Removing water from an EcoRI-noncognate DNA complex with osmotic stress.

We recently showed that a nonspecific complex of the restriction nuclease EcoRI with poly (dI-dC) sequesters significantly more water at the protein-DNA interface than the complex with the specific recognition sequence. The nonspecific complex seems to retain almost a full hydration layer at the interface. We now find that at low osmotic pressures a complex of the restriction nuclease EcoRI with a DNA sequence that differs by only one base pair from the recognition site (a 'star' sequence) sequesters about 70 waters more than the specific one, a value virtually indistinguishable from nonspecific DNA. Unlike complexes with oligo (dI-dC) or with a sequence that differs by two base pairs from the recognition sequence, however, much of the water in the 'star' sequence complex is removed at high osmotic pressures. The energy of removing this water can be calculated simply from the osmotic pressure work done on the complex. The ability to measure not only the changes in water sequestered by DNA-protein complexes for different sequences, but also the work necessary to remove this water is a potentially powerful new tool for coupling inferred structural changes and thermodynamics.