Mitosis and protein synthesis. 2. Synthesis of protein and RNA in synchronous HeLa S-3 cell populations entering and leaving M-phase of the cell cycle.

Mitotic HeLa cells collected by shake-off synchronization with minimum perturbation showed a reduced rate of protein synthesis which began to fall 15-30 min after metaphase, and at its lowest fell to 55-65% of the average interphase value in late metaphase. Similar rate determination with 3H-leucine incorporation in synchronous M-phase cells moving out of mitosis indicated that the protein synthetic rate had returned to nearly normal interphase levels by the end of telophase. RNA synthesis measured with 3H-uridine pulses showed a similar pattern of suppression, with a low of 25-35% the interphase rate in metaphase/anaphase and a return to control levels within about 15-30 min of the start G1. The downturns in both protein and RNA synthesis were progressive and greatly pronounced in cells prevented from leaving metaphase by alkaloid treatment. Further studies are reported on the separate and combined effects of action of actinomycin D and alkaloids on macromolecular synthesis in cells entering and leaving M-phase. The findings suggest that only a transient and partial suppression of protein and RNA synthesis occurs during mitosis, a situation which can lead to exaggerated and progressive decline of these activities in cells which are not permitted to complete mitosis on schedule.