New Cell Fate Potentials and Switching Kinetics Uncovered in a Classic Bistable Genetic Switch

Bistable switches are common gene regulatory motifs directing two mutually exclusive gene expression states, and consequently distinct cell fates. Theoretical studies suggest that the simple circuitry of bistable switches is sufficient to encode more than two cell fates due to the non-equilibrium, heterogeneous cellular environment, allowing a high degree of adaptation and differentiation. However, new cell fates arising from a classic bistable switch without rewiring the circuitry have not been experimentally observed. By developing a new, dual single-molecule gene-expression reporting system in live E. coli cells, we investigated the expression dynamics of two mutually repressing transcription factors, CI and Cro, in the classic genetic switch of bacteriophage λ. We found that in addition to the two expected high-Cro and high-CI production states, there existed two new ones, in which neither CI nor Cro was produced, or both CI and Cro were produced. We constructed the corresponding potential landscape and mapped the transition kinetics between the four production states, providing insight into possible state-switching rates and paths. These findings uncover new cell fate potentials beyond the classical picture of λ switch, and open a new window to explore the genetic and environmental origins of the cell fate decision-making process in gene regulatory networks.