Taxonomic Structure of the Family
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Bromoviridae |
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Virions are either spherical (Fig. 1), having T = 3 icosahedral symmetry, and a diameter of 26-35 nm (members of the genera Bromovirus, Cucumovirus, and Ilarvirus) or bacilliform (members of the genera Alfamovirus, Ilarvirus, and Oleavirus) within a species having constant diameters of 18-26 nm and lengths from 30 to 85 nm. Genomic RNAs are packaged in separate virions that may also contain sgRNAs, defective RNAs or satellite RNAs.
Physicochemical and Physical Properties
The Mr of the virions varies from 3.5-6.9 106, depending on the nucleic acid content. Virion Mr is constant among members of the genera Bromovirus, Cucumovirus, and some Ilarvirus members, and varies among the remaining members of the family. The buoyant density of formaldehyde-fixed virions ranges from 1.35-1.37 g/cm3 in CsCl. The S20w varies from 63S to 99S. Virion integrity is dependent on RNA-protein interactions and virion RNA is susceptible to RNase degradation in situ at neutral pH. Heat inactivation occurs at 60°C in some genera, others have not been tested. In most cases virions are unstable in the presence of cations. Virions are generally stable in the presence of chloroform, but not stable in the presence of phenol. Virions are unstable in the presence of strong anionic detergents such as SDS, but can tolerate low concentrations of mild detergents such as Triton X-100.
Total genome length is approximately 8 kb. Genomes consist of three linear, positive sense ssRNAs with 5-terminal cap structures. The 3-termini are not polyadenylated, but generally are highly conserved within a species or isolate, and form strong secondary structures. They are either tRNA-like and can be aminoacylated (genera Bromovirus and Cucumovirus) or form other structures that are not aminoacylated (genera Alfamovirus, Ilarvirus and Oleavirus) (Table 1).
A single capsid protein (CP) of Mr 20-24 103 is expressed from a sgRNA. The CP is generally required for systemic movement and may be required for cell to cell spread in some cases.
There are at least three nonstructural ORFs that encode the replicase (consisting of the 1a and 2a proteins, along with host factors) and the movement protein (3a) (Table 2). Cucumovirus RNA-2 also encodes 2b proteins involved in movement. TSV RNA-2 also encodes an ORF for a 2b protein.
There are no lipids associated with the virions.
There are no carbohydrates associated with the virions.
Genome Organization and Replication
The genomic RNAs 1 and 2 each encode a single large ORF, and in some genera RNA-2 also encodes a second ORF that is apparently translated from a sgRNA. RNA-3 encodes a 5 protein, the movement protein (MP), and the CP which is translated from a sgRNA. There is no clear evidence of proteolytic or other post-translational processing. Virus replication ocurrs on cytoplasmic membranes via full length minus (-) strand synthesis and subsequent plus (+) strand synthesis. The sgRNAs are synthesized from the (-) template, and may or may not be found in the virions. The CP accumulates to high levels in infected cells, whereas the nonstructural proteins accumulate to much lower levels. Virions accumulate in the cytoplasm. The life cycle of the virus takes place predominantly in the cytoplasm (Fig. 2).
Native virions are generally poor immunogens and require stabilization with formaldehyde prior to use as antigens. There are little or no serological relationships between the genera, and weak relationships between species of the same genus.
The natural host range of the viruses ranges from very narrow (genus Bromovirus) to extremely broad (genus Cucumovirus). They are predominantly transmitted by insects, in a non-persistant manner, or mechanically. Vectors have not been identified for some of the members of the family. They are distributed worldwide, and several are responsible for major disease epidemics in crop plants.
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