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The trp Operon: a repressible operon

E. coli can synthesize tryptophan using enzymes that are encoded by five structural genes located next to each other in the trp operon ( [link] ). When environmental tryptophan is low, the operon is turned on. This means that transcription is initiated, the genes are expressed, and tryptophan is synthesized. However, if tryptophan is present in the environment, the trp operon is turned off. Transcription does not occur and tryptophan is not synthesized.

When tryptophan is not present in the cell, the repressor by itself does not bind to the operator; therefore, the operon is active and tryptophan is synthesized. However, when tryptophan accumulates in the cell, two tryptophan molecules bind to the trp repressor molecule, which changes its shape, allowing it to bind to the trp operator. This binding of the active form of the trp repressor to the operator blocks RNA polymerase from transcribing the structural genes, stopping expression of the operon. Thus, the actual product of the biosynthetic pathway controlled by the operon regulates the expression of the operon.

Diagram of the trp operon. The top image shows the operon in the absence of tryptophan. The trp repressor dissociates from the operator and RNA synthesis proceeds. RNA polymerase is bound to the promoter and an arrow indicates that transcription will occur. The repressor is not bound ot anything. The bottom image shows the operon in the presence of tryprophan. When tryptophan is present, the trp repressor binds to the operator and RNA synthesis is blocked. Tryptophan is shown bound to the repressor which is bound to the operator. RNA polymerase is bound to the promoter but is blocked from moving forward by the repressor.
The five structural genes needed to synthesize tryptophan in E. coli are located next to each other in the trp operon. When tryptophan is absent, the repressor protein does not bind to the operator, and the genes are transcribed. When tryptophan is plentiful, tryptophan binds the repressor protein at the operator sequence. This physically blocks the RNA polymerase from transcribing the tryptophan biosynthesis genes.

The lac Operon: an inducible operon

The lac operon is an example of an inducible operon that is also subject to activation in the absence of glucose ( [link] ). The lac operon encodes three structural genes necessary to acquire and process the disaccharide lactose from the environment, breaking it down into the simple sugars glucose and galactose. For the lac operon to be expressed, lactose must be present. This makes sense for the cell because it would be energetically wasteful to create the enzymes to process lactose if lactose was not available.

In the absence of lactose, the lac repressor is bound to the operator region of the lac operon, physically preventing RNA polymerase from transcribing the structural genes. However, when lactose is present, the lactose inside the cell is converted to allolactose. Allolactose serves as an inducer molecule, binding to the repressor and changing its shape so that it is no longer able to bind to the operator DNA. Removal of the repressor in the presence of lactose allows RNA polymerase to move through the operator region and begin transcription of the lac structural genes.

A diagram of the lac operon. The top image shows what occurs in the absence of lactose. In the absence of lactose, the lac repressor binds the operator and transcription is blocked. The repressor is not bound to lactose but is bound to the operator. RNA polymerase is bound to the promoter but is blocked from transcription by the repressor. The bottom image shows the presence of lactose. In the presence of lactose, the lac repressor is released from the operator and transcription proceeds at a slow rate. The image shows lactose bound to the repressor which is no longer bound to the operator. RNA polymerase is bound to the promoter and an arrow indicates that transcription is occurring.
The three structural genes that are needed to degrade lactose in E. coli are located next to each other in the lac operon. When lactose is absent, the repressor protein binds to the operator, physically blocking the RNA polymerase from transcribing the lac structural genes. When lactose is available, a lactose molecule binds the repressor protein, preventing the repressor from binding to the operator sequence, and the genes are transcribed.

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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