The residue mass of L-pyrrolysine in three distinct methylamine methyltransferases.

Single in-frame amber (UAG) codons are found in the genes encoding MtmB, MtbB, or MttB, the methyltransferases initiating methane formation from monomethylamine, dimethylamine, or trimethylamine, respectively, in certain Archaea. The crystal structure of MtmB demonstrated that the amber codon codes for pyrrolysine, the 22nd genetically encoded amino acid found in nature. Previous attempts to visualize the amber-encoded residue by mass spectrometry identified only lysine, leaving information on the existence and structure of pyrrolysine resting entirely on crystallography of a single protein.