Ch 10 Pt 1

Chapter 10 Part 1: Gene Regulation in Prokaryotes

Learning Objectives

• List and describe the levels of control of gene expression in prokaryotes

• Differentiate between induction and repression

• Define the role of transcriptional activators in gene expression

• Explain the mechanisms of control of the lactose operon and the components involved

• Explain the mechanisms of control of the tryptophan operon and the components involved

Conditions Leading to Altered Gene Expression

• A cell can alter its gene expression based on external and internal signals such as:

  • External Environment Factors:

  • Temperature

  • pH

  • O2 levels

  • Nutrients

  • Solute concentrations

  • Internal Signals:

  • Sensor proteins detect external signals leading to adjustments in gene expression.

Levels of Control of Gene Expression

• Control can occur at multiple levels:

  • DNA Level:

  • Modification of nucleotides (e.g. methylation).

  • Rearrangement of DNA sequences to alter expression (phase variation).

  • Transcription Level:

  • Regulation of RNA polymerase activity.

  • mRNA Stability:

  • Influences half-life of mRNA.

  • Translational Level:

  • Affects ribosome function in protein synthesis.

  • Post-translational Level:

  • Affects protein function through modification or degradation.

• Constitutive Genes:

  • Genes that are always expressed and not subject to regulation.

Regulatory Proteins in Gene Expression

• Types of Regulatory Proteins:

  • Repressors:

  • Inhibit gene expression.

  • Activators:

  • Promote gene expression.

• Mechanisms:

  • Induction: Gene expression is initiated (turned ON).

  • Repression: Gene expression is halted (turned OFF).

• Co-regulator Mechanisms:

  • Corepressor: A molecule that binds to a repressor, leading to gene repression.

  • Derepression: The process of turning gene expression ON after it has been repressed.

Transcriptional Activators

• Function of Transcriptional Activators:

  • Form an Activator-Ligand Complex which increases transcription.

  • Supports a higher level of gene expression.

  • Distinction between low basal expression and high expression due to the presence of activators.

Control of the Lactose Operon (lacZYA)

• Components:

  • Regulatory Gene: lacI (codes for the repressor that interacts with operators).

• Transcription Levels:

  • A low level of lacZYA transcription always occurs, leading to:

  • Low levels of β-galactosidase (LacZ):

    • Converts lactose to allolactose (inducer).

  • Permease (LacY):

    • Transports lactose into the cell.

• Lactose Transport and Catabolism:

  • Presence of LacY:

  • Detects lactose and transports it into the cell.

  • Conversion of Lactose by LacZ:

  • Produces allolactose which binds the repressor, allowing transcription.

• Repression in Absence of Lactose:

  • When lactose is not present, the lac repressor is active, blocking transcription of the lac operon.

Mechanisms Explaining the Lactose Operon Control

• cAMP and CRP Interaction:

  • cAMP binds to the cAMP regulatory protein (CRP), forming a complex that binds to lac promoter, enhancing transcription.

  • Cellular cAMP levels fluctuate based on energy:

  • High levels of glucose present: Repression of the lactose operon (catabolite repression).

  • Diauxic growth: Observed when both glucose and lactose are present; growth occurs in two phases due to preference for glucose.

• Inducer Exclusion by Glucose:

  • Presence of glucose inhibits lactose transport, blocking access to the operon unless glucose is absent.

Control of the Tryptophan Operon

• Function: Regulates genes responsible for tryptophan biosynthesis.

• Key Components:

  • Repressor (TrpR) and Co-repressor (tryptophan).

• Mechanism of Action:

  • Active holorepressor forms when tryptophan is present, which blocks transcription of the Trp operon.

  • When tryptophan is low, derepression occurs, allowing transcription to proceed.

Practical Application Scenario

• Case Study with E. coli (lac+/trp+) in Minimal Medium:

  • Conditions: Grown in minimal medium containing both lactose and tryptophan; no glucose is present.

  • Regarding Operon Expression:

  • Lac operon: Expressed (YES)

  • Trp operon: Not Expressed (NO)

  • State of Repressors:

  • Lac operon repressor: Inactive (INACTIVE)

  • Trp operon repressor: Active (ACTIVE)

  • cAMP Levels:

  • Lac operon: High (HIGH)

  • Trp operon: Not applicable (N/A)

Summary of Chapter 10 Part 1

• Gene expression is influenced by both cytoplasmic and external conditions, governed by sensing proteins and regulatory proteins.

• Different levels of gene expression control are evident.

• The operon model describes prokaryotic gene organization.

• Regulatory proteins play key roles: repressors (inducers and co-repressors) and activators, allowing both inducible and repressible operons.

• The lactose operon is inducible with cAMP-CRP control leading to catabolite repression.

• The tryptophan operon is repressible, regulated by a co-repressor.