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| Modern Genetic Analysis Chapter 14. Regulation of Gene Transcription Regulation of the Lactose System Figure 14-14. Negative and positive control of the lac operon by the Lac repressor and catabolite activator protein (CAP), respec-tively. (a) In the absence of lactose to serve as an inducer, the Lac repressor is able to bind the operator; regardless of the levels of cAMP and the presence of CAP, mRNA production is repressed. (b) With lactose present to bind the repressor, the repressor is unable to bind the operator; however, only small amounts of mRNA are produced because the presence of glucose keeps the levels of cAMP low, and thus the cAMPCAP complex does not form and bind the promoter. (c) With the repressor inactivated by lactose and with high levels of cAMP present (owing to the absence of glucose), cAMP binds CAP. The cAMPCAP complex is then able to bind the promoter; the lac operon is thus activated, and large amounts of mRNA are produced. (d) When CAP binds the promoter, it creates a bend greater than 90° in the DNA. Apparently, RNA polymerase binds more effectively when the promoter is in this bent configuration. (e) CAP bound to its DNA recognition site. This part is derived from the structural analysis of the CAPDNA complex. [Parts ad redrawn from B. Gartenberg and D. M. Crothers, Nature 333, 1988, 824. (See H. N. Lie-Johnson et al., Cell 47, 1986, 995.) Adapted from H. Lodish, D. Baltimore, A. Berk, S. L. Zipursky, P. Matsudaira, and J. Darnell, Molecular Cell Biology, 3d ed. Copyright © 1995 by Scientific American Books. Part e from L. Schultz and T. A. Steitz.]

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