The starting amino acid in the synthesis of most protein chains is methionine (eukaryotes) or N-formyl methionine (prokaryotes).
Methionyl or N-formyl methionyl-tRNA specifically binds to initiation sites containing the AUG codon.
This codon is therefore called the initiation codon. Less often, G U G also serves as the initiation codon in bacterial protein synthesis.
Normally
GUG
is
the
codon
for
valine.
In
the
phage
MS2,
GUG
is
the
initiation
codon
for
the
A
protein.
GUG
has
been
found
to
initiate
protein
synthesis
when
the
normal
AUG
codon
is
lost
by
deletion.
However,
initiation
by
GUG
is
less
efficient,
since
it
has
a
lower
affinity
for
fMet-tRNA. Both AUG and GUG codons show ambiguity in one sense, since each of them codes for two different amino acids.
When these two codons are at initiation positions of mRNA they code for N-formyl methionine. In internal positions AVG codes for methionine and GUG for valine.
