Ribonucleoside diphosphate reductase and thymidylate synthase activities follow a virtually identical peak pattern during the cell cycle of Scenedesmus obliquus, coinciding with DNA synthesis. Both enzyme change in parallel when cultures are treated with inhibitors specific for one of them: 5-fluoro-2′-deoxyuridine (a thymidylate synthesis inhibitor) stimulates 30-fold, and hydroxyurea, inhibitory to eukaryotic ribonucleotide reduction, also suppresses thymidylate synthase. We conclude that the two enzymes of deoxyribonucleotide formation are subject to one common intracellular control mechanism.
