Final answer:
Experiments by Jacob, Monod, and Pardee showed that the lacI gene produces a repressor protein that controls the expression of the lac operon in E. coli. In the absence of lactose, the repressor binds to the operator to block RNA polymerase, but when lactose is present, allolactose binds to the repressor, causing it to release from the operator allowing gene expression.
Step-by-step explanation:
Jacob, Monod, and Pardee demonstrated that the lacI gene encodes a diffusible repressor protein through experiments on E. coli and its use of lactose as a carbon source. Their work revealed that this repressor protein binds to the operator region of the lac operon, which prevents RNA polymerase from transcribing the lac operon genes in the absence of lactose. However, when lactose is present, an inducer molecule (allolactose) binds to the repressor, causing a conformational change and its release from the operator, allowing for transcription of the lac genes.
The operon model proposes that the activation and repression mechanisms are a means to effectively coordinate the expression of related genes, allowing the cell to conserve energy and resources by only producing the enzymes required for lactose metabolism when lactose is available as a food source. This dynamic control of the lac operon serves as a model for understanding bacterial gene regulation.