Department of Plant Sciences, University of Cambridge, Cambridge, CB2 3EA, UK.
Nat Commun. 2020 Mar 6;11(1):1221. doi: 10.1038/s41467-020-14995-6.
Silencing of transposable elements (TEs) is established by small RNA-directed DNA methylation (RdDM). Maintenance of silencing is then based on a combination of RdDM and RNA-independent mechanisms involving DNA methyltransferase MET1 and chromodomain DNA methyltransferases (CMTs). Involvement of RdDM, according to this model should decrease with TE age but here we show a different pattern in tomato and Arabidopsis. In these species the CMTs silence long terminal repeat (LTR) transposons in the distal chromatin that are younger than those affected by RdDM. To account for these findings we propose that, after establishment of primary RdDM as in the original model, there is an RNA-independent maintenance phase involving CMTs followed by secondary RdDM. This progression of epigenetic silencing in the gene-rich distal chromatin is likely to influence the transcriptome either in cis or in trans depending on whether the mechanisms are RNA-dependent or -independent.
转座元件 (TEs) 的沉默是由小 RNA 指导的 DNA 甲基化 (RdDM) 建立的。沉默的维持然后基于 RdDM 和涉及 DNA 甲基转移酶 MET1 和染色质域 DNA 甲基转移酶 (CMTs) 的 RNA 独立机制的组合。根据该模型，RdDM 的参与应该随着 TE 年龄的增加而减少，但在这里我们在番茄和拟南芥中显示出不同的模式。在这些物种中，CMTs 沉默远端染色质中比 RdDM 影响的年轻长末端重复 (LTR) 转座子。为了解释这些发现，我们提出，在原始模型中建立主要 RdDM 之后，存在涉及 CMTs 的 RNA 独立维持阶段，然后是二次 RdDM。这种在富含基因的远端染色质中发生的表观遗传沉默的进展可能会影响顺式或反式转录组，具体取决于这些机制是否依赖 RNA。
