Efficient RNA silencing suppression activity of Potato Mop-Top Virus 8K protein is driven by variability and positive selection

Abstract: Previously, we investigated the evolution of Potato mop-topvirus (PMTV) ORFs. Results indicate that positive selection actsexclusively on an ORF encoding the 8K protein, a weak viral suppressor ofRNA silencing (VSR). However, how the extraordinary variabilitycontributes to 8K-mediated RNA silencing suppression remains unknown.Here, we characterized the RNA silencing suppression activity of the 8Kprotein from seven diverse isolates. We show that 8K encoded by isolateP1 exhibits stronger RNA silencing suppression activity than the 8Kprotein from six other isolates. Mutational analyses revealed that Ser-50is critical for these differences. By comparing small RNA profiles wefound a lower abundance of siRNAs with U residue at the 5'-terminus afterexpression of the P1 8K compared to expression of 8K from isolate P125,an isolate with weak VSR activity. These results provide new clues as tothe role of positive selection in shaping activities of VSR