紫云英矮缩病毒(MDV)是危害我国豆科蔬菜，烟草和紫云英等作物的重要病毒，近年来，在安徽、山东、云南、浙江和甘肃等地陆续发生。MDV是多组分单链环状DNA病毒，前期研究发现MDV的U2蛋白具有抑制转录后基因沉默功能，通过双分子荧光互补试验分析表明U2能自身互作，并能与NbSGS3互作，但其作用机制尚不清楚。本研究拟在此基础上，通过构建U2突变体库，分析其亚细胞定位，自身互作能力和核酸结合能力与抑制转录后基因沉默的联系。同时，通过深入研究U2与NbSGS3相互作用的关键结构域和位点，明确该互作是否影响NbSGS3亚细胞定位、结合核酸活性和表达量，并进一步分析U2，NbSGS3和NbRDR6三者之间的互作影响，进而明确U2与NbSGS3互作抑制转录后基因沉默功能的作用机制。研究结果有望解析U2抑制转录后水平基因沉默的机理，探明多组分DNA病毒沉默抑制子的作用方式，为该类病毒的防治提供理论。

Recently, Milk Vetch Dwarf virus(MDV) disease has been occurring severely in Anhui, Shandong, Yunnan, Zhejiang and Gansu provinces, and led to huge production loss of tobacco, legumes and Chinese milk vetch. The genome of MDV has several circle single strand DNAs. Our preliminary research confirmed that U2 is a viral suppressor of post-transcriptional gene silencing. Its self-interaction and interaction with NbSGS3 were verified by bimolecular fluorescence complementation. However, the underlying mechanism of U2 suppressing RNA silencing remains unclear. On this basis, the key domains and sites of U2 to bind 'dsRNA', subcellular localization, self-interaction and inhibit RNA silencing will be analyzed based on U2 mutant library. And the relationship among above domains and sites of U2 will be studied to reveal the key sequences of U2 suppressing RNA silencing through bind 'dsRNA'. On the other hand, the mechanism of U2 to influence on the gene silencing function of NbSGS3 will be clarified by analyzing the interaction domain between U2 and NbSGS3, the effects of U2 and its mutants to the subcellular localization of NbSGS3, the interaction between NbSGS3 and NbRDR6, the activity of NbSGS3 to bind 'dsRNA' and the expression amount of NbSGS3. The objective of carrying out this project is expected to unveil the molecular mechanism of U2 PTGS suppression, which will also contribute to our better understanding of the mechanism of RNA silencing suppressor of multipartite genomic DNA virus. The research results provide theoretical basis for prevention of nanovirus.