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#UGA

2 public questions tagged with this topic.

Eukaryotes gene regulation

SECIS element allows UGA to function as

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.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: SECIS Converts UGA to Selenocysteine Codon

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Eukaryotes gene regulation

Selenocysteine incorporation requires which stop codon?

Universal code designates UGA as termination codon yet selenoproteins conserve in-frame UGA at catalytic centers for Sec insertion requiring recoding. Surveys show all confirmed vertebrate, bacterial, archaeal selenoproteins use UGA exclusively; UAA or UAG never encode selenium. Reason stems from anticodon compatibility, tRNASec UCA pairs perfectly with UGA requiring single modification, other stops would need different isoacceptors. Conservation allows kinetic competition control between release factor and SelB or EFSec. Fidelity depends on SECIS presence and selenium status, ensuring efficiency regulated by nutrient availability and release factor competition dynamics.

Ref: NCBI Bookshelf, Codon Usage: UGA as Selenocysteine Codon in Selenoproteins

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