Nanoviruses have multipartite, circular, single-stranded DNA genomes and cause huge production losses in legumes and other crops. No viral suppressor of RNA silencing (VSR) has yet been reported from a member of the genus Nanovirus. Here, we demonstrate that the nanovirus U2 protein is a VSR. The U2 protein of milk vetch dwarf virus (MDV) suppressed the silencing of the green fluorescent protein (GFP) gene induced by single-stranded and double-stranded RNA, and the systemic spread of the GFP silencing signal. An electrophoretic mobility shift assay showed that the U2 protein was able to bind double-stranded 21-nucleotide small interfering RNA (siRNA). The cysteine residues at positions 43, 79 and 82 in the MDV U2 protein are critical to its nuclear localization, self-interaction and siRNA-binding ability, and were essential for its VSR activity. In addition, expression of the U2 protein via a potato virus X vector induced more severe necrosis symptoms in Nicotiana benthamiana leaves. The U2 proteins of other nanoviruses also acted as VSRs, and the three conserved cysteine residues were indispensable for their VSR activity.

中文翻译：

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纳米病毒 U2 蛋白通过三个保守的半胱氨酸残基抑制 RNA 沉默

纳米病毒具有多部分、环状、单链 DNA 基因组，会导致豆类和其他作物的巨大产量损失。尚未报道纳米病毒属成员存在 RNA 沉默 (VSR) 病毒抑制因子。在这里，我们证明纳米病毒 U2 蛋白是一种 VSR。紫云英矮缩病毒(MDV)的U2蛋白抑制单链和双链RNA诱导的绿色荧光蛋白(GFP)基因的沉默以及GFP沉默信号的全身传播。电泳迁移率变动分析表明 U2 蛋白能够结合双链 21 核苷酸小干扰 RNA (siRNA)。MDV U2 蛋白中 43、79 和 82 位的半胱氨酸残基对其核定位、自身相互作用和 siRNA 结合能力至关重要，并且对其 VSR 活性至关重要。此外，通过马铃薯病毒X载体表达U2蛋白会在本塞姆氏烟草叶片中诱导更严重的坏死症状。其他纳米病毒的U2蛋白也充当VSR，三个保守的半胱氨酸残基对于它们的VSR活性是不可或缺的。