When both lactose and glucose are available to E. coli, which part of the lac operon regulation assures that glucose will be metabolized first? When both lactose and glucose are available to E. coli, which part of the lac operon regulation assures that glucose will be metabolized first? Permease allows lactose to enter the cell when lactose enters the cell. The repressor binds to the operator in the absence of lactose. The repressor cannot bind to the operator in the presence of lactose. RNA polymerase transcribes the lac operon to make a single RNA molecule. CAP binds to cAMP and then to the promoter only in the presence of low glucose.