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What is the significance of and biological mechanisms demonstrated in lac operon?

What is the significance of and biological mechanisms demonstrated in lac operon?


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I would appreciate it someone could explain clearly how the genes in the lac operon of E coli are activated to allow the bacteria to metabolize lactose?

The part that I really don't understand is the activation of the lac operon. A good answer would make clear to me how the introduction or removal of lactose from the envirnment activates and deactivates the different genes there.

Also, what are the specific functions of each of the genes (lacA, lacI, lacY, lacZ).

I am in AP Biology and I will not see the light of day again if I do poorly on this test. Please help!


The lac operon in E coli is a set of four genes which work together to allow the bacterium to make use of lactose for energy. An Operon is a set of genes which are co-transcribed on a single mRNA, controlled from a common promoter. While operons are nearly always found in bacteria, eukaryotes (and viruses that infect eukaryotes) do have some sets of genes organized into operons.

lacZ and lacY is the business end of the lac operon. They are the only 2 genes necessary for lactose usage in the cell.

lacZ codes for beta-galactosidase, an enzyme that cleaves the lactose disaccharide into D-galactose and D-glucose. Most of know that glucose is an important source or energy via the glycolysis pathway, which is highly connected energy usage in the cell. Galactose is readily converted by an enzyme (galactose mutarotase) into glucose, making lactose an efficient source of energy.

lacY codes for a protein found in the cell membrane called lactose permease which pumps lactose into the cell from the outside using the proton gradient inside the cell for its motive force.

The lacA gene codes for an enzyme which transfers an acetyl group to galactose, which may prevent the buildup of galactose in the cell.

The way the operon works is more important than what it does. the lac operon was one of the first examples of transcription regulation every studied. lacI is not in the lac operon, but it is the single gene which controls whether the operon is copied to RNA from the chromosome. lacI is translated to the lac repressor protein (LacI), which binds to one of three sites in front of the lac operon.

lacI is a repressor gene - when it is active, it prevents the lac operon from being transcribed into RNA. LacI is a tetramer - four copies of the protein form a complex. LacI binds to allolactose, a derivative of lactose, when it is present in sufficient quantities, which then causes it to release itself from the chromosomal DNA, allowing transcription to occur and the lac genes may be expressed.


12.1: The lac Operon

  • Contributed by Todd Nickle and Isabelle Barrette-Ng
  • Professors (Biology) at Mount Royal University & University of Calgary

Early insights into mechanisms of transcriptional regulation came from studies of E. coli by researchers Francois Jacob & Jacques Monod. In E. coli, and many other bacteria, genes encoding several different proteins may be located on a single transcription unit called an operon. The genes in an operon share the same transcriptional regulation, but are translated individually. Eukaryotes generally do not group genes together as operons (exception is C. elegans and a few other species).


An operon is a complete package for gene expression and synthesis of polypeptides. By combining the related genes, all polypeptides required for a specific function are synthesized in response to a single stimulus. For example, the bacterium Escherichia coli contains a number of genes clustered into operons and regulons: the Lac operon which is involved in lactose degradation, the Trp operon which is involved in tryptophan biosynthesis, and the His operon which is involved in histidine biosynthesis. These operons are turned on when the gene products are needed.

Operons can be under negative or positive control. Negative control involves turning off the operon in the presence of a repressor this can be either repressible or inducible. A repressible operon is one that is usually on but which can be repressed in the presence of a repressor molecule. The repressor binds to the operator in such a way that the movement or binding of RNA polymerase is blocked and transcription cannot proceed. An inducible operon is one that is usually off. In the absence of an inducer the operator is blocked by a repressor molecule. When the inducer is present it interacts with the repressor protein, releasing it from the operator and allowing transcription to proceed. Repressible operons are generally involved in anabolic pathways, or the synthesis of an essential component, while inducible operons are generally involved in catabolic pathways, or the breakdown of a nutrient. Positive control of an operon is when gene expression is stimulated by the presence of a regulatory protein.


RNA Transcription and Control of Gene Expression

Regulation of the Lac Operon

The lack of a nuclear membrane in prokaryotes gives ribosomes direct access to mRNA transcripts, allowing their immediate translation into polypeptides. This makes transcription the rate-limiting step in prokaryotic gene expression and, therefore, a major point of regulation. The classic example of prokaryotic gene regulation is that of the lac operon . This operon is a genetic unit that produces the enzymes necessary for the digestion of lactose ( Fig. 16-13 ).

The lac operon consists of three contiguous structural genes that are transcribed as continuous mRNA by RNA polymerase. An operator sequence located at the 5′ end serves as a binding site for a repressor protein that blocks RNA polymerase. The repressor protein is produced constitutively (continuously) by the i gene, which is not under regulatory control. The repressor itself is formed from subunits that self-assemble to form the active tetramer. When present, the inducer, allolactose, binds to the repressor subunits, preventing their assembly into an active tetramer. Allolactose is produced from lactose by β-galactosidase at a steady low rate and thus serves as a lactose signal. Another regulatory component is the catabolite activator protein (CAP). CAP forms an active complex with intracellular cyclic adenosine monophosphate (cAMP), which accumulates in the absence of glucose (cAMP is a starvation signal). RNA polymerase binds to the lac promoter effectively only when the CAP-cAMP complex is also bound. This ensures that the lac operon will be expressed only when glucose is absent.

The lac operon exhibits both negative control and positive control. Under negative control, a regulatory factor is needed to prevent expression of the lac operon, whereas under positive control, a regulatory factor is needed to permit expression of the lac operon.

Negative control (conditions: glucose only prevent expression of lac operon). If lactose is absent and glucose is present (see Fig. 16-13A ), the gene products from the lac operon are not needed. Thus a regulatory factor, the repressor protein, prevents lac operon expression. Since the repressor is produced constitutively and spontaneously assembles as its active tetrameric form, it is available to bind to the operon and prevent transcription.

Positive control (conditions: lactose only permit expression of lac operon). If no glucose is present and lactose is present (see Fig. 16-13B ), the gene products from the lac operon are needed to use the lactose for energy. Thus a regulatory factor, the CAP-cAMP complex is needed to permit expression of the operon. Because cAMP is a starvation signal indicating an absence of glucose, it is available to form the CAP-cAMP complex and permit transcription.

Positive control (conditions: lactose and glucose do not permit expression of the lac operon even if not prevented by repressor). If both lactose and glucose are present (see Fig. 16-13C ), the regulatory mechanisms act to avoid wasteful expression of the lac operon. Even though the repressor is inactivated by the presence of lactose, RNA polymerase cannot bind to the promoter, since the CAP-cAMP complex is absent owing to the presence of glucose.


Tight regulation

Normally, the lac operon is turned off. A repressor protein binds the operator (control) region upstream of the operon preventing transcription.

When lactose is present outside the cell, it crosses the cell membrane and acts as an inducer of the operon. It does so once lactose is broken down to create allolactose. The lac operon is then transcribed and translated into proteins including permease, which embeds itself into the cell membrane facilitating lactose transport into the cell, and &beta-galactosidase, which eats up lactose to make glucose molecules. &beta-galactosidase also makes allolactose. This leads to a positive feedback loop.

This operon takes the stage when glucose levels are low. This is because of another protein, called catabolite activator protein (CAP), and cyclic adenosine monophosphate (cAMP) molecules. When glucose levels drop, cAMP levels increase until there is sufficient cAMP to bind and activate more CAP. CAP promotes RNA polymerase transcription of genes leading to an increase of lac operon expression.

Together, you have a negative regulator, the repressor protein, which is bound and deactivated by allolactose and a positive regulator, which is promoted by low glucose levels ensuring that when glucose levels are low, but lactose is present, the cell will switch to this alternative source of dinner.


Conclusion

By using operons to teach students to reason with models of complex systems and understand broad themes, we equip them with powerful skills and ideas that form a solid foundation for their future learning in biology. These skills and ideas have broad application in biology, but they also potentially have application in other areas. Constructs, such as negative feedback and natural selection, that are used to explain changing and self-organizing systems constitute what Ohlsson (1993) refers to as “abstract schemas,” which encode the structure of discourse rather than its content. Their abstract nature allows for the possibility of cross-domain transfer. For example, the schema for negative feedback was developed originally in the context of specific engineering problems and was later found to have application in biology. Similarly, natural selection was developed to explain the origin of adaptations in organisms but has subsequently been applied to the development of the immune and nervous systems, computer programming, and artificial intelligence. Mastering abstract schemas enables students to develop into mature thinkers with powerful minds who can imagine solutions to the world’s future problems.


Other Factors Affecting Gene Expression in Prokaryotes and Eukaryotes

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.

DNA-Level Control

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.

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.

Clinical Focus: Travis, Resolution

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.

Laboratory testing revealed the causative agent of Travis’s infection was a strain of group A streptococcus (Group A strep). As required by law, Travis’s case was reported to the state health department and ultimately to the Centers for Disease Control and Prevention (CDC). 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. Unfortunately, these bacterial toxins limit the effectiveness of intravenous antibiotics in clearing infection from the skin and underlying tissue, meaning that antibiotic resistance alone does not explain the ineffectiveness of Travis’s treatment. 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.

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. This allows transcription of the structural genes.
  • The lac operon is also subject to activation. When glucose levels are depleted, some cellular ATP is converted into cAMP, which binds to the catabolite activator protein (CAP). The cAMP-CAP complex activates transcription of the lac operon. When glucose levels are high, its presence prevents transcription of the lac operon and other operons by catabolite repression.
  • Small intracellular molecules called alarmones are made in response to various environmental stresses, allowing bacteria to control the transcription of a group of operons, called a regulon.
  • Bacteria have the ability to change which σ factor of RNA polymerase they use in response to environmental conditions to quickly and globally change which regulons are transcribed.
  • Prokaryotes have regulatory mechanisms, including attenuation and the use of riboswitches, to simultaneously control the completion of transcription and translation from that transcript. These mechanisms work through the formation of stem loops in the 5’ end of an mRNA molecule currently being synthesized.
  • There are additional points of regulation of gene expression in prokaryotes and eukaryotes. In eukaryotes, epigenetic regulation by chemical modification of DNA or histones, and regulation of RNA processing are two methods.

Multiple Choice

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?

Which of the following conditions leads to maximal expression of the lac operon?

  1. lactose present, glucose absent
  2. lactose present, glucose present
  3. lactose absent, glucose absent
  4. lactose absent, glucose present

Which of the following is a type of regulation of gene expression unique to eukaryotes?

  1. attenuation
  2. use of alternate σ factor
  3. chemical modification of histones
  4. alarmones

Fill in the Blank

The DNA sequence, to which repressors may bind, that lies between the promoter and the first structural gene is called the ________.

The prevention of expression of operons encoding substrate use pathways for substrates other than glucose when glucose is present is called _______.


C. Negative control

The lac operon is under both negative and positive control. The mechanisms for these will be considered separately.

1. In negative control, the lacZYAgenes are switched off by repressor when the inducer is absent (signalling an absence of lactose). When the repressor tetramer is bound to o, lacZYAis not transcribed and hence not expressed.

Figure 4.1.2. Repressed lacoperon

2. When inducer is present (signalling the presence of lactose), it binds the repressor protein, thereby altering its conformation, decreasing its affinity for o, the operator. The dissociation of the repressor-inducer complex allows lacZYAto be transcribed and therefore expressed.

Figure 4.1.3. Induction of the lac operon by derepression.


The Lac Operon

Jacob and Monod proposed the lac operon model to explain the regulation of the synthesis of an enzyme, β-galactosidase, the lac operon in e.coli, only when lactose is available as a substrate and Glucose is not available. This was the first study of metabolic regulation based on genetic analysis of the lac system. They called this the Operon Model ( Lac Operon Concept ).

  • Structural Genes: Genes that code for proteins required by the cell (for functioning as an enzyme or to be structural proteins) are called Structural genes
  • Regulatory Genes: an overwhelming majority of the genes are in this category. Genes that code for proteins which, in turn, regulate the expression of other genes, are called Regulatory genes.

The crux of regulation is that a regulator gene codes for a regulator protein that controls transcription by binding to a particular site(s) on DNA. Regulation can be positive, when the interaction turns the gene on, or negative when the gene is turned off by the interaction.

The classic model of control of gene expression is negative the most common mode of control in eukaryotes is positive. But, no type of regulation is exclusive to any one genome there are examples of positive regulation in bacteria and negative regulation in eukaryotes.


Experimental evolution and proximate mechanisms in biology

Biological functions – studied by molecular, systems and behavioral biology – are referred to as proximate mechanisms. Why and how they have emerged from the course of evolution are referred to as ultimate mechanisms. Despite the conceptual and technical schism between the disciplines that focus on each, studies from one side can benefit the other. Experimental evolution is an emerging field at the crossroads of functional and evolutionary biology. Herein microorganisms and mammalian cell lines evolve in well-controlled laboratory environments over multiple generations. Phenotypic changes arising from the process are then characterized in genetics and function to understand the evolutionary process. While providing empirical tests to evolutionary questions, such studies also offer opportunities of new insights into proximate mechanisms. Experimental evolution optimizes biological systems by means of adaptation the adapted systems with their mutations present unique perturbed states of the systems that generate new and often unexpected output/performance. Hence, learning about these states not only adds to but also might deepen knowledge on the proximate processes. To demonstrate this point, five examples in experimental evolution are introduced, and their relevance to functional biology explicated. In some examples, from evolution experiments, updates were made to known proximate processes – gene regulation and cell polarization. In some examples, new contexts were found for known proximate processes – cell division and drug resistance of cancer. In one example, a new cellular mechanism was discovered. These cases identify ways the approach of experimental evolution can be used to ask questions in functional biology.


Watch the video: Lac OPERON Concept - Class 12 - Simplified (December 2022).