Cycle dependent variation of X-ray effects on synchronous mitosis and thymidine kinase induction in Physarum polycephalum

Mitotic delays and macromolecular synthesis in G2 phase-irradiated plasmodia of Physarum polycephalum

Mitotically synchronous surface plasmodia of Physarum polycephalum were irradiated at different times of G2 phase with doses of UV ranging from 350 to 2800 Jm-2. The UV sensitivity, measured in terms of delay in progression towards mitosis, gradually declined through G2 phase to almost zero in early prophase, when irradiated with 350 Jm-2 of UV. However, with higher doses, 700 and 1400 Jm-2, late G2 was found to be even more susceptible than early G2. The delay in these cases rises for the last 0.1 part of the cycle up to a UV-transition point in early prophase. Thus, processes which were not sensitive to a lower dose of UV are affected by higher doses. Above 700 Jm-2, delay was not directly proportional to dose. With doses above 1400 Jm-2, the delays obtained were more variable, although some tendency for an increased delay is observed in plasmodia irradiated in late G2 when compared with those irradiated in middle G2. The least UV-sensitive phase is between -0.2 and -0.4 part of the cycle in relation to mitosis (0 point). Both RNA synthesis and protein synthesis are inhibited in plasmodia irradiated at any time during G2 phase with 1400 Jm-2 of UV. However, on an average, irradiation in late G2 caused the most inhibitions.

Cycloheximide-induced mitotic delay in Physarum polycephalum

Cycloheximide pulses applied to Physarum polycephalum surface plasmodia delay mitosis. Pulses applied in G2 cause a delay of mitosis which is linearly dependent on the phase in the cell cycle at which the pulse is applied. A 30 min pulse of 10 micrograms/ml cycloheximide starting in G2 at time t after mitosis induces an excess delay (delay in excess of pulse duration) of the next mitosis of (0.55) t-1.3 h. The excess delays induced by 7 h pulses during G2 are at most 1 h larger. Pulses applied less than 30 min before mitosis induce only small delays.