ROLE OF MACROMOLECULAR HYDRATION IN THE BINDING OF THE ESCHERICHIA-COLI CYCLIC-AMP RECEPTOR TO DNA

The osmotic stress technique was used to measure the changes in macrom olecular hydration that accompany binding of the Escherichia coli CAP protein to its transcription-regulatory site (C1) in the lactose promo ter and that accompany the transfer of CAP from site C1 to nonspecific genomic DNA. Formation of the C1 complex is accompanied by the net re lease of 79 +/- 11 water molecules. If all water molecules were releas ed from macromolecular surfaces, this result would be consistent with a net reduction of solvent-accessible surface area of 711 +/- 189 Angs trom(2). This area is only slightly smaller than the solvent-inaccessi ble macromolecular interface in crystalline CAP-DNA complexes. The tra nsfer of CAP from site C1 to nonspecific sites is accompanied by the n et uptake of 56 +/- 10 water molecules. Taken with the water stoichiom etry of sequence-specific binding, this value implies that formation o f a nonspecific complex is accompanied by the net release of 2-44 wate r molecules, The enhanced stabilities of CAP-DNA complexes with increa sed osmolality (decreased water activity) may contribute to the abilit y of E. coli cells to tolerate dehydration and/or high external salt c oncentrations.