When is a repressible operon transcribed

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.

A riboswitch may bind to a small intracellular molecule to stabilize certain secondary structures of the mRNA molecule. The binding of the small molecule determines which stem-loop structure forms, thus influencing the completion of mRNA synthesis and protein synthesis. Figure 8. Click for a larger image.

Riboswitches found within prokaryotic mRNA molecules can bind to small intracellular molecules, stabilizing certain RNA structures, influencing either the completion of the synthesis of the mRNA molecule itself left or the protein made using that mRNA right.

Although the focus on our discussion of transcriptional control used prokaryotic operons as examples, eukaryotic transcriptional control is similar in many ways. As in prokaryotes, eukaryotic transcription can be controlled through the binding of transcription factors including repressors and activators. Interestingly, eukaryotic transcription can be influenced by the binding of proteins to regions of DNA, called enhancers , rather far away from the gene, through DNA looping facilitated between the enhancer and the promoter Figure 9.

Overall, regulating transcription is a highly effective way to control gene expression in both prokaryotes and eukaryotes. However, the control of gene expression in eukaryotes in response to environmental and cellular stresses can be accomplished in additional ways without the binding of transcription factors to regulatory regions. Figure 9. In eukaryotes, an enhancer is a DNA sequence that promotes transcription. As with all operons, the trp operon consists of the repressor, promoter, operator and the structural genes.

In this system, though, unlike the lac operon, the gene for the repressor is not adjacent to the promoter, but rather is located in another part of the E. Promoter; operator sequence is found in the promoter.

Monod provided much of our foundational knowledge of the mechanisms of lactose metabolism in bacteria. In their research, Jacob and Monod noted that the lacI repressor, formed by a tetramer of the protein encoded by the lacI gene, binds to specific nucleotides in the operator lacO. When that O sequence is mutated, the repressor can no longer bind, leaving the entire operon induced or "unrepressed.

Thus, there is no induction time, as described in Figure 1. When investigators tried to rescue this phenotype by adding a wild-type copy of the operon to the bacteria, they were unable to change the behavior of the endogenous mutated operon.

Here, the researchers placed the wild-type O c operon on a plasmid that was separate from the bacterial chromosome , and both were present in the same cells. Even when a wild-type copy was present in the cells and there was no lactose present, the cells expressed the lac operon, so the mutant O c was dominant.

This suggested that the operator region controls only the genes adjacent to it, on the same piece of DNA. In other words, the operator functions in a cis-dominant fashion. The case of the lacI repressor mutant, denoted lacI - , was quite different. Constitutive expression of the operon is also seen in lacI - cells.

But, contrary to O c mutants, the lacI - phenotype can be overcome by the addition of a wild-type lacI gene on a plasmid. This is because the wild-type lacI repressor protein is made correctly from the gene encoded by the plasmid.

The wild-type lacI protein can then bind to any lac operon operator sequence , including the endogenous version; thus, the repressor can act in trans. Because the wild-type lacI can rescue lacI - , the mutant version is recessive. In the case of a third mutant, lacI s , the result is a repressor that is constitutively bound to the operator. Normally, the repressor protein has two conformations, or shapes.

In one conformation, it is bound to the operator. When lactose is present, however, the lactose binds to the repressor, causing a change in conformation, and releasing the repressor from the operator. In lacI s mutants, the binding site for lactose is lost in the repressor protein. As a result, no matter how much lactose is in the system, the operon stays in the "off" state.

Moreover, if wild-type lacI is added on a plasmid, it cannot rescue this mutant. Thus, the mutation is dominant. Interestingly, the relatively simple mechanisms of gene expression in prokaryotic cells, as exemplified by the trp and lac operons, provide insight into several general principles involved in regulation in eukaryotes.

For example, specific sequences in DNA serve as binding sites for specific proteins that modulate the binding of RNA polymerase, the enzyme required for mRNA transcription. These operator sequences in DNA act in cis ; in other words, they control the expression of genes on the same contiguous piece of DNA, generally in fairly close proximity.

In contrast, the proteins that bind those sites act in trans; this means they can be produced by a gene elsewhere in the genome and act wherever the consensus sequence is located. Furthermore, the ability of E. Jacob, F. The operon: A group of genes with expression coordinated by an operator. Comptes Rendus Biologies , — Genetic regulatory mechanisms in the synthesis of proteins.

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Chromatin Remodeling and DNase 1 Sensitivity. Chromatin Remodeling in Eukaryotes. RNA Functions. Citation: Shaw, K. Nature Education 1 1 How do bacteria avoid wasteful production of unnecessary proteins when their genes are always on?

The answer lies in regulating the operon. Aa Aa Aa.