The plasmodia of Physarum polycephalum, an elegant system to demonstrate the importance of genome integrity in traversing the G2/M checkpoint of the cell cycle

In this communication we show the usefulness of the plasmodia of Physarum polycephalum to demonstrate important concepts in cell cycle regulation, such as the need for an undamaged chromosomal DNA to allow the transition from late G2 to prophase (G2/M checkpoint). In Physarum, chromosomal DNA synthesis is confined approximately to the first one-third of the cell cycle which comprises the S phase, and rDNA and mitochondrial DNA are synthesized throughout the cycle except during mitosis and early S phase. Taking advantage of these facts, DNA of the plasmodia was substituted with 5-bromo-2′-deoxyuridine at a concentration where it had no effect on DNA synthesis per se during different phases of the cell cycle before exposure to UV during late G2. A strong synergistic effect between BrdU and UV, in terms of mitotic delay, was observed only in plasmodia treated with the base analogue during the early S phase, thereby showing that the integrity of chromosomal DNA is an important prerequisite for the initiation of mitosis. It is suggested that this system is ideal for molecular level studies on this aspect of checkpoint operation.