Interaction of Crp with the pla promoter region revealed by DNase I footprinting. The precise positions of the nucleotides protected by Crp were determined by comparison with the results of the Maxam-Gilbert G+A cleavage reaction shown. Purified Crp in the presence of 100 μM cAMP was used in all of the binding reactions except the control reaction shown in lane 2. Lane 1, G+A cleavage reaction; lane 2, control reaction with 0.9 μM Crp but no cAMP added; lane 3, control reaction with no Crp added; lane 4, 0.9 μM Crp; lane 5, 0.45 μM Crp; lane 6, 0.22 μM Crp; lane 7, 0.11 μM Crp; lane 8, 0.05 μM Crp; lane 9, 0.025 μM Crp. The DNA sequence of the pla promoter region is indicated on the left, and the protected nucleotide sites are indicated by bold type. The region protected from DNase I cleavage due to Crp binding of the DNA is indicated by the box.