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Operons

Operons

Prokaryotes can control gene expression through operons—DNA sequences consisting of regulatory elements and clustered, functionally related protein-coding genes. Operons use a single promoter sequence to initiate transcription of a gene cluster (i.e., a group of structural genes) into a single mRNA molecule. The terminator sequence ends transcription. An operator sequence, located between the promoter and structural genes, prohibits the operon’s transcriptional activity if bound by a repressor...

Prokaryotic Transcriptional Activators and Repressors

Prokaryotic Transcriptional Activators and Repressors

The organization of prokaryotic genes in their genome is notably different from that of eukaryotes. Prokaryotic genes are organized, such that the genes for proteins involved in the same biochemical process or function are located together in groups. This group of genes, along with their regulatory elements, are collectively known as an operon. The functional genes in an operon are transcribed together to give a single strand of mRNA known as polycistronic mRNA.
Transcription of prokaryotic...

Co-activators and Co-repressors

Co-activators and Co-repressors

Gene transcription is regulated by the synergistic action of several proteins that form a complex at a gene regulatory site. This is observed in eukaryotes, where the regulation of gene expression is a complex process. Regulatory proteins in eukaryotes can broadly be classified into two types – regulators that bind directly to specific DNA sequences and co-regulators that associate with regulatory proteins but cannot directly bind to the DNA. These co-regulators are further divided into...

Prokaryotic Transcriptional Activators and Repressors

Prokaryotic Transcriptional Activators and Repressors

The organization of prokaryotic genes in their genome is notably different from that of eukaryotes. Prokaryotic genes are organized, such that the genes for proteins involved in the same biochemical process or function are located together in groups. This group of genes, along with their regulatory elements, are collectively known as an operon. The functional genes in an operon are transcribed together to give a single strand of mRNA known as polycistronic mRNA.
Transcription of prokaryotic...

Co-activators and Co-repressors

Co-activators and Co-repressors

Gene transcription is regulated by the synergistic action of several proteins that form a complex at a gene regulatory site. This is observed in eukaryotes, where the regulation of gene expression is a complex process. Regulatory proteins in eukaryotes can broadly be classified into two types – regulators that bind directly to specific DNA sequences and co-regulators that associate with regulatory proteins but cannot directly bind to the DNA. These co-regulators are further divided into...

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 (β-galactosidase), lacY (lactose permease), and lacA (thiogalactoside...

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Related Experiment Video

Updated: May 7, 2026

Single-molecule Imaging of Gene Regulation In vivo Using Cotranslational Activation by Cleavage (CoTrAC)

Single-molecule Imaging of Gene Regulation In vivo Using Cotranslational Activation by Cleavage (CoTrAC)

Published on: March 15, 2013

Lac repressor is a transient gene-activating protein.

S B Straney¹, D M Crothers

¹Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06511.

|December 4, 1987

Summary

The lac repressor significantly enhances initial RNA polymerase binding to E. coli lac UV5 promoter DNA. This binding stabilization, contrary to prior beliefs, facilitates rapid system response upon repression release.

Area of Science:

Molecular Biology
Genetics
Biochemistry

Background:

The lac repressor is traditionally understood to inhibit gene transcription.
Its precise role in the initial stages of transcription initiation at the E. coli lac UV5 promoter is debated.

Purpose of the Study:

To investigate the effect of lac repressor on the initial binding of RNA polymerase to the E. coli lac UV5 promoter.
To elucidate the mechanism by which lac repressor influences transcription initiation.

Main Methods:

Combination of experimental methods.
Kinetic studies of transcription initiation.
Analysis of RNA polymerase-DNA binding.

Main Results:

Lac repressor increases RNA polymerase binding to the lac UV5 promoter by over 100-fold.

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Related Experiment Videos

Last Updated: May 7, 2026

Single-molecule Imaging of Gene Regulation In vivo Using Cotranslational Activation by Cleavage (CoTrAC)

Single-molecule Imaging of Gene Regulation In vivo Using Cotranslational Activation by Cleavage (CoTrAC)

Published on: March 15, 2013

In vivo Application of the REMOTE-control System for the Manipulation of Endogenous Gene Expression

In vivo Application of the REMOTE-control System for the Manipulation of Endogenous Gene Expression

Published on: March 29, 2019

Monitoring Protein-RNA Interaction Dynamics In Vivo at High Temporal Resolution Using χCRAC

Monitoring Protein-RNA Interaction Dynamics In Vivo at High Temporal Resolution Using χCRAC

Published on: May 9, 2020

The repressor blocks the isomerization step in transcription initiation.

Addition of IPTG allows open complex formation and transcription, with repressor and inducer increasing the rate of the first productive transcription round at low polymerase concentrations.

Conclusions:

Lac repressor plays a dual role: stabilizing pre-transcriptional complexes and blocking initiation.
This dual role suggests a broader regulatory model than simple repression.
The findings challenge the conventional view of lac repressor function in transcription.