THE MECHANISM OF CAP-LAC REPRESSOR BINDING COOPERATIVITY AT THE ESCHERICHIA-COLI LACTOSE PROMOTER

The cyclic AMP receptor protein (CAP) and lactose repressor bind their regulatory sites in the lactose promoter with moderate cooperativity (omega(C101)=11.8(+/-3.7)). This cooperativity is significantly reduce d by the removal of DNA located upstream of the CAP binding site or by substitution of the dimeric lacI-18 mutant repressor for the wild-typ e tetrameric protein. These results are consistent with a mechanism of interaction in which CAP bends the DNA and the inc repressor binds si multaneously to its operator site and to promoter-distal sequences. Si milar values of omega(C101) were obtained with a promoter truncation c ontaining the O3 pseudooperator site and one in which the site is dest royed, suggesting that DNA contacts distal to the O3 site are necessar y for cooperative binding. (C) 1996 Academic Press Limited