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

The monopartite genome of Nudaurelia beta virus, the type species of the Be tatetravirus genus of the family Tetraviridae, consists of a single-strande d positive-sense RNA (ss+RNA) of 6625 nucleotides containing two open readi ng frames (ORFs). The 5' proximal ORF of 5778 nucleotides encodes a protein of 215 kDa containing three functional domains characteristic of RNA-depen dent RNA polymerases of ss+RNA viruses. The 3' proximal ORF of 1836 nucleot ides, which encodes the 66-kDa capsid precursor protein, overlaps the repli case gene by more than 99% (1827 nucleotides) and is in the +1 reading fram e relative to the replicase reading frame. This capsid precursor is express ed 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 featur e common to both genera of tetraviruses. Comparison of the sequences of Nud aurelia beta virus and another member of the Tetraviridae, Helicoverpa armi gera stunt virus, which is in the genus Omegatetravirus, shows identities o f 31.6% for the replicase and 24.5% for the capsid protein. The viruses in the genera Betatetravirus and Omegatetravirus of the Tetraviridae are clear ly related hut show significant differences in their genome organization. I t is concluded that the ancestral virus with a bipartite genome, as found i n the genus Omegatetravirus, likely evolved from a virus with an unsegmente d genome, as found in the genus Betatetravirus, through evolution of the su bgenomic RNA into a separate genomic component with the accompanying loss o f the capsid gene from the longer genomic RNA. (C) 1999 Academic Press.