In E. coli, the lac operon is turned on by: A derivative of lactose (allolactose) binding to the gene for a structural protein, which when produced binds to the regulator of the lac operon and initiates its transcription. A derivative of lactose (allolactose) binding to the operator of the lac operon and causing release of the regulatory protein from the operon. A derivative of lactose (allolactose) binding to a regulatory protein which is bound to the operator of the lac operon and resulting in the release of the regulatory protein from the operon. A derivative of lactose (allolactose) binding to the gene for a regulatory protein which when produced binds to the operator of the lac operon and initiates its transcription. A derivative of lactose (allolactose) binding to a regulatory protein which is bound to the operator of the lac operon which can only be transcribed when the lactose-regulatory protein complex is bound to the operator.