hpaR, a PutativemarRFamily Transcriptional Regulator, Is Positively Controlled by HrpG and HrpX and Involved in the Pathogenesis, Hypersensitive Response, and Extracellular Protease Production ofXanthomonas campestrisPathovar campestris

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Abstract
The MarR family of transcriptional regulators of bacteria are involved in the regulation of many cellular processes, including pathogenesis. In this work, we have demonstrated genetically thathpaR(hpa,hrpassociated), which encodes a putative MarR family regulator, is involved in the hypersensitive response (HR), pathogenicity, and extracellular protease production of the phytopathogenic bacteriumXanthomonas campestrispathovar campestris. A mutation inhpaRresulted in complete loss of virulence in the host plant cabbage, a delayed and weakened HR in the nonhost plant pepper ECW-10R, and an increase in extracellular protease production. Detection of the β-glucuronidase activity of a plasmid-drivenhpaRpromoter-gusAreporter revealed that the expression ofhpaRis positively controlled by HrpG and HrpX and is suppressed in rich medium while being strongly induced in minimal andhrp-inducing media and inside the host. These findings indicate thathpaRbelongs to thehrpGandhrpXregulon and that HrpX regulates the extracellular protease production viahpaRinX. campestrispv. campestris.