Induction kinetics of a conditional pH stress response system in Escherichia coli

The analysis of stress response systems in microorganisms can reveal molecular strategies for regulatory control and adaptation. Here, we focus on the Cad module, a subsystem of E. coli’s response to acidic stress, which is conditionally activated at low pH only when lysine is available. When expressed, the Cad system counteracts the elevated H+ concentration by converting lysine to cadaverine under the consumption of H+, and exporting cadaverine in exchange for external lysine. Surprisingly, the cad operon displays a transient response, even when the conditions for its induction persist. To quantitatively characterize the regulation of the Cad module, we have experimentally recorded and theoretically modeled the dynamics of important system variables. We establish a quantitative model that adequately describes and predicts the transient expression behavior for various initial conditions. Our quantitative analysis of the Cad system supports a negative feedback by external cadaverine as the origin of the transient response. Furthermore, the analysis puts causal constraints on the precise mechanism of signal transduction via the regulatory protein CadC.

💡 Research Summary

The paper investigates the transient induction of the Cad module, a conditional pH‑stress response system in Escherichia coli that is activated only when external pH is low and lysine is available. The Cad operon encodes CadA, a lysine decarboxylase, and CadB, a lysine/cadaverine antiporter. Together they convert lysine to cadaverine while consuming intracellular protons, thereby mitigating acid stress. Paradoxically, even under sustained inducing conditions, cad operon expression rises sharply and then declines, producing a transient response.

To elucidate the underlying regulatory mechanism, the authors performed a series of time‑course experiments. E. coli MG1655 cultures were shifted to pH 5.5 medium containing 10 mM lysine. At intervals from 0 to 180 min they measured cadBA mRNA levels by quantitative RT‑PCR, extracellular cadaverine concentration by HPLC, and bulk pH with a calibrated electrode. The data showed a rapid transcriptional peak at ~30 min, followed by a return to baseline within two hours, while extracellular cadaverine accumulated to ~0.5 mM during the same period and the medium pH rose modestly.

Guided by these observations, the authors constructed a deterministic ordinary‑differential‑equation (ODE) model that captures three core variables: the active form of the membrane‑bound regulator CadC (A), cadBA transcript abundance (m), and extracellular cadaverine (C). CadC activation depends on low pH and lysine binding, whereas cadaverine provides a negative feedback that reduces CadC activity once C exceeds a threshold (C_thr). The model equations are:

dA/dt = k₁·f(pH)·g(Lys)·