BACKGROUND Telomeric small RNAs related to PIWI-interacting RNAs (piRNAs) have been described in various eukaryotes; however, their role in germline-specific telomere function remains poorly understood. Using a Drosophila model, we performed an in-depth study of the biogenesis of telomeric piRNAs and their function in telomere homeostasis in the germline. RESULTS To fully characterize telomeric piRNA clusters, we integrated the data obtained from analysis of endogenous telomeric repeats, as well as transgenes inserted into different telomeric and subtelomeric regions. The small RNA-seq data from strains carrying telomeric transgenes demonstrated that all transgenes belong to a class of dual-strand piRNA clusters; however, their capacity to produce piRNAs varies significantly. Rhino, a paralog of heterochromatic protein 1 (HP1) expressed exclusively in the germline, is associated with all telomeric transgenes, but its enrichment correlates with the abundance of transgenic piRNAs. It is likely that this heterogeneity is determined by the sequence peculiarities of telomeric retrotransposons. In contrast to the heterochromatic non-telomeric germline piRNA clusters, piRNA loss leads to a dramatic decrease in HP1, Rhino, and trimethylated histone H3 lysine 9 in telomeric regions. Therefore, the presence of piRNAs is required for the maintenance of telomere chromatin in the germline. Moreover, piRNA loss causes telomere translocation from the nuclear periphery toward the nuclear interior but does not affect telomere end capping. Analysis of the telomere-associated sequences (TASs) chromatin revealed strong tissue specificity. In the germline, TASs are enriched with HP1 and Rhino, in contrast to somatic tissues, where they are repressed by Polycomb group proteins. CONCLUSIONS piRNAs play an essential role in the assembly of telomeric chromatin, as well as in nuclear telomere positioning in the germline. Telomeric arrays and TASs belong to a unique type of Rhino-dependent piRNA clusters with transcripts that serve simultaneously as piRNA precursors and as their only targets. Telomeric chromatin is highly sensitive to piRNA loss, implying the existence of a novel developmental checkpoint that depends on telomere integrity in the germline.

中文翻译：

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piRNA在果蝇种系端粒染色质维持和端粒核定位中的关键作用。

背景技术在各种真核生物中已经描述了与PIWI相互作用RNA（piRNA）有关的端粒小RNA。然而，它们在种系特异性端粒功能中的作用仍知之甚少。我们使用果蝇模型对端粒piRNA的生物发生及其在种系端粒稳态中的功能进行了深入研究。结果为了全面表征端粒piRNA簇，我们整合了从内源性端粒重复序列以及插入不同端粒和亚端粒区域的转基因分析获得的数据。来自携带端粒转基因的菌株的小RNA-seq数据表明，所有转基因都属于一类双链piRNA簇。但是，它们产生piRNA的能力差异很大。犀牛 仅在种系中表达的异色蛋白1（HP1）的旁系同源物与所有端粒转基因相关，但其富集与转基因piRNA的丰度相关。这种异质性很可能是由端粒逆转座子的序列特性决定的。与异色非端粒种系piRNA簇相反，piRNA丢失导致端粒区域HP1，Rhino和三甲基化组蛋白H3赖氨酸9急剧降低。因此，在种系中维持端粒染色质需要piRNA的存在。此外，piRNA的丢失会导致端粒从核外围向核内部移位，但不会影响端粒末端封端。端粒相关序列（TASs）染色质的分析显示强大的组织特异性。在生殖细胞系中，TAS富含HP1和Rhino，而体细胞组织则被Polycomb组蛋白抑制。结论piRNA在端粒染色质的组装以及种系中核端粒的定位中起着至关重要的作用。端粒阵列和TAS属于独特的犀牛依赖性piRNA簇，其转录本同时充当piRNA前体和它们的唯一靶标。端粒染色质对piRNA丢失高度敏感，这意味着存在一个新的发育检查点，该检查点取决于种系中端粒的完整性。以及种系中的核端粒定位。端粒阵列和TAS属于独特的犀牛依赖性piRNA簇，其转录本同时充当piRNA前体和它们的唯一靶标。端粒染色质对piRNA丢失高度敏感，这意味着存在一个新的发育检查点，该检查点取决于种系中端粒的完整性。以及种系中的核端粒定位。端粒阵列和TAS属于独特的犀牛依赖性piRNA簇，其转录本同时充当piRNA前体和它们的唯一靶标。端粒染色质对piRNA丢失高度敏感，这意味着存在一个新的发育检查点，该检查点取决于种系中的端粒完整性。