campestris pv. campestris. This led already to the discovery of an unexpected wealth of TonB-dependent receptors [62]. A detailed genomic analysis revealed now the presence of further genes coding for components of TonB systems (Figure 1A). In total, five copies of tonB, two copies of exbB and four copies Selleckchem KU-60019 of exbD were identified within the genome. Downstream of the previously characterized tonB-exbB-exbD1-exbD2 genes, which are located close to the chromosomal origin of replication, a third exbD gene was identified (Figure 1B). While the presence of different TonB-dependent receptors has been attributed

to their distinct binding specificities, where different molecules are bound at the outer cell surface to be either transported inside or to signal their presence to the cell interior, so far it has been assumed that only one set of tonB-exbB-exbD genes is required to build a TonB protein complex Decitabine cell line that interacts with all the different TonB-dependent receptors. Results of previous mutational analyses [64] suggest that the newly identified genes of TonB system core components are not involved in iron uptake. To shed more light on the multiplicity of these genes, we concentrated on analyzing the function of exbD2, which had already been shown to be involved in plant interaction, despite being not important for iron uptake [66]. A genomic comparison showed that this gene was present

and well conserved in all complete Xanthomonas genomes (Additional file 1). Figure 1 Genomic organization of the TonB-related genes in X. campestris pv. campestris B100. (A) A circular genome plot indicates the locations of the TonB-related genes on the chromosome. The core of the TonB system is encoded by the genes tonB, exbB and exbD. In X. campestris pv. campestris B100 multiple isoforms of these genes were identified. Their genomic

locations on the circular chromosome are indicated. So far, this multiplicity was only known for tonB genes in Pseudomonas[68] and for the exbD genes in Flavobacterium psychrophilum, where two paralogous Cytidine deaminase genes were found in tandem in a cluster combined with tonB and exbB[64] close to the chromosomal origin of replication (B). Size and direction of transcription is illustrated by arrows for this gene cluster. Genes that were predicted with convincing evidence are symbolized by shaded arrows, while an open arrow indicates a putative protein-coding sequence (CDS) that was predicted with less confidence. Now a third copy of exbD was found downstream of exbD2, separated from exbD2 only by a hypothetical gene for which nor functionality neither expression could be indicated. Further copies of tonB and the genes exbB-exbD were found at different chromosomal positions. To facilitate discriminating the individual genes, unique numbers were added to their names. The exbD2 gene is involved in pectate lyase activity X. campestris pv.