Practice a College Board-style free response question on Gene Expression & Regulation. Write your response, then reveal the model answer to see exactly what earns each point.
The lac operon in E. coli contains genes needed to metabolize lactose. A repressor protein normally binds to the operator and blocks RNA polymerase from transcribing the operon's genes. Researchers grow E. coli in four different media and measure the relative transcription rate of the lac genes:
| Growth medium | Relative transcription rate of lac genes |
|---|---|
| Glucose only (no lactose) | Very low |
| Lactose only (no glucose) | Very high |
| Glucose and lactose | Low |
| No glucose, no lactose | Very low |
When only glucose is present, there is no lactose to inactivate the lac repressor protein. The repressor therefore stays bound to the operator sequence, physically blocking RNA polymerase from binding the promoter and transcribing the structural genes. Since the cell already has glucose (its preferred energy source) and has no lactose to break down anyway, keeping the operon off conserves energy and resources that would otherwise be wasted making unneeded enzymes.
In lactose-only medium, lactose (via a metabolite) binds to and inactivates the repressor, so it releases from the operator and RNA polymerase can transcribe the lac genes at a high rate — the cell needs those enzymes to use lactose as its energy source. When BOTH glucose and lactose are present, the repressor is still inactivated by lactose, but the cell preferentially uses glucose. The presence of glucose lowers the activity of a separate activator protein (CAP) that normally boosts lac transcription, so even though the operon isn't fully repressed, it is transcribed at a much lower rate than when lactose is the only sugar available.
Transcription of the lac genes would occur even in glucose-only medium, despite the absence of lactose — the operon would be transcribed when it normally should be OFF. Normally, the repressor binds the operator (when lactose is absent) and blocks RNA polymerase from transcribing the operon. If the operator is mutated so the repressor can no longer bind it, RNA polymerase is no longer blocked and can transcribe the lac genes regardless of whether lactose is present. This produces lac enzymes the cell doesn't need, wasting energy and resources.