Selenoprotein synthesis: an expansion of the genetic code.

A number of enzymes employ the unusual amino acid selenocysteine as part of their active site because of its high chemical reactivity. Selenocysteine is incorporated into these proteins co-translationally: biosynthesis occurs on a specific tRNA and insertion into a growing polypeptide is directed by a UGA codon in the mRNA. In E. coli, this requires a specific translation factor. Selenocysteine thus represents a unique expansion of the genetic code.