activator inactive

the basic unit of transcriptional control in bacteria. Despite the fact that, at the time, no methods were available for determining nucleotide sequences, Jacob and Monod deduced the structure of the operon genetically by analyzing the interactions of mutations that interfered with the normal regulation of lactose metabolism. We will examine the effects of some of these mutations after seeing how the lac operon regulates lactose metabolism.

Lactose (a disaccharide) is one of the major carbohydrates found in milk; it can be metabolized by E. coli bacteria that reside in the gut of mammals. Lactose does not easily diffuse across the E. coli cell membrane and must be actively transported into the cell by the enzyme permease (< Figure 16.6). To utilize lactose as an energy source, E. coli must first break it into glucose and galactose, a reaction catalyzed by the enzyme p-galactosidase. This enzyme can also convert lactose into allolactose, a compound that plays an important role in regulating lactose metabolism. A third enzyme, thiogalactoside transacetylase, also is produced by the lac operon, but its function in lactose metabolism is not yet known.

The enzymes p-galactosidase, permease, and transacety-lase are encoded by adjacent structural genes in the lac operon of E. coli. p-Galactosidase is encoded by the lacZ gene, permease by the lacY gene, and transacetylase by the lacA gene (Figure 16.7a). When lactose is absent from the medium in which E. coli grows, only a few molecules of each enzyme are produced. If lactose is added to the medium and glucose is absent, the rate of synthesis of all three enzymes simultaneously increases about a thousandfold within 2 to 3 minutes. This boost in enzyme synthesis results from transcription of lacZ, lacY, and lacA and examplifies coordinate induction, the simultaneous synthesis of several enzymes, stimulated by a specific molecule, the inducer (Figure 16.7b). Although

Extracellular lactose

Cell membrane

Extracellular lactose

Cell membrane

also converts lactose into the related compound allolactose...




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