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    The lac operon (article)

    Regulation of genes for lactose utilization. lac repressor, catabolite activator protein, and cAMP.

    Regulation of gene expression and cell specialization

    The lac operon

    Regulation of genes for lactose utilization. lac repressor, catabolite activator protein, and cAMP.

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    Key points:

    The lac operon of E. coli contains genes involved in lactose metabolism. It's expressed only when lactose is present and glucose is absent.

    Two regulators turn the operon "on" and "off" in response to lactose and glucose levels: the lac repressor and catabolite activator protein (CAP).

    The lac repressor acts as a lactose sensor. It normally blocks transcription of the operon, but stops acting as a repressor when lactose is present. The lac repressor senses lactose indirectly, through its isomer allolactose.

    Catabolite activator protein (CAP) acts as a glucose sensor. It activates transcription of the operon, but only when glucose levels are low. CAP senses glucose indirectly, through the "hunger signal" molecule cAMP.


    Lactose: it's what's for dinner! While that may not sound delicious to us (lactose is the main sugar in milk, and you probably don't want to eat it plain), lactose can be an excellent meal for E. coli bacteria. However, they'll only gobble up lactose when other, better sugars – like glucose – are unavailable.

    With that for context, what exactly is the lac operon? The lac operon is an operon, or group of genes with a single promoter (transcribed as a single mRNA). The genes in the operon encode proteins that allow the bacteria to use lactose as an energy source.

    What makes the lac operon turn on?

    E. coli bacteria can break down lactose, but it's not their favorite fuel. If glucose is around, they would much rather use that. Glucose requires fewer steps and less energy to break down than lactose. However, if lactose is the only sugar available, the E. coli will go right ahead and use it as an energy source.

    To use lactose, the bacteria must express the lac operon genes, which encode key enzymes for lactose uptake and metabolism. To be as efficient as possible, E. coli should express the lac operon only when two conditions are met:

    Lactose is available, and

    Glucose is not available

    How are levels of lactose and glucose detected, and how how do changes in levels affect lac operon transcription? Two regulatory proteins are involved:

    One, the lac repressor, acts as a lactose sensor.

    The other, catabolite activator protein (CAP), acts as a glucose sensor.

    These proteins bind to the DNA of the lac operon and regulate its transcription based on lactose and glucose levels. Let's take a look at how this works.

    Structure of the lac operon

    The lac operon contains three genes: lacZ, lacY, and lacA. These genes are transcribed as a single mRNA, under control of one promoter.

    Genes in the lac operon specify proteins that help the cell utilize lactose. lacZ encodes an enzyme that splits lactose into monosaccharides (single-unit sugars) that can be fed into glycolysis. Similarly, lacY encodes a membrane-embedded transporter that helps bring lactose into the cell. [More details]

    In addition to the three genes, the lac operon also contains a number of regulatory DNA sequences. These are regions of DNA to which particular regulatory proteins can bind, controlling transcription of the operon.

    Structure of the lac operon. The DNA of the lac operon contains (in order from left to right): CAP binding site, promoter (RNA polymerase binding site), operator (which overlaps with promoter), lacZ gene, lacY gene, and lacA gene. The activator protein CAP, when bound to a molecule called cAMP (discussed later), binds to the CAP binding site and promotes RNA polymerase binding to the promoter. The lac repressor protein binds to the operator and blocks RNA polymerase from binding to the promoter and transcribing the operon.

    _Image modified from "Prokaryotic gene regulation: Figure 3," by OpenStax College, Biology (CC BY 4.0)._

    The promoter is the binding site for RNA polymerase, the enzyme that performs transcription.

    The operator is a negative regulatory site bound by the lac repressor protein. The operator overlaps with the promoter, and when the lac repressor is bound, RNA polymerase cannot bind to the promoter and start transcription.

    The CAP binding site is a positive regulatory site that is bound by catabolite activator protein (CAP). When CAP is bound to this site, it promotes transcription by helping RNA polymerase bind to the promoter.

    Let's take a closer look at the lac repressor and CAP and their roles in regulation of the lac operon.

    The lac repressor

    The lac repressor is a protein that represses (inhibits) transcription of the lac operon. It does this by binding to the operator, which partially overlaps with the promoter. When bound, the lac repressor gets in RNA polymerase's way and keeps it from transcribing the operon. [Where does the lac repressor come from?]

    When lactose is not available, the lac repressor binds tightly to the operator, preventing transcription by RNA polymerase. However, when lactose is present, the lac repressor loses its ability to bind DNA. It floats off the operator, clearing the way for RNA polymerase to transcribe the operon.

    Source : www.khanacademy.org


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    52 Cards in this Set

    An ___ is a stretch of DNA consisting of an operator, a promoter, and genes for a related set of proteins, usually making up an entire metabolic pathway.


    The ___ is/are arranged sequentially after the promoter.

    Genes of an operon

    A ___ is a specific nucleotide sequence in DNA that binds RNA polymerase, positioning it to start transcribing RNA at the appropriate place.


    A ___ codes for a protein, such as repressor, that controls the transcription of another gene or group of genes

    regulatory gene

    5. Regulatory proteins bind to the ___ to control expression of the operon.


    A ___ is a protein that inhibits gene transcription. In prokaryotes, this protein binds to the DNA in or near the promoter.


    An ___ is a specific small molecule that binds to a bacterial regulatory protein and changes its shape so that it cannot bind to an operator, thus switching an operon on.


    What molecule binds to promoters in bacteria and transcribes the coding regions of the genes?

    RNA polymerase

    What is allosteric regulation?

    In allosteric regulation, a small molecule binds to a large protein and causes it to change its shape and activity.

    Under which conditions are the lac structural genes expressed most efficiently?

    No glucose, high lactose

    What happens to the expression of the lacI gene if lactose is not available in the cell?

    There is no change—the lacI gene is constitutively expressed.

    Which of the following enzymes converts ATP to cAMP?

    Adenylyl cyclase

    Regulatory proteins bind to _____.

    the operator

    operon is not transcribed

    lac operon: lactose absent; trp operon: tryptophan present

    operon is transcribed, but not sped up by the positive control

    trp operon: tryptophan absent; lac operon: lactose present, glucose present

    operon is transcribed quickly through postive control

    lac operon: lactose present, glucose absent

    Negative control:When lactose is absent

    the repressor protein is active, and transcription is turned off.

    Negative control:When lactose is present

    the repressor protein is inactivated, and transcription is turned on.

    Positive control:When glucose is absent

    another regulatory protein (CAP) binds to the promoter of the lac operon, increasing the rate of transcription if lactose is present

    Modification of chromatin structure:Methylation of histone tails in chromatin can promote condensation of the chromatin.

    T or F TRUE

    Modification of chromatin structure:DNA is not transcribed when chromatin is packaged tightly in a condensed form.


    Modification of chromatin structure:Some forms of chromatin modification can be passed on to future generations of cells.

    Source : www.cram.com

    The lac repressor

    Few proteins have had such a strong impact on a field as the lac repressor has had in Molecular Biology. Over 40 years ago, Jacob and Monod [Genetic r…

    Comptes Rendus Biologies

    Volume 328, Issue 6, June 2005, Pages 521-548

    Review / Revue The lac repressor

    Presented by Stuart Edelstein

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    Few proteins have had such a strong impact on a field as the lac repressor has had in Molecular Biology. Over 40 years ago, Jacob and Monod [Genetic regulatory mechanisms in the synthesis of proteins, J. Mol. Biol. 3 (1961) 318] proposed a model for gene regulation, which survives essentially unchanged in contemporary textbooks. It is a cogent depiction of how a set of ‘structural’ genes may be coordinately transcribed in response to environmental conditions and regulates metabolic events in the cell. In bacteria, the genes required for lactose utilization are negatively regulated when a repressor molecule binds to an upstream cis activated operator. The repressor and its operator together form a genetic switch, the lac operon. The switch functions when inducer molecules alter the conformation of the repressor in a specific manner. In the presence of a particular metabolite, the repressor undergoes a conformational change that reduces its affinity for the operator. The structures of the lac repressor and its complexes with operator DNA and effector molecules have provided a physical platform for visualizing at the molecular level the different conformations the repressor and the molecular basis for the switch. The structures of lac repressor, bound to its operator and inducer, have also been invaluable for interpreting a plethora of biochemical and genetic data. To cite this article: M. Lewis, C. R. Biologies 328 (2005).

    Previous articleNext article


    lac repressorThree-dimensional structureOperator and inducer binding sitesT and R sites

    1. Background

    All organisms respond to changing conditions in their environment by controlling the expression of their genes. Depending upon the particular circumstances, cells can efficiently regulate metabolic pathways by appropriately increasing or decreasing the concentration of specific enzymes. The concentration of these regulated enzymes in turn controls the flux through a given pathway. Escherichia coli, like all organisms, can meet its energy demands by altering enzyme concentrations to take full advantage of the fluctuating food supplies in their environment. When glucose is abundant, the bacterium utilizes it exclusively as its food source, even when other sugars are present in the surroundings. However, when the glucose supplies become exhausted, E. coli has the ability to take up and metabolize alternative sugars such as lactose. The ability of the bacteria to switch from one metabolite to another was described by Monod as diauxic growth [2]. The diauxic growth pattern, illustrated in Fig. 1, occurs when metabolites are used sequentially rather than simultaneously. Monod observed that bacteria prefer glucose as an energy source and only when the glucose supplies are depleted will the bacteria switch to an alternate carbon sources. Understanding at the molecular level how an organism sequentially utilizes metabolites has been a fundamental problem in biology that has attracted tremendous interest over the last fifty years.

    Download : Download full-size image

    Fig. 1. Diagram of diauxic growth of bacterial cultures adapted from Monod [2].

    Jacob and Monod [3] conceptually outlined how bacterial cultures could switch their mode of growth from one state to the other so rapidly and completely. They described the operon as a group of structural genes that are coordinately regulated. The structural genes of an operon correspond to a group of proteins or enzymes that are responsible for a particular task or metabolic process. In the operon, the genes are regulated depending upon the metabolic needs of the cell. In order to regulate a gene, or a family of genes in a coordinated fashion, the operon requires a master switch. The switch of the operon is a repressor molecule, which itself is the product of a regulatory gene (R). The repressor associates with a regulatory element, called the operator (O), and controls the synthesis of the structural genes (A, B). A schematic representation of the operon is shown in Fig. 2. Binding of the repressor to the operator negatively regulates or blocks the expression of structural genes of the operon. If the repressor is to function as a switch, it must be inducible; the switch must be able to turn on or turn off in response to a given chemical signal. In this model the repressor not only binds to the operator it also binds an inducer (I), a metabolite that monitors the metabolic state. The inducer is a chemical signal that either directly or indirectly modulates the affinity of the repressor for operator. In the presence of the inducer the repressor dissociates from the operator, which relieves the negative regulation and allows the expression of the structural genes. The switch can also be controlled by positive regulation where a metabolite or co-repressor increases the binding affinity of the repressor for its operator. In both cases, the repressor controls the rate of a metabolic process by increasing or decreasing the concentrations of the structural proteins. Jacob and Monod proposed two possible models for gene regulation. The first model acts as described above while the repressor in Model II acts on messenger RNA rather than operator DNA. Model II, in fact was favored by Jacob and Monod; at the time it was not known that the repressor was a protein. Jacob and Monod based their general model on the system, which they had long studied, the lactose metabolism of Escherichia coli (E. coli).

    Source : www.sciencedirect.com

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