Selenium is a trace element, the importance of which has become increasingly clear in the recent past. It is essentially found in proteins in the form of selenocysteine which is an aminoacid incorporated cotranslationally into proteins. Selenocysteine is not contained in the pool of natural aminoacids. Rather, its manufacturing and transfer to polypeptide chains are mediated by a complex, original machinery constituting a variation around the theme of protein synthesis. In bacteria, four gene products are involved to perform this function. These consist in: (1) an enzyme which activates the inorganic form of selenium into a phosphoselenoate compound acting as the selenium donor; (2) a selelocysteine tRNA which is charged by serine; (3) an enzyme which converts serine to selenocysteine on the tRNA; (4) lastly, a specific translation factor different from, but playing the role of elongation factor EF-Tu. In addition, and perhaps most fascinating, selenocysteine is encoded by a UGA codon (being normally one of the three stop codons) lying immediately upstream from a stem-loop structure located in messenger RNAs coding for selenoproteins. In eukaryotes, much less is known. However, it looks as if the mechanism parallels that of bacteria with the interesting peculiarity that the stem-loop structure resides in the 3' untranslated region of the mRNA, not within the coding region. Eight types of selenoproteins have been identified in prokaryotes and eukaryotes. In eukaryotes, they include the glutathione peroxidase family and the type I tetraiodothyronine 5'-deiodinase. The former constitutes antioxidant enzymes acting as scavengers against free radicals, the latter being involved in deiodination of thyroxine. These two examples, with more in the text, illustrate nicely the crucial role devoted to selenium in the protection of biological macromolecules against oxidative damages, on the one hand, and mediating metabolic and developmental effects, on the other.
