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J. Biol. Chem., Vol. 277, Issue 52, 50676-50682, December 27, 2002

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Role of Hydration in the Binding of lac Repressor to DNA^*

Michael G. Fried^§, Douglas F. Stickle^¶, Karen Vossen Smirnakis, Claire Adams, Douglas MacDonald^**, and Ponzy Lu^**

From the  Department of Biochemistry and Molecular Biology, Penn State University College of Medicine, Hershey, Pennsylvania 17033, the ^¶ Department of Pathology and Microbiology, Nebraska Medical Center, Omaha, Nebraska 68198, the  Department of Medicine, Massachusetts General Hospital, Boston, Massachusetts 02114, and the ^** Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104

The osmotic stress technique was used to measure changes in macromolecular hydration that accompany binding of wild-type Escherichia coli lactose (lac) repressor to its regulatory site (operator O1) in the lac promoter and its transfer from site O1 to nonspecific DNA. Binding at O1 is accompanied by the net release of 260 ± 32 water molecules. If all are released from macromolecular surfaces, this result is consistent with a net reduction of solvent-accessible surface area of 2370 ± 550 Å². This area is only slightly smaller than the macromolecular interface calculated for a crystalline repressor dimer-O1 complex but is significantly smaller than that for the corresponding complex with the symmetrical optimized O_sym operator. The transfer of repressor from site O1 to nonspecific DNA is accompanied by the net uptake of 93 ± 10 water molecules. Together these results imply that formation of a nonspecific complex is accompanied by the net release of 165 ± 43 water molecules. The enhanced stabilities of repressor-DNA complexes with increasing osmolality may contribute to the ability of Escherichia coli cells to tolerate dehydration and/or high external salt concentrations.

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