Characterization of silencing suppressor 2b of cucumber mosaic virus based on examination of its small RNA-Binding abilities

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).