Info

\ \

Gene B

Transcription on.

Inducer binds to the repressor and alters its shape, preventing the repressor from binding to the operator.

Transcription

Figure 7.18 Transcriptional Regulation by Repressors

Transcription on.

Inducer binds to the repressor and alters its shape, preventing the repressor from binding to the operator.

Transcription

Figure 7.18 Transcriptional Regulation by Repressors

Activators

An activator is a regulatory protein that facilitates transcription. Genes that are controlled by an activator have an ineffective promoter that is preceded by an activator-binding site. The binding of the activator to the DNA enhances the ability of RNA polymerase to initiate transcription at that promoter. Regulation involving an activator is sometimes called positive control.

Like repressors, activators are allosteric proteins whose function can be modulated by the binding of other molecules. A molecule that binds to an activator and alters its shape so it can effectively bind to the activator-binding site functions as an inducer (figure 7.19). Thus, the term inducer applies to a molecule that turns on transcription, either by stimulating the function of an activator or interfering with the function of a repressor.

The lac Operon As a Model for Control of Metabolic Pathways

Originally elucidated in the early 1960s by Francois Jacob and Jacques Monod, the lac operon has served as an important model for understanding the control of gene expression in bacteria. The operon, which consists of three genes involved with lactose degradation, along with regulatory components, is subject to dual control by both a repressor and an activator (figure 7.20). The net effect is that the genes are expressed only when lactose is present but glucose is absent.

The Effect of Lactose on the Control of the Lactose Operon

The lac operon employs a repressor that prevents transcription of the genes when lactose is unavailable. When lactose is not present, the repressor binds to the operator, effectively blocking transcription. When lactose is present in the cell, however, some of the molecules are converted into a compound called allolactose. This compound binds to the repressor, altering its shape so that it can no longer bind to the operator. Thus, when lactose is present, the repressor no longer prevents RNA polymerase from transcribing the operon. Note, however, that the activator described in the next section is needed for successful transcription.

The Effect of Glucose on the Control of the Lactose Operon

Escherichia coli preferentially uses glucose over other sugars such as lactose. This can readily be demonstrated by observing growth and sugar utilization of E. coli in a medium containing glucose and lactose. Cells actively grow, metabolizing only glucose until its supply is exhausted (figure 7.21). Growth then ceases for a short period until the cells begin utilizing lactose. At this point, the cells start multiplying. This two-

Transcription off (blocked). Corepressor binds to the repressor and alters its shape, enabling the repressor to bind to the operator.

Gene B

Activator . RNA polymerase ft. X \ ,-if . \ ? \ / I V

Transcription off (not activated). Activator cannot bind to the activator-binding site, thus RNA polymerase cannot bind to the promoter and initiate transcription.

Activator-binding site Promoter

Activator-binding site Promoter

Inducer

Inducer

Transcription

Transcription on (activated). Inducer binds to the activator and changes its shape, enabling the activator to bind to the site. RNA polymerase can then bind to the promoter and initiate transcription.

Gene C

Transcription

7.6 Regulating Gene Expression 185

step growth response, called diauxic growth, represents the ability of glucose to repress the enzymes of lactose degrada-tion—a phenomenon called catabolite repression.

The regulatory mechanism of catabolite repression does not directly sense glucose in a cell. Instead, it recognizes the concentration of a nucleotide derivative, called cyclic AMP (cAMP), which is low when glucose is present and high when it is absent. cAMP is an inducer of the operon; it binds to an activator that facilitates transcription of the lac operon. This activator, called CAP (catabolite activator protein), is only able to bind to the lac promoter when cAMP is bound to it. The higher the concentration of cAMP, the more likely it is to bind to CAP. Thus, when glucose concentrations are low (and therefore cAMP levels are high), the lac operon can be transcribed. Note, however, that even in the presence of a functional activator, the repressor prevents transcription unless lactose is present. Catabolite repression is significant biologically because it forces the cells to first use the carbon source that is most easily metabolized. Only when the

Figure 7.19 Transcriptional Regulation by Activators

Figure 7.19 Transcriptional Regulation by Activators

Activator- Promoter Operator

(CAP) binding site

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