Pyrrolysine (Pyl)

Pyrrolysyl-tRNA synthetase–tRNAPyl structure reveals the molecular basis of orthogonality is the title of a papaer published in Nature and authored by Kayo Nozawa, Patrick O'Donoghue, Sarath Gundllapalli, Yuhei Araiso, Ryuichiro Ishitani, Takuya Umehara, Dieter Söll and Osamu Nureki (doi:10.1038) (nature07611); Published online 31 December 2008). The paper is focused on the 22nd amino acid which is less well known than the canonical 20. It is called pyrrolysine (Pyl) and its corresponding codon is UAG. The related transfer RNA is dubbed tRNAPyl and the pyrrolysyl-tRNA synthetase (PylRS).

As the paper notes organisms known as methanosarcinaceae produce Pyl and have Pyl-containing methyltransferases. A functional correlate for PylRS exists. It is found in organisms which "utilize utilize methylamines as energy sources." A special aaRS–tRNA interaction surface, relevant to Pyl, characterizes molecular orthogonality.