Final answer:
The initiating methionine in prokaryotes is formylated, known as fMet, distinguishing it from the regular, non-formylated methionine added later during polypeptide chain elongation.
Step-by-step explanation:
In prokaryotes, the methionine that initiates the formation of a polypeptide chain differs from the subsequently added methionine because a formyl group is attached to the initiating methionine. This modified methionine is referred to as N-formylmethionine (fMet). The initiator tRNA, known as tRNAfMet, is charged with fMet instead of a regular methionine. This is the only time during protein synthesis that fMet is used, as all other methionines incorporated into a growing polypeptide chain are added by regular Met-tRNAMet and are not formylated.
The fMet serves a crucial role in the initiation of protein synthesis, interacting with the start codon AUG through its anticodon, and is facilitated by the initiation factor IF-2. Its presence signals the ribosome to start the translation process. After translation, in bacteria like E. coli, the formyl group—and sometimes also the methionine itself—is removed from the polypeptide. Contrastingly, in eukaryotic translation, the initiating methionine is not formylated.