The cholera disease still represents a dangerous fatal disease in developing countries. To gain control of this diarrheal infection one has to understand the virulence expression system of the gram-negative pathogen Vibrio cholerae, the so called ToxR-regulon. Despite extensive research on the ToxR-regulon still many questions remain unanswered. Especially the mechanism of action of the membrane regulator proteins ToxR and ToxS is yet unresolved. The ToxR protein is denoted as an important part of the virulence expression system in V. cholerae, having the ability to positively and negatively influence the expression respectively. In the current model ToxR is activated by external stimuli sensed by its periplasmic C-terminal domain (pToxR), which contains two cysteine residues. It is furthermore supposed to interact with the periplasmic part of the membrane attached ToxS protein. The interaction is proposed to stabilize the active state of ToxR and additionally enhances the DNA-binding ability of ToxR. A down regulation of the ToxR-regulon is achieved by the external addition of the hydrophobic dipeptide cyclo(Phe-Pro). The negative feedback mechanism seems to be dependent on ToxR, more precisely the periplasmic part, leading to the assumption that the pToxR sensor domain may interact directly with the inhibitor molecule. This work focuses on the interaction between the periplasmic parts of ToxR and ToxS. Moreover, experiments are made to reveal information about the inhibiting mechanism of cyclo(Phe-Pro).