PylRS-tRNA complex

Pyrrolysine (Pyl), the 22nd natural amino acid, is genetically encoded by UAG and inserted into proteins by the unique suppressor tRNA^Pyl. The Methanosarcinaceae produce Pyl and express Pyl-containing methyltransferases that allow growth on methylamines. Homologous methyltransferases and the Pyl biosynthetic and coding machinery are also found in two bacterial species. Pyl coding is maintained by pyrrolysyl-tRNA synthetase (PylRS), which catalyses the formation of Pyl-tRNA^Pyl. Pyl is not a recent addition to the genetic code. PylRS was already present in the last universal common ancestor; it then persisted in organisms that utilize methylamines as energy sources. Recent protein engineering efforts added non-canonical amino acids to the genetic code. This technology relies on the directed evolution of an 'orthogonal' tRNA synthetase-tRNA pair in which an engineered aminoacyl-tRNA synthetase (aaRS) specifically and exclusively acylates the orthogonal tRNA with a non-canonical amino acid. For Pyl the natural evolutionary process developed such a system some 3 billion years ago. When transformed into Escherichia coli, Methanosarcina barkeri PylRS and tRNA^Pyl function as an orthogonal pair in vivo.

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