MerD and MerR from Tn4378 found in Ralstonia metallidurans CH34 were purified to homogeneity after overexpression in Escherichia coli. Using electrophoretic mobility shift assays and footprinting experiments, we found that MerD cannot bind to DNA. However, in vitro MerD can form a ternary complex in association with merOP and MerR. The presence of MerD in this complex was demonstrated by Western analysis with antibodies to MerD. To our knowledge, this is the first description of such a ternary complex between MerD-MerR and DNA. The formation and stability of this ternary complex are dependent on the relative concentration of the two proteins and modulated by the presence of mercury. We postulate that MerD could displace Hg-bound MerR from the mer operator to allow new synthesis of metal-free MerR able to switch off the induction of the mer genes when the external mercury is exhausted. This could fully explain how MerD can be a co-regulator repressing the induction of the mer operon.