During initiation
[5], the first step to occur is the binding of a specific initiator methionyl tRNA and mRNA to the small ribosomal subunit. After that, the large ribosomal subunit merges with the complex to form a ribosome where the polypeptide chain will start to elongate.
The process of translation is always initiated by the start codon AUG (encodes the amino acid methionine). Alternatively, it can be substituted by the codon GUG (encodes the amino acid valine).
a. Prokaryotic cells
In prokaryotes like bacteria, the initiation of translation begins with the binding of the initiation factors 1, 2, and 3 (IF-1, IF-2, and IF-3) to the 30S ribosomal subunit.
- This is then followed by the binding of the mRNA and initiator tRNA to the complex, triggering the release of the IF-3 to allow the 50S ribosomal subunit to join the previous complex.
- The GTP (guanosine triphosphate) is then hydrolyzed to release IF-1 and IF-2, following the formation of the 70S initiation complex for the next step–elongation.
b. Eukaryotic cells
On the other hand, the process of initiation in eukaryotic cells is more complex and requires more initiation factors. For instance, the eukaryotic initiation factors such as eIF-1, eIF-1A, and eIF-3 attach to the 40S ribosomal subunit, while the eIF-2 joins the initiator methionyl tRNA.
- The eIF-4 factors then search for the mRNA and bring it to the 40S ribosomal subunit.
- Unlike prokaryotic initiation factors, eukaryotic initiation factors can recognize the 5′ and 3′ ends of the mRNA, which is specifically responsible for polyadenylation during translation.
- The 40S ribosomal subunit, together with the associated eIFs and methionyl tRNA the browses the mRNA sequence to look for the start codon (AUG).
- After reaching AUG, eIF-5 triggers the release of both eIF-5 and eIF-2 initiation factors, resulting to the binding of the 40S subunit to the 60S subunit to form the 80S initiation complex unique to eukaryotes.
