 Pyrrolysine was discovered in 2002 at the active site of methyltransferase enzyme from a methane-producing archeon, "Methanosarcina barkeri".This amino acid is encoded by UAG (normally a stop codon), and its synthesis and incorporation into protein is mediated via the biological machinery encoded by the "pylTSBCD" cluster of genes.As determined by X-ray crystallography and MALDI mass spectrometry, pyrrolysine is made up of 4-methylpyrroline-5-carboxylate in amide linkage with the εN of lysine.