The translation of mRNA is initiated by a specific methionine-accepting tRNA at a specific initiation codon, usually AUG (complementary to the tRNA anticodon). However translation at suitable (albeit unphysiological) conditions in vitro does not require a specific initiation codon or a specific amino acid, so it is not difficult to envisage an earlier stage in the evolution of the translation apparatus in which this specific initiation system was absent. Instead there might have been random initiation or initiation from the 5′ end of the mRNA.
A specific initiation codon puts constraints on the sequence of the mRNA and adds an unnecessary amino acid to the N-terminus — one that is sometimes unwanted and needs to be removed.
What advantages can one envisage for initiation at a specific internal site, rather than at, say, the 5′ end of the mRNA?
I appreciate it is impossible to prove which factor was actually responsible during evolution, but by soliciting concrete explanations I trust that this question is as valid as the original from which it is derived (see Footnote).
FOOTNOTE
This question has been expanded from one part of a three-part question posted some time ago on “Why is AUG the Initiation Codon?”. This recently resurfaced, and as nobody had answered this particular part, and as it is site policy to ask one question at a time, I decided to remove it from the original and repost it here so it could be considered on its own merits, without the distraction of other questions such as the choice of codon or of methionine over other amino acids. I shall contribute an answer with some thoughts of my own, but, of course, anyone can join in.
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