Yours

2.3.2: Prokaryotic Gene Regulation

BIOS220- Introduction to Genetics · LibreTexts

0%

Contents

    Text

    Size
    100%
    Theme
    Typeface
    Line length
    Spacing

    From LibreTexts, under CC BY 4.0, as the source states it; its equations are written in TeX, which the PDF shows typeset. This page at LibreTexts.

    The DNA of prokaryotes is organized into a circular chromosome, supercoiled within the nucleoid region of the cell cytoplasm. Proteins that are needed for a specific function, or that are involved in the same biochemical pathway, are encoded together in blocks called operons. An operon is a functional genetic unit composed of a promoter, an operator, and a continuous cluster of structural genes. The promoter is the upstream sequence where RNA polymerase physically binds to initiate transcription, while the operator is a downstream regulatory DNA sequence that acts as a gatekeeper. By organizing genes this way, all of the necessary enzymes for a specific pathway, such as the genes needed to use lactose as an energy source in the lactose (lac) operon, are arranged right next to each other and transcribed into a single, continuous polycistronic mRNA molecule.

    To control when operons are active, cells use three types of regulatory molecules that affect expression: repressors, activators, and inducers. Repressors and activators are specialized regulatory proteins encoded by separate regulatory genes, and they control expression by physically binding to the specific DNA sites adjacent to the structural genes. Activators bind to or near the promoter site to recruit RNA polymerase, effectively increasing transcription in response to external stimuli. Conversely, repressors bind directly to the operator region. When a repressor locks onto the operator, it acts as a physical roadblock that prevents RNA polymerase from moving downstream along the DNA strand, halting transcription. The activity of these structural proteins is modulated by inducers, which are small effector molecules present in the cell or its environment. Inducers bind directly to the regulatory proteins, altering their physical shape to either trigger or stop their binding to the DNA.

    File:Lactose operon.png

    Figure 5.3 The lactose operon. The far left (P) is the promoter for the repressor gene (i). The Lac operon consists of the operon promoter (pink p), the operator (o), and the three structural genes: lac z, lac y, and lac a. The repressor gene codes for the repressor protein that sits on the operator preventing transcription until it is removed.

    The trp Operon: A Repressible Operon

    Bacteria such as Escherichia coli need a constant supply of amino acids to survive, and are able to synthesize many of them when they are scarce in the environment. Tryptophan is one such amino acid that E. coli can either ingest from the environment or synthesize using enzymes that are encoded by five genes. These five genes are next to each other in what is called the tryptophan (trp) operon (Figure 16.4). The genes are transcribed into a single mRNA, which is then translated to produce all five enzymes. If tryptophan is present in the environment, then E. coli does not need to synthesize it and the trp operon is switched off. However, when tryptophan availability is low, the switch controlling the operon is turned on, the mRNA is transcribed, the enzyme proteins are translated, and tryptophan is synthesized.

    The trp operon is divided into three key regions: the upstream promoter sequence, the operator sequence, and the downstream protein-coding region. Positioned immediately between the promoter and the transcription start site, the operator sequence acts as the physical binding site for the trp repressor protein. Because it is an anabolic pathway designed to build a molecule, the trp operon operates as a negative repressible system, meaning it defaults to an "on" state and is actively switched "off" only when its end product accumulates. The word "negative" here refers strictly to the type of regulatory protein involved, meaning that transcription is controlled by a repressor that turns the system off, rather than an activator that turns it on. The word "repressible" refers to the operon's default state, meaning the system defaults to an "on" position and must be actively pushed "off" or repressed by an external stimulus, in this case, when its metabolic end product accumulates.

    The repressor protein is synthesized in an inactive conformation and cannot bind to the operator on its own. However, when tryptophan levels rise within the cell, two tryptophan molecules physically bind to the repressor protein as co-repressors. This binding induces an allosteric conformational shift, altering the repressor's shape into an active state that tightly clamps onto the trp operator. This physical roadblock prevents RNA polymerase from transcribing the downstream structural genes. When intracellular tryptophan availability drops, the co-repressor molecules dissociate, the repressor reverts to its inactive shape and falls off the DNA, allowing RNA polymerase to resume transcription and replenish the cell's amino acid supply.

    The t r p operon has a promoter, an operator, and five genes named t r p upper case E; t r p upper case D, t r p upper case C, t r p upper case B, and t r p upper case A that are located in sequential order on the D N A. R N A polymerase binds to the promoter. When tryptophan is present, the t r p repressor binds the operator and prevents the R N A polymerase from moving past the operator; therefore, R N A synthesis is blocked. In the absence of tryptophan, the repressor dissociates from the operator. R N A polymerase can now slide past the operator, and transcription begins.
    Figure 5.4 The tryptophan operon. The five genes that are needed to synthesize tryptophan in E. coli are located next to each other in the trp operon. When tryptophan is plentiful, two tryptophan molecules bind the repressor protein at the operator sequence. This physically blocks the RNA polymerase from transcribing the tryptophan genes. When tryptophan is absent, the repressor protein does not bind to the operator and the genes are transcribed.

    Link to Learning

    Watch this video to learn more about the trp operon.

    The lac Operon: An Inducible Operon

    While repressible operons control anabolic pathways to stop production when a nutrient is abundant, inducible operons typically regulate catabolic pathways, remaining turned "off" until a specific substrate arrives to be broken down. The classic example for this mechanism is the lactose (lac) operon in E. coli, which encodes the enzymes required to acquire and digest lactose from the environment. The operon features three structural genes: lacZ, which encodes \(\beta \)-galactosidase to cleave lactose into glucose and galactose; lacY, which encodes lactose permease to pump the sugar into the cell; and lacA, which encodes a transacetylase enzyme.

    Because synthesizing these enzymes is energetically costly, the lac operon defaults to an "off" state via a negative inducible control mechanism. In the absence of lactose, a constitutively expressed lac repressor protein binds tightly to the operator, physically blocking RNA polymerase from moving forward. However, when lactose becomes available, a small amount is converted into its metabolic isomer, allolactose. Allolactose acts as an inducer, binding to the lac repressor and forcing an allosteric structural change that causes the protein to release from the operator sequence, clearing the way for transcription.

    Even when the repressor is removed, the cell faces a secondary energetic dilemma because glucose remains its preferred, most efficient fuel source. To prevent the wasteful digestion of lactose when glucose is plentiful, the lac operon is also controlled by a secondary, positive regulatory mechanism, the Catabolite Activator Protein (CAP). When environmental glucose is abundant, intracellular signaling molecules called cyclic AMP (cAMP) remain low. However, when glucose levels drop, cAMP rapidly accumulates inside the cytoplasm and binds directly to CAP, activating it.

    Once activated by cAMP, the CAP complex binds to a specialized consensus site located immediately upstream of the lac promoter. The physical presence of CAP stabilizes the docking of RNA polymerase, increasing its binding affinity to an otherwise weak promoter sequence and boosting transcription rates exponentially. Full activation of the lac operon requires 2 things to happen: glucose must be completely absent (triggering cAMP/CAP activation to accelerate the engine) and lactose must be present (triggering allolactose induction to remove the physical brake). If glucose is present, the operon is restricted to a negligible, basal rate of background transcription, protecting the cell from wasting energy.

    The lac operon consists of a promoter, an operator, and three genes named lac Z, lac Y, and lac A that are located in sequential order on the D N A. In the absence of c A M P, the CAP protein does not bind the D N A. R N A polymerase binds the promoter, and transcription occurs at a slow rate. In the presence of c A M P, a CAP, c A M P complex binds to the promoter and increases R N A polymerase activity. As a result, the rate of R N A synthesis is increased.
    Figure 5.5 Transcriptional activation by the CAP protein. When glucose levels fall, E. coli may use lactose for fuel but must transcribe new genes to do so. As glucose supplies become limited, cAMP levels increase. This cAMP binds to the CAP protein, a positive regulator that binds to a promoter region upstream of the genes required to use lactose.

    The lac operon serves as a textbook example of a system that is both negative inducible and positive inducible. The system is "negative" because its primary control switch is a repressor protein that blocks transcription, and it is "inducible" because its default metabolic state is "off" and must be actively turned "on" by an environmental inducer (allolactose). The system is also "positive" because it relies on an activator protein (CAP) to increase transcription to high levels, and it is "inducible" because this activator remains dormant until it is turned "on" by a signaling molecule (cAMP) indicating that glucose is absent.

    Visual Connection

    The lac operon consists of a promoter, an operator, and three genes named lac Z, lac Y, and lac A. R N A polymerase binds to the promoter. In the absence of lactose, the lac repressor binds to the operator and prevents RNA polymerase from transcribing the operon. In the presence of lactose, the repressor is released from the operator, and transcription proceeds at a slow rate. Binding of the c A M P plus sign CAP complex to the promoter stimulates R N A polymerase activity and increases R N A synthesis. However, even in the presence of the c A M P plus sign CAP complex, R N A synthesis is blocked if the repressor binds to the promoter.
    Figure 5.6 Regulation of the lac operon. Transcription of the lac operon is carefully regulated so that its expression only occurs when glucose is limited and lactose is present to serve as an alternative fuel source.
    Signals that Induce or Repress Transcription of the lac Operon
    Glucose CAP binds Lactose Repressor binds Transcription
    + - - + No
    + - + - Some
    - + - + No
    - + + - Yes
    Table 5.1
    Link to Learning

    Watch an animated tutorial about the workings of lac operon here.

    Key Terms

    Operon
    Genetic unit found in prokaryotes composed of a promoter, an operator, and a continuous cluster of functionally related structural genes transcribed together into a single mRNA molecule.

    Operator
    A downstream regulatory DNA sequence situated between the promoter and the structural genes that acts as a physical gatekeeper by binding repressor proteins.

    Negative Regulation
    A gene control mechanism in which transcription is governed by a repressor protein that physically blocks RNA polymerase from moving down the DNA strand.

    Positive Regulation
    A gene control mechanism in which transcription is governed by an activator protein that physically binds to the DNA to recruit and stabilize RNA polymerase at a promoter.

    Repressible Operon
    An operon whose deault metabolic state is "on", which must be actively switched "off" or repressed when an environmental stimulus or metabolic end product accumulates (e.g., the trp operon).

    Inducible Operon
    An operon whose default metabolic state is "off", which must be actively switched "on" or induced when a specific environmental substrate arrives to be processed (e.g., the lac operon).

    Catabolite Activator Protein (CAP)
    A positive regulatory activator protein that, when bound to cyclic AMP (cAMP), locks onto a consensus site upstream of a promoter to exponentially accelerate transcription rates.

    Check Your Understanding

    1. In a standard prokaryotic operon, what is the linear sequence of the three primary regions from upstream to downstream?
    2. How does a repressor protein stop a gene from being expressed?
    3. If a system is described as a negative repressible operon, what does the word "negative" tell you about the protein involved, and what does "repressible" tell you about its default state?
    4. Predict the status of the lac repressor protein, the CAP protein, and the overall level of transcription (None, Basal/Low, or High) for the following environmental conditions:
      1. Glucose is highly abundant, and lactose is completely absent from the environment.
      2. Both glucose and lactose are highly abundant in the environment.
      3. Glucose is completely absent, but lactose is highly abundant in the environment.

    This page titled 2.3.2: Prokaryotic Gene Regulation was last modified on Thu, 20 Aug 2026 01:22:08 GMT and is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by OpenStax via source content that was edited to the style and standards of the LibreTexts platform.