Prokaryotic Gene Expression Regulation

Prokaryotic Gene Expression Regulation

Overview of Gene Expression in Prokaryotes

  • Structural Genes: Encode metabolic and structural proteins of the cell.

  • Regulatory Genes: Control the expression of structural genes and respond to metabolic and environmental cues.

Transcription Factors

  • Bind to DNA and modify RNA polymerase activity.

  • Types of Transcriptional Control:

    • Positive Control: Activators promote transcription.

    • Negative Control: Repressors inhibit transcription.

    • Combinatorial Control: Involves both activators and repressors working together.

Types of Transcription Factors

  • Motif: A structural feature that determines the function of the transcription factor.

  • Examples of Transcription Factor Motifs:

    • Helix-turn-Helix Proteins: Characterized by a helix structure that allows binding to DNA.

    • Zinc Finger Proteins: Use zinc ions to stabilize their structure for binding to DNA.

    • Leucine Zipper Proteins: Contain leucine residues that facilitate dimerization and DNA binding.

Operons

  • Definition: A group of genes that are transcribed together, producing polycistronic mRNA.

  • Components of an Operon:

    • Promoter: A single promoter regulates multiple protein-coding regions.

    • Operator: Located between the promoter and the transcribed region; binds the repressor (negative control).

    • Transcription occurs when the repressor is NOT bound to the operator.

Types of Operons
1. Inducible Operons

  • Default State: OFF (no transcription).

  • Mechanism: Repressor is bound to the operator and is functional by default.

  • Repressor: Coded by a separate regulatory gene.

  • Inducer: A metabolite that binds to the repressor and disables its function (acts as an allosteric inhibitor).

  • Example: Lac Operon:

    • Lactose, a disaccharide, is metabolized by enzymes of the lac operon.

    • The LacR repressor protein keeps the lac operon "off" when no lactose is present.

    • When lactose is present in the cell, it is converted to allolactose.

    • Allolactose binds to LacR, disabling it and allowing transcription to occur.

2. Repressible Operons

  • Default State: ON (transcription is active).

  • Mechanism: No repressor is bound to the operator; the repressor is non-functional when synthesized.

  • Co-repressor: A metabolite that binds to the repressor, activating it (acts as an allosteric activator).

  • Example: Trp Operon:

    • Tryptophan is an amino acid synthesized by the enzymes of the trp operon.

    • The TrpR repressor is produced but is non-functional initially, allowing for transcription.

Mechanism of Repressible Operons

  • Activation: When exogenous tryptophan is available in high concentrations, it binds to the TrpR repressor, activating it by changing its conformation.

  • Binding: The activated TrpR (TrpR + Tryptophan) binds to the operator, halting transcription.

Positive Control in Operons

  • Operons can be influenced by transcriptional activators that enhance transcription.

  • Lac Operon and Positive Control:

    • When the lac operon is induced, RNA polymerase needs assistance to effectively interact with the promoter.

    • CAP Protein: A transcription factor requiring cAMP for activation.

    • The binding of CAP + cAMP results in a sharp bend in the DNA near the promoter, increasing accessibility for RNA polymerase.

    • Transcription of the lac operon is enabled when both lactose is present and CAP is active.

Complex Regulation of Metabolism

  • Scenario Analysis:

    • If only glucose is high, cells will preferentially metabolize glucose.

    • If only lactose is high, cells will metabolize lactose.

    • If both glucose and lactose are high, cells will again metabolize glucose preferentially (high glucose results in low cAMP, disabling CAP).

Attenuation

  • Attenuation provides an additional regulatory mechanism even when the repressor is active; transcription can still occur in small amounts due to dynamic interactions.

  • Concept of Leakage: Small amounts of transcript may still be synthesized in the presence of a repressor.

Example: Attenuation in the Trp Operon

  • Leader Peptide: The trp transcript includes a leader peptide sequence rich in tryptophan.

    • The recruitment of ribosomes initiates translation of this leader peptide.

  • Low Tryptophan Scenario:

    • When tryptophan is limited, the ribosome stalls at the tryptophan-rich region.

    • During stalling, the ribosome covers Region 1 of the transcript, allowing Regions 2 and 3 to hydrogen bond, forming a weak hairpin structure known as an anti-terminator.

    • The anti-terminator allows transcription to continue despite the presence of the repressor.

  • High Tryptophan Scenario:

    • When tryptophan is abundant, the ribosome does not stall and continues through the leader peptide region.

    • The ribosome covers Region 2, leading Regions 3 and 4 to form a strong hairpin structure that acts as a transcription terminator, halting transcription of the trp operon.