Is the lac operon a negative or positive control system?
The lac operon is a negative inducible operon. Basically, when lactose is absent, the regulator protein (a repressor) binds to the operator, inhibiting transcription. (Basal (very low) levels of enzymes are still produced at this time.) In the presence of lactose, some is converted into allolactose (by beta-galactosidase), which binds to the repressor. The repressor is rendered inactive when bound, and “falls off” the operator, allowing regular transcription and translation of the enzymes to occur.
