Binding of Escherichia coli LexA repressor to the RecA operator

Equilibrium binding of Escherichia coli LexA repressor to the recA operator was studied by the polyacrylamide gel mobility shift assay as a function of solution conditions. In the presence of NaCl at 20 degrees C, there was a significant salt dependence in binding to the recA operator, typical for protein-nucleic acid interactions with some electrostatic contribution to the binding free energy. In preliminary experiments in which the anion of the Na+ salt was changed from chloride to fluoride, little change was found with anion identity. This indicates that the salt effect on the binding interaction arises solely from the polyelectrolyte effect, not from anion binding or release by the protein upon complex formation. Increasing the temperature to 37 degrees C changed the binding affinity for complex formation at any given salt concentration and resulted in a change in the sensitivity of complex formation to NaCl concentration. Quantitative analysis of the data to obtain equilibrium binding constants is discussed.