Translation 2 Flashcards
Where do IF-1 and IF-3 bind in prokaryotic initiation?
The small 30S subunit. IF-1 binds to the A site.
What does IF-3 binding do?
Prevents premature assembly of the 30S subunit with the 50S subunit.
How does mRNA bind to the 30S subunit?
By using the Shine Dalgarno sequence.
Where is the Shine Dalgarno sequence found?
8 bases upstream of AUG.
How is the first Met-charged tRNA brought into the ribsome?
Brought into the P site on the small subunit with IF2-GTP.
What does GTP hydrolysis drive during initiation?
A major conformational change that allows the large subunit and the small subunit to join. All initiation factors are then released.
What are the differences between eukaryotic and prokaryotic translation initiation?
- Eukaryotes require more initiation factors.
- Eukaryotes don’t have a Shine Dalgarno sequence.
How is the ribosome positioned in eukaryotic translation?
By interactions with both ends of the mRNA- the 5’cap and the poly(A) tail.
How is the second charged tRNA added?
It is brought into the A site by EF-TuGTP. GTP hydrolysis causes a conformational change, locking the tRNA in place and releases EF-TuGDP.
How is EF-TuGTP regenerated during translation elongation?
Regenerated by EF-Ts, a guanine exchange factor.
How is peptide bond formation catalysed during elongation?
Catalysed by 23S rRNA. Occurs via nucleophilic attack of the C=O group of the peptide in the P site, by the NH2 group of the amino acid in the A site.
How is the ribosome moved along to the next codon after peptide bond formation?
Requires EF-G and GTP hydrolysis. Moves the uncharged tRNA into the E site and the peptidyl-tRNA into the P site, leaving the A site available for the next charged tRNA.
Compare translation elongation in prokaryotes and eukaryotes.
Occurs via the same mechanism. Eukaryotes have analogous elongation factors; eEF1α, eEF1βγ and eEF2.
How is translation terminated?
A release factor binds to the A site and the ester bond between the C=O of the amino acid and the OH of the tRNA is hydrolysed. All of the ribosome subunits, uncharged tRNA and release factors dissociate.
Compare translation termination in prokaryotes and eukaryotes.
In prokaryotes there are different release factors to recognise different stop codons- RF1 for UAA, RF2 for UAA and UGA.
In eukaryotes there is only one release factor- eRF.
How can translation be inhibited?
By antibiotics that mimic translation factors, e.g. Tetracyclins block the A site.
What suggests that there are structural differences between the eukaryotic and prokaryotic ribosomes?
There are antibiotics that specifically inhibit only one. For example, chloramphenicol blocks peptidyl-transferase but cycloheximide only blocks the eukaryotic peptidyl-transferase.
How does EF-Tu improve translation accuracy?
As it binds the charged tRNA, it checks that the amino acid and the tRNA are correctly matched. Charged tRNAs are in a bent conformation when bound to EF-TuGTP:
- allows codon pairing, but not addition of the amino acid to the peptide
- GTP hydrolysis will only occur if the codon and anticodon are correctly matched.
What is the level of translation accuracy in a ribosome?
99.99% accuracy.
What happens to incorrectly matched tRNAs?
They dissociate quicker than correctly matched tRNAs due to weaker binding to the codon and are not used in protein synthesis.
Why can EF-G bind the A site of the ribosome?
The EF-G protein, also called translocase, can bind the A site of the ribosome because its structure resembles the EF-Tu/tRNA complex.
Why is the EF-G (translocase) protein needed for movement of the ribosome along the mRNA?
In binding the A site, EF-G protein displaces the peptidyl-tRNA bound there.
