SELENIUM, SELENOPROTEINS - AN ALTERNATE R EADING OF THE GENETIC-CODE

Selenium is a trace element, the importance of which has become increa singly clear in the recent past. It is essentially found in proteins i n the form of selenocysteine which is an aminoacid incorporated cotran slationally into proteins. Selenocysteine is not contained in the pool of natural aminoacids. Rather, its manufacturing and transfer to poly peptide chains are mediated by a complex, original machinery constitut ing a variation around the theme of protein synthesis. In bacteria, fo ur 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 selelo cysteine tRNA which is charged by serine; (3) an enzyme which converts serine to selenocysteine on the tRNA; (4) lastly, a specific translat ion factor different from, but playing the role of elongation factor E F-Tu. In addition, and perhaps most fascinating, selenocysteine is enc oded by a UGA codon (being normally one of the three stop codons) lyin g immediately upstream from a stem-loop structure located in messenger RNAs coding for selenoproteins. In eukaryotes, much less is known. Ho wever, it looks as if the mechanism parallels that of bacteria with th e interesting peculiarity that the stem-loop structure resides in the 3' untranslated region of the mRNA, not within the coding region. Eigh t types of selenoproteins have been identified in prokaryotes and euka ryotes. In eukaryotes, they include the glutathione peroxidase family and the type I tetraiodothyronine 5'-deiodinase. The former constitute s 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 oxida tive damages, on the one hand, and mediating metabolic and development al effects, on the other.