Horizontal gene transfer (HGT) has been well documented as a driving force in the evolution of bacteria.  It has been shown that a horizontally acquired gene, xoc_2868, involved in the global response against oxidative stress and pathogenicity of Xanthomonas oryzae pv. oryzicola strain BLS256.  However, as a transcriptional factor (TF), the regulatory mechanism of XOC_2868 has not yet been revealed.  Here, evolutionary analysis suggested XOC_2868 might be co-transferred with its physically proximate downstream genes from a Burkholderiaceae ancestor.  Interestingly, RNA-seq data of wild-type (BLS256) and Δxoc_2868 strains under oxidative stress showed that XOC_2868 did not regulate the expression of its adjacent genes, but remarkably influenced the expression of several genes involved in the extracellular polysaccharide (EPS) production and xanthan biosynthesis.  Chromatin immunoprecipitation-sequence (ChIP-seq) combined with transcriptome analysis revealed that XOC_2868 directly regulates a cydAB operon, encoding two subunits of cytochrome bd oxidase and involved in redox balance.  Consistent with Δxoc_2868 strain, cydA- and cydAB-knockout mutants also showed a higher sensitivity to H₂O₂ along with a reduced bacterial virulence compared with the wild-type strain.  Overall, our findings raise the possibility of regulatory circuit evolution shaped by HGT and driven by selection and reveal a novel regulatory pathway that regulates the expression of cytochrome bd oxidase and thus contributes to the virulence of BLS256.  

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