Converts O-phospho-L-seryl-tRNA(Cys) (Sep-tRNA(Cys)) to L-cysteinyl-tRNA(Cys) (Cys-tRNA(Cys)). H(+) + hydrogen sulfide + O-phospho-L-seryl-tRNA(Cys) = L-cysteinyl-tRNA(Cys) + phosphate Homodimer. Probably interacts with SepRS. Belongs to the SepCysS family.