Comparing Xanthomonas

In the May 23 Nature, da Silva et al. report the comparative analysis of genomes of two bacterial phytopathogens that differ in their host specificity and pathogenicity (Nature 2002, 417:459-463). Xanthomonas axonopodis pv citri (Xac) affects citrus cultivars and causes canker lesions. In contrast, Xanthomonas campestris pv. campestris (Xcc) affects crucifers and causes black rot. Each genome contains a circular chromosome of around 5 megabases. In addition, Xac carries two plasmids, of 34 and 65 kilobases. The chromosomes display a high degree of colinearity, suggesting as few as three major rearrangement events have occurred in the time that separates them,. and they have as many as 80% of genes in common. Regions around the putative termini of replication contain most strain-specific genes that may be related to pathogenesis. Xac-specific genes include those for fungicide synthesis, secreted toxins, and nodulation pathways. Xcc-specific genes include those implicated in colonization, antibiotic synthesis and nitrate assimilation. The Brazilian researchers estimate the divergence time between the two is 6.5-8.9 million years. Analysis of the differences between the Xanthomonasgenomes may provide clues to their different features of the pathogens. For example, differences in the numbers of proteases and endogluconases may explain the strain-specific affects on host tissue destruction; Xcc-specific genes may be related to invasion and colonization phenotypes, and Xac genes may explain the strongly localized Xacresponse.