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. Belongs to the SepSecS family.