Single Cell Tracking of Transcription Factor Dynamics in a Eukaryotic Bistable System

The ability of cells to differentiate and subsequently maintain their identity through multiple rounds of cell division relies on complex transcriptional networks. Frequently these networks exhibit bistability in which a biological system can toggle between two stable steady states. A bistable transcriptional network in the human commensal yeast Candida albicans controls an epigenetic switch between two distinct cell types, termed white and opaque. This network shows many features of those in higher eukaryotes including a complex network structure, long regulatory regions and a high degree of cell type stability. Using time-lapse fluorescent microscopy and microfluidics, we quantify single cell dynamic changes in the expression of major transcription factors determining cell state. We observe transitions in cell state over a number of cell divisions with both seemingly stochastic and deterministic phases. We investigate the direct molecular interactions between the transcription factors using synthetic inducible systems in a heterologous cellular environment. We find that both inhibitory and activating interactions are possible dependent on relative protein concentrations. We use mathematical models to explain transcription factor dynamics during rare events of cell switching. Quantitative molecular analysis of bistability in the white-opaque circuit can serve as a model for the general understanding of complex circuits.