Gregor Mendel Institute of Molecular Plant Biology, Austrian Academy of Sciences, Vienna, Austria.
PLoS Genet. 2010 Jun 10;6(6):e1000986. doi: 10.1371/journal.pgen.1000986.
Chromosome termini form a specialized type of heterochromatin that is important for chromosome stability. The recent discovery of telomeric RNA transcripts in yeast and vertebrates raised the question of whether RNA-based mechanisms are involved in the formation of telomeric heterochromatin. In this study, we performed detailed analysis of chromatin structure and RNA transcription at chromosome termini in Arabidopsis. Arabidopsis telomeres display features of intermediate heterochromatin that does not extensively spread to subtelomeric regions which encode transcriptionally active genes. We also found telomeric repeat-containing transcripts arising from telomeres and centromeric loci, a portion of which are processed into small interfering RNAs. These telomeric siRNAs contribute to the maintenance of telomeric chromatin through promoting methylation of asymmetric cytosines in telomeric (CCCTAAA)(n) repeats. The formation of telomeric siRNAs and methylation of telomeres relies on the RNA-dependent DNA methylation pathway. The loss of telomeric DNA methylation in rdr2 mutants is accompanied by only a modest effect on histone heterochromatic marks, indicating that maintenance of telomeric heterochromatin in Arabidopsis is reinforced by several independent mechanisms. In conclusion, this study provides evidence for an siRNA-directed mechanism of chromatin maintenance at telomeres in Arabidopsis.
染色体末端形成一种特殊类型的异染色质，对于染色体的稳定性很重要。最近在酵母和脊椎动物中发现端粒 RNA 转录本，这引发了一个问题，即 RNA 为基础的机制是否参与端粒异染色质的形成。在这项研究中，我们对拟南芥染色体末端的染色质结构和 RNA 转录进行了详细分析。拟南芥端粒表现出中间异染色质的特征，不会广泛扩散到编码转录活性基因的亚端粒区域。我们还发现了来自端粒和着丝粒位点的含有端粒重复序列的转录本，其中一部分被加工成小干扰 RNA。这些端粒 siRNA 通过促进端粒（CCCTAAA）（n）重复中不对称胞嘧啶的甲基化，有助于维持端粒染色质。端粒 siRNA 的形成和端粒的甲基化依赖于 RNA 依赖的 DNA 甲基化途径。rdr2 突变体中端粒 DNA 甲基化的缺失仅对组蛋白异染色质标记产生适度影响，表明拟南芥端粒异染色质的维持受到几种独立机制的加强。总之，本研究为拟南芥端粒染色质维持的 siRNA 指导机制提供了证据。
