Variations of intramolecular ligation rates allow the detection of protein-induced bends in DNA

A method requiring minute amounts of DNA and protein is proposed for the detection of DNA bending in solution. Local bending, induced by the binding of a protein at its stereospecific site, must increase the probability of circularization of short DNA fragments and hence the rate of formation of the corresponding minicircles. This effect should be highly sensitive to the location of the protein-binding site on the DNA fragment. Simple controls allow other possible interpretations to be ruled out. The deformation due to the cyclic AMP receptor protein when it interacts with its stereospecific site at the lactose control region is studied by this method. It is shown in this case that untwisting is negligible and bending significant. Further measurements are required to assess the actual value of the radius of curvature of the DNA double helix in this complex.