Inovirus

Taxonomy

Group: ssDNA

Virology

Inovirus virions consist of a non-enveloped, rod-shaped capsid with helical symmetry. The virions are between 760 and 1950 nm in length and 6-8 nm in width.

There capsid consists of 5 or more proteins: gp8 (the major capsid protein); gp6, gp7 and gp8 (minor capsid proteins); and gp3 which acts as the initial host binding protein.

The genomes are 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.

The viral protein gp2 plays an essential role in viral DNA replication. It binds to the origin of replication, and cleaves the dsDNA intermediate, allowing DNA replication to initiate at 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 new circular single-stranded molecule ready to be packed into a virion.

Life cycle

Inoviruses begin their life cycle by attaching to specific host receptors via viral protein gp3. After attachment, they insert their viral DNA into the host cell. Once inside the cell, they convert the genome into a double-stranded intermediate form which is then replicated by the host's DNA polymerase. At the same time, the host's RNA polymerase transcribes the viral genome to make mRNA and viral proteins. The replicated genomes then combine with newly-synthesized viral proteins to make more viruses, which are released from the host. This replication cycle generally takes 10-15 minutes to complete.

Replication

Genome replication 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 virion assembly begins.

Virion assembly

Virion assembly is initiated by the formation of a complex of gp1, gp7, gp9 and gp11 along with the single stranded DNA. 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 capsid protein subunits per virion). 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.

Virion release

Release of new viruses often involves host lysis but alternatively productive infection may occur by budding from the host membrane. This pattern is typically seen in the Plectivirus genus.

Notes

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.

Relevance

At least one of the viruses (Vibrio phage CTX) is medically important as it encodes the cholera toxin.

The type species is Enterobacteria phage M13. This phage has been extensively used in experimental work in microbiology.

Non biological uses

The phage M13 has been used to make nanosized (10−20 µm in diameter) fibers.