Molecular phylogeny of the nickel-resistance gene nreB and functional role in the nickel sensitive symbiotic nitrogen fixing bacterium Sinorhizobium meliloti

Heavy-metal tolerance is a widespread phenotype in bacteria, particularly occurring in strains isolated from heavy-metal contaminated sites. Concerning nickel tolerance, the nre system is one of the most common. An ortholog of the nreB gene is present in the alfalfa symbiont Sinorhizobium meliloti also, which stir the attention on its functional role in such Ni-sensitive species and on the evolutionary relationships with Ni-resistant strain orthologs. Phylogenetic reconstruction and comparative genomics were performed to analyze the phylogenetic relationships of nreB orthologs, as well aa nreB deletion mutant S. meliloti strain was constructed and subjected to phenotypic analysis. Phylogenetic analysis of nreB genes indicated horizontal gene transfer events, possibly mediated via mobile genetic elements. Phenotype Microarray, biochemical and symbiotic analyses of the deletion mutant strain (Delta nreB) showed that in S. meliloti nreB is involved in the tolerance to several stresses other than Ni (mainly urea and copper), possibly partially mediated through the modulation of urease and hydrogenase activities. Obtained results allowed us to speculate that nreB is a highly mobile gene cassette, spread in the bacterial phylogenetic tree via many HGT events, which could have been recruited to confer nickel-tolerance in strains thriving in contaminated environments, by small changes linked to its basic functions (e.g. modulation of urease and hydrogenase activity).