The Euplotes Crassus Selenoproteome

The hypotrichous ciliate Euplotes crassus genome has been analyzed for the presence of selenocysteine‐containing genes. Metabolic labeling of E. crassus revealed selenoprotein pattern that was similar to that observed in human HEK 293 and algal Ostreococcus tauri cells. Several selenoproteins were identified, including a new selenoprotein belonging to the Pfam09409 family (designated ep22 for Euplotes 22 kDa protein) with homologs in other ciliates, Tetrahymena and Paramecium , and in a primitive animal, Nematostella . Sec was encoded by UGA codon in all selenoprotein genes, which also had eukaryotic SECIS elements in their 3′‐UTRs. Four selenoprotein genes had UGA codons encoding both Sec and Cys, whereas in other selenoprotein genes, UGA only encoded Sec. Consistent with the use of UGA codon for both Sec and Cys insertion, we identified the corresponding Sec and Cys tRNAs containing UCA anticodon. This finding shows that the previous prediction of a single Cys tRNA GCA serving UGA, UGC and UGU codons in Euplotes is incorrect. Analysis of the Euplotes selenoproteome expands our understanding of selenoprotein evolution and genetic code.