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 'iron-siderophores' 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.