The present study revealed that Lrp, a leucine-responsive regulatory protein, is involved in the regulation of cadBA transcription through activation of $P_{cadBA}$. The influence of Lrp on $P_{cadBA}$ was mediated by CadC, and thereby, CadC was able to compensate for the lack of Lrp in the activation of $P_{cadBA}$. Western blot analyses and EMSA demonstrated that the cellular level of CadC was not significantly affected by Lrp, and that Lrp exerted its effect by directly binding to $P_{cadBA}$. These combined results suggested that CadC and Lrp function cooperatively to activate the $P_{cadBA}$ rather than sequentially in a regulatory cascade.