Functional context, biosynthesis, and genetic encoding of pyrrolysine

In Methanosarcina spp., amber codons in methylamine methyltransferase genes are translated as the 22nd amino acid, pyrrolysine. The responsible pyl genes plus amber-codon containing methyltransferase genes have been identified in four archaeal and five bacterial genera, including one human pathogen. In Escherichia coli, the recombinant pylBCD gene products biosynthesize pyrrolysine from two molecules of lysine and the pylTS gene products direct pyrrolysine incorporation into protein. In the proposed biosynthetic pathway, PylB forms methylornithine from lysine, which is joined to another lysine by PylC, and oxidized to pyrrolysine by PylD. Structures of the catalytic domain of pyrrolysyl-tRNA synthetase (archaeal PylS or bacterial PylSc) revealed binding sites for tRNAPyl and pyrrolysine. PylS and tRNAPyl are now being exploited as an orthogonal pair in recombinant systems for introduction of useful modified amino acids into proteins.