Practice question

Transfer RNA for selenocysteine tRNASec encoded by TRSP gene displays unusual architectural features distinguishing it from canonical tRNAs: nine base-pair acceptor plus TΨC helix with four unpaired nucleotides, long 6-base-pair D-stem, extra-long variable arm, and extensive modification preventing recognition by generic elongation factors. Anticodon UCA pairs with UGA, but decoding mediated by EFSec not eRF1, due to conformational dynamics. Its extended length and tertiary fold generate exclusive interface for modifying enzymes PSTK, SepSecS, and EFSec. This specialization underpins genetic code expansion ensuring selenocysteine not confused with serine or cysteine during elongation, highlighting structural basis for codon reassignment fidelity.