Translational Regulation Flashcards
What is the function of the IRES?
It is an internal ribosome entry site on the 5’ side of the start codon which interacts with the 40S ribosomal subunit for the initiation factor EIF4F to start transcription without the 5’ cap.
Why is the IREs beneficial?
We can still allow translation to proceed even if 5’ cap is damaged e.g. by viral infection.
How does the IRES work?
Releases a protease that can cut up one of the initiation factors on the host and uses this to begin protein synthesis.
How can IRES allow for co-expression of several genes under the control of the same promoter?
Insert a reporter sequence for a detectable protein which is expressed when the gene of interest is expressed (place it under the same promoter). The reporter gene will use IRES for translation and the gene of interest will translate as usual using 5’ cap.
What are the types of mRNA degradation for defective mRNA?
- Nonsense-mediated mRNA decay
- Non-stop mRNA decay
- No-go mRNA decay
What is a nonsense mutation in mRNA?
Gives rise to premature stop codons in the transcript and gives a truncated protein.
How can nonsense mutations be degraded?
They don’t have introns and so protein complexes will be laid down to mark the exon boundaries. The ribosomal complex removes all exon junctions as it is being translated. The transcript of a nonsense mutation has not had time to remove the junctions as it is truncated and so EJCs recruit proteins that cleave the 5’ cap and it is degraded by RNases.
What is non-stop mRNA?
Transcript has no stop codon. Ribosomal complex will translate the poly A’ tail - AAAAAA encodes for lysine so there will be a lot of lysine at the C terminal of protein.
How does degradation of non-stop mRNA products occur?
The defective polypeptide is degraded by protease that recognizes the C-terminal poly(lysine) tag.
What is no-go mRNA?
Ribosome stalls before the stop codon is reached likely due to rare codons or a secondary structure - the tRNA in the cell cant recognise the complex codon or unravel secondary structure.
How is no-go mRNA degraded?
Proteins dissociate ribosome releasing from defective mRNA.
______ RNAs regulate mRNA stability.
Small
What is RNA interference?
Process of mRNA degradation induced by double-stranded RNA.
What is exogenous dsRNA and how is it handled?
Comes from external environment e.g. viruses, experimentally. Cleaved by an RNAse called dicer into 21nt long fragments - one of the strands is cleaves and degraded.
What is the role of small interfering RNA (siRNA) in control of exogenous dsRNA.
Single stranded cleavage product that bind Argonaute proteins to form RISC (complex). This binds to the complementary strand of target mRNA, which stops translation machinery or recruits enzymes to cleave mRNA and degrade it.
