Final answer:
In E. coli, the initiator tRNA (tRNAfMet) introduces the initial formylated methionine at the beginning of protein synthesis, recognized by IF-2 and exclusive to initiation.
Step-by-step explanation:
In the context of E. coli protein synthesis, the special initiator tRNA is responsible for introducing the initial amino acid, which is a formylated methionine (N-formylmethionine or fMet), during the initiation phase of translation. The initiator tRNA, also known as tRNAfMet, is charged with fMet and is recognized by initiation factor IF-2. This tRNA is exclusively used for initiating protein synthesis by binding to the start codon AUG or, in rare cases, GUG. This is in contrast to regular Met-tRNAMet, which inserts non-formylated methionine during the elongation phase of translation when AUG codons appear in-frame. The formyl group and sometimes the entire fMet amino acid can be removed after the synthesis of the polypeptide is complete.