Characteristics of iridoviruses Virion structure

Virions are comprised of an icosahedral outer capsid, an internal lipid membrane and an electron-dense central core which contains the viral DNA. The capsid is made up of a single polypeptide of about 50 kDa in size, which is approximately 40% of the total mass of the virus. This protein has been shown to be highly conserved throughout the Iridoviridae family and therefore is of use in comparison studies. The tight packing of these icosahedral capsids can form a crystalline array (Figure 1) which diffracts

Table 1 Classification of the Iridoviridae family

Genus Iridovirus

Chloriridovirus

Rana virus

Lymphocystivirus

Goldfish virus-like

Vernacular name

Host species

Type species

Small iridescent insect virus targe iridescent insect virus Frog virus

Lymphocystis disease virus Goldfish virus

Diverse invertebrates (mostly Chilo iridescent virus (CIV) insects)

Díptera (mosquitoes)

Amphibia

Fish

Goldfish

IV Type 3 from Aedes taeniorhynchus Frog virus 3 (FV3)

Lymphocystivirus type 1 (LCDV-1)

Goldfish virus (GFV-1)

Figure 1 Electron micrograph of Wiseana iridescent virus. Cross-section of an infected insect showing virions in a crystalline array.

light, causing a distinctive iridescence; however, this phenomenon has no biological significance.

The number of proteins detected for each iridovirus varies depending on the method and conditions of analysis. CIV has been shown to consist of at least 20 polypeptides, ranging from 10 to 213 kDa, with reports of up to 50 polypeptides by two-dimensional gel electrophoresis. Structural polypeptides of TIV have been shown to be nonglycosylated. Closely associated with the capsid is the internal lipid membrane, through which proteins pass, connecting the outer capsid to the inner core polypeptides. The lipid composition of the iridovirus membrane differs significantly to the lipids of the host cell membrane, indicating that their synthesis is virally encoded. The inner corc is highly hydrated and comprises DNA packed with DNA-binding proteins. The DNA is linear, double stranded and nonmethylated and exhibits cyclically permuted terminal redundancy. CIV has a terminal redundancy in which 12% of the genomic DNA is repeated at the termini of the genome. This feature is unique for eukaryotic viruses and it is more commonly observed in prokaryotic viruses such as the bacteriophages T4 and P22.

Host range and classification

Iridoviruses vary in their host range: some exhibit a broad range of host species while others are restricted to one or two specific hosts. Both TIV and CIV have a wide host range, infecting dipteran, coleop-teran and lepidopteran hosts. CIV has been shown to infect over 100 insect species, woodlice, centipedes and two species of Crustacea. The greater wax moth Gallería mellonella (Lepidoptera: Pyralidae) is highly permissive to most iridoviruses and is used for general virus propagation. Iridoviruses infecting insects have not been found to infect any vertebrates. The current classification scheme for these viruses uses names according to the host species of initial isolation and a type number relating to the chronological order of discovery. A system using PCR sequencing and DNA-DNA hybridization methods is currently being investigated for classification of iridoviruses.

Replication

These viruses have a low pathogenicity and the frequency of subclinical infections (those not exhibiting iridescent coloration) is probably high. Cannibalism and wounding are the likely routes of infection, although entry via spiracles, transovarial transmission and ingestion have also been proposed. The iridescence of infected individuals is not a reliable diagnostic criterion and has in the past lead to many false, negative results. The primary site of infection is thought to be the hemocytes and fat bodies of the larvae, with eventual spread to the entire body via the hemolymph. Virus particles enter the insect cell via phagocytosis and are incorporated into lyso-somes where the virus is uncoated and replication initiated. Frog virus 3, isolated from the leopard frog Rana pipiens, has been shown to synthesize viral

Iridoviruses

Figure 2 Electron micrograph of a Wise-ana iridescent virus-infected Spodoptera frugiperda cell. Arrow indicates region of virus assembly in the cell cytoplasm. N, cell nucleus.

DNA in two stages. DNA is first synthesized in the eel! nucleus in genome lengths, then transported to cytoplasm where it is replicated as a large concat-emer of DNA. This two-step replication of DNA is a feature unique to the Iridoviridae family and it enables the linear genome to be replicated completely, resulting in the headful packaging of DNA and the terminal redundancy. Capsids are preformed in the cytoplasm where packaging occurs (Figure 2) and complete virions can be released via budding from the cell membrane or by cell lysis. Iridoviruses induce a shutdown of host macromolecular synthesis while utilizing some machinery for replication. This shutdown eventually leads to host death, although not until the individual is highly infected: in some cases up to one-third of the insect body weight can be virus.

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