Control Mechanisms

Four Levels of Control of Gene Expression

Type of Control Description

Transcriptional Regulates which genes are transcribed.

Controls rate of transcription.

Posttranscriptional Modification of mRNA. Introns removed & exons

spliced together.

Four Levels of Control of Gene Expression

Type of Control Description

Translational Controls rate of translation. Controls rate of mRNA

activation and destruction.

Posttranslational Controls rate at which a protein becomes active and the time it remains functional.

The lac Operon

• E. coli uses β-galactosidase to break down lactose in order to grow.

• The 3 genes for β-galactosidase are part of the lac operon.

The lac Operon

• When lactose is absent:– LacI repressor protein binds to the operator and

partially blocks the promoter.– Prevents transcription of the lac operon genes.

The lac Operon

• With sufficient lactose:– Binds to the LacI repressor protein.– LacI changes its shape, allowing transcription of

the lac operon genes.

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The trp Operon

• E. coli uses tryptophan for the production of protein.

• The trp operon consists of 5 genes that code for 3 enzymes needed to synthesize tryptophan.

The trp Operon

• When tryptophan is absent:– The shape of the trp repressor protein changes.– No longer binds to the trp operator.– RNA polymerase transcribes trp operon genes.

The trp Operon

• With sufficient tryptophan:– Binds to the trp repressor protein, causing a change

in shape.– trp repressor-tryptophan complex binds to the

operator.– Prevents transcription of the trp operon genes.

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Summary

lac Operon• Transcription induced when high levels of

lactose present.• Effector = level of lactose

trp Operon• Transcription repressed when high levels of

tryptophan present.• Effector = level of tryptophan