Overview
This lecture covers how prokaryotic cells regulate gene expression, focusing on operons and the roles of regulatory proteins and small molecules in turning genes on or off.
Organization of Prokaryotic DNA and Operons
- Prokaryotic DNA is organized into a single circular chromosome supercoiled in the nucleoid.
- Genes for related functions are grouped in operons and transcribed together into a single mRNA.
- The lac operon is an example containing genes for lactose metabolism transcribed as one unit.
Regulatory Molecules in Gene Expression
- Three main types of regulatory molecules: repressors, activators, and inducers.
- Repressors bind to operator regions to block transcription in response to stimuli.
- Activators bind to promoter regions to increase transcription in response to stimuli.
- Inducers are small molecules that can activate or repress transcription depending on cell needs.
The trp Operon: A Repressible Operon
- The trp operon contains five genes for tryptophan synthesis, regulated as a unit.
- When tryptophan is abundant, it binds to the trp repressor, which then attaches to the operator and blocks transcription.
- When tryptophan is absent, the repressor cannot bind the operator, allowing transcription to proceed.
- The trp operon is "negatively regulated" because the repressor silences gene expression.
Catabolite Activator Protein (CAP): Positive Regulation
- When glucose is scarce, cAMP accumulates and binds to CAP, forming a complex.
- The cAMP–CAP complex binds to certain promoters, increasing RNA polymerase binding and activating transcription of genes for alternate sugar metabolism.
- CAP acts as a positive regulator, turning genes on when glucose is low.
The lac Operon: An Inducible Operon
- The lac operon contains genes for lactose metabolism and is inducible.
- For the lac operon to be highly transcribed, glucose must be absent and lactose present.
- Allolactose (from lactose) binds to the lac repressor, causing it to release from the operator and permitting transcription.
- CAP–cAMP binding to the promoter further increases transcription when glucose is absent.
Comparison and Regulation Logic
- The trp operon is usually on and repressible, while the lac operon is usually off and inducible.
- Transcription of the lac operon requires both removal of the repressor (lactose present) and the presence of the activator (glucose absent).
- Table: Only when glucose is absent (CAP +), lactose is present (repressor -), is full transcription of the lac operon observed.
Key Terms & Definitions
- Operon — a set of genes transcribed together as a single mRNA under one promoter.
- Repressor — protein that binds to an operator to block transcription.
- Activator — protein that increases transcription by binding near a promoter.
- Inducer — small molecule that regulates operon activity, often by inactivating repressors or activating activators.
- trp operon — operon encoding tryptophan synthesis enzymes, repressible by tryptophan.
- lac operon — operon encoding lactose-metabolizing enzymes, inducible by lactose.
- CAP (Catabolite Activator Protein) — protein that, with cAMP, activates transcription when glucose is low.
- cAMP (Cyclic AMP) — small molecule signaling low glucose, activates CAP.
- Negative regulator — a molecule that suppresses gene expression.
- Positive regulator — a molecule that increases gene expression.
Action Items / Next Steps
- Review figures of trp and lac operons for gene arrangement and regulation points.
- Watch linked videos on trp and lac operon function for visual understanding.