Cooperative and salt-resistant binding of lexA protein to non-operator DNA.

The interaction of the lexA repressor of E. coli with poly[d(A-T)] has been studied by circular dichroism. The binding induces an about 2-fold increase of the circular dichroism intensity at 263 nm, pointing out a conformational change of the nucleic acid. The observed spectral changes are very similar to those observed for the binding of the lac repressor to poly[d(A-T)] and natural DNA. At elevated ionic strength the binding isotherms do show a pronounced sigmoidal shape indicating a cooperative mode of binding.