Ralstonia metallidurans strain CH34 is resistant to nickel and cobalt cations. Resistance is mediated by the cnr determinant located on plasmid pMOL28, which encodes a proton-driven efflux pump. The cnr genes are organized in two operons, cnrYXH and cnrCBA. As revealed by primer extension, transcription from these operons is initiated from promotors located upstream of the cnrY and cnrC genes. Half-maximum activation from the cnrCBA operon was determined using appropriate lacZ operon fusions and was shown to occur at an Ni2+ concentration of 50 µM. The CnrH gene product, which is required for expression of both operons, is a sigma factor belonging to the sigmaE family with specifity for the promotors cnrYp and cnrCp in in vitro transcription assays. CnrH activity seems to be governed by the membrane-bound CnrY and CnrX gene products in response to Ni2+ . The periplasmic sensor protein CnrX binds 3 Ni2+ per polypeptide chain. As shown with a yeast two hybrid system, the periplasmic nickel-binding protein CnrX interacts with the transmembrane protein CnrY. CnrX probably acts as a protease specific for CnrY, however, a glutamate residue in a HEXXH-motif in CnrX was not essential for CnrX activity. Finally, the CnrYX-related puative anti sigma factor complex NccYX from Achromobacter xylosoxydans 31A was shown to be able to regulate activity of CnrH, however sigma-factor NccH does not initiate transcription at cnr-promotors.