Zhang Wenli, Lee Hye-Ran, Koo Dal-Hoe, Jiang Jiming
Department of Horticulture, University of Wisconsin, Madison, Wisconsin 53706, USA.
Plant Cell. 2008 Jan;20(1):25-34. doi: 10.1105/tpc.107.057083. Epub 2008 Jan 31.
The centromere in eukaryotes is defined by the presence of a special histone H3 variant, CENH3. Centromeric chromatin consists of blocks of CENH3-containing nucleosomes interspersed with blocks of canonical H3-containing nucleosomes. However, it is not known how CENH3 is precisely deposited in the centromeres. It has been suggested that epigenetic modifications of the centromeric chromatin may play a role in centromere identity. The centromeres of Arabidopsis thaliana are composed of megabase-sized arrays of a 178-bp satellite repeat. Here, we report that the 178-bp repeats associated with the CENH3-containing chromatin (CEN chromatin) are hypomethylated compared with the same repeats located in the flanking pericentromeric regions. A similar hypomethylation of DNA in CEN chromatin was also revealed in maize (Zea mays). Hypomethylation of the DNA in CEN chromatin is correlated with a significantly reduced level of H3K9me2 in Arabidopsis. We demonstrate that the 178-bp repeats from CEN chromatin display a distinct distribution pattern of the CG and CNG sites, which may provide a foundation for the differential methylation of these repeats. Our results suggest that DNA methylation plays an important role in epigenetic demarcation of the CEN chromatin.
真核生物中的着丝粒由一种特殊的组蛋白H3变体CENH3的存在来定义。着丝粒染色质由含有CENH3的核小体区域与含有典型H3的核小体区域相间排列组成。然而，目前尚不清楚CENH3是如何精确地定位于着丝粒中的。有人提出，着丝粒染色质的表观遗传修饰可能在着丝粒身份确定中起作用。拟南芥的着丝粒由178 bp卫星重复序列的兆碱基大小阵列组成。在此，我们报道，与含有CENH3的染色质（CEN染色质）相关的178 bp重复序列与位于侧翼着丝粒周围区域的相同重复序列相比，甲基化程度较低。在玉米中也发现了CEN染色质中DNA的类似低甲基化现象。拟南芥中CEN染色质DNA的低甲基化与H3K9me2水平的显著降低相关。我们证明，来自CEN染色质的178 bp重复序列显示出CG和CNG位点的独特分布模式，这可能为这些重复序列的差异甲基化提供基础。我们的结果表明，DNA甲基化在CEN染色质的表观遗传划分中起重要作用。
