Marked effects of salt on estrogen receptor binding to DNA: biologically relevant discrimination between DNA sequences.

Avidin-biotin complexed with DNA (ABCD) assays were employed to determine the binding affinity of estrogen receptor (ER) to DNA under various salt conditions. Type and concentration of salt in the reaction buffer dramatically affected the ability of the ER to discriminate between DNA sequences. Under appropriate salt conditions, ER was able to bind to the estrogen response element from the Xenopus vitellogenin A2 gene with at least 3 orders of magnitude greater affinity than a two base pair mutant sequence, and 5 orders of magnitude greater affinity than plasmid DNA. In these studies, the best discrimination was observed under conditions of salt type and concentration that more closely approximated intracellular conditions, i.e., 100-150 mM potassium salts. Analysis of the binding affinities for ER to all three types of DNA over a range of KCl concentrations indicated that the ionic interactions upon ER binding were the same for the three DNA molecules tested. Therefore, the additional stability of ER binding to target DNA sequences was contributed by nonionic interactions.