The Biosynthesis of 4-Thiouridylate

Abstract The sulfurtransferase system which forms 4-thiouridylate in tRNA has been separated into two activities from Escherichia coli B. The system is stable and is freed of most of the other thionucleotide-forming activities. The first enzyme has absolute requirements for ATP, a divalent metal ion (optimally Mg2+), tRNA, and a sulfhydryl compound. The second enzyme requires cysteine as the sulfur donor and the product of the first reaction. β-Mercaptopyruvate is inactive either as a cofactor or as a sulfur donor when the enzymes are prepared using the dextran partition method. Both E. coli B and yeast tRNA can serve as sulfur acceptors, but no other polyribo- or polydeoxyribonucleotide sulfur acceptors have been found.