Control of Gene Expression

Control of Gene Expression

• Cells do not express all genes at all times.
• Gene expression is selective based on how strongly a gene is expressed and when it's expressed.
• Gene expression regulation is vital for efficient resource utilization and survival.
• Gene expression occurs only when a protein is actively needed and used.

Levels of Gene Expression

• Gene expression is crucial in living systems, enabling bacterial cells to respond to environmental changes.
• Gene expression can be controlled at three levels within cells:
  • Transcription
  • Translation
  • Post-translation (protein activation)
• Transcriptional control can be either negative or positive.

Negative and Positive Regulation

• Transcription is regulated via negative or positive control.
• Negative control: A regulatory protein binds to DNA and represses transcription.
• Positive control: A regulatory protein binds to DNA and activates transcription.

Activators and Repressors

• A DNA-protein interactions at a regulatory region is called the operator, and is close to the promoter.
• Activator and repressor proteins have a DNA-binding site and an allosteric site.
• Allosteric effectors bind to the allosteric site and influence the activity of the regulatory protein.

Metabolising Lactose

• The lac Operon is a model system for metabolising lactose.

Diauxic Growth

• E. coli prefers glucose but uses lactose when glucose is depleted.
• Catabolite repression occurs.

lac Operon Regulation

• The lac operon is transcriptionally regulated and responds to environmental conditions.
• It is subject to positive or negative regulation in response to different environmental conditions.

Structural Genes

• lacZ encodes β-Galactosidase, which cleaves lactose into glucose and galactose.
• lacY encodes a membrane permease that enables lactose transport.
• lacA encodes transacetylase, not required for lactose metabolism.

Regulatory Components

• lacI encodes the Lac repressor, blocking expression of lacZYA genes.
• The lac promoter site is where RNA polymerase binds to initiate transcription.
• The lac operator site is where the Lac repressor binds.

Control of the lac Operon

• The lac system operates efficiently under two conditions: presence of lactose and absence of glucose.
• Cell recognition of lactose presence is mediated by a repressor protein.
• Catabolite repression prevents activation of the lac operon via lactose breakdown products.
• Catabolite repression functions through the action of an activator protein.

Induction of the lac System

• The Lac repressor has a DNA-binding site for the operator and an allosteric site for lactose binding.
• Normally, the Lac repressor binds to the operator, preventing transcription by RNA polymerase.
• When lactose is present, it binds to the allosteric site, inducing a shape change in the repressor.
• The repressor detaches from the operator in the presence of lactose.
• This process, termed induction, exemplifies negative control.

Positive Regulation of lac Operon

• Glucose presence represses lac operon expression via catabolite repression.
• Glucose levels regulate cyclic adenosine monophosphate (cAMP) levels.
• When glucose is absent, adenylate cyclase actively converts ATP to cAMP.
• When glucose is present, adenylate cyclase is inhibited, and no cAMP is produced.
• cAMP complexes with catabolite activator protein (CAP) binds CAP site in lac promoter to activate transcription.

Positive Control

• When glucose is present, cAMP concentration is low.

cAMP Activation

• As glucose concentration decreases, cAMP concentration increases.
• High cAMP levels are necessary for lac operon activation.
• CAP (catabolite activator protein) is encoded by the crp gene.
• CAP binds to a specific DNA region known as the CAP site.
• CAP requires allosteric effector cAMP to bind the CAP site.
• CAP physically interacts with RNA polymerase to enhance the enzyme's affinity for the lac promoter.

Glucose Availability

• When glucose is present, the function of adenylate cyclase is inhibited,
• Insufficient cAMP/CAP fails to attract RNA polymerase, leading to inefficient transcription.

Terms

• lacI encodes the Lac repressor protein.
• The lac promoter site is the DNA location where RNA polymerase initiates lacZYA transcription.
• The lac operator site is the DNA location where the Lac repressor binds.
• Allolactose is a lactose derivative that binds to the Lac repressor to induce transcription.
• CAP (catabolite activator protein) requires cAMP binding and activates transcription.
• 'Inducible' genes function in a controlled manner with variations on this model system.