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19.3.5: Retroviruses

The genome of retroviruses consists of RNA not DNA. HIV-1 and HIV-2, the agents that cause AIDS, are retroviruses.

In February 1997 it was reported that pig cells contain a retrovirus capable of infecting human cells (at least, in vitro). This is troublesome because of the efforts that are being made to transplant pig tissue into humans (e.g., fetal pig cells into the brains of patients with Parkinson's disease). Transplant recipients must have their immune systems suppressed if the transplant is to avoid rejection. Could immunosuppressed patients be at risk from the retroviruses present in the transplanted cells? The probability that the original hosts for HIV-1 and HIV-2 were some other primate suggests that retroviruses can move from one species to another.

Fig. 19.3.5.1 Retrovirus

A typical, "minimal" retrovirus consists of:

  • an outer envelope which was derived from the plasma membrane of its host
  • many copies of an envelope protein embedded in the lipid bilayer of its envelope
  • a capsid; a protein shell containing
  • two molecules of RNA
  • molecules of the enzyme reverse transcriptase

Reverse transcriptase is a DNA polymerase that uses RNA as its template. Thus it is able to make genetic information flow in the reverse (RNA ->DNA) of its normal direction
(DNA -> RNA).

Infection of a host cell requires that the cell have a surface protein that can serve as a receptor for the envelope protein of the retrovirus. The envelope protein of HIV-1 binds to

  • CD4 molecules. It is this property that enables the virus to invade CD4+ T cells (and certain other cells that express CD4).
  • CCR5 (CC chemokine Receptor 5) - found on Th1 cells and macrophages.

All the proteins in the virus particle are encoded by its own genes.

Fig. 19.3.5.2 Retrovirus lifecycle

When a retrovirus infects a cell

  • Its molecules of reverse transcriptase are carried into the cell attached to the viral RNA molecules.
  • The reverse transcriptase synthesizes DNA copies of the RNA.
  • These enter the nucleus and are inserted into the DNA of the host.
  • These inserts are transcribed by the host's enzymes into fresh RNA molecules which re-enter the cytosol where
    • some are translated by host ribosomes
      • the gag gene is translated into molecules of the capsid protein
      • the pol gene is transcribed into molecules of reverse transcriptase
      • the env gene is translated into molecules of the envelope protein
    • other RNA molecules become incorporated into fresh virus particles

The genome of retroviruses


is flanked at each end by repeated sequences ("R") that enable the DNA copy of the genome to be inserted into the DNA of the host and act as enhancers, causing the host nucleus to transcribe the DNA copies of the retroviral genome at a rapid rate.

The retroviral genome also contains a packaging signal sequence ("P") which is needed for the newly-synthesized RNA molecules to be incorporated in fresh virus particles. Most retroviruses also contain one or more additional genes. Some of these represent RNA copies of genes that earlier were picked up from their eukaryotic host. Several cancers in animals are caused by retroviruses that have, at some earlier time, picked up a proto-oncogene from their mammalian host and converted it into an oncogene.

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