Bacterial Gene Regulation: Operons, Induction, and Repression

Operon

Set of operator and promoter sites and the structural genes they control.

Promoter

Segment of DNA where RNA polymerase initiates transcription of structural genes.

Operator

Segment of DNA that controls transcription of structural genes.

Polycistronic

In an operon, all structural genes are transcribed as a unit.

Repression

Inhibits gene expression and decreases enzyme synthesis, mediated by repressors.

Induction

Turns on gene expression, initiated by an inducer.

Constitutive genes

Genes expressed at a fixed rate, always 'turned on'.

Inducible operon

Structural genes are not transcribed unless an inducer is present.

Lac operon

Example of an inducible operon in E. coli needed to metabolize lactose.

Repressor

Protein that blocks transcription by binding to the operator.

Allolactose

Metabolite of lactose that acts as an inducer for the lac operon.

Repressible operon

Structural genes are transcribed until they are turned off.

Tryptophan

Excess tryptophan acts as a corepressor that stops tryptophan synthesis.

Corepressor

Substance that binds to a repressor to activate it and inhibit transcription.

Transcription

Process of copying a segment of DNA into RNA.

Translation

Process of synthesizing proteins from mRNA.

Enzyme a

Inducible enzyme that is not synthesized unless an inducer is present.

End-product C

Substance that causes the repressor to bind in a repressible operon.

Metabolic pathway

Series of reactions catalyzed by enzymes to produce a specific product.

Transcription initiation region

Region in mRNA that allows for independent translation of each gene.

Default position of a repressible gene

On; it is active until repressed.

Default position of an inducible gene

Off; it is inactive until induced.