Induction of RNA-directed DNA methylation upon decondensation of constitutive heterochromatin
Induction of RNA-directed DNA methylation upon decondensation of constitutive heterochromatin
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DOI：
10.1038/embor.2009.152
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发表时间：
2009-09-01
期刊：
影响因子：
7.7
通讯作者：
Tamaru, Hisashi
中科院分区：
文献类型：
--
作者：
Schoft, Vera K.;Chumak, Nina;Tamaru, Hisashi
拟南芥中着丝粒组成型异染色质是由DNA甲基化和二甲基化组蛋白H3 Lys 9（H3 K9 me 2）标记的。RNA指导的DNA甲基化（RdDM）是使用24个核苷酸（nt）的小干扰RNA（siRNA）诱导其同源DNA序列的从头甲基化的过程。尽管存在着丝粒24-nt siRNA，但RdDM所需基因的突变不会明显影响着丝粒重复序列的甲基化。组成型异染色质被保护免受RdDM的机制仍然令人费解。在这里，我们报告，营养细胞核（VN）的雄配子体（花粉）总是经历广泛的着丝粒异染色质的解浓缩和失去着丝粒的身份。VN显示H3 K9 me 2大大减少，表型复制细胞核携带染色质重塑DNA甲基化减少1（DDM 1）突变。然而，与ddm 1细胞核中的情况不同，去致密的异染色质保留了密集的CG甲基化和转录沉默，并且出乎意料地，在非CG背景下发生了RdDM依赖性超甲基化。这些发现揭示了沉默异染色质的两种组装顺序，并暗示浓缩形式在阻断RdDM机制。
Centromeric constitutive heterochromatin is marked by DNA methylation and dimethylated histone H3 Lys 9 (H3K9me2) in Arabidopsis. RNA-directed DNA methylation (RdDM) is a process that uses 24-nucleotide (nt) small interfering RNAs (siRNAs) to induce de novo methylation to its homologous DNA sequences. Despite the presence of centromeric 24-nt siRNAs, mutations in genes required for RdDM do not appreciably influence the methylation of centromeric repeats. The mechanism by which constitutive heterochromatin is protected from RdDM remains puzzling. Here, we report that the vegetative cell nuclei (VN) of the male gametophyte (pollen) invariably undergo extensive decondensation of centromeric heterochromatin and lose centromere identity. VN show greatly reduced H3K9me2, phenocopying nuclei carrying a mutation in the chromatin remodeller DECREASE IN DNA METHYLATION 1 (DDM1). However, unlike the situation in ddm1 nuclei, the decondensed heterochromatin retains dense CG methylation and transcriptional silencing, and, unexpectedly, is subjected to RdDM-dependent hypermethylation in non-CG contexts. These findings reveal two assembly orders of silent heterochromatin and implicate the condensed form in blocking the RdDM machinery.