Goto Kazunori, Kobori Takashi, Kosaka Yoshitaka, Natsuaki Tomohide, Masuta Chikara
Cell Biology and Manipulation Laboratory, Graduate School of Agriculture, Hokkaido University, Sapporo, 060-8589 Japan.
Plant Cell Physiol. 2007 Jul;48(7):1050-60. doi: 10.1093/pcp/pcm074. Epub 2007 Jun 13.
Double-stranded (ds) RNAs and imperfect hairpin RNAs of endogenous genes trigger post-transcriptional gene silencing (PTGS) and are cleaved by a Dicer-like nuclease into small interfering RNAs (siRNAs) and microRNs (miRNAs), respectively. Such small RNAs (siRNAs and miRNAs) then guide an RNA-induced silencing complex (RISC) for sequence-specific RNA degradation. While PTGS serves as an antiviral defense in plants, many plant viruses encode suppressors as a counter defense. Here we demonstrate that the PTGS suppressor (2b) of a severe strain (CM95R) of cucumber mosaic virus (CMV) can bind to in vitro synthesized siRNAs and even to long dsRNAs to a lesser extent. However, the 2b suppressor weakly bound to a miRNA (miR171) duplex in contrast to another small RNA-binding suppressor, p19 of tombusvirus that can effectively bind miRNAs. Because the 2b suppressor of an attenuated strain of CMV (CM95), which differs in a single amino acid from the 2b of CM95R, could barely bind siRNAs, we hypothesized that the weak suppressor activity of the attenuated strain resulted from a loss of the siRNA-binding property of 2b via a single amino acid change. Here we consider that 2b interferes with the PTGS pathway by directly binding siRNAs (or long dsRNA).
内源性基因的双链（ds）RNA和不完全发夹RNA会引发转录后基因沉默（PTGS），并分别被一种类Dicer核酸酶切割成小干扰RNA（siRNA）和微小RNA（miRNA）。这些小RNA（siRNA和miRNA）随后引导RNA诱导沉默复合体（RISC）进行序列特异性RNA降解。虽然PTGS在植物中作为一种抗病毒防御机制，但许多植物病毒编码抑制子作为一种反防御手段。在这里，我们证明黄瓜花叶病毒（CMV）的一个强毒株（CM95R）的PTGS抑制子（2b）能够结合体外合成的siRNA，甚至在较小程度上结合长dsRNA。然而，与另一种能够有效结合miRNA的小RNA结合抑制子——番茄丛矮病毒的p19相比，2b抑制子与miRNA（miR171）双链体的结合较弱。由于CMV弱毒株（CM95）的2b抑制子与CM95R的2b在一个氨基酸上存在差异，几乎不能结合siRNA，我们推测弱毒株的抑制活性降低是由于2b通过单个氨基酸变化丧失了结合siRNA的特性所致。在这里，我们认为2b通过直接结合siRNA（或长dsRNA）干扰PTGS途径。
