Gene Expression and Regulation

Importance of Energy in Gene Expression

Gene expression requires a large amount of energy.
Regulation of gene expression is crucial for cellular response to environmental changes (e.g., nutrient supply).

The lac operon in E. coli is the first fully described genetic regulatory mechanism.
It serves as a model for prokaryotic gene regulation.

An operon is a transcription unit of genes expressed together under similar conditions.
Components of an operon:
- Promoter
- Operator
- Genes (coding for proteins)

In Absence of Lactose:
- Repressor protein binds to lac operator, preventing transcription of downstream lac genes.
- The repressor protein is encoded by the lacI gene, located upstream of the lac operon.
In Presence of Lactose:
- Lactose enters the bacterium and converts to allolactose.
- Allolactose binds to the repressor, inactivating it and unblocking the operator.
- RNA polymerase can now bind to the promoter and transcribe lacZ, lacY, and lacA together as polygenic mRNA.

The three genes code for:
- β-galactosidase: Breaks down lactose into simple sugars.
- Permease: Facilitates lactose uptake by forming pores in the cell membrane.
- Transacetylase: Function related to lactose metabolism.

Glucose is the preferred energy source for E. coli.
If glucose is low, cAMP levels increase, leading to the following process:
1. cAMP binds to catabolite activator protein (CAP).
2. The cAMP-CAP complex binds to DNA near the lac promoter, enhancing RNA polymerase activity.
3. This results in increased synthesis of enzymes for lactose metabolism.

The concentration of lactose and glucose regulates the expression of lactose-metabolizing enzymes.
This regulation allows rapid adaptation to varying environmental conditions.

While bacteria use operons, eukaryotic gene regulation is more complex but shares similar concepts, including activator and repressor proteins.