Practice question

Conversion of Ser-tRNASec to phosphoseryl intermediate begins with O-phosphoseryl-tRNASec kinase PSTK, unique monomeric kinase using ATP to phosphorylate seryl hydroxyl still esterified to tRNA. PSTK harbors N-terminal kinase domain resembling phosphoserine kinases and C-terminal tRNASec-binding domain conferring strict specificity, discriminating against tRNASer via variable arm recognition. Phosphorylation generates high-energy phosphoester with enhanced leaving group capability for nucleophilic substitution by selenophosphate. In archaea and eukaryotes PSTK essential; bacteria bypass this step using SelA directly. Loss-of-function mutations abolish selenoprotein expression despite normal serine charging, underscoring priming role converting inert alcohol into activated precursor competent for selenium insertion.