与基因沉默有关的特殊基因RDM1

奥地利格雷戈尔·门德尔植物分子生物学研究所日前宣布，一个包括该研究所、中国同济大学、美国加利福尼亚大学等机构科学家在内的国际科研小组发现了一种特殊基因，没有它，植物细胞内其他一些基因就只能保持沉默。

最新一期英国《自然》杂志网络版发表了这个国际科研小组的论文。这一科研小组发现的特殊基因名为RDM1，它可以编码生成一种小蛋白，从而参与指导其他基因的表达。

科学家指出，基因一般处于被保护状态中，只有通过所谓的甲基化，即与甲基接触，才能表达并发挥作用。如果去除RDM1，被保护的基因就无法甲基化，也就无法进行表达。

由于每一个植物细胞中都存在着完整的遗传信息，因此必须让某些基因保持“沉默”，植物具体的器官才能顺利地发挥各自作用。否则，所有基因就会都来表达，植物器官也将不知道听从谁的“指令”。

一般在一个植物的上万个基因里，只有很少的一部分能够表达，RNA（核糖核酸）会对需要表达的基因进行标记。RDM1基因的任务就是让RNA标记过的基因表达。缺少了RDM1，植物中许多本该表达的基因就会保持“沉默”，植物无法正常生长。

An RNA polymerase II- and AGO4-associated protein acts in RNA-directed
DNA methylation

Zhihuan Gao 1, Hai-Liang Liu 1,2, Lucia Daxinger 3, Olga Pontes 4, Xinjian He 1,5, Weiqiang Qian 1,Huixin Lin 1, Mingtang Xie 1, Zdravko J. Lorkovic 6, Shoudong Zhang 1,5, Daisuke Miki 1, Xiangqiang Zhan 1,5, Dominique Pontier 7, Thierry Lagrange 7, Hailing Jin 8, Antonius J. M. Matzke 3, Marjori Matzke 3, Craig S. Pikaard 9 & Jian-Kang Zhu 1,5

1. Institute for Integrative Genome Biology and Department of Botany and Plant Sciences, University of California, Riverside, California 92521, USA
2. School of life science and technology, Tongji University, Shanghai 200092, China
3. Gregor Mendel Institute of Molecular Plant Biology, Austrian Academy of Sciences, 1030 Vienna, Austria
4. Biology Department, Washington University, St Louis, Missouri 63130, USA
5. Center for Plant Stress Genomics and Technology, 4700 King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia
6. Max F. Perutz Laboratory, Medical University of Vienna, 1030 Vienna, Austria
7. LGDP, CNRS/IRD/Université de Perpignan, UMR 5096, 66860 Perpignan, France
8. Institute for Integrative Genome Biology and Department of Plant Pathology and Microbiology, University of California, Riverside, California 92521, USA
9. Department of Biology and Department of Molecular and Cellular Biochemistry, Indiana University, Bloomington, Indiana 47405, USA
These authors contributed equally to this work.

【Abstract】DNA methylation is an important epigenetic mark in many eukaryotes1, 2, 3, 4, 5. In plants, 24-nucleotide small interfering RNAs (siRNAs) bound to the effector protein, Argonaute 4 (AGO4), can direct de novo DNA methylation by the methyltransferase DRM2 (refs 2, 4–6). Here we report a new regulator of RNA-directed DNA methylation (RdDM) in Arabidopsis: RDM1. Loss-of-function mutations in the RDM1 gene impair the accumulation of 24-nucleotide siRNAs, reduce DNA methylation, and release transcriptional gene silencing at RdDM target loci. RDM1 encodes a small protein that seems to bind single-stranded methyl DNA, and associates and co-localizes with RNA polymerase II (Pol II, also known as NRPB), AGO4 and DRM2 in the nucleus. Our results indicate that RDM1 is a component of the RdDM effector complex and may have a role in linking siRNA production with pre-existing or de novo cytosine methylation. Our results also indicate that, although RDM1 and Pol V (also known as NRPE) may function together at some RdDM target sites in the peri-nucleolar siRNA processing centre, Pol II rather than Pol V is associated with the RdDM effector complex at target sites in the nucleoplasm.