The amber codon in the gene encoding the monomethylamine methyltransferaseisolated from Methanosarcina barkeri is translated as a sense codon

Each of the genes encoding the methyltransferases initiating methanogenesis from trimethylamine, dimethylamine, or monomethylamine by various Methanos arcina species possesses one naturally occurring in-frame amber codon that does not appear to act as a translation stop during synthesis of the bioche mically characterized methyltransferase. To investigate the means by which suppression of the amber codon within these genes occurs, MtmB, a methyltra nsferase initiating metabolism of monomethylamine, was examined. The C-term inal sequence of MtmB indicated that synthesis of this mtmB1 gene product d id not cease at the internal amber codon, but at the following ochre codon. Antibody raised against MtmB revealed that Escherichia coli transformed wi th mtmB1 produced the amber termination product. The same antibody detected primarily a 50-kDa protein in Methanosarcina barkeri, which is the mass pr edicted for the amber readthrough product of the mtmB1 gene. Sequencing of peptide fragments from MtmB by Edman degradation and mass spectrometry reve aled no change in the reading frame during mtmB1 expression. The amber codo n position corresponded to a lysyl residue using either sequencing techniqu e. The amber codon is thus read through during translation at apparently hi gh efficiency and corresponds to lysine in tryptic fragments of MtmB even t hough canonical lysine codon usage is encountered in other Methanosarcina g enes.