On the Problem of an Enzymatic Regulation of DNA Synthesis in Chlorella

Uridine kinase and thymidylate kinase were investigated in Chlorella cultures synchronized by alternating periods of 16 hours light and 8 hours darkness. The enzymes were activated and stabilized by mercaptoethanol and glutathione. Both enzyme activities increased periodically between the 8th and 18th hour after beginning of the light period. The increase was blocked in the presence of 15 μM actidione. When the light was turned off at the 12th hour the enzyme accumulation ceased after another 90 min. DNA synthesis took place between the 10th and 18th hour and subsequently ceased completely, although high dTMP kinase activities were still observed. The results do not support the hypothesis that DNA synthesis in Chlorella is regulated by thymidylate kinase.