Retroviral

A Retrovirus is a virus which has a genome consisting of two RNA molecules, which may or may not be identical. It relies on the enzyme reverse transcriptase to perform the reverse transcription of its genome from RNA into DNA, which can then be integrated into the host's genome with an integrase enzyme. The virus itself stores its nucleic acid genome and serves as a means of delivery of that genome into targeted cells, which constitute the infection. Once in the host's cell, the RNA strands undergo reverse transcription in the cytosol and are integrated into the host's genome, at which point the retroviral DNA is referred to as a provirus.

When retroviruses have integrated their genome into the germ line, their genome is passed on to a following generation. These endogenous retroviruses (vs. exogenous) now make up 8% of the human genome. Most insertions have no known function and are often referred to as "junk DNA". However, many endogenous retroviruses play important roles in host biology, such as control of gene transcription, cell fusion during placental development, and resistance to exogenous retroviral infection. Endogenous retroviruses have also received special attention in the research of immunology-related pathologies, i.e. autoimmune diseases such as multiple sclerosis, although endogenous retroviruses have not yet been proven to play any causal role in this class of disease. The role of endogenous retroviruses in human gene evolution is explored in a recent peer-reviewed article: seeabstract.

While transcription was classically thought to only occur from DNA to RNA, reverse transcriptase transcribes RNA into DNA. The term "retro" in retrovirus refers to this reversal of the central dogma of molecular biology. Reverse transcriptase activity outside of retroviruses has been found in almost all eukaryotes, enabling the generation and insertion of new copies of retrotransposons into the host genome.

Because reverse transcription lacks the usual proofreading of DNA transcription, this kind of virus mutates very often. This enables the virus to grow resistant to antiviral pharmaceuticals quickly, and impedes, for example, the development of an effective vaccine against HIV.

Retrovirus genomes commonly contain these three genes, among others, that encode for proteins that can be found in the mature virus:

• gag (group-specific antigen) codes for core and structural proteins of the virus;
• pol (polymerase) codes for reverse transcriptase, protease and integrase; and,
• env (envelope) codes for the retroviral coat proteins.

Thus far, four human retroviruses (HTLV 1&2, HIV 1&2) have been found to attack Helper T cells.

Studies of retroviruses led to the first demonstrated synthesis of DNA from RNA templates, a fundamental mode for transferring genetic material that occurs in both eukaryotes and prokaryotes. It has been speculated that the RNA to DNA transcription processes used by retroviruses may have first caused DNA to be used as genetic material. In this model, cellular organisms adopted the more chemically stable DNA when retroviruses evolved to create DNA from the RNA templates.

The following genera are included here:

• Genus Alpharetrovirus; type species: Avian leucosis virus
• Genus Betaretrovirus; type species: Mouse mammary tumour virus
• Genus Gammaretrovirus; type species: Murine leukeamia virus; others include Feline leukemia virus
• Genus Deltaretrovirus; type species: Bovine leukaemia virus; others include Human T-lymphotropic virus
• Genus Epsilonretrovirus; type species: Walleye dermal sarcoma virus
• Genus Lentivirus; type species: Human immunodeficiency virus 1; others include Simian, Feline immunodeficiency viruses
• Genus Spumavirus; type species: Chimpanzee foamy virus

These were previously divided into three subfamilies (Oncovirinae, Lentivirinae, and Spumavirinae), but with our current knowledge of retroviruses, this is no longer appropriate.

See also

• sarcoma and leukaemia
• HIV
• gene therapy

External links

• Retroviruses
• NCBI retrovirus book online