No abstract

The unique Ns isolate of Cucumber mosaic virus (CMV) induces necrotic lesions on several Nicotiana spp. in contrast to other strains that cause systemic mosaic on these plants. By using biologically active RNA transcripts from cDNAs of Ns-CMV and a reference subgroup I strain Rs-CMV, we confined the genetic determinant solely responsible for necrosis induction to amino acid 461 of the la protein translated from genomic RNA1. An Arg to Cys change at this position (R461C) rendered Rs-CMV necrotic, whereas the reciprocal C461R mutation reverted the necrotic phenotype of Ns-CMV. Necrotic (Ns-CMV, R461C) and non-necrotic (Rs-CMV and C461R) viruses accumulated to similar levels in Nicotiana clevelandii protoplasts. Deletion of the residue at position 461 abolished replicase activity of the Ns-CMV 1a protein. The R461C mutation also was introduced into the 1a protein of Trk7-CMV, a subgroup II isolate. Symptoms induced by the Trk7/R461C mutant were identical to those caused by wild-type Trk7-CMV, even when the mutant Trk7 RNA1 was co-inoculated with RNA2 and 3 of the necrotic Ns strain.

The 2b protein of Cucumber mosaic virus has a role in nearly all steps of the viral cycle including cell-to-cell movement, symptom induction and suppression of antiviral RNA silencing. Previous studies demonstrated the presence of 2b protein in the nucleus and in cytoplasm as well. Phosphorylation site of 2b protein is conserved in all CMV isolates, including proposed constitute motifs for casein kinase II and cyclin-dependent kinase 2. To discern the impact of 2b protein phosphorylation, we created eight different mutants to mimic the non-phosporylated (serine to alanine) as well as the phosphorylated state (serine to aspartic acid) of the protein. We compared these mutants to the wild-type (Rs-CMV) virus in terms of symptom induction, gene silencing suppressor activity as well as in cellular localization. Here, in this study we confirmed the phosphorylation of 2b protein in vivo, both in infected N. benthamiana and in infiltrated patches. Mutants containing aspartic acid in the phosphorylation site accumulated only in the cytoplasm indicating that phosphorylated 2b protein could not enter the nucleus. We identified a conserved dual phosphorylation switch in CMV 2b protein, which equilibrates the shuttling of the 2b protein between the nucleus and the cytoplasm, and regulates the suppressor activity of the 2b protein.