Sequence of the Genomic RNA of Nudaurelia β Virus (Tetraviridae) Defines a Novel Virus Genome Organization

Abstract

The monopartite genome of Nudaurelia β virus, the type species of theBetatetravirusgenus 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 genusOmegatetravirus, shows identities of 31.6% for the replicase and 24.5% for the capsid protein. The viruses in the generaBetatetravirusandOmegatetravirusof 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 genusOmegatetravirus, likely evolved from a virus with an unsegmented genome, as found in the genusBetatetravirus, through evolution of the subgenomic RNA into a separate genomic component, with the accompanying loss of the capsid gene from the longer genomic RNA.