Taxonomic Structure of the Family
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Iridoviridae |
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Virions have an icosahedral symmetry and are usually 120-200 nm in diameter but may be up to 350 nm. The core is a highly hydrated electron-dense entity consisting of a long coiled nucleoprotein filament. This is surrounded by a lipid membrane through which pass transmembrane particles of uncertain function. The capsid comprises identical capsomers, the number of which depends on particle size. The capsomers are organized to form trisymmetrons and pentasymmetrons in members of the Iridovirus and Chloriridovirus genera. Capsomer dimensions are approximately 6-7 nm in diameter and 7-13 nm in height. Each capsomer is composed of an internal and external protein trimer. Fibres or short fibrils have been observed trailing from the capsid in viruses from both vertebrate and invertebrate genera. Vertebrate viruses possess an outer envelope derived by budding through the host cell membrane which may also be present in invertebrate viruses grown in cell culture (Figs. 1 and 2).
Physicochemical and Physical Properties
The Mr of virions is 1050-2000 106. The sedimentation coefficient S20,w is 2200-4450S and density is 1.35-1.6 g/cm3. Virions are stable in water at 4°C for extended periods. Sensitivity to pH and ether varies. All viruses are inactivated in 15 to 30 minutes at 55°C. Radiation stability is unknown.
The virion core contains a single linear dsDNA molecule of between 140 and 303 kbp. Invertebrate iridescent virus 1 (IIV-1) has been reported to have an additional genetic component of 10.8 kbp which exists as a free molecule in the particle core. DNA comprises 12-16% of the particle weight. GC content ranges from 28 to 54%. The DNA of several vertebrate iridoviruses is highly methylated. No evidence of methylation of DNA has been found in any virus species from the genera Iridovirus or Chloriridovirus. Structural polypeptides are required for the non-genetic reactivation of viral DNA. The complete genome sequence is known for Lymphocystis disease virus 1 (LCDV-1).
Iridoviruses are structurally complex with up to 36 polypeptides by two dimensional PAGE with Mr of 10 to 250 103. The major capsid protein (MCP) of Mr 48-55 103 comprises 40% of the total virion protein; the complete amino acid sequence is known for several viruses. The Frog virus 3 (FV-3) MCP sequence shares 52% amino acid identity with the MCP of LCDV-1, 44% identity with IIV-1 and Invertebrate iridescent virus 22 (IIV-22), and 46% identity with Invertebrate iridescent virus 6 (IIV-6). This protein also shares amino acid sequence homology to the MCP of African swine fever virus of the family Asfarviridae and Paramecium bursaria Chlorella virus 1 of the family Phycodnaviridae (Fig. 4). At least 6 DNA associated polypeptides have been identified in the core of IIV-6, with a major species of Mr 12.5 103. Immediately following infection, a viral protein elicits complete shutdown of host macromolecular synthesis. A number of virion-associated enzymatic activities have been detected including a protein kinase, nucleotide phosphohydrolase, ribonuclease which cleaves both ssRNA and dsRNA, deoxyribonucleases with pH optima of 5 and 7.5, and a protein phosphatase. A DNA dependent RNA polymerase II subunit, zinc finger proteins, a helicase, a GTP phosphohydrolase, and a thymidine kinase, are among the virus gene products that have been detected by DNA sequence analysis (see Fig. 3 and Table 1).
Non-enveloped particles contain 5-17% lipid, predominantly as phospholipid. The composition of the lipid component has led to the assumption that viral lipids are not derived from host membranes but are produced de novo. Viruses released from cells may have a plasma membrane derived outer envelope, but this is not essential for virus infectivity.
Carbohydrates are not present in purified virions.
Genome Organization and Replication
The following description applies mainly to FV-3 which is the type species of the genus Ranavirus. Virion entry occurs by pinocytosis with uncoating in phagocytic vacuoles. The replication strategy of iridoviruses is different from other DNA viruses. In the cell nucleus, host RNA polymerase II is modified by a viral polypeptide(s) and is used for viral transcription at an early stage. The parental virus genome serves as the template for early DNA replication. DNA produced during this phase is genome or less than genome size. Viral DNA so produced may serve as additional template or may be transported to the cytoplasm where it is present in the form of a large branched concatamer. A viral encoded DNA integrase-recombinase may be involved with the recombination of small DNA molecules or the resolution of concatomeric DNA prior to packaging of the genome into virus particles. Transcription occurs in the nucleus and the cytoplasm and is controlled by a sequential series of viral induced polypeptides. The DNA concatamer is processed into mature viral DNA presumably during packaging into the virion. A “headful“ mechanism of packaging is believed to occur resulting in circularly permuted and terminally redundant genomes as occurs in Enterobacterio phage T4 or Enterobacterio phage P22. The degree of terminal redundancy varies from approximately 5-50%. Virions assemble in close association with virogenic stroma in the cytoplasm.
The four genera are serologically distinct from one another. In the genus Iridovirus there exists one main group of serologically interrelated species and others which have little sero-relatedness. Several piscine isolates have serological relationships with FV-3 (genus Ranavirus). Antibodies prepared against virions are often non-neutralizing.
Iridoviruses have only been isolated from poikilothermic animals, usually associated with a damp or aquatic environments, including marine habitats. Iridovirus species vary widely in their natural host range and in their virulence. All are sensitive to desiccation or heat. Transmission mechanisms are poorly understood for the majority of these viruses, and there is no evidence for vector involvement.
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