A Mechanism for Induction
How do inducers reduce repressor’s affinity for operator? It is possible that they bind near the operator-binding site and merely interfere with repressor’s correct binding to operator. This possibility seems unlikely
in view of the genetic data and the separability of the IPTG-binding and operator-binding substructures of repressor discussed above. It seems more likely that IPTG merely causes the subunits of repressor to alter positions slightly with respect to one another. Why should this drasti-cally weaken the binding? Experiments with the N-terminal DNA-bind-ing domain contain the answer.
The DNA-binding domain binds to operator with a much lower affinity than wild-type repressor. Such reduced affinity actually is expected. The wild-type tetrameric repressor molecule possesses a rela-tively rigid structure in which pairs of the N-terminal regions are held in positions appropriate for binding to a single operator. That is, the binding of one of the N-terminal regions to half of the symmetrical operator perforce brings a second N-terminal region into position for its binding to the other half of the operator. Therefore most of the additional interaction energy between this subunit and the DNA can be used to hold the complex onto the DNA (Fig. 11.12). This is another
example of the chelate effect. Overall, the result is that the oligomeric repressor tightly binds to operator. The same is not true of the isolated N-terminal domains. The binding of one of these DNA-binding domains does not automatically bring another into position for binding to the other half of operator. As a result of their independent binding, the apparent affinity of a single DNA-binding domain for operator is low.
The chelate effect also streamlines the explanation of induction: binding of IPTG shifts pairs of subunits away from optimal relative positions for headpiece binding to operator. Consequently, the affinity of repressor for operator is greatly reduced, and repressor dissociates. Eventually, direct experiments may be able to test such ideas. In the meantime, two types of repressor mutations are consistent with this point of view. Repressor mutants can be found that are not located in the N-terminal region and that result in much tighter or much weaker repressor-operator binding. These mutants possess no discernible struc-tural alterations. Most likely these types of mutation merely shift the positions of the subunits slightly with respect to one another. The tighter-binding mutant must bring the subunits into closer complemen-tarity with operator, and the weaker-binding mutants must shift the subunits away from complementarity.
An enormous amount has been learned about the lac operon, and only a fraction was mentioned. More remains to be learned about the physiology and physical chemistry underlying regu-lation of this set of genes and others.