Sequence analysis of the bicistronic Drosophila X virus genome segment A and its encoded polypeptides.

Drosophila X virus represents the entomobirnavirus genus of the Family Birnaviridae. Segment A of this bisegmented dsRNA containing virus was cloned and sequenced. The 3360-bp-long nucleotide sequence revealed the presence of two open reading frames (ORFs). A large ORF of 3096 nucleotides, which is flanked by a 107-bp 5' and a 157-bp 3'-untranslated region, and a 711-nucleotide-long small ORF located within the carboxy half of the large ORF but in a different reading frame. The large ORF encodes a 114-kDa polyprotein which is cotranslationally processed by the virus-coded protease VP4 to generate preVP2, VP3, and VP4 (VP1 is encoded by genome segment B). N-terminal amino acid sequencing of VP3 and VP4 established the order NH2-preVP2-VP4-VP3-COOH within the polyprotein. The small ORF straddles the VP4/VP3 junction and is capable of encoding a basic, arginine-rich 27-kDa polypeptide which so far has not been detected in infected cells. The amino acid sequences specified by the two ORFs were compared to those of infectious pancreatic necrosis virus (IPNV) and infectious bursal disease virus (IBDV) that represent the two other genera (aquabirnavirus and avibirnavirus) of the Birnaviridae family. Significant sequence homology among the three viruses was found to be restricted to the amino and carboxy regions of preVP2 and to a small 21-residue-long domain near the carboxy terminal region of VP3. Significant sequence homology is exhibited by the small ORFs of the three viruses.