Regulation of mitosis onset and thymidine kinase activity during the cell cycle of Physarum polycephalum plasmodia: Effect of fluorodeoxyuridine

The effects of fluorodeoxyuridine were investigated during three events of the cell cycle: S‐phase, mitosis, and the cyclic synthesis of thymidine kinase in the synchronous plasmodium of the myxomycete Physarum. DNA synthesis was inhibited, and there was limited action on other macromolecular syntheses. When DNA synthesis was slowed down, onset of the following increase of thymidine kinase synthesis occurred at approximately the same time as in the control, but mitosis was blocked in a very early prophase stage and metaphase was never observed. These effects were suppressed when the action of fluorode‐oxyuridine was prevented by the addition of thymidine to the medium. In agreement with the action of aphidicolin and hydroxyurea, these observations show that: (1) perturbation of the S‐phase does not prevent the nuclei from entering a very early prophase stage, but it does prevent them from proceeding through metaphase; (2) blockage of DNA synthesis does not perturb the normal timing of the triggering of thymidine kinase synthesis; and (3) the signal that triggers the arrest of thymidine kinase synthesis is postmitotic and does not require extensive DNA synthesis. In contrast with hydroxyurea and aphidicolin, in the presence of fluorodeoxyuridine metaphase was not observed. Thus, the triggering of thymidine kinase synthesis is unambiguously dissociated from metaphase and postmitotic events. Because synthesis of thymidine kinase remains under the control of temperature shifts from 22 to 32°C, a simple model of the cell cycle involving two regulatory pathways could account for the triggering of thymidine kinase synthesis, early prophase stage, and metaphase.