Binding of Zur to the promoter regions of its potential target genes. The [γ-32P]-labeled upstream region of each genes (10 fmol of target DNA probes) were incubated with the purified Zur protein in the presence of 100 μM ZnCl2. 0, 1.25, 2.5, 5, 5, 5 and 0 pmol of Zur were used in lanes 1 to 4 and C1 to C3, respectively. The mixtures were directly subjected to 4% polyacrylamide gel electrophoresis. For lanes 1 to 4, the retarded DNA band with decreased mobility turned up, which presumably represented the Zur-DNA complex. To confirm the specificity of the binding complexes, either a 200-fold molar excess of nonspecific competitor (2 pmol of unlabeled znuA DNA without its predicted binding region in lane C1) or a 200-fold molar excess of specific competitor (2 pmol of unlabeled target DNA probe in lane C2) was added to the binding mixture. 2 pmol of an unrelated protein, i.e., purified rabbit anti-F1 antibody, were included in lane C3. Both znuA and znuC gave positive EMSA results. Since these two genes had overlapped upstream regions and shared a single predicted Zur site, the EMSA data of only znuA rather than znuC was presented herein.