Interaction between cyanobacterial copper chaperone Atx1 and zinc homeostasis.

Cyanobacterial Atx1 is a copper chaperone which interacts with two copper-transporting ATPases to assist copper supply to plastocyanin and cytochrome oxidase. ZiaA is a Zn(2+)-exporting ATPase and ziaA expression is regulated by ZiaR. Here we show that gene expression from the ziaA operator promoter, monitored using reverse transcriptase PCR and lacZ fusions, is elevated in Deltaatx1 mutants. Although Cu(+) tightly binds recombinant ZiaR in vitro, Cu(+) is less effective at dissociating ZiaR-DNA complexes than Zn(2+) and crucially ziaA expression responds to Zn(2+) but not copper in both wild-type and Deltaatx1 cells. Consistent with enhanced expression of ZiaA, Deltaatx1 cells have slightly elevated Zn(2+) resistance. Recombinant Zn(2+)-Atx1 is recovered from Zn(2+)-supplemented Escherichia coli and even after copper supplementation substantial amounts of Zn(2+)-Atx1 are isolated. Taken together, these data suggest that Zn(2+)-Atx1 can form in vivo.