In vitro and in vivo evidence for differences in the protease activity of two arabis mosaic nepovirus isolates and their impact on the infectivity of chimeric cDNA clones

Regulated processing of nepovirus polyproteins allows the release of mature proteins and intermediate polyproteins. Infectious cDNA clones of the mild NW isolate of arabis mosaic virus (ArMV) and chimeric clones incorporating RNA1 segments of Lv, a severe isolate, were generated. Clones containing the Lv X2-NTB cleavage site were not infectious unless the Lv protease was present. The Lv and NW X2-NTB cleavage sites differ at positions P6, P4 and P2. In vitro, processing at the X2-NTB site was undetectable or reduced in chimeric polyproteins containing the Lv X2-NTB site and the NW protease but was restored when both the Lv protease and X2-NTB site were present. In contrast, cleavage at this site was increased in polyproteins that contained the NW X2-NTB site and the Lv protease. These results show that the ArMV-Lv protease has greater activity and is active on a greater range of cleavage sites than that of ArMV-NW.