The Caenorhabditis elegans homologue of thioredoxin reductase contains a selenocysteine insertion sequence (SECIS) element that differs from mammalian SECIS elements but directs selenocysteine incorporation.

Thioredoxin reductases (TRR) serve critical roles in maintaining cellular redox states. Two isoforms of TRR have been identified in mammals: both contain a penultimate selenocysteine residue that is essential for catalytic activity. A search of the genome of the invertebrate, Caenorhabditis elegans, reveals a gene highly homologous to mammalian TRR, with a TGA selenocysteine codon at the corresponding position. A selenocysteyl-tRNA was identified in this organism several years ago, but no selenoproteins have been identified experimentally. Herein we report the first identification of a C. elegans selenoprotein. By (75)Se labeling of C. elegans, one major band was identified, which migrated with the predicted mobility of the C. elegans TRR homologue. Western analysis with an antibody against human TRR provides strong evidence for identification of the C. elegans selenoprotein as a member of the TRR family. The 3'-untranslated region of this gene contains a selenocysteine insertion sequence (SECIS) element that deviates at one position from the previously invariant consensus "AUGA." Nonetheless, this element functions to direct selenocysteine incorporation in mammalian cells, suggesting conservation of the factors recognizing SECIS elements from worm to man.