What is lac operon and how does it work?
What is lac operon and how does it work? 1 Answer. The Lac Operon is an operon in E. coli that is required for the transportation and metabolism (“digestion”) of lactose. It (and various mutations) is often used in genetics courses to demonstrate gene regulation and expression. Explaining how it works is more easily done with pictures.
What is the lac operon and why is it important?
Why is the lac operon important? 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. 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.
What is the function of the lac operator?
What is the function of the lac operator? The lac repressor protein attaches to the operator and prevents RNA polymerase from binding to the promoter and transcriptionally transcribed the operon. The promoter serves as a binding site for RNA polymerase, the enzyme responsible for transcription. The lac repressor protein binds to the operator, which is a negative regulatory site.
How is the lac operon turned on and off?
Putting it all together
- Glucose present, lactose absent: No transcription of the lac operon occurs. ...
- Glucose present, lactose present: Low-level transcription of the lac operon occurs. ...
- Glucose absent, lactose absent: No transcription of the lac operon occurs. ...
- Glucose absent, lactose present: Strong transcription of the lac operon occurs. ...
How does the lac operon control the production of enzymes?
The lac operon uses a two-part control mechanism to ensure that the cell expends energy producing the enzymes encoded by the lac operon only when necessary. In the absence of lactose, the lac repressor, lacI, halts production of the enzymes encoded by the lac operon. The lac repressor is always expressed, unless a co-inducer binds to it. In other words, it is transcribed only in the presence of small molecule co-inducer. In the presence of glucose, the catabolite activator protein (CAP), required for production of the enzymes, remains inactive, and EIIA Glc shuts down lactose permease to prevent transport of lactose into the cell. This dual control mechanism causes the sequential utilization of glucose and lactose in two distinct growth phases, known as diauxie .
What are the three genes of the lac operon?
In this case, when lactose is required as a sugar source for the bacterium, the three genes of the lac operon can be expressed and their subsequent proteins translated: lacZ, lacY, and lacA. The gene product of lacZ is β-galactosidase which cleaves lactose , a disaccharide, into glucose and galactos e. lacY encodes Beta-galactoside permease, a membrane protein which becomes embedded in the cytoplasmic membrane to enable the cellular transport of lactose into the cell. Finally, lacA encodes Galactoside acetyltransferase .
Why is lactose leaky?
However the lactose metabolism enzymes are made in small quantities in the presence of both glucose and lactose (sometimes called leaky expression) due to the fact that the LacI repressor rapidly associates/dissociates from the DNA rather than tightly binding to it, which can allow time for RNAP to bind and transcribe mRNAs of lacZYA. Leaky expression is necessary in order to allow for metabolism of some lactose after the glucose source is expended, but before lac expression is fully activated.
What is the lactose operon?
The lactose operon ( lac operon) is an operon required for the transport and metabolism of lactose in E.coli and many other enteric bacteria. Although glucose is the preferred carbon source for most bacteria, the lac operon allows for the effective digestion ...
Why are lactose genes not produced by bacteria?
The proteins are not produced by the bacterium when lactose is unavailable as a carbon source. The lac genes are organized into an operon; that is, they are oriented in the same direction immediately adjacent on the chromosome and are co-transcribed into a single polycistronic mRNA molecule.
What is the difference between wild type and lactose?
In the case of Lac, wild type cells are Lac + and are able to use lactose as a carbon and energy source, while Lac − mutant derivatives cannot use lactose. The same three letters are typically used (lower-case, italicized) to label the genes involved in a particular phenotype, where each different gene is additionally distinguished by an extra letter. The lac genes encoding enzymes are lacZ, lacY, and lacA. The fourth lac gene is lacI, encoding the lactose repressor—"I" stands for inducibility .
When lactose is absent, there is very little lac enzyme production?
When lactose is absent then there is very little Lac enzyme production (the operator has Lac repressor bound to it). When lactose is present but a preferred carbon source (like glucose) is also present then a small amount of enzyme is produced (Lac repressor is not bound to the operator).
How does the lac operon work?
The lac operon in the bacterium Escherichia coli functions by a repression mechanism in which an inhibitor protein (lacI) binds to regulatory sites (lacO) in the promoter and turns off transcription ( Fig. 59-2 ). On the addition of lactose, the lacI protein undergoes a conformational change, which changes its binding affinity for the lacO sequences. The lacI protein thereby comes off the lacO sites, and transcription can occur. E. coli uses this system to tightly control the genes required for the use of lactose, and it is completely reversible.
What is the role of the lac operon in prokaryotic gene expression?
This makes transcription the rate-limiting step in prokaryotic gene expression and, therefore, a major point of regulation.
What is the lactose operon?
The lactose operon (also known as the lac operon) is a set of genes that are specific for uptake and metabolism of lactose and is found in E . coli and other bacteria . The lac operon consists of three structural genes: lacZ, which codes for β-galactosidase, which acts to cleave lactose into galactose and glucose; lacY, which codes for lac permease, which is a transmembrane protein necessary for lactose uptake; and lacA, which codes for a transacetylase that transfers an acetyl group from coenzyme A (CoA) to the hydroxyl group of galactosides. In the 5′ end with respect to lacZ is the lacI gene, which encodes a repressor of the lac operon, which is transcribed independently from the structural genes ( Figure 1 ).
What is the negative control of lactose?
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.
How does lactose metabolize into glucose?
Next, residual LacZ metabolizes lactose to glucose and galactose, which produces energy for the bacterium. Notably, this catalytic process also generates low levels of allolactose (a rearrangement of the β-1,4 linkage between glucose and galactose to a β-1,6 linkage; Figure 2). The by-product allolactose binds to LacI and elicits a conformational change in the protein that results in release of the operator DNA sequence (induction). Consequently, RNA polymerase is freed to generate numerous copies of mRNA encoding the lac enzymes. When translated into proteins, these enzymes allow the bacterium to transport and metabolize large quantities of lactose as its carbon energy source, taking advantage of environmental opportunity. One result of the studies by Jacob and Monod was the discovery that a variety of non-natural galactoside sugars (e.g., IPTG; Figure 2) can induce LacI and relieve transcription repression of lacZYA.
What is the role of LacI?
The role of LacI is to inhibit mRNA production for proteins encoded by the lac operon. Transcription is not completely eliminated, but lacZYA mRNA is transcribed only at very low levels. This function is accomplished by specific binding of LacI protein to the lac operator DNA sequence to inhibit transcription via a variety of mechanisms. Since the lac operator (LacO) overlaps the promoter, LacI binding directly competes with RNA polymerase for binding this region. LacI can also impede transcription initiation and/or block elongation of mRNA. The LacI·LacO association and consequent transcription repression occur when no lactose is available to serve as the substrate of the lac metabolic proteins.
How many simulations are there in the Yildirim-Mackey lac operon model?
Figure 12.13. Sixteen stochastic simulations of the Yildirim–Mackey lac operon model is shown, with the same parameters used for Fig. 12.12. Eight simulations use the lower steady state value as the initial condition, while the others use the upper steady state as the initial condition. We observe that all the simulations starting from the lower branch converge to the upper steady state and the simulations initialized from the upper branched stay on that steady state (only one of the simulations is plotted here).
How is Lac-Operon controlled?
Positive Control of Lac-Operon. Firstly, a regulatory gene expresses the repressor protein. After that, repressor proteins are produced by the expression of a regulatory gene. A repressor protein has binding sites for the operator and the inducer (lactose).
Who discovered the lac-operon system?
The concept of Lac-operon was first explained by Jacob and Monad in E.coli. Lac-operon refers to the system that can regulate the metabolism or utilization of lactose (as sole carbon source). In E.coli, a lac-operon contains a gene sequence that controls the gene expression by switching on and switching off the system.
What are the elements that make up the operon?
These three, i.e. Lac Z, Y and A genes, are present adjacent to each other. Therefore, all the elements like promotor, operator, repressor and structural genes together form a unit called Operon.
What is the function of an operon?
Operon. It is a system of DNA containing a sequence of genetic code or carries a sequence of genes that code the m-RNA and directs the synthesis of enzymes for the protein synthesis. An operon is a coordinated system, in which all the genes coordinate to mediate the regulation of gene expression.
What is the promoter region of RNA polymerase?
It lies between the regulator and the operator. RNA-polymerase binds to this site, as a promoter region initiates transcription. It is 100 base pairs long. It consists of palindromic sequences. This site promotes and controls the transcription of structural genes or m-RNA. The regulatory genes of the repressor regulate the functioning of the promoter region.
When lactose is present as an inducer, it binds with the repressor protein and?
When lactose is present as an inducer, it binds with the repressor protein and forms R+I complex.
What is the regulatory gene of the repressor?
The regulatory genes of the repressor regulate the functioning of the promoter region. Operator Region: It codes the Lac-O gene. It lies between a promoter and the structural gene (Lac-Z). It contains an operator switch, which decides whether transcription should take place or not.
Which protein activates the transcription of the operon, only when glucose levels are low?
catabolite activator protein activa tes the transcription of the operon, only when glucose levels are low.
Why is the operon turned on and off?
The operon is turned on and off in response to the glucose and lactose levels: catabolite activator protein and lac repressor. The lac repressor blocks the transcription of the operon. In the presence of lactose, it stops acting as a repressor.
How do prokaryotes regulate gene expression?
Gene regulation in prokaryotes is most extensively observed at the initiation of transcription. Thus, the gene expression during transcription initiation is affected by regulation. The regulation usually takes place in the expression of the RNA polymerase at the promoter site. This affects the accessory proteins which bind to the recognition sites. These accessory proteins can regulate the promoter site in two ways: 1 Positive regulation by activators 2 Negative regulation by repressors
How to explain gene regulation in prokaryotes?
Gene regulation in prokaryotes can be explained with the help of the Lac Operon model. Here the alteration in physiological and environmental conditions can be observed leading to an alteration in expression in prokaryotes. It was observed by Jacob and Monod. The lac operon consists of:
How are genes regulated in eukaryotes?
Gene Regulation in Eukaryotes. Gene regulation in eukaryotes is regulated by transcriptional activators and repressors. The repressors bind to specific DNA sequences and inhibit transcription. In eukaryotes, transcription involves several steps. It occurs in both, nucleus (transcription) and cytoplasm (translation).
What is the Z gene?
z gene – It codes for beta-galactosidase which catalyzes the hydrolysis of lactose into glucose and galactose.
What is gene expression?
Gene expression is basically the synthesis of the polypeptide chain encoded by a particular gene. Therefore, we can say that the expression of the gene can be quantified in terms of the amount of protein synthesised by the genes. Let’s learn about the regulation of gene expression in prokaryotes with the example of Lac Operon.
Overview
The lactose operon (lac operon) is an operon required for the transport and metabolism of lactose in E. coli and many other enteric bacteria. Although glucose is the preferred carbon source for most bacteria, the lac operon allows for the effective digestion of lactose when glucose is not available through the activity of beta-galactosidase. Gene regulation of the lac operon was the first genetic re…
Structure
• The lac operon consists of 3 structural genes, and a promoter, a terminator, regulator, and an operator. The three structural genes are: lacZ, lacY, and lacA.
Only lacZ and lacY appear to be necessary for lactose catabolic pathway.
Three-letter abbreviations are used to describe phenotypes in bacteria including E. coli.
Regulation
Specific control of the lac genes depends on the availability of the substrate lactose to the bacterium. The proteins are not produced by the bacterium when lactose is unavailable as a carbon source. The lac genes are organized into an operon; that is, they are oriented in the same direction immediately adjacent on the chromosome and are co-transcribed into a single polycistronic mRNA mole…
Development of the classic model
The experimental microorganism used by François Jacob and Jacques Monod was the common laboratory bacterium, E. coli, but many of the basic regulatory concepts that were discovered by Jacob and Monod are fundamental to cellular regulation in all organisms. The key idea is that proteins are not synthesized when they are not needed—E. coli conserves cellular resources and energy by …
Use in molecular biology
The lac gene and its derivatives are amenable to use as a reporter gene in a number of bacterial-based selection techniques such as two hybrid analysis, in which the successful binding of a transcriptional activator to a specific promoter sequence must be determined. In LB plates containing X-gal, the colour change from white colonies to a shade of blue corresponds to about 20–100 β-galactosidase units, while tetrazolium lactose and MacConkey lactose media have a ra…
See also
• Catabolite repression
External links
• Lac+Operon at the US National Library of Medicine Medical Subject Headings (MeSH)
• lac operon in NCBI Bookshelf [2]
• Virtual Cell Animation Collection Introducing: The Lac Operon
• The lac Operon: Bozeman Science