BIOL213 Lecture Notes - Lecture 13: Aminoacyl-Trna, Prokaryotic Translation, Start Codon
Lecture 13 – Mechanisms of Translation
Recall:
• translation begins at the N-terminus of the polypeptide (usually at AUG) and adds
amino acids to the growing chain unit the C-terminus is reached.
• The mRNA is read in the 5’ to 3’ direction.
• Ribosoms supervise the interactions between the tRNA and mRNA and carry out
polypeptide synthesis
• Translation occurs in three stages - ignition, elongation and termination.
Initiation in prokaryotes
• Initiation requires initiation factors, ribosomal subunits, mRNA and initiation
tRNA ( a charged tRNA).
• Three initiation factors (IF1, IF2 and IF3) bind to the small ribosomal subunit.
GTP is bound to IF2 and drives these processes.
• mRNA and tRNA carrying the first amino acid bind to the small subunit to form
the 30s complex.
• IF1 binds at the aminoacyl site.
• IF3 binds at the exit site
• The start codon is positioned at the peptidyl site.
• The initiation tRNA binds to the peptidyl site.
• Steps of initiation:
1. Translation in initiated at the position of the start codon. At the start codon, the
Shine-Delgarno sequence is also found —> this allows for correct binding of the
mRNA to the small ribosome subunit. The binding of the mRNA to the small
subunit places the start codon at the ribosome P site.
2. A special methionine (tRNA^fMet) is used to initiate translation
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Lecture 13 – Mechanisms of Translation
3. The 70s initiation complex is formed. IF2-GTP is hydrolysed and the 50S subunit
binds to the 30S complex to form the 70S initiation complex. All three IFs leave,
and the initiation complex is not ready for chain elongation.
Initiation in eukaryotes:
• Very similar mechanisms to prokaryotic translation, but:
-the start codon in eukaryotes and archaea specifies methionine rather than fmet
-ribosomes have larger subunits and more proteins
-initiation requires more proteins factors
-mRNAs are monocistronic, therefore do not require an SD sequence —> the
mRNA is aligned on the ribosome.
• In eukaryotic initiation, after binding the mRNA the small ribosomal subunit
(including the initiation tRNA) scans along the transcript and begins translation at
the first AUG.
• After the initiation, tRNA is base-paired with the start codon, the large subunit
joins the complex, facilitated by GTP hydrolysis.
Elongation in prokaryotes:
• Chain elongation involves cycles of aminoacyl tRNA binding, peptide bond
formation and translocation.
• Steps of elongation:
1. Second aminoacyl tRNA binds at the aminoacyl site of the ribosome.
2. Peptide bond forms: tRNA at P site becomes uncharged and slides to the E site.
3. Translocation: With the assistance of EF-G (translocase) and energy from GTP
hydrolysis, ribosomes moved to the next codon, then empty the A site read for the
next tRNA-AA.
The first uncharged tRNA is now in the E site. This step is repeated until the stop
codon carries.
Elongation in Euaryotes:
• Similar mechanisms to prokaryotic elongation except:
• Requires additional factors to prokaryote elongation
• Eukaryotic ribosomes do not have an E site, so the uncharged tRNA is expelled
directly from the P site.
Termination in prokaryotes:
• Signalled by a stop codon at the A site
• RF1 or RF2 (releasing/terminating factors) binds to the stop codon.
• The peptidyl-tRNA bond (between the growing chain and the tRNA in the P-site)
is cleaved
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Document Summary
Initiation requires initiation factors, ribosomal subunits, mrna and initiation trna ( a charged trna): three initiation factors (if1, if2 and if3) bind to the small ribosomal subunit. Gtp is bound to if2 and drives these processes: mrna and trna carrying the first amino acid bind to the small subunit to form the 30s complex. If3 binds at the exit site: the start codon is positioned at the peptidyl site, the initiation trna binds to the peptidyl site, steps of initiation, translation in initiated at the position of the start codon. Shine-delgarno sequence is also found > this allows for correct binding of the mrna to the small ribosome subunit. The binding of the mrna to the small subunit places the start codon at the ribosome p site: a special methionine (trna^fmet) is used to initiate translation. Lecture 13 mechanisms of translation: the 70s initiation complex is formed.
