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Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000.

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An Introduction to Genetic Analysis. 7th edition.

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Positive and negative control

The inducer–repressor control of the lac operon is an example of negative control, in which expression is normally blocked. In contrast, the CAP-cAMP system is an example of positive control, because expression of the lac operon requires the presence of an activating signal—in this case, the interaction of the CAP-cAMP complex with the CAP region. Figure 11-13 distinguishes between these two basic types of control systems.

Figure 11-13. Comparison of positive and negative control.

Figure 11-13

Comparison of positive and negative control. The basic aspects of negative and positive control are depicted. (a) In negative control, an active repressor (encoded by the R gene in the example shown here) blocks gene expression of the A B C (more...)

For activators or repressor proteins to do their job, each must be able to exist in two states: one that can bind its DNA targets and one that cannot. The binding state must be appropriate for a given set of environmental conditions. For many activator or repressor proteins, the way that DNA binding is regulated is through the interaction of two different sites in the three-dimensional structure of the protein. One site is the DNA-binding domain. The other site, the allosteric site, acts as a switch that sets the DNA-binding domain in one of two modes: functional or nonfunctional. The allosteric site interacts with small molecules called allosteric effectors. In regard to the lac operon, the lac inducers are allosteric effectors. When allosteric effectors bind to the allosteric site, they cause a conformational change in the regulatory protein so that it alters the structure of the DNA-binding domain. Some activator or repressor proteins must bind to their allosteric effectors to bind DNA. Others can bind DNA only in the absence of their allosteric effectors. Figure 11-14 depicts these concepts.

Figure 11-14. The effects of allosteric effectors on the DNA-binding activities of activators and repressors.

Figure 11-14

The effects of allosteric effectors on the DNA-binding activities of activators and repressors.

By using different combinations of controlling elements, bacteria have evolved numerous strategies for regulating gene expression. Some examples follow.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, W. H. Freeman and Company.
Bookshelf ID: NBK22091

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