TC - Termination and Translational Control Flashcards
What occurs in Termination with reference to recycling factors (RFs/eRFs) ? (3)
Release of completed polypeptide when the stop codon is reached
- No tRNA corresponding to the stop/termination codon
Bring in a factor to release the completed polypeptide
Recycling (RRF in prokaryotes, ABCE1 in eukaryotes)
- Ribosomal subunits detach and are kept separated to allow new round of translation
Termination in bacteria (5)
- Elongation continues until a termination codon occupies A-site
- Ribosome fails to find cognate AA-tRNA
- The final aminoacyl-tRNA ester link is hydrolysed by reaction with H2O and the peptide is released from the ribosome
- The mRNA and the ribosomal subunits separate from each other
- RRF recycles the subunits
Termination in Eukaryotes (6)
- Elongation continues until a termination codon occupies A-site
- Ribosome fails to find cognate AA-tRNA
- Release factors eRF1 and eRF3 bind (eRF1 mimics tRNA)
- The final aminoacyl-tRNA ester link is hydrolysed by reaction with H2O and the peptide is released from the ribosome
- The mRNA and the ribosomal subunits separate from each other
- ABCE1 recycles the subunits
What is Nonsense-Mediated Decay (NMD)?
NMD is a surveillance mechanism to detect and destroy aberrant mRNAs containing Premature Termination Codons (PTCs) before translation
- Prevents the accumulation of truncated proteins
Exon Junction Complex (EJC) (3)?
- Deposited during splicing
- It is placed 20-24 nucleotides upstream of each splice site
- Contains various proteins, most important is UPF3
During export UPF2 binds to UPF3 in EJC
What is the SURF complex?
- SMG1 (kinase)
- UPF1 (RNA and ATP-dependent helicase)
- eRFs (translation termination factors)
Translation of normal mRNA vs Translation of mRNA containing PTC (nonsense mRNA)
Translation of normal mRNA
- First ribosome recruited
- EJC is swept off by first ribosome
Translation of mRNA containing PTC (nonsense mRNA)
- First ribosome recruited
- Ribosome does not reach EJC downstream of premature termination codon
- Recruitment of SURF complex
- SURF + EJC = DECID complex - causes aberrant termination
- Rapid degradation by multiple decay pathways
How does the SURF complex act to degrade mRNA (3)?
- UPF1 interacts with UPF2, bridging SURF complex to EJC
- SMG1 phosphorylates UPF1 and causes dissociation of eukaryotic Release Factors and ribosome
- Phosphorylated UPF1 recruits SMG7 and decay enzymes
What diseases are associated with the presence of PTCs? (5)
- Duchenne muscular dystrophy (DMD)
- Familial adenomatous polyposis
- Hereditary breast and ovarian cancer
- Polycystic kidney disease
- Cystic Fibrosis
Ribosomes can read through the PTC in the presence of Gentamicin
What are the three mRNA surveillance mechanisms, and when are they activated?
- NMD (Nonsense-Mediated Decay) for premature stop codons.
- NSD (Non-Stop Decay) for the absence of a stop codon.
- NGD (No Go Decay) for mutations or mRNA features that stall the ribosome.
What is the role of eIF2 in translational control, and how is it regulated? (3)
eIF2 binds to GTP and brings in first tRNA
- eIF2 can be phosphorylated at a specific serine residue (Serine 51) by various stress-responsive kinases, such as PERK, GCN2, HRI, and PKR
- Phosphorylation of eIF2 leads to the formation of an inactive ternary complex, preventing the initiation of translation and reducing overall protein synthesis
Activation of PKR in response to viral RNA (4)
The activation of PKR is triggered by the binding of dsRNA, a molecular pattern associated with viral replication
- Binding of dsRNA induces a conformational change in PKR, leading to its autophosphorylation at multiple sites
- Autophosphorylation activates PKR’s kinase domain, allowing it to phosphorylate its downstream substrates
- By phosphorylating eIF2α, PKR helps to suppress overall translation in infected cells, limiting the production of viral proteins
What controls the availability of eIF4E in translation initiation? (4)
- mTORC1 is active
- Phosphorylates 4E-BPs
- Causing their dissociation from eIF4E
- Enabling eIF4E to engage in translation initiation and interact with eIF4G
How are 16S and 18S rRNA used as phylogenetic markers?
Genetic analyses of 16S or 18S rRNA are used for the identification and classification of new species
rRNAs have conserved as well as hypervariable regions
- Hypervariable regions differ among organisms (provides taxonomic information about the organism)
