Retroviruses

Transposition is widely used by a variety of pathogens to hijack the host cell’s genome. 

One class of deadly pathogens that uses this process is called a retrovirus. Outside the host cell, a retrovirus exists as a retrovirus exists as a lipid enveloped, core protein shell or capsid. 

The capsid contains viral proteins and enzymes and a dimeric RNA genome that encodes three major protein types. 

The first are group-specific antigen or “gag” proteins that form the core structure of the viral particle. The second set code for envelope proteins that recognize specific host cell surface receptors and enable binding. Finally, it includes genes encoding Pol proteins - including a reverse transcriptase enzyme, an integrase, and RNase H.

When infecting a cell, the retrovirus fuses its lipid envelope with the host cell membrane. Once inside the cell, the protein capsid is lost, and the viral reverse transcriptase transcribes the viral genomic RNA into a single-stranded DNA that binds to the viral RNA as a DNA/RNA duplex. 

Next, RNase H degrades the RNA template, and the reverse transcriptase synthesizes the complementary DNA strand - creating a double-stranded DNA copy of the viral genome known as proviral DNA. Next, the viral enzyme integrase cleaves the host DNA and attaches the proviral DNA into the host genome.

Retroviruses can be classified as endogenous, or exogenous. Endogenous retroviruses are non-pathogenic - remaining in the cell as a harmless transposable element. The human genome contains between 100–1,000 copies of such viruses. 

The second group, Exogenous retroviruses or “exoviruses,” are pathogens. They enter a cell and take advantage of the host cell’s replication and translation machinery to create more copies of the virus and produce the virus-encoded proteins. Well-known examples of these include the AIDS virus, T-cell leukemia, and Hepatitis B.