DNA gel mobility shift demonstrating binding of purified HlyU to intergenic regions of the rtxACHBDE operon (A and B) and the vah1-plp gene cluster (C and D). DIG-labeled DNA fragments of the intergenic region between rtxH and rtxB (A) and between plp and vah1 (C) were obtained by PCR amplification. Individual DIG-labeled fragments (0.4 ng) were reacted with no additions (lane 1), 350 ng HlyU (lane 2), and 350 ng HlyU plus 100 ng unlabeled DNA fragment (lane 3). (B) Fragments a, b, and c from the intergenic regions of the rtxACHBDE operon.(D) Fragments d, e, f, g, and h from the intergenic regions of the vah1-plp gene cluster.