Discovery of a haem uptake system in the soil bacterium Bradyrhizobium japonicum

In Bradyrhizobium japonicum, the nitrogen‐fixing symbiont of soybeans, we have identified a haem uptake system, Hmu, that comprises a cluster of nine open reading frames. Predicted products of these genes include: HmuR, a TonB‐dependent haem receptor in the outer membrane; HmuT, a periplasmic haem‐binding protein; and HmuUV, an ABC transporter in the inner membrane. Furthermore, we identified homologues of ExbBD and TonB, that are required for energy transduction from the inner to the outer membrane. Mutant analysis and complementation tests indicated that HmuR and the ExbBD–TonB system, but not the HmuTUV transporter, are essential for haem uptake or haem acquisition from haemoglobin and leghaemoglobin. The TonB system seems to be specific for haem uptake as it is dispensable for siderophore uptake. Therefore, we propose the existence of a second TonB homologue functioning in the uptake of Fe‐chelates. When tested on soybean host plants, hmuT‐hmuR and exbD‐tonB mutants exhibited wild‐type symbiotic properties. Thus, haem uptake is not essential for symbiotic nitrogen fixation but it may enable B. japonicum to have access to alternative iron sources in its non‐symbiotic state. Transcript analysis and expression studies with lacZ fusions showed that expression of hmuT and hmuR is induced under low iron supply. The same was observed in fur and irr mutant backgrounds although maximal induction levels were decreased. We conclude either that both regulators, Fur and Irr, independently mediate transcriptional control by iron or that a yet unknown iron regulatory system activates gene expression under iron deprivation. An A/T‐rich cis‐acting element, located in the promoter region of the divergently transcribed hmuTUV and hmuR genes, is possibly required for this type of iron control.