E. coli make the proteins that mediate lactose metabolism only when they are needed

E. coli can metabolize a wide variety of different nutrients. One such energy source is the disaccharide lactose. Lactose enters the cell through a transporter protein called galactoside permease. Once inside the cell, the lactose is hydrolyzed by the enzyme b-galactosidase to yield glucose and galactose, which are then metabolized via common pathways to produce ATP. However, if lactose is absent, it is a waste of often scarce resources for the bacteria to synthesize large quantities of galactoside permease and b-galactosidase. Consequently, they don't do so. Instead, they have evolved a mechanism which senses the presence of lactose, and induces expression of these proteins only when they are needed.

The lac operon

The genes for b-galactosidase and galactoside permease are located next to each other in the DNA, along with a gene encoding thiogalactoside transacetylase (whose physiological role is unknown). This cluster of three genes is called the lac operon. Upstream of these genes is a site known as the promoter (P), the DNA sequence where RNA polymerase binds and initiates transcription. In between the promoter and the lac operon is a DNA sequence called the operator (O). The operator binds a repressor protein called the lac repressor. The lac repressor is encoded by the I gene, which is located just upstream of the promoter site. The binding of the lac repressor to the operator prevents RNA polymerase from initiating transcription at the promoter, thereby arresting the expression of the lac operon.

Inducers cause the lac repressor to dissociate from the operon, thereby permitting lac operon expression

When lactose is present, b-galactosidase occasionally catalyzes a transglycosylation reaction that converts lactose to 1,6-allolactose instead of hydrolyzing it into its component monosaccharides. This disaccharide specifically binds to the lac repressor and, in so doing, induces a conformational change that causes the lac repressor to dissociate from the operator. With the lac repressor gene gone, RNA polymerase can bind to the promoter and initiate transcription of the Z, Y, and A genes in a single strand of mRNA. The resulting transcripts are repeatedly translated by ribosomes, yielding multiple copies of b-galactosidase, lactose permease, and thiogalactoside transacetylase. The first two proteins then mediate the import and hydrolysis of lactose. Occasionally, the b-galactosidase will synthesize a 1,6-allolactose molecule, thereby preventing the lac repressor from binding to the operator. However, when the lactose has been used up, 1-6-allolactose is no longer produced, causing the lac repressor to once again bind to the operator and prevent the expression of the lac operon.