Hint: The lactose or lac operon of Escherichia coli is a cluster of three structural genes encoding proteins involved in lactose metabolism and also the sites on the DNA involved within the regulation of the operon.
The lac repressor acts as a lactose sensor which blocks transcription of the operon, but stops acting as a repressor when lactose is present. The lac repressor senses lactose indirectly, through its isomer allolactose.
- The lac operon is an operon, or group of genes with one promoter. The lac operon contains three genes: lac z, lac y, and lac a. These genes are transcribed as one mRNA.
- Genes within the lac operon specify proteins that help the cell utilize lactose. In addition to the three genes, the lac operon also contains a variety of regulatory DNA sequences. These are regions of DNA to which particular regulatory proteins can bind, controlling transcription of the operon.
- The promoter is the binding site for RNA polymerase, the enzyme that performs transcription.
- The operator could be a negative regulatory site bound by the lac repressor protein.
- The CAP binding site could be a positive regulatory site that's bound by catabolite activator protein. When CAP is absolute to this site, it promotes transcription by helping RNA polymerase bind to the promoter.
- The lac repressor: The lac repressor is protein that represses transcription of the lac operon. It does this by binding to the operator, which partially overlaps with the promoter. When bound, the lac repressor gets in RNA polymerase's way and keeps it from transcribing the operon.
- When lactose isn't available, the lac repressor binds tightly to the operator, preventing transcription by RNA polymerase. When lactose is present, the lac repressor loses its ability to bind DNA. It floats off the operator, clearing the way for RNA polymerase to transcribe the operon.
- This change within the lac repressor is caused by allolactose, an isomer. When lactose is there, some molecules are converted to allolactose inside the cell. Allolactose binds to the lac repressor and makes it deform so it can now not bind DNA.
- Allolactose is an example of an inducer, a little molecule that triggers expression of a gene or operon. The lac operon is taken into account as an inducible operon because it's usually turned off, but may be turned on within the presence of the inducer allolactose.
Therefore, the correct option is A, inducer.
Note: The lac operon is expressed at high levels if two conditions are met:
- Glucose must be unavailable: When glucose is unavailable, camp binds to CAP, making CAP ready to bind DNA.
- Lactose available: If lactose is offered, the lac repressor is going to be released from the operator (by binding of allolactose). This permits RNA polymerase to maneuver forward on the DNA and transcribe the operon.