Concepts

The lac operon of E. coli controls the transcription of three genes in lactose metabolism: the lacZ gene, which encodes p-galactosidase; the lacY gene, which encodes permease; and the lacA gene, which encodes thiogalactoside transacetylase. The lac operon is inducible: a regulator gene produces a repressor that binds to the operator site and prevents the transcription of the structural genes. The presence of allolactose inactivates the repressor and allows the transcription of the lac operon.

Absence of lactose

Regulator gene (lacI+)

Active repressor

Mutant repressor

Regulator gene (lacI+)

Active repressor

Mutant repressor

RNA ' polymerase

| The lac I + gene is trans dominant: the repressor produced by lacI+ can bind to both operators and repress transcription in the absence of lactose.

CannotN.

lacO+ operator

bind |

lacZ -

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