Overview
This lecture explains how gene expression is regulated in prokaryotes, focusing on the mechanisms and roles of repressors, activators, inducers, and operons such as the trp and lac operons.
Prokaryotic Gene Structure and Regulation
- Prokaryotic DNA is organized as a single circular, supercoiled chromosome in the nucleoid.
- Genes with related functions are grouped into operons, which are transcribed together as a single mRNA.
- Operons are controlled by regulatory molecules: repressors, activators, and inducers.
- Regulatory proteins bind to DNA sites near the genes to control transcription.
- Activators generally bind to promoter sites to increase transcription.
- Repressors bind to operator regions to block transcription.
- Inducers are small molecules that can either activate or repress transcription based on the cell’s needs.
The trp Operon: A Repressible Operon
- The trp operon contains five genes for tryptophan biosynthesis, transcribed as one mRNA.
- When tryptophan is present, it activates the trp repressor, which binds the operator and blocks transcription.
- When tryptophan is absent, the repressor does not bind, allowing transcription and tryptophan synthesis.
- The trp operon is negatively regulated because binding of repressor protein turns gene expression off.
Catabolite Activator Protein (CAP): Positive Regulation
- When glucose is low, cAMP accumulates and binds to CAP.
- The cAMP-CAP complex binds to promoter regions and enhances RNA polymerase binding, increasing transcription of genes for alternate sugar metabolism.
- CAP acts as a positive regulator, activating genes when glucose is scarce.
The lac Operon: An Inducible Operon
- The lac operon encodes enzymes (like beta-galactosidase) to utilize lactose when glucose is unavailable.
- Two conditions are required for full lac operon expression: glucose absence and lactose presence.
- Lactose (via allolactose) inactivates the lac repressor so transcription can occur.
- In the presence of cAMP-CAP (low glucose) and lactose, transcription is maximized.
- If either glucose is present or lactose is absent, lac operon transcription is low or off.
Key Terms & Definitions
- Operon — A cluster of genes transcribed as a single mRNA with shared regulatory sequences.
- Repressor — Protein that binds operator, blocking transcription.
- Activator — Protein that binds promoter to increase the rate of transcription.
- Inducer — Small molecule that influences regulatory protein function, activating or repressing transcription as needed.
- Negative regulation — Gene expression is inhibited by regulatory protein binding.
- Positive regulation — Gene expression is enhanced by regulatory protein binding.
Action Items / Next Steps
- Review the regulation mechanisms in trp and lac operons.
- Watch assigned videos on the trp and lac operons.
- Study Table 16.2 to understand how different glucose and lactose conditions affect lac operon transcription.