Selenium is an essential micronutrient incorporated as selenocysteine (Sec, the 21st amino acid) via a unique co-translational mechanism. Which feature of selenocysteine incorporation is biochemically unique?

• A Selenocysteine is post-translationally added to cysteine residues by a selenium transferase enzyme
• B UGA (normally a stop codon) is recoded as selenocysteine; a selenocysteine insertion sequence (SECIS) element in the 3'-UTR recruits a specialized elongation factor (EFSec/EEFSEC) with Sec-tRNA[Sec] to insert Sec at UGA ✓
• C A unique Sec-tRNA reads a quadruplet UGAA codon that does not interact with release factors
• D Selenocysteine uses a standard CGA (Arg) codon with a missense suppressor tRNA

Explanation

Selenocysteine (Sec) is the 21st proteinogenic amino acid but is NOT encoded by any dedicated sense codon. Instead, UGA (normally UAA/UAG/UGA — stop codons) is recoded as Sec in specific selenoproteins. This requires: (1) a specialized Sec-tRNA[Sec] charged with serine (then enzymatically converted to Sec-tRNA); (2) a SECIS (SElenoCysteine Insertion Sequence) stem-loop in the mRNA 3'-UTR; (3) SECIS-binding protein-2 (SBP2); (4) specialized elongation factor EFSec/EEFSEC. This multi-component machinery allows the ribosome to read UGA as Sec rather than stop. Selenoproteins include glutathione peroxidases (GPx), thioredoxin reductases (TrxR), iodothyronine deiodinases, and selenoprotein P.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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