Nonspecific binding of lac repressor to DNA. I. An absorption and circular dichroism study

Nonspecific binding of lac repressor on DNA has been studied by absorption and circular dichroism (CD) spectroscopies. In a first step, the complex formation is accompanied by an absorption difference spectrum and a change of the CD signal of the DNA. The absorption difference spectrum is mainly due to a spectral change of the DNA. The variation of the CD signal has been analyzed according to a model calculation, which takes into account the fact that the excluded site is shorter than the perturbed site. We found that in this first step one repressor can bind every 14 +/- 2 base-pairs, whereas one repressor perturbs 22 +/- 2 base-pairs. In a second step, more repressor can bind on DNA, but without further change in the absorption and CD spectrum, indicating that another binding process occurs. The model calculation developed here is general for all binding processes inducing a perturbation over a length of DNA longer than that of the excluded site.