The ColR-ColS two-component signal transduction system was originally characterized as a regulatory system involved in the capacity of root-colonizing biocontrol bacterium Pseudomonas fluorescens to colonize plant roots. There are three pairs of putative colR-colS two-component regulatory systems annotated in the phytopathogen Xanthomonas campestris pathovar campestris. Mutational studies revealed that one of them, named colR(XC1049) and colS(XC1050), is a global regulatory system involved in various cellular processes, including virulence, hypersensitive response and stress tolerance. Growth rate determination showed that, although the colR(XC1049) and colS(XC1050) mutants are not auxotrophic, colR(XC1049) and colS(XC1050) are required for the pathogen to proliferate well in standard media and host plants. Assays of beta-glucuronidase activities of plasmid-driven promoter-gusA reporters and/or semi-quantitative RT-PCR demonstrated that colR(XC1049) and colS(XC1050) positively regulate expression of hrpC and hrpE operons, and that expression of colR(XC1049) and colS(XC1050) is not controlled by key hrp regulators HrpG and HrpX.