SYNTHESIS OF 5-METHYLAMINOMETHYL-2-SELENOURIDINE IN TRANSFER-RNAS - P-31 NMR-STUDIES SHOW THE LABILE SELENIUM DONOR SYNTHESIZED BY THE SELDGENE-PRODUCT CONTAINS SELENIUM BONDED TO PHOSPHORUS

An enzyme preparation from Salmonella typhimurium catalyzes the conver sion of 5-methylaminomethyl-2-thiouridine in tRNAs to 5-methylaminomet hyl-2-selenouridine when supplemented with selenide and ATP. Similar p reparations from a Salmonella mutant strain carrying a defective selD gene fail to catalyze this selenium substitution reaction. However, su pplementation of the deficient enzyme preparation with the purified se lD gene product (SELD protein) restored synthesis of seleno-tRNAs. In the absence of the complementary enzyme(s), the SELD protein catalyzes the synthesis of a labile selenium donor compound from selenide and A TP. P-31 NMR studies show that among the products of this reaction are AMP and a compound containing selenium bonded to phosphorus. The reac tion is completely dependent on the addition of both selenide and magn esium. The dependence of reaction velocity on ATP concentration shows sigmoidal kinetics, whereas dependence on selenide concentration obeys Michaelis-Menten kinetics indicating a K(m) value of 46-mu-M for sele nide.