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| Modern Genetic Analysis Chapter 14. Regulation of Gene Transcription Regulation of the Lactose System Figure 14-10. The dominance of Is mutations is due to the inactivation of the allosteric site on the Lac repressor. In an Is / I+ diploid cell, none of the lac structural genes are transcribed, even in the presence of an inducer. In contrast with the wild-type repressor, the Is repressor lacks a functional lactose-binding site (the allosteric site) and thus is not inacti-vated by an inducer. Thus, even in the presence of an inducer, the Is repressor binds irreversibly to all operators in a cell, thereby blocking transcription of the lac operon.

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