Positive Control and Catabolite Repression

E. coli and many other bacteria will metabolize glucose preferentially in the presence of lactose and other sugars. They do so because glucose enters glycolysis without further modification and therefore requires less energy to metabolize than do other sugars. When glucose is available, genes that participate in the metabolism of other sugars are repressed, in a phenomenon known as catabolite repression. For example, the efficient transcription of the lac operon takes place only if lactose is present and glucose is absent. But how is the expression of the lac operon influenced by glucose? What brings about catabolite repression?

Catabolite repression results from positive control in response to glucose. (This regulation is in addition to the negative control brought about by the repressor binding at the operator site of the lac operon when lactose is absent.) Positive control is accomplished through the binding of a dimeric protein called the catabolite activator protein (CAP) to a site that is about 22 nucleotides long and is located within or slightly upstream of the promoter of the lac genes (< Figure 16.12). RNA polymerase does not bind efficiently to many promoters unless CAP is first bound to the DNA. Before CAP can bind to DNA, it must form a complex with a modified nucleotide called adenosine-3', 5'-cyclic monophosphate (cyclic AMP or cAMP), which is important in cellular signaling processes in both bacterial and eukaryotic cells. In E. coli, the concentration of cAMP is inversely proportional to the level of available glucose. A high concentration of glucose within the cell lowers the amount of cAMP, and so little cAMP-CAP complex is available to bind to the DNA. Subsequently, RNA poly-merase has poor affinity for the lac promoter, and little

(a) Partial diploid lacI+ lacO + lacZ / lacI+ lacOc lacZ+

(b) Partial diploid lacI+ lacO + lacZ +/ lacI+ lacO c lacZ-

Absence of lactose lac I+

Active repressor lac I

lacO+ operator bind y lacO+ operator bind y lac I+

Active repressor lac I

c lacOc operator _I_

c lacOc operator _I_

Transcription and translation

Absence of lactose lacI+

Active repressor lacO+ operator

Cannot bind ' lacZ+

lacO+ operator

Cannot bind ' lacZ+

lacI+

lacO c operator

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