pneumoniae, X. fastidiosa 29, 32 Polynucleotide phosphorylase

E. coli 33 TonB-dependent receptor X. axonopodis pv. citri 19 Specifically for X. a. pv. citri, we observed only a small overlap with recently published data that identified genes involved in biofilm formation by transposon mutagenesis [19]. The common proteins include UDP-glucose dehydrogenase and a TonB-dependent receptor proteins [19]. A possible explanation for this may be that transposon mutagenesis also identifies genes that are indirectly involved in biofilm formation, and additionally many of the identified genes may be required for the first stages of biofilm formation, such as adherence to AZD2281 the surface. Here, we focused on the proteins present in mature biofilms and for this reason many of the genes found in the genome-wide scale assay may be not differently expressed in this structure. The most enriched categories for the up-regulated proteins in X. a. pv. citri biofilm are ‘external encapsulating structure’,

find more ‘transporter activity’ and ‘receptor activity’, and include the outer membrane receptors termed TonB-dependent receptors (TBDRs). Among them, the OmpA-related protein (XAC4274, spot 103) and TonB-dependent receptors (XAC3050, spots 1, 2, 74, 219; XAC3071, spot 466 and XAC3489, spots 55 and 168) were up-regulated, while the TonB-dependent receptors (XAC3168, spot 38 and XAC3444, spot 15) were down-regulated in X. a. pv. citri biofilms. The TBDR proteins are localized in the outer membrane of gram-negative bacteria and their most prominent recognized role is the transport of iron-siderophore complexes and cobalamin into the periplasm [34]. Transport via TBDRs is an active process requiring energy that is provided by the inner membrane TonB-ExbB-ExbD protein complex [35]. Generally, expression of the genes encoding for these receptors is activated under conditions of iron starvation and repressed in the presence of iron by the ferric-uptake Methane monooxygenase regulator (Fur) repressor [36]. Several genome sequences of gram-negative

bacteria were examined to determine the number of TBDRs present in each genome, and it was demonstrated that only a number of these bacteria, among them the Xanthomonas species, have an over-representation of TBDRs [37]. Most of the analyzed bacteria with an elevated number of TBDRs share the ability to metabolize complex carbohydrates. Therefore, it was postulated that some Xanthomonas TBDRs might be involved in the transport of plant-derived molecules [37], and this hypothesis was confirmed with the characterization of two TBDRs from Xanthomonas campestris pv. campestris and Caulobacter crescentus, that transport sucrose and maltodextrins, respectively [37, 38]. It was also suggested that other TBDRs might be involved in signal transduction processes [39]. Our proteomics results suggest that TBDRs participates in X. a. pv.