Inhibition of selenocysteine tRNA[Ser]Sec aminoacylation provides evidence that aminoacylation is required for regulatory methylation of this tRNA

► Two isoforms of selenocysteine (Sec) tRNA[Ser]Sec differ by a single methyl group (Um34) and synthesize different subclasses of selenoproteins. ► The relationship between tRNA[Ser]Sec aminoacylation and Um34 synthesis was investigated. ► The data show that tRNA[Ser]Sec must be aminoacylated, most likely with Sec, for Um34 synthesis to occur. ► The data provide an explanation why selenium is essential for the function of Um34 methylase. ► The data provide further insights into the hierarchy of selenoprotein expression.