Biosynthesis of 4-Thiouridylate

The formation of 4-thiouridine through the transfer of sulfur from cysteine to a uridine in enzymatically activated tRNA is mediated in the Escherichia coli system by a sulfurtransferase which requires pyridoxal 5-phosphate. Resolution of the enzyme may be accomplished by hydroxylapatite chromatography. The sulfurtransferase is inactivated by incubation with hydroxylamine or potassium borohydride. It may be reactivated by a short treatment with pyridoxal 5'-phosphate, but not pyridoxal, pyridoxamine, or pyridoxamine 5'-phosphate. Incubation of the holoenzyme with amino acids leads to partial inactivation. This inactivation is alleviated if α-keto acids are present along with the amino acids.