mRNA Stability Flashcards
(19 cards)
Why does mRNA require stabilisation?
mRNA is inherently unstable and requres processing (capping, polyadenylation) to ensure stability for translation
What are the three main stages of mRNA processing?
1 - 5’ capping - Addition of 7-methyl guanine
2 - RNA Splicing - introns removed, exons joined
3 - 3’ Polyadenylation - Addition of 200 A residues at the 3’ end
What are the reoles of the 5’ and 3’ untranslated regions (UTRs)?
5’ UTR - Helps recruit ribosomes for translation
3’ UTR - Regulated mRNA stbality, translation, and localisation
How is mRNA stbalised?
Prokaryotic mRNA - Forms hairpin loops at the 3’ end to prevent degradation
Eukaryotic mRNA - Capped and polyadenylated, stbalising translation
What causes mRNA degradation?
Ribonucleases (enzymes that degrade RNA)
Endoribonucleases - Cut RNA internally
Exoribonucleases - Remove nucleotides
What initiates bacterial mRNA degradation?
Removal of pyrophosphate from the 5’ triphosphate cap → results in 5’ monophosphate, making mRNA a substrate for RNase E cleavage
What enzyme complex degrades bacterial mRNA?
The RNA degradosome, composed of:
- RNase E (endonuclease).
- Polynucleotide phosphorylase (PNPase) (3’→5’ exonuclease).
- RhlB helicase (unwinds structured RNA).
- Enolase (links metabolism to mRNA degradation).
How is the 5’ cap formed?
1 - RNA triphosphatase removes phosphate.
2 - Guanylyltransferase adds GTP via a 5’-5’ linkage.
3 - Methyltransferase methylates G at the 7th nitrogen (m7G).
How does capping influence mRNA stability?
- Protects mRNA from degradation by Pol II.
- Cap-binding proteins influence splicing, export, translation.
How is the 3’ poly-A tail formed?
1 - Cleavage at AAUAAA sequence by CPSF.
2 - Poly-A polymerase (PAP) adds 10 A residues.
3 - Poly-A binding protein (PBP) extends poly-A tail (~200 A’s).
4 - Upon nuclear export, Poly-A tail recruits translation initiation factor (eIF4G).
What triggers mRNA degradation?
1- Deadenylation (poly-A removal).
2 - Decapping (5’ cap removal).
What are the two major mRNA decay pathways?
1 - 5’ → 3’ degradation (Decapping followed by Xrn1 exonuclease digestion).
2 - 3’ → 5’ degradation (Exosome complex digestion).
What protein complex initiates decapping?
Dcp1-Dcp2, triggered by PABP loss after deadenylation.
What does the TRAMP complex do?
- Monitors the nucleus for faulty mRNA (unspliced, misfolded, lacking poly-A tails).
- Adds short oligo-A tails to flag mRNA for exosome degradation.
- Contains helicase to unwind RNA and remove stabilizing proteins.
What are the three major cytoplasmic RNA surveillance systems?
1 - Nonsense-Mediated Decay (NMD) – Detects premature stop codons (PTCs).
2 - Non-Stop Decay (NSD) – Targets mRNAs lacking stop codons.
3- No-Go Decay (NGD) – Resolves stalled ribosomes during translation.
What causes PTCs in mRNA?
1 - Genetic mutation introducing a premature stop codon.
2 - RNA polymerase error.
3 - Incorrect splicing event.
How does NMD recognize faulty mRNA?
- Exon junction complex (EJC) downstream of a stop codon triggers degradation.
- Yeast genes use extended 3’ UTR as a “warning signal”.
How is NMD linked to disease?
1/3 of inherited disorders are caused by PTC mutations, leading to dysfunctional or toxic truncated proteins.
Why is mRNA localization important?
Controls localized translation, critical for:
- Embryo axis formation.
- Cell migration & morphogenesis.
- Actin cytoskeleton dynamics.
