Topic 10

Topic 10: Prokaryotic Regulation

Lecture Objectives:

  1. Understand operon regulation through three examples: lac regulation by allolactose, lac regulation by glucose, and trp regulation by tryptophan.

  2. Differentiate between:

    • Positive inducible regulation

    • Positive repressible regulation

    • Negative inducible regulation

    • Negative repressible regulation

  3. Explore the usage of mutation studies and partial diploid studies to understand operon regulation.

  4. Understand how prokaryotic regulation can be applied experimentally.

Big Question: Why aren't all cells the same if they have the same genome?

Topic Outline:

1) Overview of Gene Regulation (Prokaryotic and Eukaryotic):

  • Nature of Regulation:

    • mRNA and proteins are short-lived, making gene regulation dynamic.

    • Changes in gene expression can lead to phenotypic differences within/on between species.

  • Regulatory Mechanisms:

    • Gene expression is modulated by genetic "dimmer switches" responding to intra- and extra-cellular stimuli.

  • Three Key Roles:

    1. Regulatory Genes: Control expression of other genes.

      • Produce regulatory proteins or RNAs; may contain DNA-binding domains.

      • Classified as trans-acting (functioning regardless of their location).

    2. Regulatory Elements: Unexpressed sequences impacting nearby gene expression.

      • Defined as cis-acting (affecting only linked genes).

    3. Structural Genes: Encode molecules impacting cell structure; their expression influenced by regulation.

2) Prokaryotic Regulation:

  • Operons Structure:

    • Prokaryotic genes arranged in operons (expressed together).

    • Operons start transcription from a single promoter and end at a single terminator, yielding a polycistronic mRNA.

  • Key Elements:

    • Promoter: Where RNA polymerase binds to begin transcription.

    • Operator: A regulatory element controlling operon expression via regulatory protein binding.

  • Regulatory Function:

    • Binding of regulatory proteins can activate (positive control) or inhibit (negative control) gene expression.

    • Signals or substrates can induce (turn on) or repress (turn off) operon activity.

3) Example Cases:

Example #1: Lac Operon Regulation and Allolactose

  • Metabolic Preference: E. coli favors glucose; lactose needs additional enzymes for breakdown.

  • Operon Details: Lac operon structural genes (lacZ, lacY, lacA) manage lactose metabolism.

  • In Absence of Lactose: Repressor (lacI) prevents expression by binding to operator.

  • In Presence of Allolactose: Repressor loses its binding ability, allowing structural gene expression (negative inducible).

  • Constitutive Expression: Some lac operon activity is necessary for permease production for lactose entry.

Example #2: Lac Operon Regulation in Presence of Glucose

  • Signal Functions: Glucose and cAMP interplay in operon regulation.

  • High Glucose: cAMP is low, inhibiting CAP from binding to the promoter, leading to low transcription.

  • Low Glucose: cAMP is high, activating CAP to bind and enhance transcription of the lac operon (positive repressible).

Example #3: Tryptophan Regulation of the trp Operon

  • Operon Role: Tryptophan biosynthesis via five structural genes.

  • Regulatory Mechanism: Tryptophan presence triggers TrpR to bind the operator, repressing expression (negative repressible).

Example #4: Tryptophan Attenuation

  • Secondary Regulation: Attenuation terminates transcription prematurely based on tRNA availability.

  • High Tryptophan: Ribosome efficiently moves, leading to termination due to secondary structure formation.

  • Low Tryptophan: Slowed ribosome creates conditions for antitermination, enabling full operon expression.

4) Key Themes in Bacterial Regulation:

  • Operons consist of related structural genes.

  • Signal molecules facilitate cellular environmental responses, influencing gene expression actively.