Question

With explanations, outline the steps involved in the elongation of protein synthesis in a prokaryotic cell.

Answer 1

Protein synthesis (or translation) in prokaryotes takes place in three stages:

1. Initiation
2. Elongation and
3. Termination.

Elongation of Protein Synthesis

• At the start of the first round of elongation, the initiation codon (AUG) is positioned in the P site with fMet-tRNAfMet bound to it via codon–anticodon base pairing.
• The next codon in the mRNA is positioned in the A site.
• Elongation of the polypeptide chain occurs in three steps called the elongation cycle, namely aminoacyl-tRNA binding, peptide bond formation and translocation:

Aminoacyl-tRNA binding

• The corresponding aminoacyl-tRNA for the second codon binds to the A site via codon–anticodon interaction.
• Binding of the aminoacyl-tRNA requires elongation factor EF-Tu and GTP which bind as an aminoacyl-tRNA/EF-Tu/GTP complex.
• Following binding, the GTP is hydrolyzed and the EF-Tu is released, now bound to GDP.
• Before the EF-Tu molecule can catalyze the binding of another charged tRNA to the ribosome, it must be regenerated by a process involving another elongation factor, EF-Ts.

Peptide bond formation

• Catalyzed by peptidyl transferase.
• In this reaction the carboxyl end of the amino acid bound to the tRNA in the P site is uncoupled from the tRNA and becomes joined by a peptide bond to the amino group of the amino acid linked to the tRNA in the A site.

Translocation

• In the third step, a complex of elongation factor EF-G (also called translocase) and GTP (i.e. EF-G/GTP) binds to the ribosome.
• Three concerted movements now occur, collectively called translocation:

1. the deacylated tRNA moves from the P site to the E site
2. the dipeptidyl-tRNA in the A site moves to the P site, and
3. the ribosome moves along the mRNA (5’ to 3’) by three nucleotides to place the next codon in the A site.

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