Molecular Regulation Chapter 10

Chapter 10: Molecular Regulation

• Overview:
  • Introduction to gene regulation
  • Transcription repressors and activators
  • The lactose operon

Introduction

• A bacterial genome encodes many different proteins
• Cells do not express all genes at maximum levels, this is due to:
  • Physical space limitations
  • Energy and resource conservation
  • Contradictory functions

Mechanisms of Gene Regulation

• Microbes utilize various mechanisms to sense internal/external environments and synthesize specific proteins
• Gene regulation occurs at multiple levels including:
  • Alteration of DNA sequence
  • Control of transcription
  • Control of mRNA stability
  • Translational control
  • Post-translational control

Transcription Repressors and Activators

• Transcription Initiation: Major control site in bacteria, often controlled by regulatory proteins.
• Regulatory Proteins:
  • Bind to DNA at or near gene promoters
  • Stimulate or inhibit RNA polymerase binding to promoter
  • Interact with DNA's major groove, often forming dimers
• DNA Targets:
  • Exhibit symmetry (inverted repeats)
  • Regulatory proteins often bind as dimers (each monomer to one repeat sequence)
• Mechanisms:
  • Intracellular changes detected by regulatory proteins binding specific low-molecular-weight compounds (ligands)
  • Genes encoding regulatory proteins are transcribed separately from target genes

Sensing the Extracellular Environment

• Two-Component Signal Transduction Systems:
  • Sensor Kinase: Membrane-bound enzyme, binds environmental signals and transfers a phosphate group from ATP to a target protein
    • Sensory domain interacts with the environment, kinase domain inside the cell
  • Response Regulator: Cytosolic protein, activated by sensor kinases binding to DNA, affecting gene expression
    • Can either stimulate or repress gene expression
    • Down-regulated by phosphatases removing the phosphate group

The Lactose Operon

• Historical Context:
  • 1961: Monod and Jacob proposed gene regulation in E. coli
  • 1965 Nobel Prize for gene regulation work
• Lactose Metabolism:
  • Disaccharide (glucose + galactose), carbon and energy source
• Requires:
  • Lactose permease (LacY): Transports lactose into the cell
  • β-Galactosidase (LacZ): Breaks down lactose into monosaccharides or rearranges it to allolactose

Regulation of the lac Operon

• Operon Structure:
  • lacZ, lacY, lacA genes form an operon (regulated by a single promoter (PlacZYA) and operator sequences (lacO))
  • Lactose repressor (LacI) is encoded by a regulatory gene upstream of the operon and constantly expressed

Lactose Presence

• Mechanism:
  • In absence of lactose, LacI binds operators (lacO and lacOI), preventing RNA polymerase from initiating transcription.
  • In presence of lactose, allolactose binds LacI, decreasing its affinity for the operator, allowing RNA polymerase to initiate transcription of lacZYA genes.
• cAMP and CRP
  • Mechanism:
    • cAMP accumulates when cell is starving for carbon and combines with cAMP receptor protein (CRP)
  • Maximum lactose operon expression requires removing LacI and binding of cAMP-CRP complex to DNA sequence upstream of transcription start site
    • cAMP-CRP complex activates RNA polymerase.
• Mechanism:
  • Catabolite Repression: Catabolism of one nutrient (e.g., glucose) represses operon expression in favour of a more favourable nutrient (eg glucose)
• Diauxic Growth
  • Biphasic growth curve of a culture growing on two carbon sources.
• Inducer Exclusion
  • Glucose transport (phosphotransferase system (PTS)) inhibits LacY permease
  • Prevents lactose from entering the cell and inducing the lac operon
  • In absence of glucose, phosphorylated forms of glucose-specific IIAGlc and IIBGlc accumulate, allowing LacY to transport lactose and induce lac operon.

Explore the fundamental concepts of gene regulation in this quiz based on Chapter 10. This chapter covers transcription repressors, activators, and the lactose operon, detailing how microbes adapt their gene expression. Test your understanding of the mechanisms that control gene expression levels at various stages.