Practice question

Unlike typical post-translational modifications such as phosphorylation or acetylation, selenocysteine incorporation occurs concurrently with polypeptide elongation at ribosome exit tunnel. UGA normally terminates translation via release factors, yet selenoprotein mRNAs carry downstream SECIS element recruiting SBP2 and EFSec. Sec-tRNASec delivered to ribosomal A-site pairs with UGA retained within coding frame, peptide bond formation directly inserts selenocysteine into nascent chain before chain release. This cotranslational recoding requires specialized tRNA, elongation factor, and SECIS-binding proteins, blurring distinction between regulatory signal and coding sequence. Thus selenocysteine becomes primary sequence component, not added after folding.