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Background  Heavy metal Resistance-Nodulation-Division (HME-RND) efflux systems help Gram-negative bacteria to keep the intracellular homeostasis under high metal concentrations. These proteins constitute the cytoplasmic membrane channel of the tripartite RND transport systems.  Caulobacter crescentus  NA1000 possess two HME-RND proteins, and the aim of this work was to determine their involvement in the response to cadmium, zinc, cobalt and nickel, and to analyze the phylogenetic distribution and characteristic signatures of orthologs of these two proteins.    Results  Expression assays of the  czrCBA  operon showed significant induction in the presence of cadmium and zinc, and moderate induction by cobalt and nickel. The  nczCBA  operon is highly induced in the presence of nickel and cobalt, moderately induced by zinc and not induced by cadmium. Analysis of the resistance phenotype of mutant strains showed that the Δ czrA  strain is highly sensitive to cadmium, zinc and cobalt, but resistant to nickel. The Δ nczA  strain and the double mutant strain showed reduced growth in the presence of all metals tested. Phylogenetic analysis of the  C. crescentus  HME-RND proteins showed that CzrA-like proteins, in contrast to those similar to NczA, are almost exclusively found in the Alphaproteobacteria group, and the characteristic protein signatures of each group were highlighted.    Conclusions  The  czrCBA  efflux system is involved mainly in response to cadmium and zinc with a secondary role in response to cobalt. The  nczCBA  efflux system is involved mainly in response to nickel and cobalt, with a secondary role in response to cadmium and zinc. CzrA belongs to the HME2 subfamily, which is almost exclusively found in the Alphaproteobacteria group, as shown by phylogenetic analysis. NczA belongs to the HME1 subfamily which is more widespread among diverse Proteobacteria groups. Each of these subfamilies present distinctive amino acid signatures.

Two RND proteins involved in heavy metal efflux in Caulobacter crescentus belong to separate clusters within proteobacteria