Allolactose

Allolactose is a disaccharide similar to lactose. It consists of the monosaccharides -galactose and -glucose linked through a β1-6 glycosidic linkage instead of the β1-4 linkage of lactose. It may arise from the occasional transglycosylation of lactose by β-galactosidase.
It is an inducer of the "lac" operon in "Escherichia coli" and many other enteric bacteria. It binds to a subunit of the tetrameric "lac" repressor, which results in conformational changes and reduces the binding affinity of the "lac" repressor to the "lac" operator, thereby dissociating it from the "lac" operator. The absence of the repressor allows the transcription of the "lac" operon to proceed. A non-hydrolyzable analog of allolactose, isopropyl β-D-1-thiogalactopyranoside (IPTG), is normally used in molecular biology to induce the "lac" operon.