Gammaretroviruses

Gammaretroviruses are a genus of viruses within the Retroviridae family, subfamily Orthoretrovirinae. They infect a range of vertebrates and include notable pathogens such as murine leukemia viruses (MLVs) and feline leukemia virus (FeLV). Taxonomically, gammaretroviruses are often grouped by host range and receptor usage into ecotropic, xenotropic, polytropic, and amphotropic categories.

The genome is a positive-sense, single-stranded RNA about 8 to 10 kilobases in length, enclosed by a lipid envelope and flanked by long terminal repeats (LTRs) at both ends. The canonical genome organization is 5'-LTR-gag-pol-env-3', with gag encoding the matrix, capsid, and nucleocapsid proteins; pol encoding enzymes such as reverse transcriptase, RNase H, and integrase; and env encoding the surface and transmembrane glycoproteins that mediate entry. Some gammaretroviruses also express accessory proteins from alternative start sites, such as glyco-Gag, which can influence replication and interactions with the host immune system.

Replication follows the standard retroviral scheme. The virion binds a specific cell receptor, fuses with the membrane, and releases RNA and enzymes. Reverse transcription converts the RNA genome into double-stranded DNA, which integrates into the host genome via integrase. The provirus is transcribed by host RNA polymerase II to produce viral mRNAs that are translated into Gag, Pol, and Env proteins. New virions assemble at the cell membrane, bud from the cell, and mature after proteolytic processing.

Host range varies by virus; MLVs can cause leukemia and immunodeficiency in mice, FeLV causes immunosuppression and lymphoma in cats, and many gammaretroviruses exist as endogenous elements in host genomes. Gammaretroviruses are also used as vectors in gene delivery research, with safety considerations related to insertional mutagenesis.