Editing Methionine (section)

===Encoding===
Methionine is one of only two amino acids encoded by a single [[codon]] (AUG) in the standard [[genetic code]] ([[tryptophan]], encoded by UGG, is the other). In reflection to the evolutionary origin of its codon, the other AUN codons encode [[isoleucine]], which is also a hydrophobic amino acid. In the mitochondrial genome of several organisms, including [[Vertebrate mitochondrial code|metazoa]] and [[The yeast mitochondrial code|yeast]], the codon AUA also encodes for methionine. In the standard genetic code AUA codes for isoleucine and the respective tRNA (''ileX'' in ''Escherichia coli'') uses the unusual base [[Lysidine (nucleoside)|lysidine]] (bacteria) or [[agmatidine]] (archaea) to discriminate against AUG.<ref>{{Cite journal |vauthors=Ikeuchi Y, Kimura S, Numata T, Nakamura D, Yokogawa T, Ogata T, Wada T, Suzuki T, Suzuki T |date=April 2010 |title=Agmatine-conjugated cytidine in a tRNA anticodon is essential for AUA decoding in archaea |journal=Nature Chemical Biology |volume=6 |issue=4 |pages=277–282 |doi=10.1038/nchembio.323 |pmid=20139989}}</ref><ref>{{Cite journal |vauthors=Muramatsu T, Nishikawa K, Nemoto F, Kuchino Y, Nishimura S, Miyazawa T, Yokoyama S |date=November 1988 |title=Codon and amino-acid specificities of a transfer RNA are both converted by a single post-transcriptional modification |journal=Nature |volume=336 |issue=6195 |pages=179–181 |bibcode=1988Natur.336..179M |doi=10.1038/336179a0 |pmid=3054566|s2cid=4371485 }}</ref>

The methionine codon AUG is also the most common start codon. A "Start" codon is message for a [[ribosome]] that signals the initiation of protein [[Translation (biology)|translation]] from mRNA when the AUG codon is in a [[Kozak consensus sequence]]. As a consequence, methionine is often incorporated into the ''N''-terminal position of [[protein]]s in [[eukaryote]]s and [[archaea]] during translation, although it can be removed by [[post-translational modification]]. In [[bacteria]], the derivative [[N-Formylmethionine|''N''-formylmethionine]] is used as the initial amino acid.{{cn|date=October 2024}}