Retrotransposons & Retrovirus. CA García Sepúlveda MD PhD. Laboratorio de Genómica Viral y Humana Facultad de Medicina, Universidad Autónoma de San Luis Potosí. Session #27 Retroposons Introduction. Retroposon or Retro-transposons are transposons that mobilize via an RNA form.
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CA García Sepúlveda MD PhD
Laboratorio de Genómica Viral y HumanaFacultad de Medicina, Universidad Autónoma de San Luis Potosí
The virus then replicates as part of the cell's DNA, which means:
They act as immunosupresors protecting the embryo from its mother's immune system.
Viral fusion proteins involved in the formation of placental syncytium limits cell migration (something an epithelium will not do well, as certain blood cells are able to diapedize).
Integration mediated by integrase.
Can occur at one or more sites.
Proviral DNA is transcribed by the host machinery to produce viral RNAs:
- genomic RNA.
The enzymes reverse transcriptase and integrase are carried with the genome in the viral particle.
Two copies of the RNA genome are packaged into each virion (diploid virus).
When a cell is infected by two different viruses,
it is possible to generate heterozygous virus particles carrying one genome of each type.
A typical retroviral sequence contains 3 or 4 "genes".
Coding regions give rise to multiple proteins by processing reactions.
Retroviral mRNA has a conventional structure; it is capped at the 5′ end and polyadenylated at the 3′ end.
Retroviral mRNAis represented by two subgenomic species.
1. Gag-Pol-Env (Full length)
The Gag product is translated by reading from the initiation codon to the first termination codon.
Gag derived products p6, p7, p17 & p24 are generated by post-translational processing.
Gag derived products have structural functions.
To translate the Pol product, the first termination codon must be bypassed.
Depends on the “reading frame correspondence” between Gag and Pol.
If Gag and Pol are in the same reading frame...
Termination codon usage is suppressed by a glutamyl-tRNA, generating a Gag-Pol peptide.
Gag-Pol peptide is then processed to yield:
1- Gag derived products (p6, p7, p17 & p24).
2- Pol derived products (p10, p32, p51, p66).
Pol derived products have enzymatic functions.
When Gag & Pol are in different reading frames, a ribosomal frameshift occurs to generate a single protein.
Readthrough is ~5% efficient, so Gag protein outnumbers Gag-Pol protein by 20x.
The Env polyprotein is expressed by post-transcriptional splicing generating a shorter subgenomic messenger that is translated into the Env product.
Env products gp120 & gp41 are envelope proteins.