| Gyrovirus | |
|---|---|
| Virus classification | |
| Group: | Group II (ssDNA) |
| Family: | Circoviridae |
| Genus: | Gyrovirus |
| Species | |
|
Avian gyrovirus 2 |
|
The genus Gyrovirus, the only well known species being the chicken anemia virus, is a part of the family of Circoviridae, and consists of a nonenveloped, round, icosahedral capsid, 19-27 nm in diameter. The genome contained within consists of a single molecule of circular, single-stranded negative-sense DNA that forms a closed circle. The complete genome is 2290–2320 nucleotides long. They are primarily vertebrate viruses.[1]
Contents |
Gyroviruses have an average size of 19 to 27 nanometers. They are nonenveloped and have an icosahedral capsid with T=1 symmetry. The unique, single protein, trumpet-shaped capsomeres of Gyrovirus are arranged into 12 pentomers yielding a capsid 60 units in size.[2]
The Gyrovirus genome consists of negative sense, single-stranded, circular, DNA. The genome is relatively small at 2,300 nucleotides. and contains three overlapping open reading frames that code for only three known proteins.[3]
The Gyrovirus genome codes for a single polysistronic mRNA that subsequently codes for three proteins, VP1, VP2, and VP3. VP1 is the 51kd capsid protein; in addition to its structural function, it also contains motifs for rolling circle replication in the C-terminal region. VP2 is a 23kd nonstructural protein with phosphatase activity. Virions with mutations in SP2 are still replication competent; however, their cytopathic effects were highly attenuated.[4] VP3, also called apoptin, is a 13kd protein that has been shown to independently induce apoptosis in chicken cells. Apoptin is also being researched for its ability to induce apoptosis in human tumor cells.[5][6]
Upon entering the host cell, the virus converts from single stranded to double stranded, circular DNA using host cell machinery. The circular dsDNA is then used as a template for transcription and for replication via a rolling circle mechanism similar to phiX174.[7]
Chicken anemia virus (CAV) is currently the only known member of Gyrovirus. The disease has been described worldwide in areas where chickens are produced. CAV causes severe anemia, hemorrhaging, and depletion of lymphoid tissue through the destruction of bone marrow erythroblastoid cells.[3] The disease affects mainly young chicks not protected by maternal antibodies. Age resistance to disease begins at about one week, but can be overcome however by coinfection with immunosuppressive diseases, such as bursal disease virus , Marek’s disease, and others.[8]
A second virus in this genus - Avian gyrovirus 2 - has been described.[9] The viral genome shares ~40% of its sequence with Chicken anemia virus (CAV). The genome is 2383 nucleotides long and has three partially overlapping open reading frames encoding the proteins VP1, VP2 and VP3. These proteins share 38.8%, 40.3%, and 32.2% amino acid identities between their homologs in the CAV.