Sperm chromatin is typically transformed by protamines into a compact and transcriptionally inactive state. Flowering plant sperm cells lack protamines, yet have small, transcriptionally active nuclei with chromatin condensed by an unknown mechanism. Here we show that a histone variant, H2B.8, mediates sperm chromatin and nuclear condensation in Arabidopsis thaliana. Loss of H2B.8 causes enlarged sperm nuclei with dispersed chromatin, whereas ectopic expression in somatic cells produces smaller nuclei with aggregated chromatin, demonstrating that H2B.8 is sufficient for chromatin condensation. H2B.8 aggregates transcriptionally inactive AT-rich chromatin into phase-separated condensates, thus achieving nuclear compaction without reducing transcription. H2B.8 also intermixes inactive AT-rich chromatin and GC-rich pericentromeric heterochromatin, altering higher-order chromatin architecture. Altogether, our results reveal a novel mechanism of nuclear compaction via global aggregation of unexpressed chromatin. We propose that H2B.8 is a flowering plant evolutionary innovation that achieves nuclear condensation compatible with active transcription.

中文翻译：

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组蛋白变体通过染色质相分离浓缩开花植物精子

精子染色质通常被鱼精蛋白转化为紧凑且转录失活的状态。开花植物精子细胞缺乏鱼精蛋白，但具有小的、具有转录活性的细胞核，其染色质通过未知机制凝聚。在这里我们显示组蛋白变体 H2B.8 介导拟南芥中的精子染色质和核凝聚。H2B.8 的缺失导致染色质分散的精子核增大，而体细胞中的异位表达产生较小的核和染色质聚集，表明 H2B.8 足以使染色质凝聚。H2B.8 将转录失活的富含 AT 的染色质聚合成相分离的凝聚物，从而在不减少转录的情况下实现核压实。H2B.8 还混合了无活性的富含 AT 的染色质和富含 GC 的着丝粒周围异染色质，改变高阶染色质结构。总而言之，我们的结果揭示了一种通过未表达染色质的全局聚集来实现核压实的新机制。我们提出 H2B.8 是一种开花植物进化创新，可实现与主动转录兼容的核凝聚。