Nucleotide sequence analysis of genome segment A of infectious bursal disease virus

The nucleotide sequence of genome segment A cDNA of the STC strain of infectious bursal disease virus (IBDV) was determined and compared with sequences of the homologous genome segment of the 002-73 strain of IBDV and the Jasper strain of infectious pancreatic necrosis virus (IPNV). The STC-IBDV genome segment A was determined to be 3262 base pairs (bp), which is close to the estimated total length of 3300 bp for genome segment A in IBDV, although there is no proof that it is the real length of this genome segment. The STC-IBDV genome segment A contains two major overlapping open reading frames (ORFs). The large ORF of 3036 bp predicts a polyprotein of Mr 109,358, whereas the small ORF is 435 bp and predicts a protein of Mr 16,550 in STC-IBDV. STC-IBDV and 002-73-IBDV polyproteins are closely related (97.4% amino acid homology). Most of the amino acid mismatches are in VP2 sequences, mainly within the area of the conformation-dependent epitope. Comparison with the Jasper-IPNV polyprotein reveals levels of amino acid sequence homology of about 40% in VP2, 32% in VP3 and 21% in VP4. Within the VP2 molecule the conformation-dependent epitope area is again the least homologous, but the heterogeneity is more conspicuous than between the two IBDV strains, which is not surprising since IBDV and IPNV are serologically unrelated. The small ORF proteins have about 88% amino acid sequence homology between STC-IBDV and 002-73-IBDV, and 30% between each IBDV strain and Jasper-IPNV. There is no homology at all in the non-coding regions of IBDV and IPNV. These comparative sequence data will be useful for subgrouping the Birnaviridae family.