LexA repressor forms stable dimers in solution - The role of specific DNA in tightening protein-protein interactions

Cooperativity in the interactions among proteins subunits and DNA is crucia l for DNA recognition. LexA repressor was originally thought to bind DNA as a monomer, with cooperativity leading to tighter binding of the second mon omer, The main support for this model was a high value of the dissociation constant for the LexA dimer (micromolar range). Here we show that the prote in is a dimer at nanomolar concentrations under different conditions, The r eversible dissociation of LexA dimer was investigated by the effects of hyd rostatic pressure or urea, using fluorescence emission and polarization to monitor the dissociation process. The dissociation constant lies in the pic omolar range (lower than 20 pM). LexA monomers associate with an unusual la rge volume change (340 ml/mol), indicating the burial of a large surface ar ea upon dimerization, Whereas nonspecific DNA has no stabilizing effect, sp ecific DNA induces tightening of the dimer and a 750-fold decrease in the K -d. In contrast to the previous model, a tight dimer rather than a monomer is the functional repressor. Accordingly, the LexA dimer only loses its abi lity to recognize a specific DNA sequence by RecA-induced autoproteolysis, Our work provides insights into the linkage between protein-protein interac tions, DNA recognition, and DNA repair.