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 10
6. 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.