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Abstract: 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 colRXC1049 and colSXC1050 , is a global regulatory system involved in various cellular processes, including virulence, hypersensitive response and stress tolerance. Growth rate determination showed that, although the colRXC1049 and colSXC1050 mutants are not auxotrophic, colRXC1049 and colSXC1050 are required for the pathogen to proliferate well in standard media and host plants. Assays of β-glucuronidase activities of plasmid-driven promoter-gusA reporters and/or semi-quantitative RT-PCR demonstrated that colRXC1049 and colSXC1050 positively regulate expression of hrpC and hrpE operons, and that expression of colRXC1049 and colSXC1050 is not controlled by key hrp regulators HrpG and HrpX. [Copyright &y& Elsevier]