The ZnuABC high-affinity zinc uptake system and its regulator zur in Escherichia coli

In Escherichia coli, lacZ operon fusions were isolated that were derepressed under iron repletion and repressed under iron depletion. Two fusions were localized in genes that formed an operon whose gene products had characteristics of a binding protein-dependent transport system. The growth defect of these mutants on TY medium containing 5 mM EGTA was compensated for by the addition of Zn2+. In the presence of 0,5 mM EGTA, only the parental strain was able to take up Zn-65(2+). This high-affinity transport was energized by ATP. The genes were named znuACB (for zinc uptake; former name yebLMI) and localized at 42 min on the genetic map of E. coli, At high Zn2+ concentrations, the znu mutants took up more Zn-65(2+) than the parental strain. The high-affinity Zn-65(2+) uptake was repressed by growth in the presence of 10 mu M Zn2+. A znuA-lacZ operon fusion was repressed by 5 mu M Zn2+ and showed a more than 20-fold increase in beta-galactosidase activity when Zn2+ was bound to 1.5 mu M TPEN [tetrakis-(2-pyridylmethyl) ethylenediamine]. To identify the Zn2+-dependent regulator, constitutive mutants were isolated and tested for complementation by a gene bank of E. coli. A complementing gene, yjbK of the E. coli genome, was identified and named zur (for zinc uptake regulation). The Zur protein showed 27% sequence identity with the iron regulator Fur. High-affinity Zn-65(2+) transport of the constitutive zur mutant was 10-fold higher than that of the uninduced parental strain. An in vivo titration assay suggested that Zur binds to the bidirectional promoter region of znuA and znuCB.