Birnaviridae. Birnaviridae

Description of Birnaviridae adapted from ICTVdb Virion Properties Morphology Virions consist of a capsid. Virus capsid is not enveloped, round with icosahedral symmetry (T=13) (skewed right handed lattice). The capsid. The capsid shells of virions are composed of a single layer. Capsids appear hexagonal in outline. The capsomer arrangement is clearly visible. The capsid consists of 132 capsomers. Surface projections are not present. Capsids all have the same appearance. Physicochemical and Physical Properties The molecular mass (Mr) of virions is 55 x 106. Virions have a buoyant density in CsCl of 1.33 g cm-3, or 1.3 g cm-3 (defective virions). There are 2 sedimenting component(s) found in purified preparations. The sedimentation coefficient is 435 S20w. Under in vitro conditions virions are stable in acid environment of pH 3-7; stable in alkaline environment of pH 7-9. Virions are not sensitive to treatment with heat (60C, 1 hour), or ether (and 1% SDS at 20C, pH 7.5 for 20 min). Nucleic Acid The Mr of the genome constitutes 9.7% of the virion by weight. The genome is segmented and consists of two segments of linear, double -stranded RNA. Minor species of non-genomic nucleic acid are also found in virions. The complete genome is 5880-6400 nucleotides long. The segment A is fully sequenced, complete sequence is 3100-3200 nucleotides long (A). Segment B is sequenced, but only an estimate is available. The genome has a guanine + cytosine content of 45.5-54.5 %. The 5'-end of the genome has a on both segments genome-linked protein (VPg). The multipartite genome is found in one type of particle only. Proteins The viral genome encodes structural proteins. Lipids Lipids are absent. Carbohydrates Carbohydrates are not found in virions, or are found in virions; are O-linked glycosidic side chains (according to a report for VP2 of IPNV). Genome Organization and Replication Transcription: Sub-genomic RNA is present in infected cells. The mRNA is a transcription of RNA-A and RNA-B.