Converts O-phosphoseryl-tRNA(Sec) to selenocysteinyl-tRNA(Sec) required for selenoprotein biosynthesis. H2O + O-phospho-L-seryl-tRNA(Sec) + selenophosphate = L-selenocysteinyl-tRNA(Sec) + 2 phosphate Aminoacyl-tRNA biosynthesis; selenocysteinyl-tRNA(Sec) biosynthesis; selenocysteinyl-tRNA(Sec) from L-seryl-tRNA(Sec) (archaeal/eukaryal route): step 2/2. Homotetramer composed of two homodimers. Expressed at the promastigote stage. Impaired selenocysteine incorporation into proteins, such as SelT, SelK and SelTryp (PubMed:26586914). No defect in promastigote growth rate and in their capacity to infect host macrophages (PubMed:26586914). Belongs to the SepSecS family.