. "A group of enzymes transferring sulfur to various nucleotides in a tRNA chain, producing residues such as 4-thiouridylate." . "With some enzymes mercaptopyruvate can act as sulfur donor." . . . "tRNA sulfurtransferase" . "L-cysteine + 'activated' tRNA = L-serine + tRNA containing a thionucleotide." .