Figure 1. Proposed pathway of Sec biosynthesis on its tRNA in mammalian cells. Serine (Ser, shown in blue as an oblong circle) is attached to tRNA[Ser]Sec (shown in green as a cloverleaf structure) by seryl-tRNA synthetase (SRS) to form seryl-tRNA[Ser]Sec (shown in blue as serine attached to tRNA[Ser]Sec) and is then phosphorylated by phosphoseryl-tRNA kinase (PSTK) to form the intermediate phosphoseryl-tRNA[Ser]Sec (P, shown in red as a circle attached to serine). The phosphate on phosphoseryl-tRNA[Ser]Sec is then replaced by the selenium donor that is likely activated by selenophosphate synthetase (SPS), and the compound is converted to selenocysteyl-tRNA[Ser]Sec (Sec, shown in gold as an oblong circle attached to tRNA[Ser]Sec) by Sec synthase (SST).