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.