Impact of the biased nucleotide composition of viral RNA genomes on RNA structure and codon usage.

We are interested in the influence of nucleotide composition on the fundamental characteristics of the virus RNA genome. Most RNA viruses have genomes with a distinct nucleotide composition, e.g. ranging from minimally 12.9 % to maximally 40.3 % (C- and U-count, respectively, in coronavirus HKU). We present a global analysis of diverse virus types, including plus-strand, minus-strand and double-strand RNA viruses, for the impact of this nucleotide preference on the predicted structure of the RNA genome that is packaged in virion particles and on the codon usage in the viral open reading frames. Several virus-specific features will be described, but also some general conclusions were drawn. Without exception, the virus-specific nucleotide bias was enriched in the unpaired, single-stranded regions of the RNA genome, thus creating an even more striking virus-specific signature. We present a simple mechanism that is based on elementary aspects of RNA structure folding to explain this general trend. In general, the nucleotide bias was the major determinant of the virus-specific codon usages, thus limiting a role for codon selection and translational control. We will discuss molecular and evolutionary scenarios that may be responsible for the diverse nucleotide biases of RNA viruses.