Reduction of UV-induced mitotic delay by caffeine in BUdR-substituted plasmodia of Physarum polycephalum.

Chromosomal DNA of the synchronously mitotic plasmodia of P. polycephalum was substituted with 5-bromo-2'-deoxyuridine, by growing the plasmodia during S phase, on a medium containing this nucleoside analog. A strong synergism was observed between bromodeoxyuridine and UV-irradiation, in late G2-irradiated plasmodia in that, the mitotic delay obtained in them was much more than a simple sum of the delays induced by these two agents individually. It was also observed that the mitotic delay in this system is reduced significantly by different concentrations of caffeine applied immediately after irradiation and there was a stage specificity in this effect. The reduction in mitotic delay was maximum (80%) in those plasmodia irradiated 20-30 min before control metaphase, when mitogenic factors also reach their maximum activity in this system. It is proposed that the mitotic delay reducing effect of caffeine is due to its ability to promote the activity of the mitogenic factors, largely independent of the system which is responsible for monitoring the state of the chromosomal DNA.

Differential effect of cordycepin on S and G2 phases of cell cycle in plasmodia of Physarum polycephalum Schw.

Effect of pulse treatments of cordycepin, an analog of adenosine, on S and G2 phases of the cell cycle of the mitotically synchronous plasmodia of Physarum polycephalum has been studied. Various concentrations of the drug (50-200 micrograms ml-1) were found to be effective in delaying mitosis by several hours in both the phases. However, there was a significant increase in mitotic delay in those treated during G2. It is suggested that this extra delay during G2 could be due to the transcriptive level inhibition of specific RNA types, such as that of tubulins, whose gene activity is cell cycle regulated and turned on during G2 in Physarum, or alternatively because of a deficiency for ATP and the consequent inhibition of events such as mitotic spindle assembly and phosphorylation of histones.