Name: lactose switch in the lac operon
Uploaded: 2025-06-12T09:14:20.000Z
Duration: 1 min 25 s
Channel: JoVE

E. coli lactose metabolism is controlled by the structural genes of the lac operon, which are locally regulated by lactose-dependent LacI repressor.Additionally, the catabolite repression mechanism regulates these genes based on glucose availability via a global regulatory protein, cAMP receptor protein, or CRP.When glucose is abundant, cyclic AMP, or cAMP formation, is inhibited.No CRP-cAMP complex can form without cAMP, leaving the CRP binding site upstream of the lac operon unbound. Simultaneously, the LacI remains bound to the operator.Inhibition of lac operon expression leads E. coli to first metabolize only glucose, even if lactose is available, conserving cell energy.When glucose levels drop, cAMP levels increase. The cAMP-CRP complex is formed, which binds the DNA and enhances the promoter recruitment of RNA polymerase.A small amount of available lactose generates allolactose, which acts as an inducer binding the LacI repressor.Allolactose-binding triggers an allosteric change, releasing LacI from the operator and allowing transcription of the lac operon.

inducible operons: lac operon

Inducible Operons: lac Operon

The lac operon in Escherichia coli is a model for understanding inducible gene regulation and metabolic flexibility. It integrates local control by lactose and global regulation through catabolite repression, enabling E. coli to preferentially metabolize glucose when available and switch to lactose utilization when glucose is scarce.Structure and Function of the lac OperonThe lac operon contains three structural genes: lacZ (&#946;-galactosidase), lacY (lactose permease), and lacA (thiogalactoside transacetylase), which are responsible for lactose metabolism. These genes are transcribed as a single mRNA under the control of a promoter, an operator, and a regulatory region that responds to environmental glucose and lactose levels.Local Regulation by the LacI RepressorUnder normal conditions, the LacI repressor binds to the operator, preventing RNA polymerase from transcribing the lac operon. When lactose is present, a small amount is converted into allolactose, an inducer. Allolactose binds to LacI, causing an allosteric change that detaches the repressor from the operator. This derepression allows RNA polymerase to access the promoter and initiate transcription of the structural genes.Global Regulation: Catabolite Repression and CRP-cAMP ComplexThe catabolite repression mechanism ensures that E. coli preferentially metabolizes glucose, even in the presence of lactose. When glucose is abundant, cyclic AMP (cAMP) levels are low due to inhibited adenylate cyclase activity. Without cAMP, the cAMP receptor protein (CRP) cannot bind to the CRP-binding site upstream of the lac promoter. In this state, transcription of the lac operon is suppressed because RNA polymerase recruitment to the promoter is inefficient.When glucose levels drop, cAMP levels increase, forming a cAMP-CRP complex. This complex binds to the CRP-binding site, enhancing RNA polymerase recruitment to the promoter and significantly increasing lac operon transcription. Simultaneously, allolactose derepresses the operon by inactivating LacI, ensuring full activation of the system for lactose metabolism.Integrated Control for Metabolic EfficiencyThis dual regulatory mechanism allows E. coli to efficiently manage its metabolic resources, prioritizing glucose utilization as a primary energy source. Once glucose is depleted, the system shifts to metabolizing lactose, ensuring survival and adaptability in changing environments. The lac operon exemplifies how local and global regulatory networks interact to optimize bacterial gene expression.

The lac operon in Escherichia coli is a model for understanding inducible gene regulation and metabolic flexibility. It integrates local control by ...

Education

JoVE Core

Microbiology

Unit

Bacterial Genetics and Gene Regulation

KEY TERMS AND CONCEPTS

lactose switch in the lac operon

The lac operon in Escherichia coli controls lactose metabolism with inducible gene regulation and metabolic flexibility. It helps the bacterium use glucose first when it is available, then switch to lactose when glucose becomes scarce. This system is a classic example of how bacteria match gene expression to changing food sources.

The lac operon includes three structural genes: lacZ, lacY, and lacA. These genes make beta-galactosidase, lactose permease, and thiogalactoside transacetylase, respectively. They are transcribed together as one mRNA under the control of a promoter, an operator, and a regulatory region that responds to lactose and glucose levels.

Local control depends on the LacI repressor. Under normal conditions, LacI binds to the operator and blocks RNA polymerase from transcribing the operon. When lactose is present, some of it is converted into allolactose, which acts as an inducer. Allolactose binds LacI, changes its shape, and causes it to leave the operator. This derepression lets RNA polymerase reach the promoter and start transcription.

The operon also responds to glucose through catabolite repression. When glucose is abundant, cyclic AMP, or cAMP, stays low because adenylate cyclase is inhibited. Without cAMP, the cAMP receptor protein, or CRP, cannot bind the CRP-binding site upstream of the lac promoter. As a result, RNA polymerase is recruited inefficiently, and transcription stays low.

When glucose levels drop, cAMP levels rise and form a cAMP-CRP complex. This complex binds upstream of the promoter and helps RNA polymerase bind more effectively. At the same time, allolactose removes LacI from the operator. Together, these local and global controls fully activate the lac operon for lactose use.

This dual system gives E. coli an efficient way to manage energy. It favors glucose as the main energy source, but it can shift to lactose metabolism when glucose is gone. The lac operon shows how bacteria use linked regulatory networks to adjust gene expression and survive in changing environments.

Lactose Switch in the lac Operon

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The lac operon in Escherichia coli controls lactose metabolism with inducible gene regulation and metabolic flexibility. It helps the bacterium use glucose first when it is available, then switch to lactose when glucose becomes scarce. This system is a classic example of how bacteria match gene expression to changing food sources. The lac operon includes three structural genes: lacZ, lacY, and lacA. These genes make beta-galactosidase, lactose permease, and thiogalactoside transacetylase,...

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key terms and concepts

Explore how the lac operon lets E. coli use glucose first and switch to lactose through LacI, allolactose, cAMP, and CRP control.

Lactose Switch in the lac Operon | JoVE K12

JoVE Core Microbiology provides an engaging introduction to the microbial world and its influence on health, industry, and the environment. This comprehensive collection covers key concepts such as microbial structure, metabolism, genetics, and ecology. It introduces essential laboratory techniques and microscopy methods through visually engaging animations. Corresponding scientist-in-action videos bridge classroom concepts with real-world research and applications.

microbiology

Lactose Switch in the lac Operon

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