Higher Order Categories
Family: Leviviridae Genus: Levivirus Species: Enterobacteria phage BZ13, Enterobacteria phage MS2
Description and Significance
ssRNA positive-strand viruses, no DNA stage
Levivirus is one of two known genera of the family Leviviridae, with Allolevivirus being the other. It replicates in only three bacteria genera: Escherichia, Pseudomonas, and Caulobacter. The virus attaches to the pili, sometimes the virion receptor site, and transiently exposes viral RNA while penetrating the cell . The family Leviviridae is found to have one of the fastest known mutation rates, at 10-3 bp/replication, but also one of the smallest genomes at 4,268 nt which code for 4 protein subunits . The distinguishing factor of levivirus is a cell-lysis protein coded within its genome, something that is absent in allelovirus.
In 1976 the species Enterobacteria phage MS2 became the first organism ever to have its genome completely sequenced. This species of levivirus attacks only E. Coli exhibiting the F pilus where it will bind and penetrate the cell membrane. Levivirus are exclusive to 3 genera of Gram negative bacteria, likely a result of the comparatively small size of the virus in conjunction to the thin peptidoglycan walls found in Gram negative bacteria.
The genome is 4,268 nt, and consists of 4 proteins: Lysis protein , A Protein (maturation protein), Capsid Protein and Replicase. The lysis gene overlaps the Replicase in a +1 frameshift. Once a large amount of the Capsid Protein have been made, the dimers inhibit Replicase translation via binding with an RNA hairpin structure present at the 5’ end of the gene.[2,5]
The virion structure of levivirus is a non-enveloped capsid consisting of approximately 180 Capsid Proteins, 1 A Protein, and the ssRNA genome. It is 26 nm in diameter and is icosahedrally symmetrical.
Reproductive Cycle in Host
When entering a host the levivirus attaches to the F pili, found in E. Coli, and cleaves, releasing the maturation protein (A protein) into the host. The result of this is genomic viral RNA being present in the bacteria cell and initiating translation of viral proteins. The viral Replicase protein then interacts with host proteins, ribosomal protein s1, translation elongation factor EF-Tu and EF-Ts, and synthesizes active RNA polymerase within the host . Viral genomic RNA is then replicated within the cell to serve as a template for the further production of viral RNA. When a substantial amount of Capsid protein has been replicated the CP protein dimers, fragments of the replicated protein, attach to an RNA hairpin found at the 5’ end cap. The constructed virions are then assembled within the cell and the cell-lysis coding protein initiates destruction of the host and release of the new virions.[1,4]
Viral Ecology & Pathology
Levivirus are found most commonly in waste-water, sewage, and feces. The bacteria that it targets (E. Coli) is predominantly found in these conditions, therefore making it an obvious habitat for the levivirus bacteriophage. The virus itself is harmless to humans as it only targets the cells of bacteria.
1. "ViralZone: Levivirus." ViralZone: Root. Swiss Institute of Bioinformatics. Web. 19 Mar. 2012. <http://viralzone.expasy.org/all_by_species/291.html>.doi:N/A
2. King, Andrew M. Q. Virus Taxonomy: Classification and Nomenclature of Viruses : Ninth Report of the International Committee on Taxonomy of Viruses. London: Academic, 2012. doi: N/A
3. Bollback, Johnathan P., and Johnathan P. Huelsenbeck. "Phylogeny, Genome Evolution, and Host Specificity of Single-Stranded RNA Bacteriophage (Family Leviviridae)." Journal of Molecular Evolution 52.2 (2001): 117-28. doi:10.1007/s002390010140
4. Ruokoranta, Tanja M. "Complete Genome Sequence of the Broad Host Range Single-Stranded RNA Phage PRR1 Places It in the Levivirus Genus with Characteristics Shared with Alloleviviruses." Journal of Virology 80.18 (2006): 9326-330. doi:10.1128/JVI.01005-06
5. Ruokoranta, T. M., A. M. Grahn, J. J. Ravantti, M. M. Poranen, and D. H. Bamford. "Complete Genome Sequence of the Broad Host Range Single-Stranded RNA Phage PRR1 Places It in the Levivirus Genus with Characteristics Shared with Alloleviviruses." Journal of Virology 80.18 (2006): 9326-330. doi:10.1128/JVI.01005-06