Info

RNA polymerase

Glucose present No lactose

Glucose present Lactose present

No glucose No lactose

Activator- Promoter Operator

(CAP) binding site

RNA polymerase

Glucose present No lactose

Inducer (allolactose)

Lac operon.

The lactose operon contains three genes. Transcription is controlled by regulatory proteins that bind to an activator-binding site and an operator.

Transcription not activated AND blocked.

Low cAMP; CAP cannot bind. In addition, repressor is bound to operator, blocking polymerase.

Transcription not activated.

Low cAMP; CAP cannot bind. At the same time, inducer (allolactose) prevents repressor from binding to the operator.

Transcription activated but blocked.

High cAMP; CAP/cAMP complex binds to activator-binding site. Repressor is bound to operator, however, blocking polymerase.

Transcription

No glucose

Lactose present

Transcription

No glucose

Lactose present

Figure 7.20 Regulation of the lac Operon

Transcription activated.

High cAMP; CAP/cAMP complex binds to activator-binding site. In addition, inducer (allolactose) prevents repressor from binding to the operator.

Gene C

Figure 7.20 Regulation of the lac Operon

Chapter 7 The Blueprint of Life, from DNA to Protein

Lactose exhausted

Glucose exhausted

Growth on lactose

Glucose exhausted

Glucose and lactose added

Growth on lactose

Growth on glucose

Time of incubation (hr)

Glucose and lactose added

Growth on glucose

Time of incubation (hr)

Figure 7.21 Diauxic Growth Curve of E. co// Growing in a Medium Containing Glucose and Lactose Cells preferentially use glucose. Only when the supply of glucose is exhausted do cells start metabolizing lactose. Note that the growth on lactose is slower than it is on glucose.

supply of glucose is exhausted do the cells begin degrading lactose, a carbon source that requires additional enzymatic steps to metabolize.

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