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The Chromosome II-Like Segment in Bartonella

The region in B. henselae and B. quintana with the majority of Bartonella-specific genes is located in the fourth quarter of the genome (Fig. 13.2). This segment of the genome has a slightly elevated G+C content, a low coding potential (50-60%), and a higher than normal density of short tandem repeats and tandem duplication remnants. Most of the B. henselae islets are also located here, indicative of continuous sequence degradation in this region of the B. quintana genome (from which the corresponding islets appear to have been lost). Flanked by one of the two rRNA operons, and containing the other, is a segment of 280 kbp in B. henselae and 200 kbp in B. quintana with as many as 20 genes with top hits to chromosome II (Chr II) in B.suis, compared to only two genes that phylogenetically cluster with the main chromosome (Chr I) [37]. This stands in contrast to the rest of the genome, where most genes are orthologues of B. suis Chr I genes. A suggested explanation for the observed pattern is integration of an auxiliary replicon (or parts thereof) analogous to Chr II in Brucella, by homologous recombination at the rRNA operons in an ancestor of Bartonella, followed by sequence loss and rearrangements [37].

Fig. 13.2 Genome circle. Schematic view of the genomes of B. henselae and B. quintana, showing the location of the chromosome II region, the prophage, and the genomic islands in B. henselae and remnants thereof in B. quintana, and the known virulence factors such as BadA/Vomp, Trw, and VirB-Bep.

The Chr II-like region in Bartonella harbors many genes that are not present in other sequenced a-proteobacteria. These parts may represent islands that were acquired before the divergence of B. henselae and B. quintana and may yield clues to how Bartonella evolved to become an intraerythrocytic parasite. Located in a segment with the characteristics of a genomic island are a number of surface protein homologues and many short sequences repeated in tandem. Because of their hypermutability, tandem repeats can alter the expression of associated genes, a mechanism used by pathogens to vary expression of surface-exposed antigens.

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