Kinetics of the cellular intake of a gene expression inducer at high concentrations in the media

From in vivo single-event measurements of the transient and steady-state transcription activity of a single-copy lac-ara-1 promoter in Escherichia coli, we study the intake kinetics of its inducer (IPTG) from the media. We show that the empirical data is well-fit by a model of intake that assumes a bilayer membrane coupled to a stochastic, multi-step transcription process. Next, using this model, we show that for a wide range of extracellular inducer levels (up to 1.25mM) the intake process is diffusive-like, suggesting unsaturated membrane permeability. Finally, we infer from the model that an inducer molecule takes, on average, 31.7 minutes to travel from the periplasm to the cytoplasm, which suggests that this event is a key rate-limiting process in the response time of the cells to the appearance of inducers in the media. The methodology here proposed should be of assistance to future studies of cellular intake mechanisms at the single-cell level.