Abstract
The cyclic AMP receptor protein (CAP) and lactose repressor bind their regulatory sites in the lactose promoter with moderate cooperativity (ω C101=11.8(±3.7)). This cooperativity is significantly reduced 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-type tetrameric protein. These results are consistent with a mechanism of interaction in which CAP bends the DNA and the lacrepressor binds simultaneously to its operator site and to promoter-distal sequences. Similar values of ω C101were obtained with a promoter truncation containing the O3 pseudooperator site and one in which the site is destroyed, suggesting that DNA contacts distal to the O3 site are necessary for cooperative binding.
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