3 - lac and trp operon

BIOL10221 “Molecular Biology” Module 4 - From DNA to RNA

Lecture 3: Gene Expression in Prokaryotes: The Lac and Trp Operon

Dr. David Boam

Important Concepts of Gene Expression

• Control of Gene Expression

  • Operons: Cluster of genes transcribed by a single promoter, leading to a polycistronic mRNA.

  • Operons typically consist of genes that are functionally related.

  • The ability of RNA polymerases to initiate transcription is crucial for gene regulation.

The Lac Operon

• Located at Placl: Contains genes lacI, lacZ, lacY, and lacA.

• Components:

  • lacI: Encodes the lac repressor.

  • lacZ: Encodes β-galactosidase, which hydrolyzes lactose into glucose and galactose.

  • lacY: Encodes permease, facilitating lactose uptake.

  • lacA: Encodes acetylase.

Mechanism of Action

• Repression: In the absence of lactose, the lac operon is repressed because the lac repressor binds to the operator (O), preventing RNA polymerase from initiating transcription.

• Induction: Allolactose (a lactose derivative) binds to the repressor, causing it to dissociate from the operator, allowing transcription to proceed.

• Negative Feedback Control: The lac operon operates under negative feedback regulation, thus preventing overproduction of enzyme products.

Cis-acting Elements and Trans-acting Factors

• Cis-acting: Regulatory sequences that influence genes only to which they are directly attached (e.g., promoters).

• Trans-acting: Factors (mostly proteins) that can regulate multiple genes regardless of their location on the chromosome and are typically considered dominant in action.

Mutations in the Lac Operon

• Cis-acting mutations (lacOc): Constitutive expression leads to unregulated gene action, cannot be complemented by other genes.

• Trans-acting mutations (lacI-): Mutated repressor leads to constitutive expression; can be complemented by the introduction of a wild-type lacI gene.

Summary of the Lac Operon

• Repressor prevents transcription initiation when bound to the operator.

• Lactose regulates repressor activity leading to transcription.

• Mutations affect regulatory elements differently:

  • Cis-acting mutations cannot be complemented.

  • Trans-acting mutations may affect multiple operons and can be complemented.

Catabolite Repression

• Definition: Catabolite repression is the mechanism by which the operons responsible for metabolizing alternative carbon sources (like lactose) are repressed when glucose is present.

• Mechanism:

  • Glucose is the preferred carbon source.

  • When glucose levels are low, the levels of cAMP increase, facilitating CRP binding to promote the transcription of operons, including the lac operon.

The Trp Operon

• Functions oppositely to the lac operon.

• In the presence of tryptophan, the Trp repressor binds to the operator, inhibiting transcription.

• Contains several genes involved in the synthesis of the amino acid tryptophan.

Comparison of Lac and Trp Operons

• Lac Operon: Induced by lactose, serves to metabolize it (substrate).

• Trp Operon: Repressed by tryptophan, functions in its biosynthesis (product).

Intended Learning Objectives

By the end of this lecture, students should be able to:

• Define and describe the components of a typical operon.

• Understand the mechanisms regulating the lac operon by lactose and glucose.

• Explain negative feedback regulation and how it functions.

• Describe the mechanism of catabolite repression.

• Discuss general concepts relevant to operon regulation including DNA-protein interactions, protein-protein interactions, protein-small molecule interactions, conformational changes, and the role of inducers and co-repressors.