Escherichia coli cAMP receptor protein-DNA complexes. 2. Structural asymmetry of DNA bending

The effect of DNA sequence variability and the degree of cyclic AMP receptor protein (CRP)-induced bending of the flanking ends of fluorescently labeled DNA were investigated by steady-state fluorescence and differential phase polarization studies in the presence and absence of CRP. Six sequences, including the primary CRP binding sites of lac P1 (class I) and gal P1 (class II), were studied. Excitation and emission spectra of CPM-DNA upon binding CRP were observed to be qualitatively similar to one another, regardless of the CRP binding site sequence examined or the location of the probe. This result implies that the probe is not interacting with the protein. However, the magnitude of the changes in the fluorescence intensities of sensitized emission spectra of CPM-DNA is apparently dependent on the DNA sequence, indicating that the environments of the flanking ends of DNA may be different from one another in the protein-DNA complex. Differential phase polarization results were qualitatively consistent with the fluorescence energy transfer measurements. The implication of this study supports the idea that the DNA is bent symmetrically in the lac-CRP complex but is asymmetrically bent in the gal-CRP complex. The sequence in the half-site in conjunction with the flanking sequence defines the geometry of the bent DNA. It appears that the CRP-induced bend in the DNA may also be class dependent. This may be an important feature used by the system to regulate transcription at different promoter sites.