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#SECIS element

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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

SECIS element is located in eukaryotes at

Location of SECIS diverges between domains reflecting distinct recoding strategies. Eukaryotic SECIS lies in 3' untranslated region often 500 to 2500 nucleotides downstream of UGA, acting at distance via mRNA looping facilitated by SBP2 binding ribosome and eS31 protein. Multiple isoforms share conserved secondary structure but variable primary sequence. Bacterial SECIS immediately follows UGA within open reading frame, directly contacted by SelB C-terminal domain. Distant 3' UTR placement allows single SECIS to recode multiple UGAs and integrates selenium availability regulation effectively.

Ref: NCBI Bookshelf, Eukaryotic Translation: SECIS Location in 3′ UTR and Sec Incorporation

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Eukaryotes gene regulation

UGA is recoded as selenocysteine due to presence of

Recoding UGA from stop to selenocysteine relies on cis-acting SECIS element forming kink-turn hairpin structure with conserved quartet UGA, AUGA, and apical AAR motifs. Eukaryotic SECIS resides in 3' UTR; bacterial SECIS immediately downstream within coding region positioned to contact SelB. SECIS recruits trans-acting factors SBP2 and EFSec or SelB, positioning Sec-tRNASec near ribosomal A-site and antagonizing release factor access. Interaction involves ribosomal protein L30 and SBP2 Lys-rich domain, prolonging pause at UGA and favoring incorporation. Without SECIS, UGA triggers standard termination via eRF1. Hence RNA structure contextually modifies codon semantics enabling regulated selenoprotein expression.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: SECIS Element Mediated UGA Recoding

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