Sequence of the genomic RNA of Nudaurelia β virus (Tetraviridae) defines a novel virus genome organization

The monopartite genome of Nudaurelia β virus, the type species of the Betatetravirus genus of the family Tetraviridae, consists of a single- stranded positive-sense RNA (ss+RNA) of 6625 nucleotides containing two open reading frames (ORFs). The 5' proximal ORF of 5778 nucleotides encodes a protein of 215 kDa containing three functional domains characteristic of RNA- dependent RNA polymerases of ss+RNA viruses. The 3' proximal ORF of 1836 nucleotides, which encodes the 66-kDa capsid precursor protein, overlaps the replicase gene by more than 99% (1827 nucleotides) and is in the +1 reading frame relative to the replicase reading frame. This capsid precursor is expressed via a 2656-nucleotide subgenomic RNA. The 3' terminus of the genome can be folded into a tRNA-like secondary structure that has a valine anticodon; the tRNA-like structure lacks a pseudoknot in the aminoacyl stem, a feature common to both genera of tetraviruses. Comparison of the sequences of Nudaurelia β virus and another member of the Tetraviridae, Helicoverpa armigera stunt virus, which is in the genus Omegatetravirus, shows identities of 31.6% for the replicase and 24.5% for the capsid protein. The viruses in the genera Betatetravirus and Omegatetravirus of the Tetraviridae are clearly related but show significant differences in their genome organization. It is concluded that the ancestral virus with a bipartite genome, as found in the genus Omegatetravirus, likely evolved from a virus with an unsegmented genome, as found in the genus Betatetravirus, through evolution of the subgenomic RNA into a separate genomic component, with the accompanying loss of the capsid gene from the longer genomic RNA.