Drosophila</i> embryos is associated with strong inducibility of Polycomb target genes

Polycomb group (PcG) and Trithorax group (TrxG) proteins orchestrate development of a multicellular organism by faithfully maintaining cell fate decisions made early in embryogenesis. An important chromatin mark connected to PcG/TrxG regulation is bivalent domains, the simultaneous presence of H3K27me3 and H3K4me3 on a given locus, originally identified in mammalian embryonic stem cells but considered to be absent in invertebrates. Here, we provide evidence for the existence of bivalency in fly embryos. Using a recently described PcG reporter fly line, we observed a strong reporter inducibility in the embryo and its sharp decrease in larval and adult stages. Analysis of the chromatin landscape of the reporter revealed a strong signal for the repressive PcG mark, H3K27me3, in all three developmental stages and, surprisingly, a strong signal for a transcriptionally activating H3K4me3 mark in the embryo. Using re-chromatin immunoprecipitation experiments, bivalent domains were also uncovered at endogenous PcG targets like the Hox genes.多梳蛋白家族（Polycomb group, PcG）与三胸蛋白家族（Trithorax group, TrxG）通过忠实维持胚胎早期确立的细胞命运决策，调控多细胞生物的发育进程。与PcG/TrxG调控相关的一类重要染色质表观标记为二价结构域（bivalent domains），即特定基因组位点上同时存在H3K27me3与H3K4me3两种组蛋白修饰；该标记最初在哺乳动物胚胎干细胞中被鉴定，但此前认为无脊椎动物中不存在此类结构。本研究提供了果蝇胚胎中存在二价染色质状态的实验证据。我们利用新近构建的PcG报告基因果蝇品系，观察到报告基因在胚胎中具有显著的诱导表达活性，而在幼虫与成虫阶段其表达水平急剧下调。对报告基因的染色质景观分析显示，在三个发育阶段均检测到较强的抑制性PcG标记H3K27me3信号；令人意外的是，在胚胎中同时检测到较强的转录激活型H3K4me3标记信号。通过重染色质免疫沉淀（re-chromatin immunoprecipitation）实验，我们还在同源异形基因（Hox genes）等内源性PcG靶基因位点中发现了二价结构域。