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The lac operon of E. coli consists of three genes (Figure 3-H-4): lacZ, lacY, and lacA, encoding b-galactosidase, lactose permease and thiogalactoside transacetylase, respectively. Lactose permease is located on the cell membrane, capable of pumping lactose into the cell. b-galactosidase can convert lactose into glucose and galactose (Figure 4-H-1). Thiogalactoside transacetylase is responsible for degrading small molecules. In the absence of lactose, transcription of the lac operon is inhibited by the lac repressor (Figure 4-D-3). The lactose can bind to the lac repressor, preventing it from interacting with its DNA binding site. Hence, in a medium containing lactose, the lac operon is quickly transcribed, producing the enzymes to generate glucose, which is the major energy source for E. coli.
Figure 4-H-1. b-galactosidase can convert lactose into glucose and galactose.
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