Gene expression (prokaryotes): the lac operon model

In the presence of lactose, E.coli bacteria initiate the production of galactoside permease, a membrane transport protein that traffics high levels of lactose across the cellular membrane, as well as $\beta$‍-galactosidase, an enzyme cleaves lactose into the monosaccharides glucose and galactose.

To investigate the mechanistic basis for the selective expression of lactose-processing genes, researchers created three different E.coli lines, each with mutations affecting their ability to process lactose. They cultured each of the three lines in either lactose-only media or glucose-only media and monitored intracellular accumulation of lactose and $\beta$‍-galactosidase activity. The results are shown in Table 1. Note that $\beta$‍-galactosidase activity was assayed using an indicator molecule other than lactose.

Table 1 Phenotypic observations for three different E.coli mutants when cultured in lactose-containing media (lac+) or glucose-containing media (glu+)

Scientists observing these findings proposed a set of hypotheses that have come to be known as the lac operon model of gene regulation in prokaryotes. The Jacob-Monod group hypothesized that an operon in E. coli, the lac operon, accounts for the lactose processing behavior observed in the above data. According to this model, the lac operon itself consists of two genes: lacZ, which codes for $\beta$‍-galactosidase, and lacY, which codes for galactoside permease (the model also contains a gene called lacA, not discussed here). Crucially, the model also hypothesizes that the transcription of the lac operon is controlled by the LacI gene, which codes for a DNA-binding repressor protein. This repressor protein down-regulates the transcription of the lac operon by binding immediately upstream of lacZ and lacY, at a site called the operator, which serves to impede the progress of RNA polymerase.

Extracellular lactose has the ability to enter the cell, and its metabolite then binds the repressor protein, altering its conformation in such a way that it releases from the operator and allowing for unimpeded transcription of the lac operon. Transcription of the lac operon is also regulated by cAMP levels: when cAMP attaches to the protein CAP, CAP helps RNA polymerase bind to the promoter.