A survey of TonB-dependent receptors in fluorescent pseudomonads

For bacteria with an aerobic lifestyle, iron is in the oxidized Fe(3+) form, hence poorly soluble. The solution is the synthesis and excretion of siderophores with a high affinity for iron. These ferrisiderophores are recognized by TonB-dependent outer membrane receptors in Gram-negative bacteria. Haem is also a source of iron and is captured via TonB-dependent receptors as well. In many cases bacterial genomes encode genes for receptors for siderophores produced by other microorganisms (xenosiderophores). Pseudomonads are known for their high adaptive capacity and it is therefore not surprising to find a relatively large number of genes encoding these receptors. In this study we analysed the genomes of three fluorescent pseudomonads available in the Pseudomonas genome database (http://www.pseudomonas.com; P. aeruginosa, P. putida, P. syringae) in order to extract the genes coding for TonB-dependent receptors. As expected we observed differences between species for the number of receptors. We also report differences within species, suggesting the acquisition of some genes via horizontal gene transfer, including those coding for the ferripyoverdine receptors. We also report cases where duplications of receptor genes are observed and the presence of 'receptor islands'. Our study strongly supports the notion of 'core' and 'accessory' TonB-dependent receptors within each species, with the ferripyoverdine receptors belonging to the last category.