Converts seryl-tRNA(Sec) to selenocysteinyl-tRNA(Sec) required for selenoprotein biosynthesis. H(+) + L-seryl-tRNA(Sec) + selenophosphate = L-selenocysteinyl-tRNA(Sec) + phosphate Aminoacyl-tRNA biosynthesis; selenocysteinyl-tRNA(Sec) biosynthesis; selenocysteinyl-tRNA(Sec) from L-seryl-tRNA(Sec) (bacterial route): step 1/1. Belongs to the SelA family.