The class 2 selenophosphate synthetase gene of Drosophila contains a functional mammalian-type SECIS

Synthesis of monoselenophosphate, the selenium donor required for the synth esis of selenocysteine (Sec) is catalyzed by the enzyme selenophosphate syn thetase (SPS), first described in Escherichia coli. SPS homologs were ident ified in archaea, mammals and Drosophila. In the latter, however, an amino acid replacement is present within the catalytic domain and lacks selenide- dependent SPS activity. We describe the identification of a novel Drosophil a homolog, Dsps2. The open reading frame of Dsps2 mRNA is interrupted by an UGA stop codon. The 3'UTR contains a mammalian-like Sec insertion sequence which causes translational readthrough in both transfected Drosophila cell s and transgenic embryos. Thus, like vertebrates, Drosophila contains two S PS enzymes one with and one without Sec in its catalytic domain. Our data i ndicate further that the selenoprotein biosynthesis machinery is conserved between mammals and fly, promoting the use of Drosophila as a genetic tool to identify components and mechanistic features of the synthesis pathway.