Selenocysteine (Sec, U) is the '21st'proteinogenic amino acid because it shares three key properties with the other 20 common amino acids: it has its own codon (UGA), it has its own unique tRNA, and it is co-translationally inserted at the ribosome [1, 2]. Heterologous expression and production of selenoproteins is difficult because of the complexity of the UGA recoding machinery needed by the ribosome to translate UGA as a sense codon for ...
