Molecular Studies on Bromovirus Capsid Protein: VI. Contributions of the N-Terminal Arginine-Rich Motif of BMV Capsid Protein to Virion Stability and RNA Packaging

Specific interactions are likely to occur between the highly conserved N-proximal arginine-rich motif (ARM) of Brome mosaic virus (BMV) coat protein (CP) and each of three genomic RNAs and a single subgenomic RNA during in vivo encapsidation. To characterize these interactions, three independent deletions were engineered into a biologically active clone of BMV RNA3 (B3) such that the matured CP of each B3 variant precisely lacks either the entire ARM (B3/Δ919) or two consecutive arginine residues (B3/13ΔΔ14 and B3/18ΔΔ19) within the ARM. Analysis of virion RNA for each B3 variant recovered from symptomatic leaves of Chenopodium quinoa revealed that the interactions between the N-terminal ARM of BMV CP and each of three genomic RNAs is distinct. Northern blot hybridization of B3Δ919 virion RNA revealed that the deleted ARM region specifically affected the stability of virions containing RNA1. An abundant truncated RNA species recurrently found in the virions of B3Δ919 was identified to be a derivative of genomic RNA1, lacking the 5′ 943 nucleotides. Additional Northern blot analysis of virion RNAs from B3/Δ919, B3/13ΔΔ14, and B3/18ΔΔ19, and in vitro reassembly assays revealed that the N-terminal ARM region contains crucial amino acids required for RNA4 packaging, independent of genomic RNA3. The significance of these observations in relation to Bromovirus CP-RNA interactions during virion assembly is discussed.