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Pyrrolysine, the 22nd cotranslationally inserted amino acid, was found in the Methanosarcina barkeri monomethylamine methyltransferase protein in a position that is encoded by an in-frame UAG stop codon in the mRNA. M. barkeri encodes a special amber suppressor tRNA (tRNA(Pyl)) that presumably recognizes this UAG codon. It was reported that Lys-tRNA(Pyl) can be formed by the aminoacyl-tRNA synthetase-like M. barkeri protein PylS [Srinivasan, G., James, C. M. lysine and tRNA(Lys) are not substrates of the enzyme. In view of the properties of PylS we propose to name this enzyme pyrrolysyl-tRNA synthetase. In contrast, the LysRS1:LysRS2 complex does not recognize pyrrolysine and charges tRNA(Pyl) with lysine. These in vitro data suggest that Methanosarcina cells have two pathways for acylating the suppressor tRNA(Pyl). This would ensure efficient translation of the in-frame UAG codon in case of pyrrolysine deficiency and safeguard the biosynthesis of the proteins whose genes contain this special codon.
Polycarpo et al. (Mon,) studied this question.