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 macromolecular hydration that accompany binding of the Escherichia coli CAP protein to its transcription-regulatory site (C1) in the lactose promoter and that accompany the transfer of CAP from site C1 to nonspecific genomic DNA. Formation of the C1 complex is accompanied by the net release of 79 +/- 11 water molecules. If all water molecules were released from macromolecular surfaces, this result would be consistent with a net reduction of solvent-accessible surface area of 711 +/- 189 A2. This area is only slightly smaller than the solvent-inaccessible macromolecular interface in crystalline CAP-DNA complexes. The transfer of CAP from site C1 to nonspecific sites is accompanied by the net uptake of 56 +/- 10 water molecules. Taken with the water stoichiometry of sequence-specific binding, this value implies that formation of a nonspecific complex is accompanied by the net release of 2-44 water molecules. The enhanced stabilities of CAP-DNA complexes with increased osmolality (decreased water activity) may contribute to the ability of E.coli cells to tolerate dehydration and/or high external salt concentrations.