| Inovirus | |
|---|---|
| Virus classification | |
| Group: | Group II (ssDNA) |
| Family: | Inoviridae |
| Genus: | Inovirus |
| Species | |
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Enterobacteria phage f1 |
|
Inovirus is a genus of filamentous bacteriophages. Viruses in this genus hosts with the Enterobacteriaceae, Pseudomonadaceae, Spirillaceae, Xanthomonadaceae, Clostridium and Propionibacterium. The name of the genus is derived from the Greek word 'nos' meaning 'muscle'.
At least one of the viruses (Vibrio phage CTX) is medically important as it encodes the cholera toxin.[2]
The type species is Enterobacteria phage M13. This phage has been extensively used in experimental work in microbiology.
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The virions consist of a non enveloped, filamentous capsid with helical symmetry.[3] The virons are between 760-1950 nanometers (nm) in length and 6-8 nm in width.
There are five or more proteins in the capid: gp8 (the major capid protein); gp6, gp7 and gp8 (minor capid proteins); and gp3 which acts as the initial host binding protein.
The genomes are non segmented, circular, positive-sense, single-stranded DNA 4.4-8.5 kilobases in length. They encode 4 to 11 proteins.
Replication of the genome occurs via a dsDNA intermediate and the rolling circle mechanism.
Gene transcription is by the host's cellular machinery each gene having a specific promoter.
Protein gp2 is an essential gene because of its role in viral DNA replication. It binds to the origin of replication (ori), cleaves the dsDNA replicative form I and becomes covalently bound to it via phosphotyrosine bond, generating the dsDNA replicative form II. Viral DNA replication initiates at the 3'-OH of the cleavage site. After one round of rolling circle synthesis, gp2 is linked to the newly synthesized ssDNA and joins the ends of the displaced strand to generate a circular single-stranded molecule ready to be packed into a virion.
The major coat protein gp8 has a parallel coiled-coil structure.
There are six steps in the life cycle
1. Adsorbion to the host via specific receptor(s)
2. Movement of the viral DNA into the host cell
3. Conversion of the single strand form to a double stranded intermediate
4. Replication of the viral genome
5. Synthesis of the new virons
6. Release of the new virons from the host
A typical replication cycle normally take 10-15 minutes to complete.
This is mediated by one of the viral proteins (gp3) binding to the host receptor
The conversion from single stranded to double stranded form is carried out by the host's own DNA polymerase. The host's RNA polymerase binds to the viral genome and syntheses RNA. Some of this RNA is translated and the remainder is used to initiate DNA replication.
This is initiated when a viral endonuclease (gp2) nicks the double stranded intermediate. This nicking site is specific and the sequence around the site highly symmetrical. The activity of gp2 is regulated by two other viral proteins: gp5 (single strand binding protein) and gp10. New viral genomes are produced via the rolling circle mechanism. These new single strand DNA sequences become templates for further DNA and RNA synthesis. When sufficient gp5 has accumulated within the cell, further DNA synthesis is halted and viron assembly begins.
This is a complex process. It is initiated by the formation of a complex of gp1, gp7, gp9 and gp11 along with the single stranded DNA and gp%. It begins at a specific sequence within the DNA which is predicted to have a hairpin formation. Assembly continues at the membrane where ~1500 subunits of gp5 are displaced by ~2700 subunits of gp8 (the number of major capid protein subunits per viron). This process involves both gp1 and gp11. Assembly is completed by the addition of the viral proteins gp3 and gp6. In hosts with both an inner and outer membrane adhesion zones are created by gp4, a process that may also involve gp1.
This may involve host lysis but alternatively productive infection may occur by budding from the host membrane. This pattern is typically seen in the Plectivirus genus.
The phage DNA may integrate into the host genome via site-specific homologous recombination.
A number of exceptions to this life cycle are known. Lysogenic species, which encode integrases, exist within this family.
The phage DNA may integrate into the host genome via site-specific homologous recombination. Most phages that do integrate into the host genome encode a recombinase. Inoviruses do not encode this enzyme. The phages that infect hosts in the genus Vibro highjack the chromosome dimer resolution system of their hosts in order to integrate into the genome of the host.
The phage M13 has been used to make nanosized (10 − 20 microns in diameter) fibers.[4]