The 2b protein and the C-terminus of the 2a protein of cucumber mosaic virus subgroup I strains both play a role in viral RNA accumulation and induction of symptoms

Two chimeras of cucumber mosaic virus (CMV), FCb72b-CMV and FRad352b-CMV, with the 2b genes of strains Cb7-CMV and Rad35-CMV, respectively, in an Fny-CMV background, gave different responses on Nicotiana glutinosa: FCb72b-CMV induced systemic necrosis while FRad352b-CMV caused only mild mosaic. This differential virulence was attributable to the nature of amino acid 55 of their 2b proteins. However, sequence analysis revealed that Leu55 of the 2b protein was necessary but not sufficient for FCb72b-CMV to induce systemic necrosis. Surprisingly, inhibition of translation of the 2a/2b overlapping region of the 2a protein in FCb72b-CMV led to a loss of systemic necrosis and a reduction in accumulation of viral progeny RNAs. The 2a/2b overlapping region of Fny-CMV had a similar effect on virulence and viral accumulation. Thus, the 2a protein C-terminus of subgroup I strains, as well as the 2b protein, play a role in symptom induction and accumulation of viral RNAs.