Genomic structure of the large RNA segment of infectious bursal disease virus

The larger RNA segment of infectious bursal disease virus (IBDV: Australian strain 002-73) has been characterized by cDNA cloning and nucleotide sequence analysis. We believe IBDV is the first birnavirus to be sequenced and so have confirmed the coding region by N-terminal amino acid sequence analysis of intact viral proteins and several tryptic peptide fragments. The large RNA segment encodes in order the 37-kDa, 28-kDa and 32-kDa proteins within a continuous open reading frame and the primary translation product appears to be subsequently processed into the mature viral proteins. The large protein precursor is still processed into the 32-kDa host protective immunogen when expressed as a fusion protein in E. coli. These results are in marked contrast to the predictions from in vitro translation data that birnavirus genomes are expressed as polycistronic templates. We can now propose that birnaviruses, in particular IBDV, possess monocistronic segments and that the precursor is proteolytically processed in vivo. The sequence data presented for the 32-kDa host protective immunogen may provide the basic information needed for the production of an effective subunit vaccine against this commercially important virus.