 Unlike posttranslational modifications of lysine such as hydroxylysine, methyllysine, and hypusine, pyrrolysine is incorporated during translation (protein synthesis) as directed by the genetic code, just like the standard amino acids.It is encoded in mRNA by the UAG codon, which in most organisms is the 'amber' stop codon.This requires only the presence of the "pylT" gene, which encodes an unusual transfer RNA (tRNA) with a CUA anticodon, and the "pylS" gene, which encodes a class II aminoacyl-tRNA synthetase that charges the "pylT"-derived tRNA with pyrrolysine.