How does induction by IPTG differ from induction by lactose via allolactose?

Answer 1

Lactose is eventually metabolized in the cell to allolactose, which binds to the lacI repressor, leading to the induction of the lac promoter (or derivatives of the lac promoters). The allolactose is also eventually metabolised. IPTG does essentially the same as allolactose, however, it does not get metabolized. Moreover, since lactose gets transported in the cell by the lacY gene product and metabolized to allolactose by the lacZ gene product, lactose cannot be used in lacZY- strains for induction. IPTG, however, can be used since it does not require the two genes.

Answer 2

Induction by IPTG is an artificial method that directly induces the lac operon by binding to the lac repressor protein and inhibiting its repressor activity. On the other hand, induction by lactose involves the conversion of lactose to allolactose, which then binds to the lac repressor, leading to its inactivation and subsequent induction of the lac operon. IPTG is a non-metabolizable inducer, whereas allolactose is a natural metabolite in the lactose catabolism pathway.