Genus: Gammaretrovirus

Genus: Gammaretrovirus

Distinguishing features

Viruses in the genus have simple genomes comprising just the canonical gag, pro, pol and env genes, with little or no evidence for additional regulatory genes. Instead of frameshifting, translation of the Gag-Pro-Pol polyprotein relies on readthrough of the Gag termination codon. In contrast to viruses in the Betaretrovirus and Lentivirus genera, the SU and TM subunits of Env are covalently linked by a disulfide bond that undergoes isomerization as part of the fusion and entry mechanism.

The mammalian gammaretroviruses include replication-competent viruses, which lack cell-derived oncogenes, and replication-defective viruses, which have acquired a variety of oncogenes from their hosts. Murine and feline transforming gammaretroviruses, which are invariably replication-defective, can be distinguished from members of the species Murine leukemia virus and from one another by the presence of a large variety of distinct cell-derived oncogenes (e.g. mos in isolates of Moloney murine sarcoma virus and sis in isolates of Woolly monkey sarcoma virus) and the characteristic loss of portions of gag, pol or env.

Collectively, gammaretroviruses have a very broad host range, and have been detected in, or isolated from, a wide variety of vertebrate hosts. Receptor use is correspondingly diverse, but with a very strong preference for proteins in the solute carrier (SLC) superfamily. As with alpharetroviruses, even closely related viruses may use distinct host cell receptors for viral entry (Greenwood et al., 2018, Overbaugh et al., 2001).

Virion

Virions exhibit a “C-type” morphology with barely visible surface spikes. They have a centrally located, condensed core. Capsid assembly occurs at the inner surface of the membrane at the same time as budding.

Physicochemical and physical properties

See discussion under family description.

Nucleic acid

See discussion under family description.

Proteins

Approximate protein sizes are: matrix protein (MA) 15 kDa; p12 12 kDa; capsid protein (CA) 30 kDa; nucleocapsid protein (NC) 10 kDa; protease (PR) 14 kDa; reverse transcriptase (RT) 80 kDa; integrase (IN) 46 kDa; surface envelope protein (SU) 70 kDa; and transmembrane envelope protein (TM) 15 kDa.

Genome organization and replication

The genome is about 8.3 kb (one monomer) (Figure 1.Gammaretrovirus). The pro-pol region is translated following ribosomal readthrough at the gag gene termination codon. Some gammaretroviruses additionally translate the gag region from an upstream initiation codon, yielding a larger, glycosylated form of Gag protein, glyco-Gag. There are no known additional genes. The tRNA primer is usually tRNAPro , although examples of tRNAGlu, tRNAGln and tRNAThr have been found among endogenous mouse viruses. The long terminal repeat (LTR) is about 600 nt, of which the U3 region is 500 nt, the R sequence is 60 nt and the U5 region is 75 nt.

 
Figure 1.Gammaretrovirus. Gammaretrovirus genome organization. The 8.3 kb provirus of Moloney murine leukemia virus (MMLV) is shown, with LTRs, protein-coding regions (gag, pro, pol and env) and transcripts (solid line arrows with protein names added) marked. The asterisk under a curved arrow indicates a ribosomal readthrough site.

Biology

The viruses are associated with a variety of diseases, including malignancies, immunosuppression, and neurological disorders, and many oncogene-containing members of the mammalian and reticuloendotheliosis virus groups have been isolated (Rosenberg and Jolicoeur 1997). Viruses resulting from recombination between exogenous and endogenous viruses are frequently encountered. Gammaretroviruses are widely distributed with many mammals having both exogenous viruses transmitted vertically and horizontally, as well as endogenous viruses. For example, koala retrovirus (KoRV) is found in both exogenous and endogenous forms and appears to be actively endogenizing in Australian koala populations. The reticuloendotheliosis viruses (REV) comprise a few isolates from birds, but share features with several mammalian retroviruses suggesting a possible inter-class transmission. Interestingly, REV proviruses have been found in field samples integrated into the genomes of avian DNA viruses, including Marek’s disease virus (Gallid alphaherpesvirus 2, Herpesviridae) and fowlpox virus (Avipoxvirus, Poxviridae).

Species demarcation criteria

Species demarcation criteria include differences in genome sequence and presence of viral oncogenes, differences in antigenic properties, differences in natural host range and differences in pathogenicity. Members of the species Murine leukemia virus can be distinguished from isolates of Gibbon ape leukemia virus, for example, by sequence divergence, and only show limited antigenic cross-reactivity in ELISA assays

Member species

Exemplar isolate of the species
SpeciesVirus nameIsolateAccession numberRefSeq numberAvailable sequenceVirus Abbrev.
Chick syncytial viruschick syncytial virusDQ237904NC_043196Partial genomeCSV
Feline leukemia virusfeline leukemia virusRickard subgroup AAF052723NC_001940Complete genomeFeLV
Finkel-Biskis-Jinkins murine sarcoma virusFinkel-Biskis-Jinkins murine sarcoma virusK02712NC_038858Complete genomeFBJMSV
Gardner-Arnstein feline sarcoma virusGardner-Arnstein feline sarcoma virusNo entry in GenbankGAFeSV
Gibbon ape leukemia virusgibbon ape leukemia virusM26927NC_001885Complete genomeGALV
Guinea pig type-C oncovirusguinea pig type-C oncovirusNo entry in GenbankGPCOV
Hardy-Zuckerman feline sarcoma virusHardy-Zuckerman feline sarcoma virusX03711NC_038923Partial genomeHZFeSV
Harvey murine sarcoma virusHarvey murine sarcoma virusJ02207NC_038668Partial genomeHaMSV
Kirsten murine sarcoma virusKirsten murine sarcoma virusX00984NC_043426Partial genomeKiMSV
Koala retroviruskoala retrovirusAF151794NC_039228Complete coding genomeKoRV
Moloney murine sarcoma virusMoloney murine sarcoma virusAF033813NC_001502Complete genomeMoMSV
Murine leukemia virusMoloney murine leukemia virusAF033811NC_001501Complete genomeMMLV
Porcine type-C oncovirusporcine type-C oncovirusAF356697NC_003059Complete genomePCOV
Reticuloendotheliosis virusreticuloendotheliosis virusHA9901AY842951NC_006934Complete genomeREV
Snyder-Theilen feline sarcoma virusSnyder-Theilen feline sarcoma virusM22820NC_043382Partial genomeSTFeSV
Trager duck spleen necrosis virustrager duck spleen necrosis virusNo entry in GenbankTDSNV
Viper retrovirusviper retrovirusNo entry in GenbankVRV
Woolly monkey sarcoma viruswoolly monkey sarcoma virusV01201NC_009424Complete genomeWMSV

Virus names, the choice of exemplar isolates and virus abbreviations are not official ICTV designations.