Thymidine Phosphorylating Enzymes in Acetabularia: Evidence for the Occurrence of a Thymidine Kinase

A test system allowing the estimation of the dT phosphorylating enzyme activity of one Acetabularia cell and of singular cell parts was developed. The occurrence in Acetabularia of a true dT kinase (EC. 2.7.1.75) was demonstrated. This enzyme phosphorylates dT in the presence of ATP but not of AMP. dT kinase of Acetabularia is mainly bound to the particular fraction and a high percentage is localized outside the chloroplasts and mitochondria. After removal of the Triton X 100 (1%, by vol.) soluble part of the crude extract, a strong feedback inhibition by dTTP of the enzyme found in the pellet was observed. This enzyme may be solubilized with ionic detergents. The dT phosphorylating enzyme of the sediment (dT kinase) is distinct from supernatant phosphorylating enzyme in view of substrate specificity, stability, solubility, dTTP-inhibition, molecular mass, and K$_{\text{M}}$ value for dT.