Decameric SelA•tRNA Sec Ring Structure Reveals Mechanism of Bacterial Selenocysteine Formation

Abstract

Putting Selenium in Proteins The 21st amino acid, selenocysteine (Sec), occurs in the active site of many redox enzymes. Its cognate transfer RNA (tRNA) is first loaded with Ser by seryl-tRNA synthetase and the Ser-tRNA Sec is then converted to Sec-tRNA Sec . Itoh et al. (p. 75 ) determined the crystal structures of the selenocysteine synthase, SelA, that is responsible for this conversion in bacteria, alone and in complex with tRNA. The decameric SelA complex binds to 10 tRNA Sec molecules. The structures, together with biochemistry, show how SelA discriminates tRNA Sec from tRNA Ser , give insight into the mechanism of catalysis, and show that decamerization is essential for function.