Figure 7–14 Genes can be switched on by activator proteins. An activator protein binds to its cis-regulatory sequence on the DnA and interacts with the rnA polymerase to help it initiate transcription. without the activator, the promoter fails to initiate transcription efficiently. in bacteria, the binding of the activator to DnA is often controlled by the interaction of a metabolite or other small molecule (red triangle) with the activator protein. the Lac operon works in this manner, as we discuss shortly.
DNA-bound activator proteins can increase the rate of transcription initiation as much as 1000-fold, a value consistent with a relatively weak and nonspecific interaction between the transcription regulator and RNA polymerase. For example, a 1000-fold change in the affinity of RNA polymerase for its promoter corresponds to a change in ∆G of ≈18 kJ/mole, which could be accounted for by just a few weak, noncovalent bonds. Thus, many activator proteins work simply by providing a few favorable interactions that help to attract RNA polymerase to the promoter. To provide this assistance, however, the activator protein must be bound to its cis-regulatory sequence, and this sequence must be positioned, with respect to the promoter, so that the favorable interactions can occur.
Like the tryptophan repressor, activator proteins often have to interact with a second molecule to be able to bind DNA. For example, the bacterial activator protein CAP has to bind cyclic AMP (cAMP) before it can bind to DNA. Genes activated by CAP are switched on in response to an increase in intracellular cAMP concentration, which rises when glucose, the bacterium’s preferred carbon source, is no longer available; as a result, CAP drives the production of enzymes that allow the bacterium to digest other sugars.
An Activator and a repressor Control the Lac Operon
In many instances, the activity of a single promoter is controlled by several different transcription regulators. The Lac operon in E. coli, for example, is controlled by both the Lac repressor and the CAP activator that we just discussed. The Lac operon encodes proteins required to import and digest the disaccharide lactose. In the absence of glucose, the bacterium makes cAMP, which activates CAP to switch on genes that allow the cell to utilize alternative sources of carbon— including lactose. It would be wasteful, however, for CAP to induce expression of the Lac operon if lactose itself were not present. Thus the Lac repressor shuts off the operon in the absence of lactose. This arrangement enables the control region of the Lac operon to integrate two different signals, so that the operon is highly expressed only when two conditions are met: glucose must be absent and lactose must be present (Figure 7–15). This genetic circuit thus behaves much like
RNA-cis-regulatory polymerase-start of transcription sequence binding site for CAP (promoter)
Figure 7–15 The Lac operon is controlled by two transcription regulators, the Lac repressor and CAP. LacZ, the first gene of the operon, encodes the enzyme β-galactosidase, which breaks down lactose to galactose and glucose. when lactose is absent, the lac repressor binds to a cisregulatory sequence, called the Lac operator, and shuts off expression of the operon (Movie 7.4). Addition of lactose increases the intracellular concentration of a related compound, allolactose; allolactose binds to the lac repressor, causing it to undergo a conformational change that releases its grip on the operator DnA (not shown). when glucose is absent, cyclic AMp (red triangle) is produced by the cell, and CAp binds to DnA.
a switch that carries out a logic operation in a computer. When lactose is present AND glucose is absent, the cell executes the appropriate program—in this case, transcription of the genes that permit the uptake and utilization of lactose.