2.9.1.2 L-phosphoseryl-tRNASec + selenophosphate selenocysteine is the only genetically encoded amino acid in humans whose biosynthesis occurs on its cognate transfer RNA (tRNA). O-Phosphoseryl-tRNA:selenocysteinyl-tRNA synthase catalyzes the final step of selenocysteine formation by a tRNA-dependent mechanism 2.9.1.2 O-phospho-L-seryl-tRNASec + selenophosphate L-phosphoseryl-tRNA is the crucial precursor for L-selenocysteinyl-tRNA formation in archaea and eukarya. Selenocysteine formation is achieved by a two-step process: O-phosphoseryl-tRNASec kinase (PSTK) phosphorylates the endogenous Ser-tRNASec to O-phosphoseryl-tRNASec, and then this misacylated amino acid-tRNA species is converted to L-selenocysteinyl-tRNASec by Sep-tRNA:Sec-tRNA synthase (SepSecS) 2.9.1.2 O-phospho-L-seryl-tRNASec + selenophosphate L-phosphoseryl-tRNA is the crucial precursor for L-selenocysteinyl-tRNA formation in archaea and eukarya. Selenocysteine formation is achieved by a two-step process: O-phosphoseryl-tRNASec kinase phosphorylates the endogenous L-seryl-tRNASec to O-phospho-L-seryl-tRNASec, and then this misacylated amino acid-tRNA species is converted to L-selenocysteinyl-tRNASec by Sep-tRNA:Sec-tRNA synthase 2.9.1.2 O-phospho-L-seryl-tRNASec + selenophosphate null mutants of SepSecS abolish selenoprotein synthesis, demonstrating the essentiality of the enzyme for the formation of L-selenocysteinyl-tRNASec. Growth of the knockout strain is not impaired. Thus, unlike mammals, trypanosomes do not require selenoproteins for viability 2.9.1.2 O-phospho-L-seryl-tRNASec + selenophosphate SepSecS is a key enzyme required for the synthesis of the trypanosomal selenoproteins. The enzyme does not affect growth of bloodstream forms of Trypanosoma brucei 2.9.1.2 O-phospho-L-seryl-tRNASec + selenophosphate the micronutrient selenium is present in proteins as selenocysteine. In eukaryotes and archaea, selenocysteine is formed in a tRNA-dependent conversion of O-phosphoserine by O-phosphoseryltRNA:selenocysteinyl-tRNA synthase 2.9.1.2 O-phospho-L-seryl-tRNASec + selenophosphate + H2O -