The Upstream Sequence Transcription Complex Dictates Nucleosome Positioning and Promoter Accessibility at piRNA Genes in the C. elegans germ line [ChIP-Seq]. The Upstream Sequence Transcription Complex Dictates Nucleosome Positioning and Promoter Accessibility at piRNA Genes in the C. elegans germ line [ChIP-Seq]

The piRNA pathway is a conserved germline-specific small RNA pathway that ensures genomic integrity and continued fertility. In C. elegans and other nematodes, Type-I piRNA precursor transcripts are expressed from >10,000 small, independently regulated genes clustered within two discrete domains of 1.5 and 3.5 MB on Chromosome IV. These large domains likely play a significant role in promoting germline-specific expression of piRNAs, but the underlying mechanisms are unclear. By examining the chromatin environment specifically in isolated germ nuclei, we demonstrate that piRNA clusters are located in closed chromatin, and confirm the enrichment for the inactive histone modification H3K27me3. The piRNA biogenesis factor USTC (Upstream Sequence Transcription Complex) plays two roles – it promotes a strong association of nucleosomes throughout the domain, and it organizes the local nucleosome environment to direct the exposure of individual piRNA genes. Overall, this work reveals new insight into how chromatin state coordinates transcriptional regulation over large genomic domains, which has implications for understanding global genome organization in the germ line. Overall design: Chromatin immunoprecipitation DNA-sequencing (ChIP-seq) for histone modification H3K27me3, H3K36me3, and H3K9me3 as well as Native ChIP-seq of H3 of isolated germline nuclei in C. elegans.

piRNA通路（piRNA pathway）是一类保守的生殖系特异性小RNA通路，其功能是维持基因组完整性与生殖系持续的生育能力。在秀丽隐杆线虫（C. elegans）及其他线虫物种中，I型piRNA前体转录本由超过10000个小型独立调控基因表达，这些基因聚集在四号染色体上两个分别为1.5 Mb和3.5 Mb的离散区域内。这类大型基因簇区域或在促进piRNA的生殖系特异性表达中发挥关键作用，但其背后的分子机制尚未明确。本研究通过专门分析分离获得的生殖细胞核内的染色质环境，证实piRNA基因簇位于闭合染色质区域，并确认了非活性组蛋白修饰H3K27me3在此处的富集现象。piRNA生物发生因子USTC（上游序列转录复合物，Upstream Sequence Transcription Complex）具备双重功能：一方面可促进该基因组区域内核小体的紧密结合，另一方面可重塑局部核小体环境，以介导单个piRNA基因的转录暴露。总体而言，本研究揭示了染色质状态如何协调大型基因组区域的转录调控，这一发现对于理解生殖系中的全局基因组组织具有重要参考价值。实验整体设计：对秀丽隐杆线虫的分离生殖细胞核，开展组蛋白修饰H3K27me3、H3K36me3及H3K9me3的染色质免疫沉淀测序（ChIP-seq），以及组蛋白H3的原生染色质免疫沉淀测序（Native ChIP-seq）。