Rous sarcoma virus

Rous sarcoma virus (RSV) is a retrovirus and is the first oncovirus to have been described: it causes sarcoma in chickens.

As with all retroviruses, it reverse transcribes its RNA genome into cDNA before integration into the host DNA.

History
RSV was discovered in 1911 by Peyton Rous, working at Rockefeller University in New York City, by injecting cell free extract of chicken tumour into healthy Plymouth Rock chickens. The extract was found to induce oncogenesis. The tumour was found to be composed of connective tissue (a sarcoma).

Rous was awarded the Nobel Prize for the significance of his discovery in 1966.

Structure and genome
RSV is a class VI enveloped virus with a positive sense RNA genome having a DNA intermediate.

RSV has three genes:
 * gag - encodes capsid proteins
 * pol - encodes reverse transcriptase

(It lost the env gene normally seen in avian retroviruses during the recombination events that allowed it to acquire src)


 * src - encodes a tyrosine kinase that attaches phosphate groups to the amino acid tyrosine in host cell proteins.

The RSV genome has terminal repeats enabling its integration into the host genome and also overexpression of RSV genes.

src gene
The src gene is oncogenic as it triggers uncontrolled growth in abnormal host cells. It is an acquired gene, found to be present throughout the animal kingdom with high levels of conservation between species.

The src gene was taken up by RSV and incorporated into its genome conferring it with the advantage of being able to stimulate uncontrolled mitosis of host cells, providing abundant cells for fresh infection.

The src gene is not essential for RSV proliferation but it greatly increases virulence when present.

RNA secondary structure
The RNA genome of RSV contains an extremely long 3' UTR that ranges between 5–7 kb in length which would usually direct it toward nonsense mediated decay (NMD) within the eukaryotic host cell. A conserved secondary structure element has been identified within the 3'UTR and is known as the Rous Sarcoma Virus Stability Element (RSE). This element has been shown to prevent the degradation of the unspliced viral RNA

The RSE element was first identified in the genome of the Rous Sarcoma Virus but appears to be widely conserved across the avian retrovirus family. The RSE element is ~300 bp in length and located downstream of the gag natural translational termination codon. The secondary structure of the RSE element has been determined by RNAse digestion and SHAPE chemistry analysis.

Other elements that have been identified in RSV include a primer binding site.