Bacterial initiation: During protein synthesis in prokaryotes, what is the identity of the initiating amino acid delivered by the initiator tRNA?

Introduction / Context:
Initiation differs between bacteria and eukaryotes. The initiator residue in bacteria is chemically distinct, which has implications for immune recognition, proteomics, and antibiotic targeting.

Given Data / Assumptions:

• System: prokaryotes (e.g., Escherichia coli).
• Initiator tRNA is tRNA^fMet.
• The first residue is often removed or deformylated post-translationally.

Concept / Approach:
Bacteria charge the initiator tRNA with methionine and then formylate it, producing N-formyl methionine (fMet). This fMet is inserted when AUG functions as the start codon.

Step-by-Step Solution:

Verification / Alternative check:
Enzymes peptide deformylase and methionine aminopeptidase remove the formyl group and sometimes the initial residue; this processing validates the starting presence of fMet.

Why Other Options Are Wrong:

• Methionine: correct for eukaryotic initiation; in bacteria it is formylated.
• Phenylalanine: coded by UUU/UUC, unrelated to initiation.
• None: a defined initiating residue exists.

Common Pitfalls:
Equating bacterial and eukaryotic initiation chemistry; they differ in initiator tRNAs and modifications.

Final Answer:
N-formyl methionine