Practice question

SECIS does not code amino acid but acts as recoding signal redefining upstream UGA termination into selenocysteine sense. Hairpin kink-turn formation creates platform for SBP2 binding, recruiting EFSec loaded with Sec-tRNASec to ribosome paused at UGA, excluding eRF1 and delaying hydrolysis. Reporter assays show inserting functional SECIS downstream restores activity when UGA placed at catalytic position, converting stop into Sec codon only when intact. Without SECIS, UGA releases peptide. Thus SECIS extends genetic code contextually, analogous to bacterial SECIS proximal element altering codon semantics via RNA structure-mediated recruitment.