TonB-Dependent Receptors in Nitrogen-Fixing Nodulating Bacteria

TonB-dependent receptors (TBDRs) allow Gram-negative bacteria to uptake scarce resources from competitive environments with very high affinity. Early reports on TBDRs focused on the uptake of siderophore-iron complexes but recent studies have showed that the spectrum of ligands includes sugars, vitamins, heme, and other non-ferrous cations. To investigate the possible roles of TBDRs in nitrogen-fixing, nodulating bacteria, a bioinformatics approach was adopted to identify their presence in the genome of 13 selected rhizobacteria. The number of TBDR-like genes ranged from 1 (Mesorhizobium loti MAFF303099) to 14 (Azorhizobium caulinodans ORS 571 and Methylobacterium nodulans ORS 2060). These TBDRs can be largely grouped into two clusters, the 'heme' cluster and the 'ironsiderophores' cluster. The only exceptions are a putative nickel-specific TBDR (bll6948) in Bradyrhizobium japonicum USDA110 and two putative sugar-specific TBDRs in B. japonicum USDA110 and A. caulinodans ORS571 genomes, respectively. No TBDR-like sequences lie in the 'Vitamin B12' or 'Non-Fe cations' clusters. A model of the biological roles of TBDRs in free-living and symbiotic states is proposed for B. japonicum.