A survey of TonB-dependent receptors in fluorescent pseudomonads Pierre Cornelis 1 * and Josselin Bodilis 2 1 Laboratory of Microbial Interactions, Department of Molecular and Cellular Interactions, Flanders Institute for Biotechnology (VIB), Vrije Universiteit Brussel, Building E, Pleinlaan 2, 1050 Brussels, Belgium. 2 Université de Rouen, Laboratoire M2C, UMR CNRS 6143, groupe microbiologie, Bâtiment IRESE B, UFR des Sciences, 76821 Mont Saint Aignan, France. Summary For bacteria with an aerobic lifestyle, iron is in the oxidized Fe 3+ form, hence poorly soluble. The solu- tion 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 pro- duced 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. syrin- gae) 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 hori- zontal 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 ‘acces- sory’ TonB-dependent receptors within each species, with the ferripyoverdine receptors belonging to the last category. Introduction Iron exists in two oxidation states, Fe 2+ and Fe 3+ , the latter being dominant under aerobic conditions and highly insoluble. The availability of free iron in some bacterial niches like soil or in mammalian hosts is therefore limiting for bacterial growth (reviewed by Andrews et al., 2003). Gram-negative bacteria have developed different strate- gies for acquiring iron, the most common being the pro- duction of low molecular weight iron chelating compounds termed siderophores, which are secreted to scavenge iron outside of the cell (Guerinot, 1994; Wiener, 2005). In Gram-negative bacteria the ferrisiderophores are recog- nized and taken up via outer membrane receptors, which function as gated porin channels (Koebnik et al., 2000; Ratledge and Dover, 2000; Koebnik, 2005). After binding of the ferrisiderophores, transport is mediated by a complex of inner membrane-anchored proteins TonB, ExbB and ExbD (Wiener, 2005). TonB-dependent recep- tors have an N-terminal domain known as ‘cork’ or ‘plug’ that transiently blocks the 22 sheets b-barrel domain (Wiener, 2005). Fluorescent pseudomonads respond to iron-deficiency by secreting the yellow-green fluorescent peptidic siderophores pyoverdines (Cornelis and Matthijs, 2002). Pyoverdines have a conserved chromophore and a variable peptide chain and each Pseudomonas species produces a different pyoverdine (Meyer, 2000; Ravel and Cornelis, 2003; Visca et al., 2007). Evolution of pyoverdines peptide chains is accompanied by a parallel evolution of the receptor (Smith et al., 2005; Tümmler and Cornelis, 2005; Bodilis et al., 2009). Even within the species P. aeruginosa three different pyoverdines have been described subdividing the species into three siderovars (Meyer et al., 1997). The receptors for the three different P. aeruginosa ferripyoverdines (termed FpvA) have been characterized and, in one instance, the structure elucidated (de Chial et al., 2003; Cobessi et al., 2005; Smith et al., 2005). Often a second siderophore, of lower affinity, is produced as well, such as pyochelin in P. aeruginosa, pseudomonin in some P. fluorescens, thioquinolobactin in P. fluorescens ATCC 17400, or orni- corrugatin in P. fluorescens AF76 (Cox, 1980; Mossialos et al., 2000; Mercado-Blanco et al., 2001; Cornelis and Matthijs, 2002; Matthijs et al., 2004; Matthijs et al., 2007; Matthijs et al., 2008). TonB-dependent receptors have also been described for the uptake of haem, including in Received 4 May, 2009; accepted 26 May, 2009. *For correspon- dence. E-mail pcornel@vub.ac.be; Tel. (+32) 2 629 1906; Fax (+32) 2 629 1902. Environmental Microbiology Reports (2009) 1(4), 256–262 doi:10.1111/j.1758-2229.2009.00041.x © 2009 Society for Applied Microbiology and Blackwell Publishing Ltd