Notes on Gene Regulation

Gene Regulation Overview

  • Definition of Gene Regulation:

    • The ability of cells to control their level of gene expression.

    • Essential for maintaining proper cellular functions and responses to environmental cues.

  • Types of Genes:

    • Structural Genes:

    • Regulated to ensure that proteins are produced at specific times and in specific quantities.

    • Constitutive Genes:

    • Unregulated, exhibiting essentially constant levels of expression across different conditions.

    • Housekeeping Genes:

    • Required for basic cellular functions, typically expressed in all types of cells.

Mechanisms of Gene Regulation

  1. Operon

  2. Histone Modification

  3. DNA Methylation

  4. Epigenetic Inheritance

  5. Transcription Factors

  6. Small RNA Regulation

Gene Regulation in Prokaryotes

  • Response to Environment:

    • Gene regulation is activated in response to environmental changes.

  • Example:

    • Escherichia coli and Lactose:

    • Involves repressors that inhibit transcription and activators that enhance transcription.

  • Small Effector Molecules:

    • Critical in the regulation through binding to transcription factors.

    • Two domains in regulatory transcription factors:

    1. Site for binding to DNA.

    2. Site for binding small effector molecules.

    • Binding of the small effector molecules induces a conformational change that influences the transcription factor's ability to bind to DNA.

Operons

  • Definition:

    • A set of genes controlled by a single promoter.

    • Advantage:

    • Enables coordinated control of functionally related genes via a single “on-off switch.”

    • Examples:

    • Lac Operon, Trp Operon.

Lac Operon

  • In Escherichia coli:

    • Contains genes necessary for lactose metabolism.

    • Structural Genes:

    • lacZ, lacY, and lacA encode enzymes β-galactosidase, β-galactoside permease, and β-galactoside transacetylase, respectively.

    • Function:

    • β-galactoside permease transports lactose into the cell.

    • β-galactosidase cleaves lactose into glucose and galactose.

  • Mechanism of Regulation:

    • The lac operon is a negatively controlled inducible operon.

    • Presence of Lactose:

    • The operon is expressed only when lactose is present and glucose is absent.

    • Repressor:

      • In the absence of lactose, the repressor binds to the operators, preventing RNA polymerase from transcribing the lac genes.

      • Inducer, allolactose (derived from lactose), binds to the repressor, causing its release and allowing transcription of lacZ, lacY, and lacA.

  • Lactose Availability:

    1. Without Lactose:

    • LacI gene coding for repressor is constitutively expressed.

    • The lac repressor binds to the operator, preventing transcription.

    • RNA polymerase can bind but cannot initiate transcription.

    1. With Lactose:

    • Lactose is converted to allolactose, which binds the repressor, allowing transcription of lac operon for lactose breakdown into glucose.

Regulatory Mechanisms in the Lac Operon

  • Catabolite Repression:

    • When lactose is present, the lac repressor is inactive.

    • RNA polymerase alone exhibits low transcription efficiency at the lac operon promoter.

    • Catabolite Activator Protein (CAP):

    • An activator protein that binds upstream of the lac operon promoter, facilitating the binding of RNA polymerase and enhancing transcription.

    • Regulation by Cyclic AMP (cAMP):

    • cAMP, produced when glucose levels are low, binds to CAP, activating it to enhance transcription at the lac operon promoter.

  • Transcription Activation Conditions:

    • When lactose is high and glucose is low:

    • Allolactose levels rise, preventing repressor binding to the operator.

    • cAMP activates CAP to promote transcription.

    • When both lactose and glucose are high:

    • The lac operon shuts off due to decreased cAMP levels from glucose uptake.

    • When lactose is low and glucose is high or low:

    • The operon is also shut off as the lac repressor prevents transcription.

Trp Operon

  • E. coli Tryptophan Synthesis:

    • Encodes enzymes for tryptophan production.

    • Transcription is switched "off" when tryptophan is present and "on" when tryptophan levels are low.

    • Composed of five genes for tryptophan biosynthesis along with promoter and operator sites.

  • Regulation by Repressor Protein:

    • Encoded by trpR gene, which binds to the operator to inhibit transcription when tryptophan is abundant.

    • Tryptophan acts as a corepressor, inducing conformational changes in the repressor, allowing it to bind to the operator site.

    • When tryptophan levels drop, the repressor's binding is inhibited, allowing transcription to proceed.