mRNA Surveillance & RNA interference Flashcards
What are the pathways to eliminate faulty mRNA?
- non-stop decay
- nonsense-mediated decay
What is the non-stop mRNA decay pathway?
- if mRNA is missing a stop codon, translation continutes through the poly(A) sequence
- this makes a poly lysine tail (AAA = lys)
- Ski7 protein recruited
- Ski dissociates ribosome and recruits exosome to degrade mRNA 3’-5’
- polypeptide degraded by a protease that recognizes poly(Lysine)
Nonsense-mediated mRNA decay (NMD) is a — mechanism that eliminates mRNAs containing premature translation-termination codons (PTCs…nonsense codons UAA, UAG or UGA).
translation-coupled
NMD was first discovered through studies on the genetic basis of
B-thalassemia
How does B-thalassemia disease vs no disease occur in the gene?
Disease: premature termination codon in last exon (mRNA not degraded)
No: PTC in other exons (mRNA degraded)
This is in the beta gene of hemoglobin
NMD requires a protein complex known as the
Where is this protein placed?
exon junction complex (EJC)
20-24 nt upstream of every single exon-exon junction after splicing
What is the exon junction complex made of?
many proteins: UPF1, 2, 3 (up-frameshift protein
What are the steps of the NMD pathway?
- EJCs placed on every junction
- ribosome displaces EJCs from each exon:exon junction if normal mRNA
- mRNA translated by ribosome
- if premature termination codon is more than **50-55 nts **away from the exon:exon junction, NMD occurs
- ribosome will stop at PTC
- UPF1 and UPF2 recruited by downstream EJCs
- UPF1 and UPF2 recruits protein that removes 5’ cap
- 5’ end susceptible to 5’-3’ exoribonuclease degradation
short RNAs are called — or —
miRNA (microRNAs)
siRNA (short interfering RNAs)
What are the two sources of RNA for interfering RNA? (RNAi)?
- extracellular - dsRNA viruses
- intracellular - miRNA genes
canonical miRNA biogenesis pathway?
- miRNA gene transcripbed with RNAPII
- pri-miRNA forms stem-loop structure (RNA-RNA bp)
- Drosha enzyme cleaves to make pre-miRNA
- Exportin-5 exports pre-miRNA to the cytoplasm from nucleus
- pre-mRNA processed by Dicer enzyme to remove loop and just have miRNA duplex
- AGO (slicer) protein associates with duplex which is the pre-RISC complex
- One strand (passenger strand) is lost
- RISC complex formed
pri-miRNA
What is the RISC complex?
What does it do and what does it include
RNA-Induced Silencing Complex ie guide RNA
- interferes with gene expression (mRNA degradation or translational suppression)
- AGO (slicer) protein + 1 mature, guide miRNA strand that complementary bps with the target
- complementarity of RNA to target determines functionality
What is the non-canonical pathway to generate miRNA?
introns
- pre-miRNA embedden in intron
- spliceosome releases lariat
- this is the pre microRNA
- continute to be exported
- will usually interfere with their own gene because they have the complementarity
intron generated
What are the sections of the miRNA when bound to its target?
- region at 5’ end of miRNA called the seed region that binds to target site
- bulge
- 3’ end has varied complementarity
more miRNAs bind in the — of target mRNA
3’ UTR (between ORF termination codon sequence and polyA tail)
