Department of Molecular Biology and Biochemistry, Rutgers University, Nelson Laboratories, Piscataway, New Jersey, United States of America.
PLoS Genet. 2010 Jan;6(1):e1000805. doi: 10.1371/journal.pgen.1000805. Epub 2010 Jan 8.
Polycomb (PcG) regulation has been thought to produce stable long-term gene silencing. Genomic analyses in Drosophila and mammals, however, have shown that it targets many genes, which can switch state during development. Genetic evidence indicates that critical for the active state of PcG target genes are the histone methyltransferases Trithorax (TRX) and ASH1. Here we analyze the repertoire of alternative states in which PcG target genes are found in different Drosophila cell lines and the role of PcG proteins TRX and ASH1 in controlling these states. Using extensive genome-wide chromatin immunoprecipitation analysis, RNAi knockdowns, and quantitative RT-PCR, we show that, in addition to the known repressed state, PcG targets can reside in a transcriptionally active state characterized by formation of an extended domain enriched in ASH1, the N-terminal, but not C-terminal moiety of TRX and H3K27ac. ASH1/TRX N-ter domains and transcription are not incompatible with repressive marks, sometimes resulting in a "balanced" state modulated by both repressors and activators. Often however, loss of PcG repression results instead in a "void" state, lacking transcription, H3K27ac, or binding of TRX or ASH1. We conclude that PcG repression is dynamic, not static, and that the propensity of a target gene to switch states depends on relative levels of PcG, TRX, and activators. N-ter TRX plays a remarkable role that antagonizes PcG repression and preempts H3K27 methylation by acetylation. This role is distinct from that usually attributed to TRX/MLL proteins at the promoter. These results have important implications for Polycomb gene regulation, the "bivalent" chromatin state of embryonic stem cells, and gene expression in development.
多梳（PcG）调控被认为产生稳定的长期基因沉默。然而，在果蝇和哺乳动物中的基因组分析表明，它靶向许多基因，这些基因在发育过程中可以改变状态。遗传证据表明，对于 PcG 靶基因的活性状态至关重要的是组蛋白甲基转移酶 Trithorax (TRX) 和 ASH1。在这里，我们分析了不同果蝇细胞系中 PcG 靶基因所发现的替代状态的范围，以及 PcG 蛋白 TRX 和 ASH1 在控制这些状态中的作用。通过广泛的全基因组染色质免疫沉淀分析、RNAi 敲低和定量 RT-PCR，我们表明，除了已知的抑制状态外，PcG 靶基因还可以存在于转录活跃的状态，其特征是形成富含 ASH1 的扩展结构域，TRX 的 N 端，但不是 C 端部分和 H3K27ac。ASH1/TRX N 端结构域和转录与抑制标记并不矛盾，有时会导致“平衡”状态，同时受抑制剂和激活剂的调节。然而，通常情况下，PcG 抑制的丧失反而会导致“空”状态，缺乏转录、H3K27ac 或 TRX 或 ASH1 的结合。我们得出结论，PcG 抑制是动态的，而不是静态的，靶基因切换状态的倾向取决于 PcG、TRX 和激活剂的相对水平。N 端 TRX 发挥了显著的作用，拮抗 PcG 抑制，并通过乙酰化预先阻止 H3K27 甲基化。这个作用与通常归因于 TRX/MLL 蛋白在启动子上的作用不同。这些结果对 Polycomb 基因调控、胚胎干细胞的“双价”染色质状态以及发育过程中的基因表达具有重要意义。
