Tracking operator state fluctuations in gene expression in single cells.

We report the results of operator state fluctuations in gene expression for the entire bacterial growth cycle, using single-cell analysis and synthetic unregulated and negative-feedback transcription regulatory gene circuits. In the unregulated circuit, during the cell cycle, we observe a crossover from log-normal-to-normal distribution of expressed proteins and an unusual linear dependence of their standard deviation on the mean gene expression levels. With negative-feedback circuits we find the existence of bimodality as the cell cycle progresses. We suggest that such long-tail and bimodal distributions may be used as selection mechanisms in developmental switches and for assigning cell identity.