Practice question

Eukaryotic selenocysteine delivery requires dedicated elongation factor eEFSec or EFSec, paralogous to canonical eEF1A but unable to bind standard aminoacyl-tRNAs. EFSec contains unique domain IV interacting with SBP2-SECIS complex and extended C-terminus specifically recognizing tRNASec distinct features including long variable arm and 13-base-pair acceptor-TΨC stack. GTP-bound EFSec delivers Sec-tRNASec to ribosomal A-site when UGA occupies decoding center, with GTP hydrolysis enabling accommodation and peptide transfer. This selectivity prevents misincorporation of biosynthetic intermediates serine or phosphoserine, ensuring fidelity. Regulation via GTPase cycle coordinates recoding efficiency with selenium nutritional status controlling EFSec expression.