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. 1968 Nov;96(5):1774–1782. doi: 10.1128/jb.96.5.1774-1782.1968

Paradoxical Effect of Weak Inducers on the lac Operon of Escherichia coli

Beverly Williams 1, Kenneth Paigen 1
PMCID: PMC315240  PMID: 4882023

Abstract

Previously, we reported the existence of a group of compounds whose function in the regulation of the lac operon was “paradoxical” in that they acted as either inducers or repressors depending on the circumstances. We now show that this group of compounds does not repress the lac operon by catabolite repression, transient repression, or by preventing the uptake of inducers. A model is presented which shows that “paradoxical” behavior is to be expected if a weak inducer is present at a concentration that is high relative to its binding affinity for the regulatory macromolecule. This model depends on the assumptions that the regulatory macromolecule is an allosteric protein which undergoes a transition between two conformational states and that the rate of enzyme synthesis depends on the fraction of protein molecules in each state. The previous observations on the responses of lac regulatory mutants to weak inducers have been extended to a series of such mutants. Weak inducers repress β-galactosidase synthesis in several i− mutants. When this happens, enzyme synthesis can be reinduced by using a strong inducer such as isopropyl-β-d-thiogalactoside. These compounds induce operator constitutives and the it mutant more easily than they induce a wild-type strain.

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Selected References

These references are in PubMed. This may not be the complete list of references from this article.

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