Genetic code in bacteria: In prokaryotic translation, what amino acid does the start codon AUG encode, particularly at initiation?

Introduction / Context:
The genetic code uses nucleotide triplets (codons) to specify amino acids during translation. While AUG commonly codes for methionine, in bacteria the initiating methionine is chemically modified. Understanding this nuance is essential in microbiology, molecular biology, and interpreting N-termini of prokaryotic proteins.

Given Data / Assumptions:

• The codon under consideration is AUG.
• The context is prokaryotic (bacterial) protein synthesis.
• Initiation versus internal codon usage may differ.

Concept / Approach:
In bacteria, the initiator tRNA is tRNA^fMet, which carries methionine that is enzymatically formylated to produce N-formyl methionine (fMet). Thus, when AUG serves as the start codon, the first incorporated residue is fMet. Internal AUG codons (not used for initiation) specify unmodified methionine via elongator tRNA^Met.

Step-by-Step Solution:

Verification / Alternative check:
The presence of peptide deformylase and methionine aminopeptidase in bacteria, which remove the formyl group and sometimes the initiator residue post-translationally, corroborates that initiation starts with fMet.

Why Other Options Are Wrong:

• Methionine: true for internal AUG or eukaryotic initiation, but not for bacterial initiation.
• A stop signal: stops correspond to UAA, UAG, UGA; AUG is not a stop.
• Alanine: coded by GCU, GCC, GCA, GCG, not AUG.

Common Pitfalls:
Assuming AUG always inserts the same unmodified methionine; initiation context and organism matter. Also, post-translational removal can hide the original fMet in mature proteins.

Final Answer:
N-formyl methionine