BioCoach Activity

Concept 6: The Effect of Lactose on the lac Operon

When lactose is present, the lac genes are expressed because allolactose binds to the Lac repressor protein and keeps it from binding to the lac operator.

Allolactose is an isomer of lactose. Small amounts of allolactose are formed when lactose enters E. coli.

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Allolactose binds to an allosteric site on the repressor protein causing a conformational change. As a result of this change, the repressor can no longer bind to the operator region and falls off. RNA polymerase can then bind to the promoter and transcribe the lac genes.