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| Modern Genetic Analysis Chapter 14. Regulation of Gene Transcription Regulation of the Lactose System Figure 14-15. Comparison of repression and activation. (a) In repression, an active repressor (encoded by the R gene in the example shown here) blocks gene expression of the A,B,C operon by binding to an operator site (O). An inactive repressor allows gene expression. The repressor can be inactivated either by an inducer or by mutation. (b) In activation, a functional activator is required for gene expres-sion, as shown for the X,Y,Z operon here. Small molecules can convert a nonfunctional activator into a functional one, as in the case of cyclic AMP and the CAP protein. A nonfunctional activator results in no gene expression. The activator binds to the control region of the operon, termed I in this case. (The positions of both O and I with respect to the promoter, P, in the two examples are arbitrarily drawn.)

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