Regulates the lysine- and pH-dependent expression of the lysine decarboxylase CadA and the cadaverine-lysine antiporter CadB (PubMed:1370290, PubMed:16491024, PubMed:18086202, PubMed:21216950). At low external pH, and in the presence of external lysine, CadC activates transcription of the cadBA operon by binding directly to two sites, Cad1 and Cad2, within the cadBA promoter region (Pcad) (PubMed:16491024, PubMed:28432336). Preferentially binds to AT-rich regions within the Cad1 promoter (PubMed:28432336). Activation of CadC requires two stimuli, lysine and low pH (PubMed:18086202, PubMed:21216950, PubMed:21486484, PubMed:24056175). CadC shows an extremely low affinity for lysine, and it senses the extracellular lysine not directly but indirectly via interaction with the lysine permease LysP (PubMed:18086202). At a low external lysine concentration, CadC is inactivated by an interaction with LysP. When lysine is abundantly available, the interaction between LysP and CadC is released, and CadC becomes susceptible to activation by low pH (PubMed:18086202, PubMed:24056175). Acidification of the external milieu is sensed by protonation of a patch of acidic amino acids within the periplasmic domain and associated to conformational and/or oligomerization effects (PubMed:21216950). The pH-dependent regulation may be due to the presence/absence of a disulfide bond within the periplasmic domain (PubMed:21486484). At pH 7.6, a disulfide bond is found in the inactive state of CadC. At pH 5.8, disulfide bond formation is prevented, which transforms CadC into a semi-active state with respect to both the pH and the lysine stimuli (PubMed:21486484). Activity is also feedback inhibited by cadaverine (PubMed:22999955). Cadaverine binds first to the central cavity, which putatively triggers intramolecular rearrangements to expose the binding sites for cadaverine at the dimer interface, which inactivates CadC and consequently shuts off transcription of the cadBA operon (PubMed:22999955). Homodimer (PubMed:21308846, PubMed:24946151). Dimerization of the periplasmic domain is required for activation of the cadBA operon (PubMed:24946151). Interacts strongly with the lysine permease LysP in the absence of lysine or at low lysine concentrations (PubMed:18086202, PubMed:24056175). Interaction is markedly attenuated under increasing lysine levels (PubMed:24056175). Concomitant pH-dependent protonation of periplasmic amino acids in both proteins dissolves their electrostatic connections resulting in further destabilization of the CadC/LysP interaction (PubMed:24056175). Membrane insertion requires the SecA translocase. Translation requires the translation elongation factor P (EF-P). This requirement is reduced as the number of consecutive Pro residues in the protein is decreased. Contains a cytoplasmic DNA-binding N-terminal domain, a transmembrane domain and a periplasmic C-terminal domain (PubMed:7830562, PubMed:24946151). A cluster of aromatic amino acids within the transmembrane domain is important for lysine-dependent regulation of CadC (PubMed:18086202). The transmembrane domain of CadC is important for the interaction with LysP, but the periplasmic domain of CadC is also important in facilitating correct positioning of the two proteins (PubMed:24056175). A cluster of negatively charged amino acids (Asp-198, Asp-200, Glu-461, Glu-468 and Asp-471) forms a negatively charged patch on the surface of the periplasmic domain and is crucial for pH detection (PubMed:21216950). Dimerization requires the periplasmic domain, but not the transmembrane domain (PubMed:24946151). Conformational changes at the dimer interface may functionally influence the transcriptional activity (PubMed:21308846). Contains a functionally important disulfide bond, which may provide structural support for the pH-dependent activation via a switch of the sensor between the inactive and active state. Loss of Pcad activity.