Question

How N-formylmethionine (fMet) is only associated with the 5' AUG initiation codon and not with internal AUG codons, given that methionine in both cases in encoded by an AUG in the mRNA.

Answer 1

Different transfer RNAs (tRNAs) selectively bind to N-terminal and internal AUG codons of bacterial polypeptides .

Step-by-step explanation:

In prokaryotic organisms, N-formylmethionine (fMet) is coded by the start codon (AUG), thereby generating the most N-terminal residue of bacterial proteins. This residue (fMet) is a modified methionine amino acid that contains attached a formyl group to its amino group. Remarkably, it has been shown that formylmethionyl-tRNA only can bind to this initial AUG codon, while methionyl-tRNA binds to internal AUG codons.