Different Fate of Sibling Cells upon Inhibition of Transcription in G1

Abstract In normally cycling populations of NIH3T3 cells, both sibling cells generated by a mitotic division require similar periods of time for completion of the cell cycle, with variations (intersibling times) of ⩽3 h for >90% of the population. In this study, we analyzed by time-lapse video recording the effect of the RNA polymerase II inhibitor α-amanitin on the intersibling times of cells exposed to the drug at defined postmitotic ages. Our results led to the identification of three subpopulations of NIH3T3 cells. In the first one, both sibling cells were highly sensitive to α-amanitin. These cells were exposed to the inhibitor at a postmitotic age of ≤2 h. The second subpopulation was composed of cells where one sibling showed normal kinetics of cell cycle progression, while the other sibling had an increased cell cycle length (intersibling times of up to 12 h) or did not divide at all during the period of observation. These cells were 2-8 h old at the time of treatment. In the third subpopulation, representing cells at later stages in the cell cycle, no significant increase in intersibling times was observed. These data indicate that α-amanitin increases the intersibling times of cells exposed to the drug during G 1 . In addition, our results suggest that the variable-length period of G 1 , thought to be located in late G 1 , is dependent on RNA polymerase II transcription.