IPNV is a medium-sized, unenveloped bisegmented dsRNA-containing virus in the family Birnaviridae. Genome segment A (3097 bp) contains two overlapping open-reading frames (ORFs). A large ORF encodes a 106 kDa polyprotein (NH2-pVP2-NS protease-VP3-COOH) which is cotranslationally cleaved by the protease to generate the major capsid proteins VP2 and VP3, and a second, small ORF which overlaps the amino end of the large ORF but in a different reading frame, and encodes a 17 kDa arginine-rich minor polypeptide. Genome segment B (2784 bp) encodes a minor internal capsid polypeptide VP1 (94 kDa), which based on its size, low copy number and the presence of several conserved domains associated with RNA-dependent RNA polymerases (RdRp) of other RNA viruses, is the putative virion-associated RdRp. VP1 is present in the virion in two forms: as a free polypeptide and as a genome-linked protein (VPg) covalently attached to the 5′ ends of both genome segments. During in vitro RNA transcription, VP1 serves as a primer and remains attached to the 5′ end of the RNA thereby becoming a VPg. Transcription follows a semi-conservative, strand-displacement mechanism. In infected cells two genome-length 24S viral mRNAs lacking 3′ poly A tracts are synthesized that can hybridize to the two denatured genome segments. In vivo protein synthesis involves both polyprotein processing and internal initiation of translation at some of the in-phase methionine codons. The virus-coded protease functions only in cis and its insensitivity to a number of proteinase inhibitors suggests that it may be a novel viral protease. The putative cleavage sites on the polyprotein have been mapped to within a few amino acids but the exact boundary between pVP2NS and NSVP3 has not been established. A universal, group-specific epitope has been mapped to near the amino terminus of VP2, whereas a serotype-specific epitope was found to be located in the middle of the polypeptide.

Yields of infectious pancreatic necrosis virus from fathead minnow cell cultures were maximal at 20 degrees C. The virus failed to replicate at 28 degrees C and neither virus-specific mRNA nor virus-specific polypeptides could be detected when infected cells were maintained at this temperature. Intrinsic thermolability of virus infectivity or inability to adsorb to cells at 28 degrees C could not account for the temperature-dependent block in virus morphogenesis. Analysis of infectious virus production and virus-specific polypeptide and RNA synthesis following shifts from the permissive (20 degrees C) to the non-permissive temperature (28 degrees C) at various times after infection indicated that multiple temperature-sensitive (ts) steps were involved in the inhibition of virus replication.