Control of Gene Expression Notes

Chapter 16: Control of Gene Expression

16.1 Overview of Gene Expression Control

  • Prokaryotic Regulation: Simple and rapid, responding dynamically to environmental changes.
  • Eukaryotic Regulation: Complex, allowing for cellular specialization and maintenance of homeostasis.
  • Key Mechanism: Control largely occurs at the transcription initiation level.

16.2 Regulatory Proteins

  • Role of Regulatory Proteins: They bind to specific DNA sequences to control gene expression.
    • Repressors: Prevent transcription initiation (e.g., block RNA polymerase binding).
    • Activators: Enhance transcription by promoting RNA polymerase binding.
  • DNA-Binding Motifs: Structural features that allow regulatory proteins to recognize and interact with specific DNA regions.
    • Examples of Motifs:
    • Helix-Turn-Helix: Two alpha-helices connected by a nonhelical segment.
    • Zinc Finger: Uses zinc ions to stabilize binding segments.
    • Leucine Zipper: Dimerization structure forming a "zipper" that facilitates protein interaction.

16.3 Prokaryotic Regulation

  • Operon Model: A cluster of genes regulated together by an operator; controls transcription in response to environmental signals.
    • Example: Trp Operon
    • Low Tryptophan: Operon is ON (genes transcribed).
    • High Tryptophan: Tryptophan binds to the repressor, turning the operon OFF.
  • Types of Control:
    • Negative Control: Repressors decrease transcription frequency.
    • Positive Control: Activators increase transcription frequency (e.g., CAP in lac operon).

16.4 Eukaryotic Regulation

  • Complex Control: Involves chromatin structure and is influenced by multiple transcription factors.
  • Transcription Factors (TF): General TFs are required for RNA polymerase binding, while specific TFs regulate higher transcription levels in specific tissues.
    • Enhancers: Regulatory DNA sequences that increase transcription efficiency by binding specific TFs.
    • Silencers: Decrease transcription by binding repressor-specific TFs.

16.5 Chromatin Structure Affects Gene Expression

  • Chromatin Modifications: Impact access to DNA by the transcription machinery.
    • DNA Methylation: Generally correlates with repression of gene expression.
    • Histone Acetylation: Associated with active transcription; removes positive charges on histones, thus loosening DNA winding.

16.6 Eukaryotic Post-Transcriptional Regulation

  • Controls gene expression after transcription via:
    • Small RNAs: Including miRNA and siRNA, play crucial roles in the regulation of mRNA.
    • Alternative Splicing: Produces different mRNA variants from the same gene,
    • Example: Calcitonin vs. CGRP.
    • RNA Editing: Alters mRNA to create different proteins from the same gene (e.g., serotonin receptor isoforms).

16.7 Protein Degradation

  • Continuous Process: Proteins are regularly synthesized and degraded, crucial for cellular regulation.
  • Ubiquitin-Proteasome Pathway: Specific proteins tagged with ubiquitin are directed for degradation within a proteasome.
  • Lysosomes: Contain proteases for non-specific protein digestion.

Summary of Gene Expression Control Mechanisms

  1. Transcription Initiation: Controlled predominantly via regulatory proteins.
  2. RNA Splicing: Adjusts the final mRNA product.
  3. Nuclear Transport: Regulates mRNA exit and translation efficiency.
  4. Translation Regulation: Impacted by available resources and specific proteins.
  5. RNA Interference: Utilizes miRNA and siRNA for mRNA stability and translation control.
  6. Proteolytic Control: Degradation of proteins regulates levels and timing of function within the cell.