Which initiator tRNA is used during initiation in bacteria?

• A. fMet-tRNA
• B. tRNA^Met (Met-tRNAi)
• C. tRNA^Ala
• D. tRNA^Phe

Answer: (A)

In bacteria, starting a protein synthesis round uses a specialized initiator tRNA carrying formylated methionine. This fMet-tRNA recognizes the start codon AUG and, with initiation factors, positions itself at the P site to set the reading frame for the ribosome. The formyl group helps the ribosome distinguish the initiator from tRNAs that bring amino acids during elongation, preventing premature entry into elongation. After initiation, the formyl group is typically removed by deformylase, and often the methionine itself may be removed later depending on the next amino acid.

The other tRNAs listed deliver their respective amino acids during elongation, not initiation. In contrast, in eukaryotes the initiator is Met-tRNAi^Met without a formyl group, which is why tRNA^Met is associated with initiation in those organisms but not in bacteria.