Genus Ranavirus

Type Species	Frog virus 3	(FV-3)

Distinguishing Features

Morphology

Particle diameter is approximately 130  nm in ultrathin section. Enveloped virions, released by budding, measure 160-200  nm in diameter. The capsid has a skew symmetry with a triangulation number of 133 or 147. The internal lipid bilayer contains transmembrane proteins. The nucleoprotein core consists of a long coiled filament 10  nm wide.

Physicochemical and Physical Properties

Buoyant density is 1.28  g/cm³ for enveloped particles and 1.32  g/cm³ for unenveloped particles. Infectivity is rapidly lost at pH 2.0-3.0 and at temperatures above 50°C. Particles are sensitive to treatment with ether or sodium deoxychlorate.

Nucleic Acid

Each virion contains a single linear dsDNA molecule of 150-170  kbp. The genome is circularly permuted and approximately 30% terminally redundant. The unit genome size is approximately 107  kbp. The GC content is 53%. The genome is methylated at all cytosines in the dinucleotide CpG by a virus encoded DNA methyl-transferase. There are conflicting reports concerning whether methylation of DNA occurs in the cytoplasm or the cell nucleus. Methylation may be important in protecting DNA from viral endonucleases. Non-genetic reactivation of viral DNA can be achieved in the presence of structural virion polypeptides.

Protein

By two-dimensional gel electrophoresis, 29 structural proteins have been detected in FV-3. Proteins do not undergo extensive post-translational processing and no evidence for glycosylation, sulfation or cleavage from precursors has been detected. Phosphoproteins of Mr 10-114 10³ are present in the virion core and a virion-associated protein kinase (Mr 44 10³) capable of such phosphorylation has been isolated. Other virion associated enzymes include nucleotide phosphohydrolase and a pH5 endodeoxyribonuclease which are located in the core whereas an endoribonuclease, a pH7.5 endodeoxyribonuclease, and the protein kinase are external. The location of a virion-associated protein phosphatase is uncertain. A viral encoded DNA integrase-recombinase (Mr 30.5 10³) may be involved in the recombination of small viral DNA molecules in the cytoplasm or the resolution of concatameric DNA prior to packaging of genomes into virus particles. Two proteins (VP44 and VP63) are present in the intermediate lipid layer and one protein (VP58) appears in the membrane of enveloped particles. The complete amino acid sequence (463 aa) of the MCP of Mr 49.8 10³ is known, as is the sequence of a DNA methyltransferase that shares 53.3% amino acid sequence identity with the homologous protein of LCDV-1.

Lipids

Non-enveloped particles contain 9% lipid. The composition of the lipid component differs from that of the host cell. Viruses released from cells may have a plasma membrane derived outer envelope, but this is not essential for virus infectivity. Infectivity is destroyed by treatment with ether or phospholipase A.

Genome Organization and Replication

FV-3 was the first virus in the family recognized as having a circularly permuted and terminally redundant genome. DNA replication and particle assembly occurs as described in the general description of family characteristics. FV-3 genes are expressed in infected cells in a stepwise fashion which is regulated by a sequence of trans-acting regulatory proteins that cause shutdown of host transcription, interact with host RNA polymerase II to initiate transcription of specific viral sequences and to enable host RNA polymerase to transcribe highly methylated promoters that are normally refractory to transcription. Translation of FV-3 mRNA is also a sequential and co-ordinated process. Late gene transcripts are selectively translated in the presence of immediate early and early messages. Virus induced factors are required for this to occur and 5 sequences of some late transcripts of immediate early genes are modified which may impair their translation. Messenger RNAs have dyad symmetry (hairpin structure) and appear to be rapidly degraded such that steady state levels of viral messages seen at late times postinfection require a continually high level of transcription.

Antigenic Properties

FV-3 is serologically and genetically distinct from members of other genera. Several piscine, reptilian and amphibian isolates also show serological and/or genetic relatedness to FV-3, but these have yet to be fully characterized.

Biological Properties

Transmission of members of the genus Ranavirus has been demonstrated by oral and intraperitoneal injection. The pathogenicity of the resulting infection depends on host species and life stage. FV-3 grows in a wide variety of cultured cells. Viruses reported as causing epizootic haematopoietic necrosis in fish (Epizootic haematopoietic necrosis virus, EHNV) have been reported from at least 13 genera of marine and anadromous fish. The DNA of EHNV is highly methylated at CpG sites. Bohle virus, a highly virulent pathogen from the borrowing frog Lymnodynastes ornatus in Australia, causes hepatic, renal and pulmonary necroses and can be transmitted to fish. Bohle virus differs from EHNV in terms of particle size, protein, antigenic and genomic characteristics. Other tentative Ranavirus species may cause hypertrophy of cells in organs and tissues such as the liver, vascular epithelium and lamia propria, although there is some evidence of ranaviruses that cause no overt symptoms of disease in certain hosts. These viruses are now tentatively assigned to the genus Ranavirus. EHNV should not be confused with Erythrocytic necrosis virus from fish and reptiles which is also probably an iridovirus but has yet to be isolated and characterized.

List of Species Demarcation Criteria in the Genus

Not applicable. Only one virus species is recognized. PCR primers have been designed to amplify a region of the MCP gene for tentative members of this genus (Mao, Hedrick and Chinchar, 1997).

List of Species in the Genus

Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names and assigned abbreviations ( ) are:

Species in the Genus

Frog virus 3	[U36913; U15575]	(FV-3)
(Box turtle virus 3 (TV3))
(Lucké triturus virus 1; CP4-4398B (276))
(Tadpole edema virus)
(Tortoise virus 5 (TV5))

Tentative Species in the Genus

Bohle iridovirus		(BIV)
Epizootic haematopoietic necrosis virus	[U82552, U82631, AJ007358]	(EHNV)
(Rainbow trout virus (RTV))
(Redfin perch virus (RFPV))
Redwood Park virus		(RPV)
(Tadpole virus 2)
(Stickleback virus (SBV))
Regina ranavirus	[AF080218]	(RRV)
(Tiger salamander virus)
(Ambystoma tigrinum stebbensi virus (ATV))
Santee-Cooper ranavirus	[AF080250]	(SCRV)
(Largemouth bass virus (LMBV))
(Doctor fish virus (DFV))
(Guppy virus 6 (GV6))