7.2 Transcriptional regulation in prokaryotes: lac and trp operons

Prokaryotes use operons to regulate gene expression efficiently. The lac and trp operons are prime examples, showcasing how bacteria adapt to changing environments by controlling enzyme production for lactose metabolism and tryptophan synthesis.

These operons demonstrate negative regulation through repressor proteins and DNA binding sites. They also highlight inducible versus repressible systems, illustrating how bacteria fine-tune gene expression based on nutrient availability and metabolic needs.

Lac Operon Structure and Function

Components and Regulatory Mechanisms

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• Lac operon consists of cluster of genes in E. coli responsible for lactose metabolism
  • Three structural genes (lacZ, lacY, lacA) encode enzymes for lactose breakdown
  • Regulatory elements include promoter, operator, and regulator
• LacI gene located upstream of operon encodes lac repressor protein
  • Binds to operator site when lactose absent
• Promoter region serves as RNA polymerase binding site
• Operator site overlaps promoter
  • Acts as binding site for lac repressor

Lactose-Dependent Regulation

• Lac repressor binds operator in lactose absence
  • Prevents transcription of structural genes
• Lactose presence triggers repressor release
  • Lactose binds repressor causing conformational change
  • Allows transcription to proceed
• Positive regulation occurs via catabolite activator protein (CAP)
  • CAP binds promoter when cAMP levels high
  • Enhances RNA polymerase binding and transcription
• Dual control allows fine-tuned response to glucose and lactose levels
  • Ensures efficient use of available carbon sources

Trp Operon in Tryptophan Biosynthesis

Operon Structure and Components

• Trp operon in E. coli controls tryptophan biosynthesis
  • Five structural genes (trpE, trpD, trpC, trpB, trpA) encode biosynthetic enzymes
  • Regulatory elements include operator and leader sequence
• TrpR gene located elsewhere on chromosome
  • Encodes trp repressor protein regulating operon activity
• Leader sequence contains regulatory features
  • Attenuator regions allow premature transcription termination
  • Pause sites for RNA polymerase enable regulation

Tryptophan-Dependent Regulation

• Trp repressor binds operator when excess tryptophan present
  • Inhibits transcription of structural genes
• Attenuation mechanism provides additional regulation
  • Premature transcription termination based on tryptophan availability
• Low tryptophan levels cause ribosome stalling
  • Stalling occurs at specific codons in leader sequence
  • Allows formation of antiterminator structure
  • Permits continued transcription of structural genes
• Dual regulation ensures precise control of tryptophan production
  • Prevents wasteful synthesis when tryptophan abundant

Inducible vs Repressible Operons

Characteristics and Functions

• Inducible operons typically involved in catabolic pathways
  • Activated by presence of specific substrate (lactose in lac operon)
• Repressible operons usually involved in anabolic pathways
  • Deactivated by presence of pathway end product (tryptophan in trp operon)
• Both types demonstrate negative regulation
  • Regulatory protein binding to operator inhibits transcription

Regulatory Mechanisms

• Inducible operons regulated by repressor proteins
  • Repressors dissociate from operator when inducer molecule present
  • Dissociation allows gene expression to occur
• Repressible operons controlled by repressor proteins
  • Repressors bind operator when corepressor molecule present
  • Binding inhibits gene expression
• Differential responses allow efficient resource allocation
  • Bacteria can adapt to changing environmental conditions
  • Prevents unnecessary enzyme production

Mutations and Gene Expression

Operator and Promoter Mutations

• Operator site mutations affect repressor protein binding affinity
  • Can lead to constitutive expression or reduced regulation
  • Example: mutation preventing lac repressor binding causes constant lactose metabolism enzyme production
• Promoter sequence alterations impact RNA polymerase binding
  • May change basal transcription level or regulatory protein response
  • Example: mutation strengthening RNA polymerase binding site increases basal expression of operon

Regulatory Protein Mutations

• Repressor gene mutations produce altered repressor proteins
  • Non-functional repressors lead to constitutive gene expression
  • Hyperactive repressors cause excessive gene repression
• Regulatory protein coding sequence changes alter DNA binding or interactions
  • Example: CAP protein mutation reducing cAMP binding affinity impairs glucose-dependent regulation

Structural Mutations Affecting Regulation

• Leader sequence or attenuator region mutations disrupt attenuation
  • Can lead to overproduction of gene products
  • Example: trp operon mutation preventing formation of terminator structure causes tryptophan overproduction
• Insertions or deletions in spacer regions affect regulatory element spacing
  • Alters spatial relationships necessary for proper regulation
  • Example: insertion between lac operator and promoter prevents repressor from blocking RNA polymerase
• Analysis of these mutations provides insights into regulatory element functions
  • Helps understand control of gene expression in prokaryotes