Prokaryotic operons are commonly controlled by the binding of repressors to operator regions, thereby preventing the transcription of the structural genes.
These prokaryotic operons are classified as either repressible operons or inducible operons.
Repressible operons, like the tryptophan (trp) operon, typically contain genes encoding enzymes required for a biosynthetic pathway. As long as the product of the pathway, like tryptophan, continues to be required by the cell, a repressible operon will continue to be expressed. However, when the product of the biosynthetic pathway begins to accumulate in the cell, removing the need for the cell to continue to make more, the expression of the operon is repressed. This happens when 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 the expression of the operon. Thus, the actual product of the biosynthetic pathway controlled by the operon regulates the expression of the operon.
Conversely, inducible operons, like the lac operon of E. coli, often contain genes encoding enzymes in a pathway involved in the metabolism of a specific substrate like lactose. These enzymes are only required when that substrate is available. Thus the expression of the operons is typically induced only in the presence of the substrate.
The lac operon is an example of an inducible operon that is also subject to activation in the absence of glucose. 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. 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.
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Key Points
• Gene expression in prokaryotes is largely regulated at the point of transcription. Gene expression in eukaryotes is additionally regulated post-transcriptionally.
• Prokaryotic structural genes of related function are often organized into operons, all controlled by transcription from a single promoter. The regulatory region of an operon includes the promoter itself and the region surrounding the promoter to which transcription factors can bind to influence transcription.
• Although some operons are constitutively expressed, most are subject to regulation through the use of transcription factors (repressors and activators). A repressor binds to an operator, a DNA sequence within the regulatory region between the RNA polymerase binding site in the promoter and first structural gene, thereby physically blocking transcription of these operons. An activator binds within the regulatory region of an operon, helping RNA polymerase bind to the promoter, thereby enhancing the transcription of this operon. An inducer influences transcription through interacting with a repressor or activator.
• The trp operon is a classic example of a repressible operon. When tryptophan accumulates, tryptophan binds to a repressor, which then binds to the operator, preventing further transcription.
• The lac operon is a classic example of an inducible operon. When lactose is present in the cell, it is converted to allolactose. Allolactose acts as an inducer, binding to the repressor and preventing the repressor from binding to the operator. This allows transcription of the structural genes.
Key Terms
repressor: any protein that binds to DNA and thus regulates the expression of genes by decreasing the rate of transcription
operon: a unit of genetic material that functions in a coordinated manner using an operator, a promoter, and structural genes that are transcribed together
RNA polymerase: a DNA-dependent RNA polymerase, an enzyme, that produces RNA
promoter: the section of DNA that controls the initiation of RNA transcription
repressible operon: an operon that is regulated by a co-repressor (chemical substance)
inducible operon: an operon which can be turned on by a molecule called an inducer
lac operon: The lac operon is an operon that encodes proteins that allow the bacteria to use lactose as an energy source
lactose: a large sugar molecule that is made up of two smaller sugar molecules, glucose and galactose