title: TonB-dependent receptors in nitrogen-fixing nodulating bacteria.
abstract: 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.
authors.affiliation: School of Biological Sciences, University of Hong Kong, Pokfulam, Hong Kong SAR, China. bllim@hku.hk
authors.forename: Boon L
authors.initials: BL
authors.lastname: Lim
mesh_terms: D020561:Alphaproteobacteria / Q000235:genetics* / Q000378:metabolism; D001425:Bacterial Outer Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D019295:Computational Biology / Q000379:methods*; D016680:Genome, Bacterial / Q000235:genetics; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D008954:Models, Biological; D005810:Multigene Family / Q000235:genetics*; D009586:Nitrogen Fixation / Q000235:genetics / Q000502:physiology; D011956:Receptors, Cell Surface / Q000235:genetics* / Q000378:metabolism; D053204:Root Nodules, Plant / Q000235:genetics / Q000502:physiology
chemical_list: D001425:Bacterial Outer Membrane Proteins; D001426:Bacterial Proteins; D008565:Membrane Proteins; D011956:Receptors, Cell Surface; C057616:tonB protein, Bacteria