The tryptophan (trp) operon in E. coli is a negative control repressible system. It’s negative control because the system produces a repressor which functions to turn off the operon. It’s repressible because, unlike the lac operon, repression occurs when a critical substance is abundant in the cell.his means that the cell only metabolizes lactose if glucose is low and lactose is plentiful. The cell uses a negative control system (the lac operon) to respond to the availability of lactose in the environment. Negative control. means that it’s a system in which the active substance acts to turn off function.