Host-specific symbiotic requirement of BdeAB, a RegR-controlled RND-type efflux system in Bradyrhizobium japonicum.

Multidrug efflux systems not only cause resistance against antibiotics and toxic compounds but also mediate successful host colonization by certain plant-associated bacteria. The genome of the nitrogen-fixing soybean symbiont Bradyrhizobium japonicum encodes 24 members of the family of resistance/nodulation/cell division (RND) multidrug efflux systems, of which BdeAB is genetically controlled by the RegSR two-component regulatory system. Phylogenetic analysis of the membrane components of these 24 RND-type transporters revealed that BdeB is more closely related to functionally characterized orthologs in other bacteria, including those associated with plants, than to any of the other 23 paralogs in B. japonicum. A mutant with a deletion of the bdeAB genes was more susceptible to inhibition by the aminoglycosides kanamycin and gentamicin than the wild type, and had a strongly decreased symbiotic nitrogen-fixation activity on soybean, but not on the alternative host plants mungbean and cowpea, and only very marginally on siratro. The host-specific role of a multidrug efflux pump is a novel feature in the rhizobia-legume symbioses. Consistent with the RegSR dependency of bdeAB, a B. japonicum regR mutant was found to have a greater sensitivity against the two tested antibiotics and a symbiotic defect that is most pronounced for soybean.