Production of cucumber mosaic virus RNA5 and its role in recombination.

Cucumber Mosaic Virus (CMV) is a plant infecting tripartite positive-strand RNA virus. In addition to three genomic and two known subgenomic RNAs, CMV strains of subgroup II (e.g. Q-CMV), but not subgroup I (e.g. Fny-CMV), produce and package a redundant RNA5 encompassing the 3' 304-307 nucleotides of RNAs 2 and 3. The mechanism regulating RNA5 production and its role in CMV life cycle is unknown. In this study, transient expression of Q2 or Q3 by agroinfiltration into Nicotiana benthamiana plants resulted in efficient accumulation of RNA5 suggesting that its production is independent of CMV replication. Deletion and point mutations engineered into a highly conserved region (Box1) adjacent to the 5' end of RNA5 identified sequences required for its efficient production. An experimental system, involving a chimera of Q3 (Q3B3) characterized by having a 3' tRNA-like structure (3'TLS) from Brome mosaic virus (BMV) and RNA5 defective variants of Q1 (Q1Delta), Q2 (Q2Delta) and Q3B3 (Q3DeltaB3), was used to evaluate in vivo the contribution of RNA5 in promoting RNA recombination. Generation of precise homologous recombinants was strictly dependent on sequence identity. When both parental RNAs carried the Box1, recombination occurred preferentially within the Box1. In contrast, generation of non-homologous recombinants occurred only when Q1 and Q2 were competent to produce RNA5. A mechanistic model explaining the functional role played by the RNA5 in generating CMV recombinants was presented.