Facultative heterochromatin controls development and differentiation in many eukaryotes. In metazoans, plants, and many filamentous fungi, facultative heterochromatin is characterized by transcriptional repression and enrichment with nucleosomes that are trimethylated at histone H3 lysine 27 (H3K27me3). While loss of H3K27me3 results in derepression of transcriptional gene silencing in many species, additional up- and downstream layers of regulation are necessary to mediate control of transcription in chromosome regions enriched with H3K27me3. Here, we investigated the effects of one histone mark on histone H4, namely H4K20me3, in the fungus Zymoseptoria tritici, a globally important pathogen of wheat. Deletion of kmt5, the gene encoding the sole methyltransferase responsible for H4K20 methylation, resulted in global derepression of transcription, especially in regions of facultative heterochromatin. Derepression in the absence of H4K20me3 not only affected known genes but also a large number of novel, previously undetected transcripts generated from regions of facultative heterochromatin on accessory chromosomes. Transcriptional activation in kmt5 deletion strains was accompanied by a complete loss of Ash1-mediated H3K36me3 and chromatin reorganization affecting H3K27me3 and H3K4me2 distribution in regions of facultative heterochromatin. Strains with H4K20L, M or Q mutations in the single histone H4 gene of Z. tritici recapitulated these chromatin changes, suggesting that H4K20me3 is important for Ash1-mediated H3K36me3. The ∆kmt5 mutants we obtained were more sensitive to genotoxic stressors than wild type and both, ∆kmt5 and ∆ash1, showed greatly increased rates of accessory chromosome loss. Taken together, our results provide insights into an unsuspected mechanism involved in the assembly and maintenance of facultative heterochromatin.

中文翻译：

---

H4K20me3 对于 Ash1 介导的 H3K36me3 和真菌病原体中兼性异染色质的转录沉默非常重要。

兼性异染色质控制许多真核生物的发育和分化。在后生动物、植物和许多丝状真菌中，兼性异染色质的特征是转录抑制和组蛋白 H3 赖氨酸 27 (H3K27me3) 三甲基化的核小体富集。虽然 H3K27me3 的缺失会导致许多物种中转录基因沉默的去抑制，但需要额外的上下游调控层来介导富含 H3K27me3 的染色体区域的转录控制。在这里，我们研究了小麦发酵锈斑菌（一种全球重要的小麦病原体）中的一个组蛋白标记对组蛋白 H4（即 H4K20me3）的影响。kmt5（编码负责 H4K20 甲基化的唯一甲基转移酶的基因）的缺失导致整体转录抑制，特别是在兼性异染色质区域。在 H4K20me3 缺失的情况下，去抑制不仅影响已知基因，还影响副染色体上兼性异染色质区域产生的大量新的、以前未检测到的转录本。kmt5 缺失菌株中的转录激活伴随着 Ash1 介导的 H3K36me3 的完全丧失和染色质重组，影响兼性异染色质区域中 H3K27me3 和 H3K4me2 的分布。小麦单组蛋白 H4 基因中具有 H4K20L、M 或 Q 突变的菌株重现了这些染色质变化，表明 H4K20me3 对于 Ash1 介导的 H3K36me3 很重要。我们获得的 Δkmt5 突变体比野生型对遗传毒性应激源更敏感，并且 Δkmt5 和 Δash1 都显示出副染色体丢失率大大增加。总而言之，我们的结果提供了对兼性异染色质组装和维护所涉及的未曾怀疑的机制的见解。