Icosahedral cytoplasmic deoxyribovirus

Synonym for Iridovirus.

icosahedral cytoplasmic deoxyriboviruses of amphibians Several viruses in the genus Ranavirus, isolated from frogs, newts and toads. The only recognized species is Frog virus 3, but there are a number of tentative species. Frog viruses do not cause disease in their natural adult host, Rana pipiens, but are lethal for frog embryos and larvae and Fowler toads. Grow in piscine, amphibian, avian and mammalian cells at 12-32°C. The genome is a single linear double-stranded DNA of about 170 kb which is highly methylated, terminally redundant and circularly permuted. Replication of DNA occurs in both the nucleus and the cytoplasm, but virus assembles in the cytoplasm.

Synonym: cytoplasmic amphibian viruses.

Goorha RM and Granoff A (1999) In Encyclopedia of Virology, Second edition, edited by A Granoff and RG Webster. London: Academic Press, p. 582

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icosahedral symmetry One of the two types of symmetry found in viral capsids, the other being helical symmetry. Crystallo-graphic considerations prescribe that the identical units forming the capsid of an isometric particle must be arranged with cubic symmetry. Of the possible forms that this may take, icosahedral symmetry provides the facility to make a range of viral capsids with different numbers of structural units. An icosahedron has 20 triangular faces and 12 vertices. The


simplest has 60 identical structural units in regular relation to each other, three to a triangular face. To make a large virus in this simple form from 60 units would require a large protein, which raises difficulties with genome coding capacity, and an alternative is to use a larger number of small units (i.e. more than 60). This inevitably means that the units cannot all have identical relationships to each other. Those not surrounding a vertex form groups of six called 'hexons', and those at each vertex are in groups of five called 'pentons'. Only certain multiples of 60 units are possible, and the numbers which make up different viral capsid structures are defined by the triangulation number, T. There are always 60T units, where T = h2 + hk + k2 (h and k are integers having no common factors). Examples are T = 3 (caliciviruses), T = 4 (alphaviruses), T =13 (rotaviruses and orbiviruses) and T =16 (herpesviruses). The structural units form into morphological units on the virus surface. In general, the number of morphological units (capsomeres) which can be visualized on the surface of an icosahedral virion is 10T + 2 (e.g. 162 for herpesviruses).

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Harrison SC et al (1996) In Fields' Virology, Third edition, edited by BN Fields et al. Philadelphia: Lippincott-Raven, p. 59 Johnson JE and Speir JA (1999) In Encyclopedia of Virology, Second edition, edited by A Granoff and RG Webster. London: Academic Press, p. 1946

icosahedron A solid with 20 triangular faces and 12 vertices. In a regular icosa-hedron the faces are equilateral triangles and there are axes of two-fold, three-fold and five-fold rotational symmetry.

ICR 2A cells (CCL 145) A haploid frog cell line established from androgenetic hap-loid embryos of the grass frog, Rana pipiens.

ICR 134 cells (CCL 128) A cell line established from the tissue of stage 17 gynogenetic haploid embryos of the grass frog, Rana pipiens, and cloned at the 11th passage.

'Ictalurid herpes-like viruses' An unas-signed genus in the family Herpesviridae. The only member is Ictalurid herpesvirus 1 (channel catfish virus).

Ictalurid herpesvirus 1 (IcHV-1) The type species of the genus 'Ictalurid herpes-like viruses' (which is unassigned to a subfamily) in the family Herpesviridae. Isolated from channel catfish, Ictalurus punctatus, in which it causes a severe hemorrhagic disease which may have a mortality rate in excess of 95%. The first fish herpesvirus to be isolated, it has been extensively studied. Experimentally, the virus will infect blue catfish, Ictalurus fur-catus, but other species of fish are not affected. The virus replicates in ictalurid and clariid fish cell lines, between 10°C and 33°C, and about 50% of the progeny is released into the culture medium. Morphologically, IcHV-1 is a typical her-pesvirus. The complete DNA sequence has been determined as 134226 base pairs, with G+C 56%. The predicted proteins are quite different from those of mammalian and avian herpesviruses. The virus may become latent in adult fish, and is perpetuated by this means in channel catfish populations. No related species have yet been recognized. Synonym: channel catfish virus.

Chinchar VG et al (1996) Dis Aquat Org 33, 77 Davison AJ (1992) Virology 186, 9 Davison AJ (1999) In Encyclopedia of Virology, Second edition, edited by A Granoff and RG Webster. London: Academic Press, p. 553

ICTV International Committee on Taxonomy of Viruses. A committee of the Virology Division of the International Union of Microbiological Societies (IUMS) which decides upon the classification and nomenclature of viruses affecting all species of animals, plants, fungi, bacteria and archea. Since their work began in 1968, the ICTV has issued seven Reports, the most recent of which was presented to the International Congress of Virology held in Sydney, Australia in August 1999. The ICTV is composed of an 18-member Executive Committee supported by numerous subcommittees and study groups.

van Regenmortel et al (editor) (2000) In Virus Taxonomy, Seventh Report of the ICTV. London: Academic Press

ID50 The 50% infective dose. The dose that on average will infect 50% of the individuals to which it is administered. They may be human volunteers, experimental animals, tissue cultures or eggs. When eggs are used the term EID50 is often used.

idiotype The structural features of the variable regions of a particular antibody from a single individual. Anti-idiotype antibodies combine with these structures, and may resemble the epitope to which the first antibody reacts. See also isotype.

idoxuridine (IDU) 5-Iodo-2'-deoxyuri-dine. An antiviral agent and an analog of thymidine. Its action is due to its incorporation into DNA, when it becomes active against DNA viruses such as Human herpesvirus types 1, 2 and 3, vaccinia and Suid herpesvirus 1. The drug is probably inactive until enzymatic conversion into the nucleotide that is a competitive inhibitor for the incorporation of thymidine nucleotides into DNA. This occurs during both cellular and viral DNA synthesis. Because of its toxi-city it is used mainly as a topical application. Has been used with success in the treatment of herpes simplex ker-atitis, particularly dendritic ulcers of the cornea, but is now largely supplanted by acyclovir.

Synonyms: dendrid; herpid; idoxyuridine; kerecid; stoxil.

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