PREFERENTIAL INTERACTIONS OF THE ESCHERICHIA-COLI LEXA REPRESSOR WITHANIONS AND PROTONS ARE COUPLED TO BINDING THE RECA OPERATOR

The binding of Escherichia coli LexA repressor to the recA operator wa s examined as a function of the concentration of NaCl, KCl, NaF, and M gCl2 at pH 7.5, 21 degrees C. The effects of pH at 100 mM NaCl were al so examined. Changes both in the qualitative appearance of the binding isotherms and in the magnitude of the apparent binding affinity with changes in solution conditions suggest that binding of anions and prot ons by LexA repressor is linked to oligomerization and/or operator bin ding. Binding of LexA repressor to the recA operator in the presence o f NaCl ranging from 25 to 400 mM at picomolar DNA concentration showed a broad, apparently noncooperative, binding isotherm. Binding of LexA repressor in NaF at the same [DNA] yielded binding isotherms with a n arrow transition, reflecting an apparently cooperative binding process . Also, the apparent binding affinity was weaker in NaF than in NaCl F urthermore, the binding affinity and also the apparent binding mode, c ooperative vs noncooperative, were pH dependent. The binding affinity of LexA repressor for operator was greatest near neutral pH. The appar ent binding mode was noncooperative at pH 7-9 but was cooperative at p H 6 or 9.3. These observations suggest that the specific cation and an ion composition and concentrations must be considered in understanding the details of regulation of the SOS system.