An E. coli cell growing on a carbon source such as glucose or glycerol contains fewer than 10 molecules of β -galactosidase. In contrast, the same cell contains several thousand molecules of the enzyme when grown on lactose. The presence of lactose in the culture medium induces a large increase in the amount of β -galactosidase by eliciting the synthesis of new enzyme molecules rather than by activating a preexisting but inactive precursor. A crucial clue to the mechanism of gene regulation is that two other proteins are synthesized in concert with β -galactosidase namely, galactoside permease and thiogalactoside transacetylase . The permease is required for the transport of lactose across the bacterial cell membrane. The transacetylase is not essential for lactose metabolism but appears to play a role in the detoxification of compounds that also may be transported by the permease. Thus, the expression levels of a set of enzymes that all contribute to the adaptation to a given change in the environment change together . Such a coordinated unit of gene expression is called an operon .
Although repressors are free to diffuse through the cell, how does — for example — the lac repressor find the single stretch of 24 base pairs of the operator out of the million base pairs of DNA in the E. coli genome ? It turns out the repressor is free to bind anywhere on the DNA using both