Prokaryotic Gene Regulation Study Notes

CH 10 Part 1 Learning Objectives

• Levels of Control of Gene Expression in Prokaryotes
    - Understand the various mechanisms that regulate gene expression in prokaryotic organisms.

• Induction vs. Repression
    - Differentiate between these two fundamental mechanisms of gene regulation.

• Role of Transcriptional Activators
    - Define the importance of transcriptional activators in regulating gene expression.

• Lactose Operon Mechanisms
    - Explain the mechanisms controlling the lactose operon along with its components.

• Tryptophan Operon Mechanisms
    - Explain the mechanisms controlling the tryptophan operon along with its components.

Gene Expression Adjustments

• When does a cell alter gene expression?
    - Cells adapt gene expression in response to changes in the external environment, which includes:
      - Temperature
      - pH
      - O₂ levels
      - Nutrients
      - Solute concentrations

• Sensor Proteins
    - Detect external signals that trigger internal responses leading to changes in gene expression.

• Internal Signals
    - Internal functions respond to external signals, influencing gene expression.

Control of Gene Expression Levels

• Multiple Levels of Gene Expression Control
    - DNA Level:
      - Modifications such as methylation of nucleotides.
      - Rearrangement of DNA sequences to alter expression.
      - Phase variation mechanisms.
    - Transcription Level:
      - Factors affecting the function of RNA polymerase.
    - mRNA Stability:
      - Half-life of mRNA can influence expression.
    - Translational Level:
      - Ribosome activity can be altered.
    - Post-translational Level:
      - Modifications or degradation of proteins can affect their function.

• Constitutive Genes:
    - These genes are always expressed regardless of external conditions.

Regulatory Proteins

• Types of Regulatory Proteins:
    - Repressors:
      - Stop gene expression (OFF state) when active.
      - Can be inactive, thus not blocking transcription.
    - Activators:
      - Promote gene expression (ON state) when bound to their ligands.
      - Enhance transcription rates compared to basal levels.

• Induction and Repression Mechanisms:
    - Induction:
      - Results in the initiation of gene expression.
    - Repression:
      - Halting gene expression, usually activated by a corepressor.
    - Active vs. Inactive Repressor States:
      - Active repressors block transcription, while inactive do not.

Control of the Lactose Operon (lacZYA)

• Key Components:
    - Regulatory Gene: lacI
      - Codes for the repressor, which interacts with operators involved in the operon.

• Transcription Activity:
    - A low level of transcription occurs even in the absence of lactose.
      - Low levels of β-galactosidase (LacZ) convert lactose into allolactose (the inducer).
      - Permease (LacY):
        - Responsible for transporting lactose into the cell.

Lactose Transport & Catabolism

• Mechanism of Action:
    - β-Galactosidase (LacZ):
      - Converts lactose to allolactose which binds the repressor.
    - LacY's Role:
      - Functions as a lactose detector.
      - Facilitates the transport of lactose across the cell membrane.

Lactose Operon Control Mechanism

• Induction Vs Repression:
    - Absence of Lactose:
      - Active repressor prevents transcription.
    - Presence of Lactose (Allolactose):
      - Induces transcription by inactivating the repressor.
      - RNA polymerase can bind and initiate transcription.

• Role of cAMP:
    - cAMP and CRP (cAMP Regulatory Protein):
      - The complex enhances transcription of the lac operon by binding to its promoter.
    - Cellular cAMP Levels:
      - Fluctuate based on energy availability and substrates present.
      - Glucose presence leads to catabolite repression, inhibiting the lactose operon.
      - Preference for glucose over lactose results in diauxic growth patterns.

Tryptophan Operon Control

• Essential Components:
    - Regulatory Gene: trpR
      - Encodes the repressor protein that inhibits transcription.
    - Co-repressor:
      - Tryptophan acts as a co-repressor, enhancing the repressor's activity.

• Mechanism of Action:
    - When tryptophan is present, it binds to the aporepressor to form the active holorepressor, blocking transcription.
    - In low tryptophan conditions, derepression occurs, allowing transcription to proceed.

Effects of Growth Conditions on Operons

• Experiment with E. coli (lac+/trp+):
    - Growth in minimal medium with both lactose and tryptophan, excluding glucose.

• Operon Expression Status:
    - lac Operon: Expressed
    - trp Operon: Not expressed

• Repressor Status:
    - lac Repressor: Inactive
    - trp Repressor: Active

• cAMP Levels:
    - High levels of cAMP, with cAMP-CRP complex formation, promote lac operon activity.

Summary of Gene Regulation in Prokaryotes

• Key Factors Influencing Gene Expression:
    - External and cytoplasmic conditions determine gene expression levels via sensing and regulatory proteins.

• Levels of Control
    - Operon model as fundamental to prokaryotic gene organization.

• Regulatory Proteins:
    - Include repressors, inducers, co-repressors, and activators, crucial for regulation.

• Operons Characteristics:
    - Lactose operon is termed inducible and controlled by cAMP during catabolite repression.
    - Tryptophan operon is repressible, with tryptophan acting as a co-repressor.