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Lac Operon

The lac operon of E. coli contains genes coding for several proteins which are involved in metabolism of the disaccharide lactose. One protein of the lac operon, called a permease, imports lactose through the otherwise-impermeable cell membrane. Another protein is an enzyme called β-galactosidase, which can hydrolyze the disaccharide to its two constituent monosaccharides, galactose and glucose, which the cell can then process further. Because lactose is rarely available in the environment, the bacterial cell switches off synthesis of the permease and β-galactosidase to conserve energy until lactose is available. The switch is controlled by another protein called a repressor, whose gene is located next to the operon. Ordinarily the repressor binds to the lac operon, shutting it off by physically interfering with expression of the operon. In the presence of the natural "inducer" allolactose (a by-product of lac β-galactosidase activity) or the artificial chemical inducer isopropylthiogalactoside (IPTG), however, the repressor binds to the inducer and releases the operon, allowing the lac operon enzymes to be synthesized by the cell.

*This entry taken from: Michael Behe's A Response to Critics of Darwin's Black Box

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