Taxonomic Structure of the Family
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Leviviridae |
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Genus |
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Genus |
Virions are spherical and exhibit icosahedral symmetry (T = 3); with a diameter of about 26 nm. There is no envelope (Fig. 1).
Physicochemical and Physical Properties
Virion Mr varies between 3.6 and 4.2 106 depending on the genus. The range in S20w value is from 80 to 84S; buoyant density in CsCl is 1.46 g/cm3. Infectivity is ether, chloroform and low pH resistant but is sensitive to detergents. Inactivation by UV light and chemicals is comparable to that of other icosahedral viruses containing ssRNA.
Virions contain one molecule of positive sense ssRNA ranging in size from 3,466 to 4,276 nts; size and gene arrangement vary with genus. The RNA makes up 30% of the virion weight. The 5 nucleotide carries a triphosphate, while on the 3 side a non-templated A residue is added by the replicase.
The capsid contains 180 copies of the capsid protein (Mr 14 103), arranged in 60 identical triangular units which are related by the symmetry elements of an icosahedron. The structure of the protein shell of Enterobacteria phage MS2 (MS2) has been resolved by X-ray crystallography. The capsid protein has no structural similarity to that of other icosahedral RNA viruses. The capsid contains one copy of the A protein (Mr 35-44 103), which is required for maturation of the virion and for pilus attachment.
None reported.
None reported.
Genome Organization and Replication
Phages infect by adsorption to the sides of pili. The specificity of this adsorption is determined by a wide variety of different plasmids. The enterobacteria phages attach to F pili, which leads to cleavage of the A protein and release of the RNA from the virion. The infecting RNA encodes a replicase, which assembles with four host proteins (ribosomal protein S1, EF-Tu, EF-Ts and host factor, the product of the hfq gene) to form the active replicase holoenzyme. This enzyme synthesizes a free negative strand which is the template for positive strand synthesis. Positive strand synthesis requires only the translational elongation factors EF-Tu and EF-Ts as cofactors. Late in infection the capsid protein acts as a translational repressor of the replicase gene. Capsids assemble in the cytoplasm around phage RNA. Infection usually results in cell lysis releasing some thousand phages per cell.
The viruses infect enterobacteria, species of the genera Caulobacter and Pseudomonas and possibly many other gram-negative bacteria, provided they express appropriate pili on their surface.
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