| Members of the Methanosarcinaceae family can in addition to hydrogen/carbon dioxide utilize several methylated compounds and convert them to methane. Methanogenesis from methylamines involves methylamine specific methyltransferases that transfer the methyl group from the methylamines to a corrinoid protein. The methylamine specific methyltransferase genes contain a single in-frame amber codon that is not read as a translational stop. The residue encoded by the amber codon, has been found to be a novel amino acid, pyrrolysine in MtmB. Multiple copies of monomethylamine methyltransferase genes (mtmB) containing a single amber codon within their open reading frames, along with the genes encoding their cognate corrinoid proteins (mtmC), exist within the genomes of the members of the Methanosarcinaceae family. The two copies of mtmCB genes from M. barkeri MS are differentially transcribed. Editing of the mtmB2 transcript was not detected suggesting a mechanism of amber codon readthrough occurring in the organism.; Similar to selenocysteine incorporation at UGA codons, the Methanosarcinaceae also appear to have a unique mechanism for amber codon readthrough. An amber decoding tRNA gene, pylT, along with its cognate lysyl tRNA synthetase, pylS, are found near the MMA methyltransferase gene cluster. The pylT and pylS genes are co-transcribed. Homologs of pylS and pylT are found in a Gram-positive bacterium that also contains a trimethylamine methyltransferase gene homolog with a single in-frame amber codon in its open reading frame.; Two additional lysyl-tRNA synthetases, LysK and LysS, occur in M. barkeri. Comparison of tRNA substrate specificity of the three lysyl-tRNA synthetases showed that LysK and LysS have slower rate of charging of the in vitro transcript of pylT (the gene encoding the amber decoding tRNA) as compared to PylS. This suggested that PylS is involved in acylating tRNACUA and cellular tRNA Lys while LysK and LysS are involved in acylating only cellular tRNA Lys.; Charging tRNACUA with lysine by PylS is possibly an initial step in the mechanism of readthrough of UAG (amber) codons and encoding pyrrolysine within methylamine methyltransferases in Methanosarcinaceae. |
