The Interplay of AphB and CadC to Activate Acid Resistance of .

Bacteria have evolved different systems to sense and adapt to acid stress. For example, Vibrio campbellii, a marine pathogen for invertebrates, encounters acidic conditions in the digestive glands of shrimp. The main acid resistance system of is the Cad system, which is activated when cells are in a low-pH, amino acid-rich environment. The Cad system consists of the pH-responsive transcriptional activator CadC, the lysine decarboxylase CadA, and the lysine/cadaverine antiporter CadB. In many species, the LysR-type transcriptional regulator AphB is involved in the regulation of the Cad system, but its precise role is unclear. Here, we examined AphB of and in the context of Cad activation. At low pH, an deletion mutant was less able to grow and survive compared with the wild-type because it did not excrete sufficient alkaline cadaverine to increase the extracellular pH. AphB was found to upregulate the transcription of thereby increasing its protein copy number per cell. Moreover, AphB itself was shown to be a pH-sensor, and binding to the promoter increased under low pH, as shown by surface plasmon resonance spectroscopy. By monitoring the activation of the Cad system over a wide range of pH values, we found that AphB-mediated upregulation of not only adjusts CadC copy numbers depending on acid stress strength, but also affects the response of individual cells and thus the degree of heterogeneous Cad system activation in the population. Acid resistance is an important property not only for neutralophilic enteric bacteria such as Escherichia, , and Salmonella, but also for . To counteract acidic threats, the marine Vibrio campbellii, a pathogen for various invertebrates, activates the acid-resistance Cad system. The transcriptional activator of the Cad system is CadC, an extracellular pH-sensor. The expression of is upregulated by the transcriptional regulator AphB to achieve maximum expression of the components of the Cad system. studies demonstrate that AphB binds more tightly to the DNA under low pH. The interplay of two pH-responsive transcriptional activators allows tight control of the activity of the Cad system.