20 Questions
What does the GAG gene encode in retroviruses?
core protein
Which gene is responsible for encoding reverse transcriptase in retroviruses?
POL
Which part of the retroviral genome functions as promoter or enhancer sequences?
LTRs
In retroviruses capable of causing malignant transformation, which additional gene is found?
SRC
What do the envelope glycoproteins in retroviruses do?
Facilitate entry into host cells
What was discovered about the SRC oncogene in the Rous sarcoma virus?
It was picked up from the host genome.
Which process describes how a retrovirus integrates a host gene into its own genome?
Transduction
What role does reverse transcriptase play in the life cycle of a retrovirus?
It copies the viral RNA genome into duplex DNA.
Following reinfection, how does a virus containing a transduced oncogene affect host cells?
It transforms host cells, potentially causing malignancy.
What is generally true about viruses that have kidnapped a host gene through recombination?
They are often defective in replication.
What happens to host genes when transduced by the retrovirus?
They are activated.
What is one result of a gene being altered by retrovirus transduction?
It produces a protein with abnormal activity.
What brings a transduced gene under the control of a viral promoter?
Viral promoters
What does the viral promoter lead to in terms of gene expression?
Aberrant, high levels of expression
What drives the high levels of gene expression when a gene is under the control of a viral promoter?
Retroviral transcriptional enhancers
What process causes the activation of proto-oncogenes by retroviruses?
Insertional mutagenesis
Which gene is demonstrated to be activated in breast cancer in mice infected with the mouse mammary tumor virus?
INT1
What role do the LTRs play in insertional mutagenesis?
Stimulation of gene expression via promoter action
Where in the cellular genome does the insertion of a DNA copy of the retrovirus occur to activate proto-oncogenes?
Close to a proto-oncogene
Retroviruses can activate proto-oncogenes by stimulating gene expression through which type of action?
Promoter action
Study Notes
Retrovirus Structure and Genes
- Retroviruses are RNA viruses that encode three genes: GAG, POL, and ENV, which produce a core protein, a reverse transcriptase, and envelope glycoproteins, respectively.
- Long terminal repeats (LTRs) are found at either end of the viral genome, functioning as promoter or enhancer sequences.
- Some retroviruses capable of causing malignant transformation have a fourth gene, SRC, which is an oncogene.
How Retroviruses Cause Cancer
- The Rous sarcoma virus, which causes malignancy in chickens, was found to carry the SRC oncogene, which was picked up from the host genome through transduction.
- Retroviruses use reverse transcriptase to copy their RNA genome into duplex DNA, which can integrate reversibly into the host genome.
- Through recombination, retroviruses can "kidnap" a host gene, making it part of the viral genome and expressing it under viral promoter control.
- The resulting virus is often defective in replication but can transform host cells following reinfection.
- Transduced host genes can be activated due to alteration, resulting in abnormal protein activity, or because they are brought under viral promoter control, leading to aberrant expression.
Retroviruses and Proto-Oncogenes
- Retroviruses can activate proto-oncogenes through insertional mutagenesis, where the insertion of a DNA copy of the retrovirus into the cellular genome near a proto-oncogene causes abnormal activation.
- This is demonstrated by the activation of the INT1 gene in breast cancer in mice infected with the mouse mammary tumor virus, due to the promoter action of the LTRs.
Learn about the genes and proteins encoded by retroviruses, including GAG, POL, and ENV, and their roles in viral replication and transformation.
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