Pyrrolysine

Pyrrolysine is an amino acid used by some bacteria and archaea in enzymes that are part of their methane-producing metabolisms. It is coded for by a UAG codon, which is normally a stop codon named "amber" but whose meaning is modified by the presence of a specific downstream sequence which forms a loop in the mRNA, forcing the incorporation of pyrrolysine instead of terminating transcription.

See also selenocysteine, an amino acid with a similar mechanism of encoding in other organisms.