Lac operon

The lac operon is a DNA sequence that governs the production of proteins and enzymes for transporting and metabolizing lactose in bacteria such as E. coli. In the absence of lactose, the lac repressor substance binds to the operator (a part of the DNA sequence), inhibiting the production of three proteins. Lactose, however, represses/inhibits the repressor, allowing the enzymes to be produced. When the mRNA of the lac operon is transcribed, a polycistronic mRNA, three proteins will be produced by ribosomes: β-galactosidase, lactose permease and transacetylase. The lac operon is considered the canonical example of prokaryotic gene regulation.

Contents

1 Discovery 2 The operon 2.1 Regulation by glucose 2.2 Regulation by lactose

Discovery

It is traditional to give three letter codes for genes; "Lac" refers to a group of three enzymes for metabolism of the sugar lactose that are together as a group on the bacterial chromosome. The experimental system used by Jacob and Monod was a common bacterium, E. coli, but the basic regulatory concept (described below) that was discovered by Jacob and Monod is fundamental to cellular regulation for all organisms. The key idea is that E. coli conserves cellular resources and energy by not making the three "Lac" proteins when there is no need to metabolize lactose, such as when other sugars like glucose are available. How do bacteria "know" when to activate certain genes in response to metabolic needs?

During World War II, Monod was testing the effects of combinations of sugars as nutrient sources for E. coli. He found that bacteria grown with two different sugars often displayed two phases of growth. For example, if glucose and lactose were both provided, glucose would be metabolized first (growth phase I, see Figure 2) and then lactose (growth phase II). But why was there a delay between the two growth phases?

The operon

Regulation by glucose

At high glucose concentrations, the production of the cellular signal cAMP is very low. When the glucose has been removed from the culture medium at the end of growth phase I, cAMP levels begin to increase. The bacteria make an activator protein called CAP which is able to dimerize when bound by cAMP. The CAP homodimer binds to the upstream element and recruits RNA polymerase.

Regulation by lactose

There is also a second level of regulation. The bacteria also make a specific repressor protein, lacI, which binds to the operator located downstream of the promoter, preventing expression of lactose metabolism genes. However, when lactose is present it binds to the repressor protein and allostericly prevents the repressor from binding to the operator site.

This dual regulation causes the lactose metabolism enzymes to be made only when there is no glucose available and when there is lactose available (Phase II in Figure 2).

So why is there a delay between the two growth phases? The CAP regulatory protein has to assemble on the Lac operator, resulting in an increase in the production of Lac mRNA. More available copies of the Lac mRNA results in the production (see translation) of significantly more copies of the enzymes that metabolize lactose. After a delay needed to increase the level of the lactose metabolizing enzymes, the bacteria enter into a new rapid phase of cell growth.