Stability of epigenetic state: the lysogen of phage λ

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In this work, we present a new perspective of understanding the stability of epigenetic states. The systen we examined is one of classical systems of epigenetic regulation: the lysogeny state of bacteriophage λ. We use single-molecule-resolution mRNA measurements in individual cells to characterize the fluctuations around the steady lysogenic state and predict the rate of escape from lysogeny. The predictions exhibit excellent agreement with experiments over 8 decades of time scales. Epigenetic stability shows a simple exponential dependence on the number of gene-activity events during the life of the cell. This independence from physiological details suggests that a quantitative prediction of epigenetic stability may also be possible in more complex systems.