【2016/2/26】近日,北京协和医院分子医学研究室李龙承课题组在Cell Research发表题为“saRNA-guided Ago2 targets the RITA complex to promoters to stimulate transcription”的论著,揭示了RNA激活现象的分子机制。 RNA激活,也称RNAa,是一种由小非编码RNA介导的基因激活现象,能够介导RNAa的小非编码RNA被称为小激活RNA(saRNA)。RNAa为李龙承教授于2006年在世界上首次发现并命名。该文采用生物化学、分子生物学及蛋白质谱等手段对RNAa的机制进行了深入研究,回答了有关RNAa是如何工作的几个关键问题并首次解析了RITA复合物(RNA-induced transcriptional activation, RITA)的构成。该文结果显示,saRNA进入细胞后与其共因子Ago蛋白形成活性复合物,然后结合于靶基因的启动子上的靶位点。进而,Ago蛋白募集另外至少两个分别与DNA双螺旋打开和转录激活有关的蛋白来装配RITA复合物。新装配的RITA直接促进转录起始及延伸,并导致组蛋白2B的单泛素化,最终导致整个基因转录活化。该文的结论代表了RNAa领域的重要进展。阐明RNAa机制对于促进RNAa技术的应用及研究RNAa在疾病中的生物学作用具有重要意义。

另一方面,北京大学分子医学研究所梁子才教授几乎同一时间在Nucleic Acids Research(《核酸研究》发表了RNAa机制研究的重要成果(见北京大学新闻报道)。梁教授课题组的文章揭示了saRNA种子序列对于靶点匹配的重要性,并且用十分可信的证据质疑了现有的“启动子区反义转录本(antisense transcript)介导RNA激活”的观点。这一结果对于设计saRNA分子具有重要指导意义。

RNAa机制

RNAa机制示意图

论文摘要:

【摘要】Small activating RNAs (saRNAs) targeting specific promoter regions are able to stimulate gene expression at the transcriptional level, a phenomenon known as RNA activation (RNAa). It is known that RNAa depends on Ago2 and is associated with epigenetic changes at the target promoters. However, the precise molecular mechanism of RNAa remains elusive. Using human CDKN1A (p21) as a model gene, we characterized the molecular nature of RNAa. We show that saRNAs guide Ago2 to and associate with target promoters. saRNA-loaded Ago2 facilitates the assembly of an RNA-induced transcriptional activation (RITA) complex, which, in addition to saRNA-Ago2 complex, includes RHA and CTR9, the latter being a component of the PAF1 complex. RITA interacts with RNA polymerase II to stimulate transcription initiation and productive elongation, accompanied by monoubiquitination of histone 2B. Our results establish the existence of a cellular RNA-guided genome-targeting and transcriptional activation mechanism and provide important new mechanistic insights into the RNAa process.

论文全文:

http://www.nature.com/cr/journal/vaop/ncurrent/full/cr201622a.html