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 and for the exbD genes in Flavobacterium psychrophilum, where two paralogous genes were found in tandem in a cluster combined with tonB and exbB 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.