Gal operon
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The gal operon is a prokaryotic operon, which encodes enzymes necessary for galactose metabolism.

The gal operon contains two operators, O_E (for external) and O_I. The former is just before the promoter, and the latter is just after the galE gene (the first gene in the operon).

Repression of gene expression works via binding of repressor molecules to the two operators. These repressors dimerize, creating a loop in the DNA. The loop as well as hindrance from the external operator prevent RNA polymerase from binding to the promoter, and thus prevent transcription.

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