Regulation of gene expression

Goals

1. To understand how different kinds of metabolic pathways in prokaryotes are regulated by different control mechanisms

2. To understand how these control mechanisms allow the cell to respond to changes in the environment

3. To understand how gene regulation in eukaryotes differs from that in prokaryotes

Outline:

Gene control in prokaryotes

The need for regulation

in single-celled organisms

in multicellular organisms -- cell differentiation

Metabolic pathways

Carried out in a series of steps, each step controlled by an specific enzyme

Pathway will be functional if enzymes are present (if genes for those enzymes are expressed)

Anabolic pathways:

synthesize essential end-products from raw materials

only need to function if product is lacking from environment

Catabolic pathways:

breakdown complex molecules (substrates) into simpler ones, releasing energy

only need to function is substrate is present

Operon structure

• promoter
• operator
• structural genes

Negative gene regulation -- stops transcription

Repressible (e.g., trp operon)

• Regulatory gene makes mRNA for repressor protein
• Repressor only works if corepressor (product of anabolic pathway) is bound to it
• If so, they will bind to operator and prevent transcription
• Prevents too much product from being made

Inducible (e.g., lac operon)

• Regulatory gene makes mRNA for repressor protein
• Repressor works fine alone, but if inducer (substrate of catabolic pathway) is present, repressor is inactivated
• Lack of repression means transcription will happen
• Allows cell to make catabolic enzymes only if there is something to break down

Positive gene regulation -- enhances transcription

Presence of a substance increases activity of RNA polymerase

coactivator and activator protein bind ot promotor

Regulons

families of operons

Gene control in eukaryotes

Differences in genomes

• circular vs. linear DNA
• Histones
• Lots of eukaryotic DNA doesn't code for polypeptides
  • introns
  • repetitive sequences at centromeres and telomeres
  • nonsense
• No operons in eukaryotes

Gene structure

------enhancer------//------promoter--transcriptional unit-------

Gene regulation

• Transcription factor binds to enhancer
• DNA coils so this is close to promoter sequence
• RNA polymerase is stimulated

  Hormones may facilitate this process (steroids)

Three levels of gene regulation in eukaryotes:

1. Physical organization of DNA

• euchromatin vs. heterochromatin
• methylation

2. Enhancers and transcription factors (see above)

3. Post-transcriptional control

• RNA processing
• RNA lifespan
• polypeptide modification
• etc.

Vocabulary

click here to go to regulation vocabulary