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. Figure 2. The five structural genes needed to synthesize tryptophan in E. 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 is an example of an inducible operon that is also subject to activation in the absence of glucose Figure 3. 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. Figure 3. The three structural genes that are needed to degrade lactose in E. 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. Figure 4. When grown in the presence of two substrates, E. Then, enzymes needed for the metabolism of the second substrate are expressed and growth resumes, although at a slower rate. Bacteria typically have the ability to use a variety of substrates as carbon sources. However, because glucose is usually preferable to other substrates, bacteria have mechanisms to ensure that alternative substrates are only used when glucose has been depleted.
Additionally, bacteria have mechanisms to ensure that the genes encoding enzymes for using alternative substrates are expressed only when the alternative substrate is available. In the s, Jacques Monod was the first to demonstrate the preference for certain substrates over others through his studies of E. Such studies generated diauxic growth curves, like the one shown in Figure 4. Although the preferred substrate glucose is used first, E. However, once glucose levels are depleted, growth rates slow, inducing the expression of the enzymes needed for the metabolism of the second substrate, lactose.
Notice how the growth rate in lactose is slower, as indicated by the lower steepness of the growth curve. As a result, cAMP levels begin to rise in the cell Figure 5. Figure 5. Thus, increased cAMP levels signal glucose depletion. The lac operon also plays a role in this switch from using glucose to using lactose. The complex binds to the promoter region of the lac operon Figure 6. In the regulatory regions of these operons, a CAP binding site is located upstream of the RNA polymerase binding site in the promoter.
Binding of the CAP-cAMP complex to this site increases the binding ability of RNA polymerase to the promoter region to initiate the transcription of the structural genes.
Thus, in the case of the lac operon, for transcription to occur, lactose must be present removing the lac repressor protein and glucose levels must be depleted allowing binding of an activating protein. When glucose levels are high, there is catabolite repression of operons encoding enzymes for the metabolism of alternative substrates.
See Table 1 for a summary of the regulation of the lac operon. Figure 6. In prokaryotes, there are also several higher levels of gene regulation that have the ability to control the transcription of many related operons simultaneously in response to an environmental signal. A group of operons all controlled simultaneously is called a regulon. When sensing impending stress, prokaryotes alter the expression of a wide variety of operons to respond in coordination. They do this through the production of alarmones , which are small intracellular nucleotide derivatives.
Alarmones change which genes are expressed and stimulate the expression of specific stress-response genes. The use of alarmones to alter gene expression in response to stress appears to be important in pathogenic bacteria. On encountering host defense mechanisms and other harsh conditions during infection, many operons encoding virulence genes are upregulated in response to alarmone signaling.
Knowledge of these responses is key to being able to fully understand the infection process of many pathogens and to the development of therapies to counter this process. Although most gene expression is regulated at the level of transcription initiation in prokaryotes, there are also mechanisms to control both the completion of transcription as well as translation concurrently. Since their discovery, these mechanisms have been shown to control the completion of transcription and translation of many prokaryotic operons.
Because these mechanisms link the regulation of transcription and translation directly, they are specific to prokaryotes, because these processes are physically separated in eukaryotes. Beyond the transcriptional repression mechanism already discussed, attenuation also controls expression of the trp operon in E. The trp operon regulatory region contains a leader sequence called trpL between the operator and the first structural gene, which has four stretches of RNA that can base pair with each other in different combinations.
However, when an antiterminator stem-loop forms, this prevents the formation of the terminator stem-loop, so RNA polymerase can transcribe the structural genes. Figure 7. Click to view a larger image. When tryptophan is plentiful, translation of the short leader peptide encoded by trpL proceeds, the terminator loop between regions 3 and 4 forms, and transcription terminates. When tryptophan levels are depleted, translation of the short leader peptide stalls at region 1, allowing regions 2 and 3 to form an antiterminator loop, and RNA polymerase can transcribe the structural genes of the trp operon.
The stability of an mRNA molecule depends upon its nucleotide sequence and the length of its poly-A tail, or the long sequence of adenines added to one end of the mRNA after transcription. After a gene has been transcribed, control mechanisms can still regulate its expression during the translation process. Within eukaryotes, special repressor proteins can bind to mRNA molecules and physically block their translation.
In addition, after translation, unneeded proteins may be marked for degradation by certain molecules before they have the opportunity to do their job. This page appears in the following eBook. Aa Aa Aa. Promoters and proteins. How do prokaryotes regulate gene expression? Turning genes on. Turning genes off. Turning genes up or down. How do eukaryotes regulate gene expression?
Control at the DNA level. Figure 1: Eukaryotic cells must tightly fold their DNA so that it fits within the cellular nucleus. Control at the transcription level. Control via RNA splicing. More on gene expression. Can the central dogma be reversed? How can the environment affect gene expression? How does eukaryotic DNA unfold and open?
How, exactly, does RNA splicing occur? Control via RNA stability. Control at the translation level. Key Questions What else is there to know about operons? How do environmental influences affect gene expression? What role does noncoding RNA play in gene expression? How do genes express and regulate themselves? Key Concepts intron exon splicing transcription factor.
Topic rooms within Genetics Close. No topic rooms are there. Browse Visually. Other Topic Rooms Genetics. Dicistronic transcription units specify a messenger RNA mRNA encoding two separate genes that is transported to the cytoplasm and translated in that form. Presumably, internal ribosome entry sites IRES , or some form of translational re-initiation following the stop codon, are responsible for allowing translation of the downstream gene.
In the other type, the initial transcript is processed by 3' end cleavage and trans-splicing to create monocistronic mRNAs that are transported to the cytoplasm and translated.
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