This compound is used as an analogue of allolactose, a lactose metabolite that activates the transcription of lactose operon and in particular the β-galactosidase gene, lacZ. Like allolactose or lactose, IPTG binds to the suppressor of the lactose operon, LacI, which prevents its binding to the operator and thus induces the transcription of the β-galactosidase gene. Unlike allolactose, IPTG is not a substrate for this enzyme because the presence of a sulfur atom in IPTG makes the chemical bond with the isopropyl part non-hydrolysable, so IPTG is what is called a free inducer. Not being metabolizable, the concentration of IPTG does not vary in the culture, which makes IPTG induction constant and stable.

 

In the laboratory, IPTG is used to induce the expression of recombinant proteins under the control of the lake promoter or its derivatives in Escherichia coli. Bacteria carrying a plasmid containing the gene of interest are cultured in the absence of IPTG until cell density reaches a sufficient level. The addition of IPTG to the culture allows the production of the protein to be induced at the desired time.