Pyrrolysine, known as the 22nd amino acid, is found in Methanosarcina barkeri (M.barkeri) methylamine methyltransferases. It comes from the sense decoding of the UAG amber stop codon. It has specific pyrrolysyl-tRNA synthetase and tRNA^Pyl. tRNA^Pyl has noncanonical secondary structure. M.barkeri has two routes: direct and indirect routes to synthesize pyrrolysyl-tRNA^Pyl. The special structure in the mRNA and other unknown mechanisms may control the decoding of UAG as pyrrolysine or termination signal. Pyrrolysine was compared with the 21st amino acid: selenocysteine.