Comprehensive Genomics and Functional Validation Reveal Evolutionary Insights into Xanthan Gum Biosynthesis and Pathogenesis of Xanthomonas campestris pv. campestris

We performed comparative genomics on 66 Xanthomonas campestris pv. campestris (Xcc) strains to investigate the interplay between pathogenicity and xanthan production. Our analysis revealed an open pan-genome and a striking evolutionary dichotomy: the xanthan biosynthesis (gum) cluster is highly conserved to maintain polymer integrity, while the O-antigen (wxc) cluster is hypervariable, likely driven by immune evasion. Functional validation showed that variants of Rmd exhibit a 22-fold activity span with minimal impact on xanthan gum yield. Conversely, wxc diversity significantly influences xanthan yield via metabolic feedback without affecting viscosity. Furthermore, we demonstrated that host restriction–modification systems drive exogenous DNA instability by targeting specific methylation motifs. Site-directed mutagenesis of these motifs successfully restored plasmid stability. These findings provide a population-scale framework for Xcc evolution and offer validated strategies for overcoming barriers to genetic engineering.