Final answer:
In bacteria, a modified methionine called formylmethionine (fMet) is used to begin each polypeptide, and it is later removed during post-translational processing.
Step-by-step explanation:
In bacteria, the modified form of methionine used to begin each polypeptide is known as formylmethionine (fMet). The initiator tRNA, charged with fMet, recognizes the start codon AUG and initiates protein synthesis.
After the polypeptide chain is synthesized, the formyl group is removed from the N-terminal formylmethionine, and often the methionine residue itself is also cleaved off.
The process of adding and later removing the formyl group from the N-terminal methionine is part of post-translational processing.
This phenomenon is specific to prokaryotes; eukaryotic organisms start their polypeptides with an unmodified methionine, as their initiator tRNA does not carry a formyl group.