A New UAG-Encoded Residue in the Structure of a Methanogen Methyltransferase

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Abstract
Genes encoding methanogenic methylamine methyltransferases all contain an in-frame amber (UAG) codon that is read through during translation. We have identified the UAG-encoded residue in a 1.55 angstrom resolution structure of the Methanosarcina barkeri monomethylamine methyltransferase (MtmB). This structure reveals a homohexamer comprised of individual subunits with a TIM barrel fold. The electron density for the UAG-encoded residue is distinct from any of the 21 natural amino acids. Instead it appears consistent with a lysine in amide-linkage to (4 R ,5 R )-4-substituted-pyrroline-5-carboxylate. We suggest that this amino acid be named l -pyrrolysine.