Protein 5. Enzymes 6. Cell Respiration 9. Photosynthesis 3: Genetics 1. Genes 2. Chromosomes 3. Meiosis 4. Inheritance 5. Genetic Modification 4: Ecology 1. Energy Flow 3. Carbon Cycling 4. Climate Change 5: Evolution 1. Evolution Evidence 2. Natural Selection 3. Classification 4. Cladistics 6: Human Physiology 1. Digestion 2. 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. Each enhancer is made up of short DNA sequences called distal control elements. Activators bound to the distal control elements interact with mediator proteins and transcription factors.
Two different genes may have the same promoter but different distal control elements, enabling differential gene expression. In eukaryotes, the DNA molecules or associated histones can be chemically modified in such a way as to influence transcription; this is called epigenetic regulation. Methylation of certain cytosine nucleotides in DNA in response to environmental factors has been shown to influence use of such DNA for transcription, with DNA methylation commonly correlating to lowered levels of gene expression.
Additionally, in response to environmental factors, histone proteins for packaging DNA can also be chemically modified in multiple ways, including acetylation and deacetylation, influencing the packaging state of DNA and thus affecting the availability of loosely wound DNA for transcription.
These chemical modifications can sometimes be maintained through multiple rounds of cell division, making at least some of these epigenetic changes heritable. Although Travis survived his bout with necrotizing fasciitis, he would now have to undergo a skin-grafting surgery, followed by long-term physical therapy.
Based on the amount of muscle mass he lost, it is unlikely that his leg will return to full strength, but his physical therapist is optimistic that he will regain some use of his leg. At the CDC, the strain of group A strep isolated from Travis was analyzed more thoroughly for methicillin resistance. Methicillin resistance is genetically encoded and is becoming more common in group A strep through horizontal gene transfer.
In necrotizing fasciitis, blood flow to the infected area is typically limited because of the action of various genetically encoded bacterial toxins. This is why there is typically little to no bleeding as a result of the incision test. Nevertheless, intravenous antibiotic therapy was warranted to help minimize the possible outcome of sepsis, which is a common outcome of necrotizing fasciitis.
Through genomic analysis by the CDC of the strain isolated from Travis, several of the important virulence genes were shown to be encoded on prophages, indicating that transduction is important in the horizontal gene transfer of these genes from one bacterial cell to another. An operon of genes encoding enzymes in a biosynthetic pathway is likely to be which of the following?
An operon encoding genes that are transcribed and translated continuously to provide the cell with constant intermediate levels of the protein products is said to be which of the following? Skip to main content. Mechanisms of Microbial Genetics. Search for:. Gene Regulation: Operon Theory Learning Objectives Compare inducible operons and repressible operons Describe why regulation of operons is important.
What types of regulatory molecules are there? Watch this video to learn more about the trp operon. Watch an animated tutorial about the workings of lac operon here. Think about It What affects the binding of the trp operon repressor to the operator? How and when is the behavior of the lac repressor protein altered?
In addition to being repressible, how else is the lac operon regulated? Think about It What is the name given to a collection of operons that can be regulated as a group? This video describes how epigenetic regulation controls gene expression.
Think about It What stops or allows transcription to proceed when attenuation is operating? What determines the state of a riboswitch? Describe the function of an enhancer. Describe two mechanisms of epigenetic regulation in eukaryotes.
The ability to turn lactose-metabolizing genes on or off as a group therefore provides an efficient way to quickly adapt to environmental changes. The terminator, on the other hand, instructs the transcriptional machinery to terminate transcription. As such, the promoter serves as a transcriptional start site, the terminator serves as a stop site, and the operator helps determine whether transcription will occur. Because of the common control mechanism for all of the genes in the lac operon, mutations in this operon can have multiple, or pleiotropic, effects.
For example, mutations affecting the promoter can prevent all of the operon's genes from being expressed, because RNA polymerase will be unable to bind and commence transcription. Other mutations may affect expression of only some of the genes in the operon.
For instance, nonsense mutations in the lac z gene encoding beta-galactosidase might interfere with expression of downstream permease and transacetylase genes by causing the ribosome to fall off prior to their translation. This kind of mutation is said to have a "polar" effect on the pathway in that it affects downstream genes but not upstream genes.
Thus, although bacteria may be considered simpler organisms than humans, it is clear that bacterial gene regulation is extremely efficient and that the bacterial genome is highly organized.
Bacteria appear to be perfectly adapted to a variety of environments, and they are ready to respond to whatever environmental changes they encounter by employing elegant and complex regulatory mechanisms.
Jacob, F. On the regulation of gene activity. Lawrence, J. Shared strategies in gene organization among prokaryotes and eukaryotes. Cell , — Atavism: Embryology, Development and Evolution. Gene Interaction and Disease. Genetic Control of Aging and Life Span. Genetic Imprinting and X Inactivation. Genetic Regulation of Cancer. Obesity, Epigenetics, and Gene Regulation. Environmental Influences on Gene Expression. Gene Expression Regulates Cell Differentiation.
Genes, Smoking, and Lung Cancer. Negative Transcription Regulation in Prokaryotes. Operons and Prokaryotic Gene Regulation. Regulation of Transcription and Gene Expression in Eukaryotes. The Role of Methylation in Gene Expression. DNA Transcription. Reading the Genetic Code. Simultaneous Gene Transcription and Translation in Bacteria. Chromatin Remodeling and DNase 1 Sensitivity.
Chromatin Remodeling in Eukaryotes. RNA Functions. Citation: Ralston, A. Nature Education 1 1 How do bacteria adapt so quickly to their environments?
Part of the answer to this question lies in clusters of coregulated genes called operons. Aa Aa Aa. The lac Operon. References and Recommended Reading Jacob, F. Article History Close. Share Cancel.
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