lac Operon

What is lac operon?

The lac operon is a group of genes with one promoter that encode proteins to use lactose, the chemical structure is shown in Figure 1 below, as an energy source for enteric bacteria. Enteric bacteria are bacteria found in the intestines. Remember that bacteria prefer to use glucose as their fuel of choice, so in order for the lac operon to turn on, there needs to be no glucose for them available.

Figure 1. Lactose chemical structure. Source: Wobble via commons.wikimedia.org

Lac operon function

Since glucose is much easier to break down than lactose, the lac operon will only turn on if there is no glucose present and only lactose present! In order to determine when to turn on, the lac operon contains two regulatory proteins that function as sensors. The lac repressor is able to sense the amount of lactose, and the catabolite activator protein (CAP) is able to sense the amount of glucose. Both the lac repressor and the CAP are bound to the lac operon DNA and assist with transcription based on the amount of lactose and glucose.

Lac operon inducer

Inducers are small molecules that are able to turn on genes or operons. The inducer for lac operon is allolactose, an isomer of lactose, which means the presence of allolactose is able to turn on the lac operon. Lactose is able to be converted to allolactose, and the allolactose is able to bind to the repressor and change its shape, which prevents it from binding to DNA (Fig. 2).

Lac operon regulation

There are two major lac operon regulation parts: the lac repressor and catabolite activator protein (CAP).

The lac repressor is a protein that prevents the transcription of the lac operon. It binds to the operator, which partially covers the promoter and stops RNA polymerase from being able to bind to the promoter. The lac repressor is bound to the operator when lactose is not present, but when lactose is present, it is no longer able to bind to the operator, which allows RNA polymerase to begin transcription. It is important to note that the gene, lacl, that aids in the transcription of the lac repressor is continuously on and is part of a different promoter and not part of the lac operon.

RNA polymerase does not bind as well to the promoter as expected, so it needs CAP to assist by binding to a region of DNA next to the promoter. The CAP bound next to the promoter will help the RNA polymerase bind to the promoter. The gene for CAP is found in the bacterial chromosome, and it is not located near the lac operon, but it is constantly "on" so CAP is always able to monitor glucose levels. CAP is not always able to bind to DNA and is instead regulated by cyclic AMP (cAMP). E. coli uses cAMP as a signal when glucose levels are low, and cAMP is able to change the shape of CAP in order to allow it to bind to DNA.

Lac operon structure

There are three genes found in the lac operon, lacZ, lacY, and lacA. These genes each have their own different functions but are considered a single mRNA due to having one promoter. lacZ uses an enzyme, β-galactosidase, to turn lactose into the monosaccharides glucose and galactose. lacY uses the membrane protein lactose permease to help lactose get inside of the cell. lacA uses the enzyme transacetylase to attach chemical groups to target molecules.

Alongside the three genes, the lac operon also contains the promoter, operator, and the CAP binding site. The promoter is where RNA polymerase, the enzyme that performs transcription, is able to bind. The operator is bound by lac repressor protein and partially covers the promoter. If the lac repressor protein covers the promoter, then RNA transcriptase is unable to bind to the promoter. The CAP binding site is where CAP binds, and when it is there, it helps RNA polymerase bind to the promoter.

Lac operon diagram

As you can see in Figure 3 below, it shows the three genes used in the lac operon and the scenarios when it is and is not activated.

Figure 3. lac operon diagram. Source: G3pro via commons.wikimedia.org