Question

What causes the lac operon to shut off?

Answer 1

This change in the lac repressor is caused by the small molecule allolactose, an isomer (rearranged version) of lactose. When lactose is available, some molecules will be converted to allolactose inside the cell. Allolactose binds to the lac repressor and makes it change shape so it can no longer bind DNA.

Answer 2

here is your answer

The lac operon in E. coli , is under both negative and positive transcriptional controls by the repressor protein and catabolite activator protein ( CAP) respectively.

because the working of the lac operon is explained very well