Defective and Helper Viruses

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Defective and Helper Viruses

Defective and Helper Viruses - Viruses in which a part of the genome is missing are called defective viruses.
Most sarcoma viruses, including the murine, feline and simian sarcoma viruses and some strains (e. g. the Bryan strain) of the rous sarcoma virus (RSV) are defective.
The Bryan strain of RSV appears to lack envelope glycoproteins and does not form infectious progeny.
If cells infected with RSV are super-infected with a leukemia virus, the RSV becomes infectious.
The leukemia virus codes for envelope glyco proteins and is known as a helper virus.

The RSV viruses bearing leukemia virus envelopes are known as pseudo types. The helper virus genetically complements the RSV.
One Bryan strain of RSV is defective for both envelope antigens and reverse transcriptase. A helper virus complements both these defects
Sometimes the helper virus associated with the defective virus may be replaced by another helper.
This causes a change in the serological classification of the virus complex, since some or all of the antigens of the virus particles are provided by the helper virus.

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