Original sequence divergence among Pseudomonas putida CadRs drive specificity

Bacteria, especially those living in soils, are in constant contact with metals. Transition metals like Fe or Zn, are required for proper growth. Some other metals like Cd or Hg are only toxic. Several systems exist to detoxify cells when these metals are present in concentrations harmful to biological systems. The expression of these systems is under control of specialized regulatory proteins able to detect metals and to regulate cognate detoxifying systems. In this work we report on the characterisation of the metallo-regulator CadR from Pseudomonas putida KT2440. By using gene reporter assays, we investigated the repertoire of metals detected by CadR. We show that CadR is much more responsive to Hg than to Cd, as compared to CadR from P. putida 06909. CadR from P. putida KT2440 differs in only 3 amino-acids in its metal-binding domain with respect to CadR from P. putida 06909. We show that these residues are important determinants of metal selectivity by engineering a modified CadR.