Molecular Genetics: Codon Exceptions: Selenocysteine
- Several enzymes (for example, glutathione peroxidase, tetraikiodothyronine 5' deiodinase and formate dehydrogenase) contain the unusual amino acid selenocysteine.
- The genes for glutathione peroxidase and formate dehydrogenase contain TGA codons in the middle of their coding regions (ref). The TGA follows the codon for the amino acid that precedes the selenocysteine in the polypeptide chain.
- In the absence of selenium, protein synthesis from these mRNAs terminates prematurely.
- Mutations at four distinct loci selectively prevent the formation of selenocysteine-containing enzymes (ref). The functions of the four genes have been worked out (see Table).
- In the formate dehydrogenase mRNA, a region 40 bp downstream of the UGA codon is also needed for selenocysteine incorporation (ref).
- In the tetraikiodothyronine 5' deiodinase mRNA, a 250 bp region in the 3' untranslated region (about 1 kbp removed from the UGA) is also required for selenocysteine incorporation (ref).