  Multistable gene regulatory systems sustain different levels of gene
expression under identical external conditions. Such multistability is used to
encode phenotypic states in processes including nutrient uptake and persistence
in bacteria, fate selection in viral infection, cell cycle control, and
development. Stochastic switching between different phenotypes can occur as the
result of random fluctuations in molecular copy numbers of mRNA and proteins
arising in transcription, translation, transport, and binding. However, which
component of a pathway triggers such a transition is generally not known. By
linking single-cell experiments on the lactose-uptake pathway in E. coli to
molecular simulations, we devise a general method to pinpoint the particular
fluctuation driving phenotype switching and apply this method to the transition
between the uninduced and induced states of the lac genes. We find that the
transition to the induced state is not caused only by the single event of
lac-repressor unbinding, but depends crucially on the time period over which
the repressor remains unbound from the lac-operon. We confirm this notion in
strains with a high expression level of the repressor (leading to shorter
periods over which the lac-operon remains unbound), which show a reduced
switching rate. Our techniques apply to multi-stable gene regulatory systems in
general and allow to identify the molecular mechanisms behind stochastic
transitions in gene regulatory circuits.
