Mitotic delay following inhibition by 5′-fluorodeoxyuridine of S-phase in Physarum is not due to delay in termination of S-phase

It has been known for several years that inhibition by 5′-fluorodeoxyuridine (FdUrd) of DNA synthesis in plasmodia of Physarum polycephalum delays subsequent nuclear mitosis. To test whether this delay is due to delay in the termination of S-phase, we blocked DNA synthesis with FdUrd + uridine for 3 h at different stages of S-phase, and in plasmodia with different cycle times. The results show that in short-cycling plasmodia the delay in mitosis can be as long as 9 h, despite little delay in termination of S-phase, and is longest when plasmodia are blocked in early S-phase. In plasmodia with long cycle times, no mitotic delay following 3 h inhibition by FdUrd of S-phase is observed. Our results suggest that mitotic delay after pulses of FdUrd is not due to delay in termination of S-phase, which therefore does not appear to ‘gate’ entry into a G2 period of fixed length. The fact that delay is longest after FdUrd blocks in early S-phase suggests that normal progress through S-phase, not its termination, is critical for the timing of the subsequent mitosis. This may reflect an obligate coupling between replication and transcription of specific genes needed for progress toward mitosis. The lack of mitotic delay in long-cycling plasmodia shows that S-phase-coupled processes need not act as ‘timers’ if other processes become rate-limiting.