The lac operon is a model system for understanding how effector molecules regulate transcription and are necessary for allosteric transitions. The crystal structures of the lac repressor bound to inducer and anti-inducer molecules provide a model for how these small molecules can modulate repressor function.
The structures of the apo repressor and the repressor bound to effector molecules are compared in atomic detail. All effectors examined here bind to the repressor in the same location and are anchored to the repressor through hydrogen bonds to several hydroxyls of the sugar ring. Inducer molecules form a more extensive hydrogen bonding network compared to anti-inducers and neutral effector molecules.
The structures of these effector molecules suggest that the O6 hydroxyl on the galactoside is essential for establishing a water mediated hydrogen bonding network that bridges the N-terminal and C-terminal subdomains.
The altered hydrogen bonding can account in part for the different structural conformations of the repressor, and is vital for the allosteric transition. Proteins that perform specific metabolic tasks are often regulated to meet the needs of the organism.
In many instances, regulating the flux through a pathway is achieved by adjusting the concentration of an enzyme that controls the rate determining step. Transcriptional regulation is one of the most effective ways to modulate enzyme concentrations. In both prokaryotic and eukaryotic organisms, transcription is frequently controlled by repressors and activators. These molecules either directly or indirectly monitor the accumulation or diminution of a metabolite; they respond like a molecular switch, turning transcription on or off.
Effector molecules are the chemical signals that convey the metabolic state of the cell to the genetic machinery.
The lac repressor, which is constitutively expressed, binds to an upstream cis-activated operator and consequently blocks transcription of the genes necessary for the cell to utilize lactose as an energy source. In this particular case, the negative regulation is relieved in the presence of a particular effector, allolactose, which binds to the repressor and activates the expression of the genes necessary for lactose metabolism. Understanding how effector molecules alter the properties of the repressor at the molecular level is essential for establishing a detailed understanding of gene regulation.
Here we describe the structures of the apo repressor as well as the repressor bound to three effector molecules, and propose a molecular mechanism for how the effector molecules alter the conformation of the repressor, which in turn attenuates the rate of transcription of the lac operon.
The repressor and the operator are the two key macromolecular components of the molecular switch that regulates lactose metabolism.
The lac repressor is a protein with a modular structure composed of four distinct structural units [ 23 ]. Connecting the headpiece to the body of the repressor is a hinge-helix residues 50— In the absence of DNA, the hinge region is unfolded, allowing residues 1—61 to move freely with respect to the rest of the protein [ 4 ]. However, when the operator is present, the hinge-helix becomes ordered and binds to the central portion of the operator in the minor groove [ 3 ].
The core of the repressor residues 62— is structurally divided into two subdomains. At the interface of these two domains is the effector binding site that is biologically responsible for monitoring the concentration of a metabolite. The repressor binds to an operator that is located between the lacI gene and the beginning of the lacZ gene and prevents transcription of the structural genes of the operon [ 1 ].
The operator sequence is pseudo-symmetric, possessing an approximate dyad axis through the central base pair [ 5 ]. Key amino acids located on the HTH motif recognize bases in the major groove of the operator, and, amino acids on the hinge helix interact with the bases in the minor groove of the operator, conferring further specificity of operator binding [ 3 ].
This binding to the minor groove of the operator also distorts the conformation of the operator [ 3 ]. Effector molecules alter the affinity of the repressor for the operator.
When the inducer is present, the repressor binds less efficiently to the operator, which allows RNA polymerase to recognize its promoter and transcribe the genes necessary for lactose utilization.
In an effort to unravel the switching mechanism, Monod et. Some of these compounds were able to mimic the natural inducer allolactose, while others bound but failed to induce, indicating that binding to the repressor was not sufficient to cause the allosteric response [ 8 — 11 ].
Muller-Hill extended the list of effector molecules and classified them as inducers or anti-inducers based upon how they modulated bacterial growth [ 8 ], and Riggs performed extensive in vitro binding assays in an attempt to understand how a group of chemically similar compounds can produce such distinct phenotypes [ 17 ].
These observations were subsequently quantified by Barkley et. All other inducers analyzed also destabilize the repressor-operator complex by fold, suggesting a single mechanism of induction [ 8 ]. It was also shown that all inducers bind with higher affinity to the free repressor than to the repressor-operator complex [ 8 ].The protein played a central role in Jacob and Monod's development of the operon model for the regulation of gene expression.
Determination of the crystal structure made it possible to understand why deletion of certain residues toward the amino-terminus not only caused the full enzyme tetramer to dissociate into dimers but also abolished activity. The enzyme is well known to signal its presence by hydrolyzing X-gal to produce a blue product. That this reaction takes place in crystals of the protein confirms that the X-ray structure represents an active conformation. Extensive kinetic, biochemical, mutagenic, and crystallographic analyses have made it possible to develop a presumed mechanism of action.
Substrate initially binds near the top of the active site but then moves deeper for reaction. The first catalytic step called galactosylation is a nucleophilic displacement by Glu to form a covalent bond with galactose.
This is initiated by proton donation by Glu The second displacement degalactosylation by water or an acceptor is initiated by proton abstraction by Glu Both of these displacements occur via planar oxocarbenium ion-like transition states. The acceptor reaction with glucose is important for the formation of allolactose, the natural inducer of the lac operon.
It played a central role in Jacob and Monod's 1 development of the operon model for the regulation of gene expression. Also, its ability to signal its presence by producing an easily recognizable blue reaction product has made it a workhorse in cloning and other such molecular biology procedures. Second, the enzyme can catalyze the transgalactosylation of lactose to allolactose, and, third, the allolactose can be cleaved to the monosaccharides. The enzyme can hydrolyze lactose to galactose plus glucose, it can transgalactosylate to form allolactose, and it can hydrolyze allolactose.
The presence of lactose results in the synthesis of allolactose which binds to the lac repressor and reduces its affinity for the lac operon. Thus, it hydrolyzes X-gal, releasing the substituted indole that spontaneously dimerizes to give an insoluble, intensely blue product.
On growth medium containing X-gal, colonies of E.
A Deep Dive Into Induction with IPTG
As shown in Figure 2the X-gal reaction can readily be performed in single crystals of the enzyme. The solvent-filled channels that extend throughout the crystal are much larger than the substrate and allow substrate to freely diffuse throughout the crystal.
In early experiments on the nature of protein crystals, Wyckoff et al. When the concentration of ammonium sulfate surrounding the crystal was rapidly changed, the half-time for re-equilibration within the crystal was 90 s.Lac operon
Based on these experiments, it can be estimated that a molecule the size of X-gal will diffuse through a 0. It also tends to suggest, but does not prove, that catalysis proceeds via relatively modest changes in the conformation of the enzyme, that is, there is no suggestion of major structural changes which might destroy the crystals.
Within each monomer, the amino acids form five well-defined structural domains. As noted below, critical elements of the active site are also contributed by amino acids from elsewhere in the same polypeptide chain as well as from other chains within the tetramer. The overall structure of the tetramer is illustrated in Figure 3 a and in the associated interactive images. Domain 1, blue; Domain 2, green; Domain 3, yellow; Domain 4, cyan; Domain 5, red. Lighter and darker shading is used to differentiate equivalent domains in different subunits.
Interactive views are available in the electronic version of the article see below. The four active sites each highlighted with an asterisk are located toward the center of the figure. In each case a loop including residues — extends from one subunit to complete the active site of a neighboring subunit. Residues 13—50, shown as thick lines, pass through a tunnel between the first domain labeled D1 and the rest of the protein.
Magnesium ions small solid circles bridge between the complementation peptide and the rest of the protein from Ref. An interactive view is available in the electronic version of the article. Furthermore, some of these additional elements might promote the production of the inducer, allolactose.
It can now be rationalized in terms of the three-dimensional structure. As can be seen in Figure 3 a see also the interactive imageresidues from about 13 to 20 in adjacent subunits contact each other at the bottom of the figure. An equivalent interaction between the other two subunits occurs at the top of the figure.A non-metabolizable allolactose analogue, widely used in molecular biology for overexpression of recombinant proteins from inducible systems under the control of lac promoter.
IPTG binds to the LacI repressor and causes its release from the lac operator, allowing gene expression to take place. USA toll-free Ordering Information. Worldwide Locations. For requirements greater than the catalogue prepacks email: enquiries carbosynth. For custom synthesis or special requests email: enquiries carbosynth. To order this product please fill out our contact formemail enquiries carbosynth. This product is restricted and not available to purchase online.
Please contact us at sales carbosynth. Isopropyl b-D-thiogalactopyranoside - non-animal origin. Price on application. Please contact: sales carbosynth. Add Check Stock. There is no hazardous surcharge associated with this product. References Barco A and Carrasco L Cloning and inducible synthesis of poliovirus non-structural proteins in Saccharomyces cerevisiae. Gene 1 Modulation of expression of the human gamma interferon gene in E. Biochem Biophys Res Commun Date T, et al Expression of active rat DNA polymerase.
Biochemistry 27 8 : — IPTG-dependent vaccinia virus: identification of a virus protein enabling virion envelopment by Golgi membrane and egress. Nucleic Acids Res 18 18 Goeddel DV, et al Expression in Escherichia coli of chemically synthesized genes for human insulin.
Hansen LH, et al Curr Microbiol 36 6 Vaccinia virus morphogenesis is interrupted when expression of the gene encoding an kilodalton phosphorylated protein is prevented by the Escherichia coli lac repressor. J Virol 65 11 : — Itakura K, et al Expression in Escherichia coli of a chemically synthesized gene for the hormone somatostatin. Science Further Information IPTG is a chemical inducer of gene expression used for the production of recombinant proteins from bacterial hosts and mammalian cells.
How does IPTG – Induced gene expression work at a molecular level?
The IPTG-inducible genetic systems are versatile and controllable tools for production of proteins for therapeutic, diagnostic or research purposes. Mechanism of action in biological systems IPTG is a molecular mimic of lactose metabolite allolactose. Lactose is an important carbon source for bacteria and has a regulatory role in the expression of lactose-metabolising proteins encoded by the lac operon. Given the molecular similarity between allolactose and IPTG, the latter is being used in molecular biology to activate lactose-responsive genetic elements.The lactose operon lac operon is an operon required for the transport and metabolism of lactose in E.
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. It is often discussed in introductory molecular and cellular biology classes for this reason. Their work on the lac operon won them the Nobel Prize in Physiology in Bacterial operons are polycistronic transcripts that are able to produce multiple proteins from one mRNA transcript.
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: lacZlacYand lacA. Finally, lacA encodes Galactoside acetyltransferase. It would be wasteful to produce enzymes when no lactose were available or if a preferable energy source such as glucose were available. 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 other words, it is transcribed only in the presence of small molecule co-inducer. In the presence of glucose, the catabolite activator protein CAPrequired 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. Only lacZ and lacY appear to be necessary for lactose catabolism. 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 lacZlacYand lacA.
The fourth lac gene is lacIencoding the lactose repressor—"I" stands for inducibility. One may distinguish between structural genes encoding enzymes, and regulatory genes encoding proteins that affect gene expression. Various short sequences that are not genes also affect gene expression, including the lac promoter, lac pand the lac operator, lac o. Although it is not strictly standard usage, mutations affecting lac o are referred to as lac o cfor historical reasons.
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 molecule.
This protein can only be removed when allolactose binds to it, and inactivates it. The protein that is formed by the lacI gene is known as the lac repressor. The type of regulation that the lac operon undergoes is referred to as negative inducible, meaning that the gene is turned off by the regulatory factor lac repressor unless some molecule lactose is added.
Because of the presence of the lac repressor protein, genetic engineers who replace the lacZ gene with another gene will have to grow the experimental bacteria on agar with lactose available on it.
If they do not, the gene they are trying to express will not be expressed as the repressor protein is still blocking RNAP from binding to the promoter and transcribing the gene. Each of the three genes on the mRNA strand has its own Shine-Dalgarno sequenceso the genes are independently translated.
The lacI gene coding for the repressor lies nearby the lac operon and is always expressed constitutive. If lactose is missing from the growth medium, the repressor binds very tightly to a short DNA sequence just downstream of the promoter near the beginning of lacZ called the lac operator.
When cells are grown in the presence of lactose, however, a lactose metabolite called allolactose, made from lactose by the product of the lacZ gene, binds to the repressor, causing an allosteric shift. Thus altered, the repressor is unable to bind to the operator, allowing RNAP to transcribe the lac genes and thereby leading to higher levels of the encoded proteins.
Cyclic adenosine monophosphate cAMP is a signal molecule whose prevalence is inversely proportional to that of glucose. More recently inducer exclusion was shown to block expression of the lac operon when glucose is present. Glucose is transported into the cell by the PEP-dependent phosphotransferase system. The unphosphorylated form of EIIA Glc binds to the lac permease and prevents it from bringing lactose into the cell.
Therefore, if both glucose and lactose are present, the transport of glucose blocks the transport of the inducer of the lac operon.
IPTG Induction Protocol
The lac repressor is a four-part protein, a tetramer, with identical subunits. The operator site where repressor binds is a DNA sequence with inverted repeat symmetry.Don't have a profile? If you are viewing this page as a nonregistered user, the price s displayed is List Price.
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Biological Buffers. Custom Services and Products. Enzymes and Inhibitors.This compound is a molecular mimic of allolactose, a lactose metabolite that triggers transcription of the lac operon, and it is therefore used to induce E. Like allolactose, IPTG binds to the lac repressor and releases the tetrameric repressor from the lac operator in an allosteric manner, thereby allowing the transcription of genes in the lac operon. But unlike allolactose, the sulfur S atom creates a chemical bond which is non-hydrolyzable by the cell, preventing the cell from metabolizing or degrading the inducer.
The lac operon is one of the most commonly used systems for creating proteins in E. Usually your gene of interest is inserted into a commercial vector pET vectors are common that contains:.
The lac repressor protein LacI evolved to sense the presence of lactose a combined galactose-glucose disaccharide.
Both the host chromosome and the insert have copies of the lac repressor gene to ensure that there is always enough LacI protein to titrate all DNA operator sites.
This blocks access of T7 RNA polymerase to the promoter site and thus prevents leaky transcription of your gene before induction. When lactose binds to LacI it induces a conformational change in the protein structure that renders it incapable of binding to the operator DNA sequence.
IPTG is a structural mimic of lactose it resembles the galactose sugar that also binds to the lac repressor and induces a similar conformational change that greatly reduces its affinity for DNA. Unlike lactose, IPTG is not part of any metabolic pathways and so will not be broken down or used by the cell. This ensures that the concentration of IPTG added remains constant, making it a more useful inducer of the lac operon than lactose itself. Contact Us.
Structural Analysis of Lac Repressor Bound to Allosteric Effectors
Protein Expression. Monoclonal Antibody.
Polyclonal Antibody. Recombinant Antibody. Protein Refolding. Protein Purification.IPTG induction is a longstanding technique in molecular biology. No time to read the whole thing? Look to this list as a reference for topics covered and scroll to the piece of information you need most.
IPTG is the structural analog of lactose; however within a cell, it is not part E. Think of the word induce on its own.
Rather than flood you with these words throughout the article, it might be helpful to look at each thing and its role individually. The first part of induction is just making sure you have all your ducks in a row. That means having your vector prepared and ready, making sure your cells are competent and then getting your cells to take up the vector.
Inside the E. Your E. Below is what starts to happen in these cells and why IPTG is so important:. The answer is because IPTG is not part of the metabolic pathway. Therefore, the induction process is more efficient when using IPTG instead of lactose. When it comes to the optimal optical density, the key is finding the point where all your cells are alive and very healthy log phase or exponential phasewhich is usually going to be an OD of around 0. Lower ODs than 0.
If you are interested in learning about other aspects of protein expression, make sure to check out this troubleshooting articleand take a look at some of our high-quality products for expression and purification, including IPTG.
Arur, S. IPTG Induction. Beel, C.
A closer view of the conformation of the Lac repressor bound to operator. Gay, G. Rapid modification of the pET expression vector for ligation independent cloning using homologous recombination in Saccharomyces cerevisiae. Lewis, M. Lu, P.