The Zur of Xanthomonas campestris functions as a repressor and an activator of putative zinc homeostasis genes via recognizing two distinct sequences within its target promoters

It has been long considered that zinc homeostasis in bacteria is maintained by export systems and uptake systems, which are separately controlled by their own regulators and the uptake systems are negatively regulated by Zur which binds to an about 30-bp AT-rich sequence known as Zur-box present in its target promoters to block the entry of RNA polymerase. Here, we demonstrated in vivo and in vitro that in addition to act as a repressor of putative Zn(2+)-uptake systems, the Zur of the bacterial phytopathogen Xanthomonas campestris pathovar campestris (Xcc) acts as an activator of a Zn(2+) efflux pump. The Xcc Zur binds to a similar Zur-box with approximately 30-bp AT-rich sequence in the promoters of the genes encoding putative Zn(2+)-uptake systems but a 59-bp GC-rich sequence with a 20-bp inverted repeat overlapping the promoter's -35 to -10 sequence of the gene encoding a Zn(2+)-export system. Mutagenesis of the inverted repeat sequence resulted in abolishment of the in vitro binding and the in vivo and in vitro activation of the export gene's promoter by Zur. These results reveal that the Xcc Zur functions as a repressor and an activator of putative zinc homeostasis genes via recognizing two distinct sequences within its target promoters.