Understanding the role of tRNA modifications in UGA recoding as selenocysteine in eukaryotes

Selenocysteine (Sec), the 21st proteogenic amino acid, is a key component of selenoproteins, where it performs critical roles in redox reactions. Sec incorporation during translation is unique and highly sensitive to selenium levels. Encoded by the UGA codon, typically a termination signal, its insertion necessitates the presence of a selenocysteine insertion sequence (SECIS) within the 3' untranslated region (UTR) of selenoprotein mRNAs. This SECIS element orchestrates the recruitment of specialized molecular factors, including SECISBP2, the unique tRNA^[Ser]Sec, and its dedicated elongation factor, EEFSEC. The extended variable arm of tRNA^[Ser]Sec permits its specific recognition by EEFSEC. While the structure of the ribosome-bound complex is known, the precise mechanism by which EEFSEC-tRNA^[Ser]Sec recodes UGA in the presence of SECIS and SECISBP2 remains unclear. tRNA^[Ser]Sec has relatively few epitranscriptomic modifications, but those at the anticodon loop are crucial. Key modifications include N6-isopentenyladenosine (i6A) at position 37 and two forms of 5-methoxycarbonylmethyluridine (mcm⁵U and mcm⁵U_m) at position 34. The ratio of these isoforms varies with tissue type and selenium levels, influencing mRNA-specific Sec recoding. A C65G mutation in the acceptor stem, identified in patients, disrupts these modifications at position 34, impairing selenoprotein synthesis. This highlights the essential role of wobble position modifications in anticodon function. tRNA^[Ser]Sec exemplifies the complex regulation of UGA codon recoding and underscores the interplay of structural and epitranscriptomic factors in selenoprotein translation.