Biosynthesis of 5-methylaminomethyl-2-selenouridine, a naturally occurring nucleoside in Escherichia coli tRNA.

A selenium-containing nucleoside, 5-methylaminomethyl-2-selenouridine (mnm5se2U), is present in lysine- and glutamate-isoaccepting tRNA species of Escherichia coli. The synthesis of mnm5se2U is optimum (4 mol/100 mol tRNA) when selenium is present at about 1 microM concentration and is neither decreased by a high (8 mM) level of sulfur in the medium nor increased by excessive (10 or 100 microM) levels of selenium. Lysine- and glutamate-isoaccepting tRNA species that contain 5-methylaminomethyl-2-thiouridine (mnm5s2U) coexist with the seleno-tRNAs in E. coli cells and a reciprocal relationship between the mnm5se2U- and the mnm5s2U-containing species is maintained under a variety of growth conditions. The complete 5-methylaminomethyl side chain is not a prerequisite for introduction of selenium at the 2-position. In E. coli mutants deficient in the ability to synthesize the 5-methylaminomethyl substituent, both the 2-thiouridine and the corresponding 2-selenouridine derivatives of intermediate forms are accumulated. Broken cell preparations of E. coli synthesize mnm5se2U in tRNAs by an ATP-dependent process that appears to involve the replacement of sulfur in mnm5s2U with selenium.