Histone 3 K4 trimethylation (depositing H3K4me3 marks) is typically associated with active promoters yet paradoxically occurs at untranscribed domains. Research to delineate the mechanisms of targeting H3K4 methyltransferases is ongoing. The oocyte provides an attractive system to investigate these mechanisms, because extensive H3K4me3 acquisition occurs in nondividing cells. We developed low-input chromatin immunoprecipitation to interrogate H3K4me3, H3K27ac and H3K27me3 marks throughout oogenesis. In nongrowing oocytes, H3K4me3 was restricted to active promoters, but as oogenesis progressed, H3K4me3 accumulated in a transcription-independent manner and was targeted to intergenic regions, putative enhancers and silent H3K27me3-marked promoters. Ablation of the H3K4 methyltransferase gene Mll2 resulted in loss of transcription-independent H3K4 trimethylation but had limited effects on transcription-coupled H3K4 trimethylation or gene expression. Deletion of Dnmt3a and Dnmt3b showed that DNA methylation protects regions from acquiring H3K4me3. Our findings reveal two independent mechanisms of targeting H3K4me3 to genomic elements, with MLL2 recruited to unmethylated CpG-rich regions independently of transcription.

组蛋白 3 K4 三甲基化（沉积 H3K4me3 标记）通常与活性启动子相关，但自相矛盾地发生在未转录的结构域。描述靶向 H3K4 甲基转移酶机制的研究正在进行中。卵母细胞为研究这些机制提供了一个有吸引力的系统，因为广泛的 H3K4me3 采集发生在非分裂细胞中。我们开发了低输入染色质免疫沉淀来询问整个卵子发生过程中的 H3K4me3、H3K27ac 和 H3K27me3 标记。在非生长卵母细胞中，H3K4me3 仅限于活性启动子，但随着卵子发生的进展，H3K4me3 以不依赖转录的方式积累，并靶向基因间区域、推定的增强子和沉默的 H3K27me3 标记的启动子。H3K4甲基转移酶基因Mll2的消融导致转录非依赖性 H3K4 三甲基化的丧失，但对转录偶联的 H3K4 三甲基化或基因表达的影响有限。删除Dnmt3a和Dnmt3b表明 DNA 甲基化保护区域免于获得 H3K4me3。我们的研究结果揭示了两种将 H3K4me3 靶向基因组元件的独立机制，其中 MLL2 独立于转录而被招募到未甲基化的富含 CpG 的区域。