This document summarizes different cytoplasmic mRNA surveillance systems that ensure mRNA fidelity: 1) Nonsense-mediated decay degrades mRNAs containing premature termination codons to prevent defective proteins from being synthesized. 2) Nonstop decay degrades mRNAs lacking termination codons to prevent polylysine tails from stalling ribosomes. 3) No-go decay degrades mRNAs when ribosomes stall due to secondary structures, ensuring only full-length proteins are produced.

1. CYTOPLASMIC
SURVEILLANCE SYSTEM
CENTRAL UNIVERSITY OF KASHMIR
DEPARTMENT OF BIOTECHNOLOGY
PRESENTED BY: SHAMS UN NAZIR
1716CUKmr029

2. INTRODUCTION
 Simply Regulation of mRNA
 Mechanism to ensure mRNA fidelity
 Defective transcripts are degraded
NORMAL ABNORMAL
PROTEINS
DECAY

3. mRNA DECAY
NONSTOP
DECAY

4. EXON
SPLICING
AUG EXON EXON EXON
EJC EJC
UAG AAAAAAA
EJC
EXONUCLEASES
EXON
AUG EXON EXON EXON
EJC EJC
UAG AAAAAAA
PTC
TERMINATION
CODON
EJC

5. NONSENSE MEDIATED DECAY (NMD)
 Found in all eukaryotes
 Decay as a result of nonsense mutation
 mRNA having PTC (premature termination codon) are degraded
Image source: Google

6. EXON
AUG EXON EXON EXON
EJC EJC
UAG AAAAAAA
PTC
TERMINATION
CODON
PABP
CNOT
PAN1/2

7. NONSTOP DECAY
 Occurs due to lack of an in-frame termination codon
 Formation of poly lysine (AAA) tail and hence stalling of Ribosome
 Occurs due to Premature termination of Transcription
EXON
AUG EXON EXON EXON AAAAAAA
L
L
L
SKI7
L
L
L
EXOSOME

8. NO-GO DECAY
 Occurs due to stalled ribosomes
 Stalling is due to secondary structure of mRNA
EXON
AUG EXON EXON EXON UAG AAAAAAA
RIBOSOME
DOM31
Hbs1

9. References
 Cell and Molecular Biology: Concepts and Experiments. Karp. John Wiley& Sons
 The Cell: A Molecular Approach. Cooper, G.M. and Hausman
 Yahoo.com