SECISStruktur

SECISStruktur, or selenocysteine insertion sequence structure, refers to an RNA secondary structure that enables the incorporation of selenocysteine at UGA codons during translation. It is a key element in the biosynthesis of selenoproteins, proteins that contain the amino acid selenocysteine and often participate in redox biology and antioxidant defense.

Location and general features

The SECIS element is located in the messenger RNA downstream of the UGA codon that is to be recoded. In bacteria it resides in the 3' untranslated region of the mRNA; in archaea and eukaryotes it is also typically found in the 3' UTR. The structure is a stem-loop RNA motif that presents an apical loop and a conserved core, allowing it to be recognized by specialized translation factors rather than the standard termination machinery.

Mechanism and factors

SECIS elements recruit dedicated protein and tRNA components to enable insertion of selenocysteine. In bacteria, the Sec-tRNA[Sec] is delivered to the ribosome by the GTPase SelB in conjunction with the SECIS element. In archaea and eukaryotes, the process uses a different set of factors, most notably SBP2 (selenocysteine insertion–binding protein 2) and the specialized elongation factor eEFSec to deliver Sec-tRNA[Sec] to the ribosome. The result is decoding of UGA as selenocysteine rather than as a stop signal, contingent on the presence of a functional SECIS structure.

Diversity and study

SECIS elements vary in sequence and length across domains and among individual genes, but share the core function of enabling Sec incorporation. They are studied through comparative genomics, RNA structure probing, and structural biology, with high-resolution structures providing insight into protein-RNA interactions that govern recoding.

Significance

Proper SECIS function is essential for the expression of selenoproteins. Disruption of SECIS integrity or its interacting factors can impair selenoprotein synthesis and affect cellular redox balance and health.