Bacterial and eukaryotic selenocysteine biosynthesis and co-translational incorporation

A reusable module for the bacterial and eukaryotic synthesis of selenocysteyl-tRNA(Sec) and recoding of UGA as selenocysteine. Selenophosphate synthetase activates selenium, and seryl-tRNA synthetase charges tRNA(Sec) with serine. Bacteria convert Ser-tRNA(Sec) directly with SelA, whereas the represented eukaryotic route first phosphorylates it with PSTK and then uses SepSecS. The completed Sec-tRNA(Sec) is delivered by a lineage-specific elongation system to a UGA codon in a SECIS-dependent translation context.

This revision generalizes the previous human-only document while preserving its curated eukaryotic route. The shared SerS and selenophosphate steps are modeled once. SelA versus PSTK/SepSecS and SelB versus SECISBP2/EEFSEC are explicit implementation variants. Archaeal Sec-tRNA synthesis overlaps the PSTK-SepSecS chemistry, but archaeal recoding machinery is not represented, so the module title is explicitly limited to bacteria and eukaryotes. tRNA(Sec) and SECIS RNA elements are required non-protein substrates/context, not missing gene parts. Selenium uptake, selenide production, tRNA transcription and maturation, ribosome biogenesis, and downstream selenoprotein function are outside the boundary.