SelB

SelB is a bacterial translation factor and a specialized GTPase that mediates the incorporation of selenocysteine into proteins. In bacteria that encode selenoproteins, the UGA stop codon is recoded as selenocysteine when a selenocysteine insertion sequence (SECIS) element is present in the mRNA’s 3' untranslated region. SelB binds Sec-tRNASec and escorts it to the ribosome at the UGA codon, enabling the insertion of selenocysteine during elongation.

Mechanism: In the GTP-bound form, SelB forms a complex with Sec-tRNASec and recognizes the SECIS element. This ternary complex associates with the ribosome, delivering Sec-tRNASec to the A site when the UGA codon is encountered. GTP hydrolysis induces a conformational change that releases Sec-tRNASec into the ribosome, allowing peptide elongation to continue with selenocysteine.

Structure and gene context: SelB is typically about 450–500 amino acids long and comprises three major regions: an N-terminal GTPase domain, a central helical domain, and a C-terminal tRNA-binding domain that contacts Sec-tRNASec. The SECIS element and Sec-tRNASec recognition position SelB to coordinate translation at the UGA codon. In most bacteria, SelB works with other selenocysteine biosynthesis components, such as SelA (selenocysteine synthase) and SelD (selenophosphate synthetase).

Distribution and evolution: The SelB pathway is found in many, but not all, selenoprotein-encoding bacteria. The selB gene is often located in operons with selA and selD and is absent in organisms that do not utilize selenocysteine. Studies of SelB have helped illuminate how cells repurpose a stop codon to encode an amino acid during translation.