Control of Gene Expression

Control of Gene Expression

• A cell only expresses some of its genes at any given time
• Gene expression is selective in how strongly genes are expressed and when
• Regulation of gene expression ensures efficient use of resources by organisms
• Gene expression occurs when a gene product, such as a protein, is required, and actively used in a cell
• Gene expression can be controlled at 3 levels: transcription, translation, and post-translation
  • Transcription has an energetic advantage, but is slower
• Transcriptional control is either negative or positive

Negative & Positive Regulation

• Transcription is regulated via negative or positive control
• Negative control occurs when a regulatory protein binds to DNA to repress transcription
• Positive control occurs when a regulatory protein binds to DNA to activate transcription

Activators and Repressors

• DNA-protein interactions occur at the operator, a regulatory region close to the promoter
• Activator and repressor proteins have both a DNA binding site and an allosteric site
• Allosteric effectors bind to the allosteric site, influencing the regulatory protein's activity

Metabolising Lactose - A Model System

• The lac operon consists of the lacI gene, CAP site, and control region

Diauxic Growth

• E. coli prefers glucose as a carbon source and only uses lactose once glucose is depleted

lac Operon Regulation

• The lac operon is transcriptionally regulated, responding to differing conditions via positive or negative regulation
• The lac operon has evolved for maximal efficiency in two environmental conditions
  • Presence of lactose
  • Absence of glucose
• Cell recognition that lactose is present is accomplished by a repressor protein
• Catabolite repression is when the breakdown product of lactose (glucose) prevents the activation of the lac operon
• Catabolite repression functions through the action of an activator protein

lac Structural Genes

• lacZ encodes β-Galactosidase, which breaks down lactose into glucose and galactose
• lacY encodes a membrane permease that helps transport lactose
• lacA encodes a transacetylase that is unessential for lactose metabolism

lac Regulatory Components

• lacI encodes the Lac repressor, which can block the expression of the lacZYA genes
• The lac promoter site is the DNA region where RNA polymerase binds to start transcription
• The lac operator site is the DNA region where the Lac repressor binds

Induction of the lac System

• The Lac repressor protein has a DNA binding site to interact with the operator, and an allosteric site that binds lactose
• Normally, the Lac repressor binds the operator, preventing transcription from the promoter by RNA polymerase
• When lactose is present, it binds to the allosteric site, causing an allosteric transition (shape change)
• The repressor detaches from the operator when lactose is present
• This process, called induction, is an example of negative control

Positive Regulation of lac Operon

• When glucose is present, the expression of the lac operon will be repressed via catabolite repression
• Glucose levels regulate the levels of cyclic adenosine monophosphate (cAMP)
• In the absence of glucose, adenylate cyclase activity converts ATP to cAMP
• In the presence of glucose, adenylate cyclase is inhibited, and no cAMP is produced
• cAMP complexes with CAP (catabolite activator protein) to activate transcription by binding to the CAP site in the lac promoter

Terms

• The lacI gene produces the Lac repressor protein
• The lac promoter site is the DNA where RNA polymerase starts transcription of lacZYA
• The lac operator site is the DNA where the Lac repressor binds
• Allolactose, a lactose derivative, binds to the Lac repressor to induce transcription
• CAP, a catabolite activator protein, requires cAMP binding to activate transcription
• 'Inducible' genes all function under basic principles with fine control, but with significant variation