Gene Expression Control in Prokaryotes

Chapter 16 Control of Gene Expression in Prokaryotes

Key Concepts

  • Regulatory Protein: Proteins that regulate the expression of genes by binding to specific DNA sequences.

  • Regulatory Element: A DNA sequence that influences gene expression but is not transcribed.

  • Inducible Operon: An operon that is normally off but can be turned on (e.g., Lac operon).

  • Repressible Operon: An operon that is normally on but can be turned off.

  • Lac Operon: A well-studied example of an inducible operon that regulates lactose metabolism in E. coli.

  • Catabolite Repression: A phenomenon where the presence of glucose represses the genes required to metabolize other sugars, including lactose.

Levels of Gene Control

  • Gene expression can be controlled at multiple levels, including:

    • Transcription Control: Regulation during the transcription of DNA to mRNA.

    • RNA Processing: Modifications of mRNA before translation.

    • Translation Control: Regulation of the translation process that affects the synthesis of proteins.

    • Posttranslational Modification: Changes to the protein after synthesis, which may influence its activity.

Structural and Regulatory Genes

  • Structural Gene: Encodes proteins or RNAs with cellular function.

  • Constitutive Genes: Constantly expressed for essential functions.

  • Regulatory Gene: Encodes proteins or products affecting other gene sequences.

Mechanisms of Gene Regulation

  • Gene regulation can be positive (stimulates expression) or negative (inhibits expression).

  • Trans Factors: Regulatory proteins affecting gene transcription.

  • Cis Elements: Non-coding DNA sequences that regulate neighboring gene expression.

  • Regulatory DNA-binding proteins usually consist of functional domains that interact with DNA.

Operon Model

  • Operon: A cluster of related genes transcribed together under a single promoter.

    • Example: Lac Operon.

  • Components of an Operon:

    • Promoter: Initiates transcription.

    • Operator: Sequence where regulatory proteins bind.

    • Structural Genes: Genes that are regulated by the operon.

Types of Operons

  • Inducible Operon:

    • Transcription is usually off but can be turned on by an inducer.

    • Negative Inducible Operon: Repressor blocks transcription until induced.

    • Inducer: Molecule that inactivates the repressor, allowing transcription to proceed.

  • Repressible Operon:

    • Transcription is typically on but can be turned off.

    • Negative Repressible Operon: Requires a corepressor to activate the repressor to block transcription.

Lac Operon Overview

  • Jacob, Monod, and L’Woff described the Lac Operon. It includes structural genes for:

    • LacZ: Encodes β-galactosidase (breaks down lactose).

    • LacY: Encodes permease (transport protein for lactose).

    • LacA: Encodes transacetylase (role in detoxifying compounds).

  • Without Lactose:

    • Repressor binds to operator, blocking transcription.

  • With Lactose:

    • Lactose (or allolactose) binds to the repressor, causing it to change shape and release from the operator, allowing transcription.

Catabolite Repression

  • Positive Control:

    • Low glucose levels stimulate lac operon expression through the CAP-cAMP complex.

    • CAP binds to the lac promoter enhancing RNA polymerase binding when cAMP is present.

  • Inverse Concentration Relationship:

    • Higher glucose means lower cAMP and less expression of the lac operon.

Conclusions

  • Understanding gene regulation mechanisms, including the Lac operon model, is crucial in microbiology and genetics.

  • Regulation can be both negative and positive, influencing metabolic pathways based on environmental availability of substrates.

Homework Questions

  • Key questions related to chapter concepts, particularly on gene regulation mechanisms.