Final answer:
In E. coli, protein synthesis initiation involves a unique initiator tRNA carrying a formylated methionine (fMet), which binds to the start codon. This modified methionine is essential for initiation but removed after translation. In eukaryotes, methionine is not formylated, showing evolutionary distinctions in protein synthesis mechanisms.
Step-by-step explanation:
Initiator tRNA and Formylated Methionine
In E. coli, protein synthesis begins with the initiation complex formation. A special initiator tRNA known as tRNA Met, which is only utilized during initiation, binds to the AUG or occasionally GUG start codon. The bound amino acid is methionine, modified by formylation, to create N-formylmethionine (fMet). This formylation is a unique feature for the initiator tRNA, distinguishing it from other tRNAs that deliver methionine during the elongation phases of protein synthesis. In eukaryotes, however, the methionine on the initiator tRNA is not formylated. Translation initiation in E. coli requires several other components, including the small 30S ribosome subunit, mRNA, and initiation factors (IFs).
Early in evolution, formylation of the N-terminal methionine may have played a vital role in ensuring accurate translation initiation, but in modern eukaryotes, other mechanisms have taken over this function. Although formylated methionine is typically removed from the growing polypeptide chain once translation is underway, it serves as an essential initiation signal in prokaryotes such as E. coli. This evolutionary legacy reflects the nuanced differences in protein synthesis between prokaryotic and eukaryotic organisms.