DNA loops induced by cooperative binding of λ repressor

It has been shown by Hochschild and Ptashne1 that λ repressers bind cooperatively to operator sites separated by five or six turns of the helix. Cooperative binding is not observed if the sites are separated by a nonintegral number of turns, unless a four-nucleotide gap is introduced into one of the strands between the two sites. These and other facts suggested that repressers at the separated sites touch each other, the DNA bending smoothly so as to accommodate the protein–protein interaction. Here we use electron microscopy to visualize the predicted protein-DNA complexes.