Cooperativity in Repressor Binding and its Measurement
As seen in the preceding section, repressor does not bind with equal affinity to the three operators on the right. More detailed experiments also reveal that repressor molecules do not bind independently to the
Table 14.4 The Binding Energies of Repressor Di-mers to the Various Operators and the Interaction Energies that Generate Cooperative Binding of Re-pressor Dimers
three operators (Table 14.4). Not surprisingly, repressors bound at the operators interact with one another and change the overall binding energy of repressor for an operator. Most unexpectedly, however, the results show that a repressor molecule bound at the middle operator can interact either with a repressor bound at OR1 or with a repressor bound at OR3, but it does not simultaneously interact with both. This is termed a pairwise interaction.
Operator mutants were used to measure the intrinsic affinity of repressor for the three operator sites as well as to reveal interactions between adjacently bound repressor molecules. By eliminating re-pressor binding to OR2, the binding energy of repressor to OR1 and to OR3could be measured without complications caused by interactionswith adjacent repressors. Similarly, eliminating binding at OR1 or OR3 permitted determination of the other binding energies. Ultimately, this series of measurements provided enough data to permit calculation of the interaction energies of adjacent repressor molecules. The measure-ments showed that repressor binding at OR2 has a dramatic effect on the binding of adjacent repressors.