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. Key Concepts and Summary Gene expression is a tightly regulated process. 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 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. The metabolic benefit of operon organization increases when the number of different proteins in the complex is larger Fig.
This is because having more proteins in the complex means that there are more chances for the level of one protein to fall below those of the others and thus to become limiting. Thus, for E. Effect of the number of genes carried by the operon. Stochastic simulations were performed as shown for Fig. Conversely, operon organization has the potential disadvantage of increasing fluctuations in the level of the complex Fig.
Thus, it may be significant in larger systems which require complex regulation. This effect and the reduced regulatory flexibility of cotranscription may favor independent transcription units in eukaryotes.
In bacteria and archaea, the metabolic savings in the production of protein complexes seem to dominate, promoting operon formation. In our simulations, the individual RNA production, degradation, and translation events occur randomly, with rates which secure a preset average protein number in a cell. Throughout the paper, we assume that each ribosome binding site initiates an average of 20 proteins before the mRNA is inactivated by degradation factors.
We assume that mRNA has a much shorter lifetime than the encoded proteins and, accordingly, simulate protein production as an instant event happening immediately after the production of each mRNA. We implement this by assigning each newly synthesized mRNA a protein production capacity c drawn from an exponential distribution with a mean number of Subsequently, we increase the concentration of each protein encoded by the mRNA by an amount drawn from a Poisson distribution with mean c.
In this way, protein production by subsequent genes carried by a given polycistronic mRNA will vary to an extent, given by the variations in the number of random translation initiations.
Finally, protein dilution upon cell division was taken into account by randomly distributing each protein between the daughter cells.
In our simulations, we assign identical protein production capacity to each ribosome binding site on a polycistronic mRNA. Assuming equal protein production capacity from different parts of the same mRNA seems appropriate to estimate the gain of cotranscription, because a different average protein production capacity would obviously lead to systematic wastage if the proteins are required in equimolar amounts in a functional complex.
Such a systematic difference in production capacity was present in a previous study 7 and resulted in the underestimation of the noise reduction due to cotranscription alone. Any systematic differences in protein production caused by a time delay in transcription or the directionality of mRNA decay 7 can easily be compensated by altering the translation initiation sites of the genes without affecting the advantage of the operon arrangement.
Premature termination of an RNA polymerase within an operon can produce systematic decreases in protein production from distal genes 11 and can be compensated for in the same way to maintain equal numbers of the components of the complex. However, this kind of polarity reduces the transcriptional coupling between the genes and reduces the noise benefits of operon organization.
In our paper, we focus on the amount of protein complex formed relative to the amount that would be produced if protein production and degradation were noise free. In our simulations, we assume that noise is independent of the mRNA lifetime. This is true when the mRNA lifetime is much shorter than the protein lifetime. Even in a more general case, where we do not make such an assumption, we can calculate noise in the protein number as follows:. The estimate of overall synthetic gain of at least 0.
This gain is a minimum estimate, as it ignores the fraction of the protein mass comprising numerous different complex-forming proteins with lower expression levels whose encoding genes are cotranscribed. These could make a large contribution to the gain Table 2. We thank Stanley Brown of CMOL for suggesting the examination of the evolutionary conservation of cotranscription of complex-encoding genes.
Citation Sneppen, K. Pedersen, S. Krishna, I. Dodd, and S. Economy of operon formation: cotranscription minimizes shortfall in protein complexes. National Center for Biotechnology Information , U. Journal List mBio v. Published online Sep Author information Copyright and License information Disclaimer. Address correspondence to Kim Sneppen; kd. This is an open-access article distributed under the terms of the Creative Commons Attribution-Noncommercial-Share Alike 3.
This article has been cited by other articles in PMC. Abstract Genes of prokaryotes and Archaea are often organized in cotranscribed groups, or operons. Introduction What are the evolutionary forces that drive operon formation in prokaryotes but not in eukaryotes? Open in a separate window. Article History Close. Share Cancel.
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