Molecular Genetics: Regulation of Gene Expression

Gene Regulation

• Gene regulation controls gene expression in response to cell/environment conditions.
• Constitutive gene = always expressed; facultative gene = transcribed when needed.

Regulation in Prokaryotes

• Occurs during transcription (most common), translation, or after protein synthesis.
• Promoter = where RNA polymerase II binds to initiate transcription.

Operon

• Operon = gene cluster under ONE promoter.
• Genes in same metabolic pathway are often in same operon.
• One promoter = one mRNA strand (polycistronic mRNA).
• Multiple proteins from single mRNA.

E. Coli and the lac Operon

• E. coli uses different sugars for energy/carbon.
• Gene expression adjusts based on sugar type.
  • Example: Lactose present = increased enzymes for lactose metabolism.
  • Lactose removed = decreased enzyme production.

The lac Operon

• An inducible operon.
• Coding region: genes for 3 enzymes for lactose metabolism.
• Regulatory region:
  • Promoter: RNA polymerase binds.
  • Operator: repressor protein binds to inhibit transcription.

lac Operon Enzymes

• lacZ: encodes β-galactosidase (lactose to glucose/galactose).
• lacY: encodes lactose permease (imports lactose).
• lacA: function in lactose breakdown unclear.

lac Operon - No Lactose

• lac repressor binds to operator, blocking RNA polymerase II.
• No RNA polymerase binding = no transcription.

lac Operon - Lactose Present

• Lactose binds to repressor, inactivating it.
• Repressor cannot bind to operator.
• RNA polymerase II binds = transcription.

The trp Operon

• Normally active until repressor turns it off.
• Coding region: genes for 5 enzymes for tryptophan synthesis.
• Regulatory region:
  • Promoter for transcription
  • Operator

trp Operon - No Tryptophan

• Repressor (no trp) does not bind to operator = transcription.

trp Operon - Too Much Tryptophan

• Tryptophan binds to repressor = activates repressor = binds to operator = no transcription.
• Tryptophan is a corepressor.