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.
How is siRNA/RISC complex formed?
It is a duplex in its natural form - dicer cleaves dsRNA into shorter - referred to as siRNA. Then in cytoplasm sense strand is degraded and antisense strand forms a complex with RISC.
How are endogenous dsRNA handled?
Generated from larger transcripts made by RNA Pol II or III that can fold into themselves and form hairpin structures. These are chipped by dicer which form small sections of dsDNA - processed to ss then into RISC complex.
What are the proteins that control cellular iron? What are their individual roles?
Transferrin - carries Fe in blood.
Ferritin - stores excess iron in the liver and kidneys
What does the transferrin receptor do?
Binds transferrin and enables entry into the cells
How are transferrin and ferritin receptor levels related?
Low Fe: transferrin receptor increased and ferritin decreased
High Fe: transferrin receptor decreased and ferritin increased
How is Ferritin synthesized?
Ferritin mRNA has a stem loop called the IRE in the 5’ untranslated region which binds to the IRE binding protein.
What happens to ferritin when iron is low?
IRE binding protein binds to ferritin mRNA and block initiation of translation.
What happens to ferritin when iron is high?
IRE binding proteins binds to E so it cant binds to ferritin mRNA - Ferritin stores excess Fe
How is transferrin receptor synthesized?
Transferrin receptor mRNA has several IRE regions in the 3’ untranslated region that bind to IRE binding protein. mRNA can still be translated even when IRE protein is bound.
