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

[Temporary table - until VMR updated]

Exemplar isolate of the species

Species Virus name Isolate Accession number RefSeq number Available sequence Virus abbrev.
Chick syncytial virus chick syncytial virus DQ237904 NC_043196 Partial genome CSV
Feline leukemia virus feline leukemia virus Rickard subgroup A AF052723 NC_001940 Complete genome FeLV
Finkel-Biskis-Jinkins murine sarcoma virus Finkel-Biskis-Jinkins murine sarcoma virus K02712 NC_038858 Complete genome FBJMSV
Gardner-Arnstein feline sarcoma virus Gardner-Arnstein feline sarcoma virus No entry in Genbank GAFeSV
Gibbon ape leukemia virus gibbon ape leukemia virus M26927 NC_001885 Complete genome GALV
Guinea pig type-C oncovirus guinea pig type-C oncovirus No entry in Genbank GPCOV
Hardy-Zuckerman feline sarcoma virus Hardy-Zuckerman feline sarcoma virus X03711 NC_038923 Partial genome HZFeSV
Harvey murine sarcoma virus Harvey murine sarcoma virus J02207 NC_038668 Partial genome HaMSV
Kirsten murine sarcoma virus Kirsten murine sarcoma virus X00984 NC_043426 Partial genome KiMSV
Koala retrovirus koala retrovirus AF151794 NC_039228 Complete coding genome KoRV
Moloney murine sarcoma virus Moloney murine sarcoma virus AF033813 NC_001502 Complete genome MoMSV
Murine leukemia virus Moloney murine leukemia virus AF033811 NC_001501 Complete genome MMLV
Porcine type-C oncovirus porcine type-C oncovirus AF356697 NC_003059 Complete genome PCOV
Reticuloendotheliosis virus reticuloendotheliosis virus HA9901 AY842951 NC_006934 Complete genome REV
Snyder-Theilen feline sarcoma virus Snyder-Theilen feline sarcoma virus M22820 NC_043382 Partial genome STFeSV
Trager duck spleen necrosis virus trager duck spleen necrosis virus No entry in Genbank TDSNV
Viper retrovirus viper retrovirus No entry in Genbank VRV
Woolly monkey sarcoma virus woolly monkey sarcoma virus V01201 NC_009424 Complete genome WMSV
Virus names, the choice of exemplar isolates and virus abbreviations are not official ICTV designations.