Cell cycle arrest and morphological alterations following microinjection of NIH3T3 cells with Pura alpha

Levels of Pur alpha, a protein implicated in control of both DNA replicatio n and gene transcription, fluctuate during the cell cycle, being lowest in early S phase and highest just after mitosis. Here we have employed a new v ideo time-lapse technique enabling us to determine the cell cycle position of each cell in an asynchronous culture at a given time and to ask whether introduction of Pur alpha protein at specific times can affect cell cycle p rogression. Approximately 80% of all NIH3T3 cells injected with Pur alpha w ere inhibited from passing through mitosis, Cells injected with Puree durin g S or G2 phases were efficiently blocked with a 4N (G2 phase) DNA level, a s determined by quantitative DNA photometry of individual cells. Of the cel ls injected with Pur alpha. during G1 phase, 40% experienced a rapid cell d eath characterized by extreme cellular fragmentation. Of those G1 injected cells which remained viable, approximately equal numbers were arrested with either 2N or 4N DNA levels. Cells arrested by Pur alpha in G2 phase grew t o cover a large surface area. These results link fluctuations in Pur alpha levels to aspects of cell cycle control.