Description of Tombusviridae
adapted from
ICTVdb
Virion Properties
Morphology
Virions consist of a
capsid. Virus capsid is not enveloped, round with icosahedral
symmetry (T=3). The capsid is isometric and has a diameter of
28-35 nm. Capsids appear round, or hexagonal in outline, or
slightly angular. The capsid surface structure reveals a regular
pattern with distinctive features (of granular appearance). The
capsomer arrangement is clearly visible, or is not obvious. The
capsid consists of 32 capsomers (virion composed of 180 protein
subunits).
Physicochemical and Physical Properties
Virions have a buoyant density in
CsCl of 1.28-1.346-1.36 g cm
-3. There are 1
sedimenting component(s) found in purified preparations, or 2
sedimenting component(s) found in purified preparations. The
sedimentation coefficient is 130-134.2-140
S20w. Isoelectric point pH is 3.9-4-4.1.
A
260/A
280 ratio is 1.64. The thermal
inactivation point (TIP) is at 75-85-90°C. The longevity
in vitro (LIV) is 21-104.5-300 days. Although the titer is
dependent on the host, the decimal exponent (DEX) of the dilution
end point is usually around 4-5, or 6, or 7. The infectivity is
retained when deproteinized with proteases; retained when
deproteinized with phenol or detergent.
Nucleic Acid
The Mr of the genome constitutes 14-16.92-18% of the
virion by weight. The genome is monopartite, only one particle
size is recovered, or segmented; bipartite, segements are
distributed among 2 particle types of linear, positive-sense,
single-stranded RNA. Minor species of non-genomic nucleic acid
are also found in virions, or not found in virions. The
encapsidated nucleic acid is mainly of genomic origin, but
virions may also contain subgenomic RNA, or subgenomic RNA and
satellite RNA (occasionally), or nucleic acid of host origin and
subgenomic mRNA. Virions may also contain defective RNA species
arising from a deletion of full-length genomic RNA. The genome is
sequenced, and complete sequence is about 4600-4702-4800
nucleotides long. The genome has a base ratio of 27.8-28.13-28.6
% guanine; 24.9-26.06-27 % adenine; 21.2-21.5-22 % cytosine;
23-25.13-26.3 % uracil. The 5'-end of the genome has a probably
methylated nucleotide cap. The terminus is protected, however,
the presence of a cap was demonstrated only in 74.0.2.0.004
carnation mottle virus, Genus 74.0.2.
Carmovirus). Each
virion contains a full length copy and defective interfering
copies (are known to occur).
Proteins
Proteins constitute about
82-82.66-84% of the particle weight.
The viral genome encodes structural proteins and
non-structural proteins. Virions consist of 1 structural
protein(s).
Non-Structural Proteins: Virus-coded
non-structural proteins have been isolated and 3 non-structural
protein(s) are found.
Lipids
Lipids are not reported.
Genome Organization and Replication
By itself, genomic nucleic acid is infectious (and addition
of a cap analogue to
in vitro RNA transcripts enhances
infectivity little or not at all).
Transcription:
Sub-genomic RNA is present in infected cells. The genome
expression is based on RNA production which can be analyzed by
the dsRNA patterns found in the infected tissues. Usually there
are 3-3.5-4 virus specified dsRNA species found in infected
cells. Size of largest virus specified dsRNA 4.7-4.716-4.733 kbp.
2nd largest 2.1-2.144-2.188 kbp. 3rd largest 0.9-0.918-0.936 kbp.
4th largest 0.62 kbp. 5th largest 0.499 kbp.
Translation: The genome
replicates in the cytoplasm.
Virions may provide helper functions to dependent virus during
replication. Virion acts as helper for a satellite RNA.