Bacteria must control which genes are active so they do not waste energy making proteins they do not need. The lac operon is a classic example of gene regulation in Escherichia coli. It controls genes that help the cell use lactose as a food source.
This system matters because it shows how DNA, proteins, and environmental signals work together to control cell behavior.
The lac operon has a promoter, an operator, and three structural genes called lacZ, lacY, and lacA. When lactose is absent, a repressor protein binds the operator and blocks transcription. When lactose is present, a lactose-derived molecule binds the repressor, causing it to release the operator so RNA polymerase can transcribe the genes.
The operon is strongest when lactose is present and glucose is low, because the cell then needs lactose metabolism most.
Key Facts
- The lac operon includes the promoter, operator, lacZ, lacY, and lacA regions on bacterial DNA.
- lacZ codes for beta-galactosidase, which helps break lactose into glucose and galactose.
- lacY codes for lactose permease, which helps lactose enter the bacterial cell.
- When lactose is absent, the lac repressor binds the operator and transcription is blocked.
- When allolactose is present, it binds the repressor and changes its shape so it cannot bind the operator.
- Gene expression level depends on signals: lactose present plus low glucose gives high lac operon transcription.
Vocabulary
- Operon
- An operon is a group of bacterial genes controlled together by one promoter and regulatory region.
- Promoter
- A promoter is a DNA sequence where RNA polymerase binds to begin transcription.
- Operator
- An operator is a DNA sequence where a regulatory protein such as a repressor can bind to control transcription.
- Repressor
- A repressor is a protein that reduces gene expression by binding DNA and blocking transcription.
- Inducible operon
- An inducible operon is usually off but can be turned on when a specific molecule, called an inducer, is present.
Common Mistakes to Avoid
- Saying the promoter and operator are the same thing is wrong because RNA polymerase binds the promoter, while the repressor binds the operator.
- Assuming lactose directly turns on transcription is incomplete because allolactose, a lactose-derived inducer, binds the repressor and prevents it from blocking the operator.
- Forgetting the role of glucose gives an incomplete prediction because the lac operon is expressed most strongly when lactose is present and glucose is low.
- Calling the lac operon repressible is wrong because it is an inducible operon that is normally off and turns on when lactose is available.
Practice Questions
- 1 In a culture of 1,000 E. coli cells, 80% have lactose present and low glucose. If each of those cells makes 120 beta-galactosidase molecules per minute, how many beta-galactosidase molecules are made per minute in the whole culture?
- 2 A lac operon DNA segment is 6,000 base pairs long. The promoter and operator together take up 300 base pairs. What percentage of the segment is promoter plus operator?
- 3 Predict whether the lac operon will be mostly ON or OFF in each condition: lactose absent and glucose low, lactose present and glucose high, lactose present and glucose low. Explain the reason for each prediction.