Effects of amino-acid substitutions in the Brome mosaic virus capsid protein on RNA encapsidation

Brome mosaic virus (BMV) packages its genomic RNAs (RNA1, RNA2, and RNA3) and subgenomic RNA4 into three different particles. However, since the RNAs in the virions have distinct lengths and electrostatic charges, we hypothesize that subsets of the virions should have distinct properties. A glutamine to cysteine substitution at position 120 of the capsid protein (CP) was found to result in a mutant virus named QC that exhibited a dramatically altered ratio of the RNAs in virions. RNA2 was far more abundant than the other RNAs, although the ratios could be affected by the host plant species. RNAs with the QC mutation were competent for replication early in the infection, suggesting that they were either selectively packaged or degraded after packaging. In support of the latter idea, low concentrations of truncated RNA1 that co-migrated with RNA2 were found in the QC virions. Spectroscopic analysis and peptide fingerprinting experiments showed that the QC virus capsid interacted with the encapsidated RNAs differently than did the wild type. Furthermore, wild-type BMV RNA1 was found to be more susceptible to nuclease digestion relative to RNA2 as a function of the buffer pH. Other BMV capsid mutants also had altered ratios of packaged RNAs.