Corynebacterium glutamicum Zur acts as a zinc-sensing transcriptional repressor of both zinc-inducible and zinc-repressible genes involved in zinc homeostasis.

Zur is a zinc-dependent transcriptional repressor of zinc uptake systems in bacteria. In the present study, we examined the role of Corynebacterium glutamicum Zur in the zinc-inducible expression of two genes: one encoding a cation diffusion facilitator (zrf) and the other a metal-translocating P-type ATPase (zra). Both genes were shown to be involved in zinc resistance. Disruption of the zur gene encoding Zur resulted in constitutive expression of zrf and zra mRNAs. An electrophoretic mobility shift assay revealed that the Zur protein binds to the zrf and zra promoters, for which the in vivo activities were up-regulated in response to excess zinc. Interestingly, the in vitro DNA binding activity of Zur was inhibited by zinc, in contrast to its zinc-dependent binding to the promoter region of a zinc-repressible ABC transporter gene znuB2. A 21-bp motif found in the Zur binding site overlaps the putative -35 region of both the zrf and zra promoters. This new motif is a 10-1-10 direct repeat sequence distinct from the 10-1-10 inverted repeat sequence of a previously identified Zur box for zinc-dependent binding. Nevertheless, their 10-bp elements share some sequence similarities. Overexpression of zur in the zur deletion mutant background, as well as deletion of zur in the zrf and zra double deletion mutant background, resulted in decreased resistance to zinc. These results suggest that the direct negative control of both zinc uptake and export systems by Zur is central to C. glutamicum zinc homeostasis and is effected in distinct ways.