Protein Synthesis in prokaryotes Flashcards
What is peptidyl tRNA?
tRNA which has a peptide attached to it
What is a svedberg unit?
A non-metric unit for density, a measure of the size of the particle
Which direction is mRNA translated in?
5’ to 3’
Which amino acid is used in intiation of protein synthesis in prokaryotes?
N-formyl methionine
What does polycistronic mean?
mRNA that encodes several proteins, all sit on the same mRNA but all have their own start and stop codons
What is the ribosome binding site in E.coli called?
Shine dalgarno sequence
What is required for translation initation in prokaryotes?
Ribosome binding sequences
How far are ribosome binding sites from start codons?
3-11 bases upstream (from the 5’ end)
Which part of the ribosome recognises the shine dalgarno sequence?
The 16s ribosomal sequence
How does the ribosome bind to the shine dalgarno sequence?
Watson-Crick (complementary) base pairing
Which site on the ribosome is the AUG start codon placed in?
The P site
What is required for the tRNA to associate to the small ribosomal subunit?
3 Initiation factors (IF1, IF2, IF3) and GTP
What ensures the specific modified methionine tRNA is recruited to the mRNA in initiation?
The initiation factors
How does the large ribosomal subunit bind during initiation?
The hydrolysis of GTP into GDP and Pi produces a conformational change within the large subunit to make it bind
What is required for elongation?
The elongation factor EF-Tu
Which site does aminoacyl tRNA bind to on the ribosome?
The A site
Outline the steps of elongation
- Aminoacyl tRNA binds to empty A site
- Proofreading and EF-Tu dissociates
- Growing polypeptide chain is tranferred from peptidyl tRNA to aminoacyl tRNA
- Large subunit translocates
- Small subunit translocates
What is being proofread in elongtion?
If base pairing between tRNA and mRNA is correct (therefore if the correct tRNA is bound)
What is the primary function of EF-Tu?
To transport the correct aa-tRNA to the A site of the ribosome
How is the peptide chain transferred from the tRNA in the p site to the tRNA in the A site?
- Nucleophilic attack of bond between peptide and tRNA breaks it
- Growing polypeptide chain transfers it to the amino group of the incoming aminocyl tRNA
Does the transfer of the polypeptide require ATP? Why?
No as ATP has already been consumed by synthetase to attach the amino acid to the tRNA
How does the ribosome move to free up the A site?
- The large subunit translocates 3 nucleotides forward and is stabilised by elongation factor G
- The small subunit then translocates
Why does the large subunit have to be stabilised by elongation factor G when translocating?
The movement of the ribosome is energetically unfavourable
What is used to stabilise the movement of the large ribosomal subunit?
Elongation factor G
When is GTP used in translation?
- At initiation
- In proofreading during elongation
- In the movement of the ribosome during elongation
What happens when the small ribosomal subunit translocates?
- Full E, P, A sites are reformed
- Elongation factor hydrolyses GTP to provide energy
- EF-G is released
How does termination of translation occur in prokaryotes?
- Stop codon is put in the A site
- Release factor binds to stop codon
- This causes conformational change in ribosome causing ribosome to come apart
What is release factor?
A protein which mimicks tRNA
What is the effect of release factor binding?
- causes large and small ribosomal subunits to come apart
- mRNA is released
- Causes cleavage of final aminoacyl bond between tRNA and polypeptide
In prokaryotes are transcription and translation coupled?
Yes
