Lead(II) resistance in Cupriavidus metallidurans CH34: interplay between plasmid and chromosomally-located functions

Proteome and transcriptome analysis, combined with mutagenesis, were used to better understand the response of Cupriavidus metallidurans CH34 against lead(II). Structural Pb(II)-resistance genes of the pMOL30-encoded pbrUTRABCD operon formed the major line of defense against Pb(II). Several general stress response mechanisms under the control of alternative sigma factors such as r24/rpoK, r32/rpoH and r28/fliA were induced. The expression of the pbrR2 cadA pbrC2 operon of the CMGI-1 region and the chromosomally encoded zntA were clearly induced in the presence of Pb(II), although their respective gene products were not detected via proteomics. After inactivation of the pbrA, pbrB or pbrD genes, the expression of the pbrR2 cadA pbrC2 operon went up considerably. This points towards synergistic interactions between pbrUTRABCD and pbrR2 cadA pbrC2 to maintain a low intracellular Pb(II) concentration, where pbrR2 cadA pbrC2 gene functions can complement and compensate for the mutations in the pbrA and pbrD genes. This role of zntA and cadA to complement for the loss of pbrA was further confirmed by mutation analysis. The pbrB::Tn(Km2) mutation resulted in the most significant decrease of Pb(II) resistance, indicating that Pb(II) sequestration, avoiding re-entry of this toxic metal ion, forms critical step in the pbr-encoded Pb(II) resistance mechanism.