COLD DENATURATION OF A REPRESSOR-OPERATOR COMPLEX - THE ROLE OF ENTROPY IN PROTEIN-DNA RECOGNITION

The mechanisms by which regulatory proteins recognize specific DNA seq uences are not fully understood. Here we examine the basis for the sta bility of a protein-DNA complex, using hydrostatic pressure and low te mperature. Pressure converts the DNA-binding Arc repressor protein fro m a native state to a denatured, molten-globule state. Our data show t hat the folding and dimerization of Arc repressor in the temperature r ange 0-20 degrees C are favored by a large positive entropy value, so that the reaction proceeds in spite of an unfavorable positive enthalp y. On binding operator DNA, Arc repressor becomes extremely stable aga inst denaturation. However, the Arc repressor-operator DNA complex is cold-denatured at subzero temperatures under pressure, demonstrating t hat the favorable entropy increases greatly when Arc repressor binds t ightly to its operator sequence but not a nonspecific sequence. We sho w how an increase in entropy may operate to provide the protein with a mechanism to distinguish between a specific and a nonspecific DNA seq uence. It is postulated that the formation of the Arc-operator DNA com plex is followed by an increase in apolar interactions and release of solvent which would explain its entropy-driven character, whereas this solvent would not be displaced in nonspecific complexes.