DIETARY SELENIUM AFFECTS METHYLATION OF THE WOBBLE NUCLEOSIDE IN THE ANTICODON OF SELENOCYSTEINE TRANSFER RNA[SER]SEC

We reported previously that the presence of selenium in culture media of mammalian cells influences both the steady-state levels and distrib utions of two tRNA isoacceptors involved in the insertion of selenocys teine into protein in response to certain UGA codons. In this study, w e demonstrate an increase in the levels of these isoacceptors in rats fed a selenium-adequate diet compared to animals fed a selenium-defici ent diet, as well as a shift in the relative distribution toward the t RNA which elutes later from an RPC-5 column. These effects were found to occur in a tissue-specific manner. Both selenocysteine tRNAs were i solated from rat liver, sequenced, analyzed by mass spectrometry, and shown to differ only by ribose 2'-O-methylation of 5-methylcarboxymeth yluridine that occurs in the wobble position of the anticodon. This mo dified nucleoside has been documented previously only in yeast tRNA wh ile the corresponding 2'-O-methylribose derivative has not been observ ed. The structure of these nucleosides was established by mass spectro metry and confirmed by chemical synthesis. Although the role of methyl ation of the wobble nucleotide is not known, the differences in elutio n properties from RPC-5 columns are consistent with other experimental observations indicating that a change in tRNA conformation accompanie s this methylation.