As its concentration builds up, the protein binds to its mRNA and blocks translation. As the concentration drops, RNA is free and translation resumes. If glucose is present, even with lactose, enzyme is not produced. Our model system must account for both things, that is lactose is required for induction, and glucose must not be present. Also the glucose effect is pleotrophic.
Lactose operon forms the basis for many genetic engineering expression systems.
When tryptophan levels in the cell are low, the operon is induced and high levels shut the system down. In addition to on-off functions, the operon is regulated over a large range of trp concentrations. In cells growing in minimal medium, the cells use are making a small fraction of their total enzyme synthetic ability. Thus we have two types of regulation, on-off and attenuation. Stage 2 operates over a ten-fold range when starvation is severe.