Microbiology Chapter #11 - Regulation of Gene Expression at the Transcriptional Level

Housekeeping Genes

- Always present and produced

- Expressed continuously

Constitutive Genes

- Always turned ON

Inducible Genes

- Usually OFF but can be turned ON by an inducer

- Involved in catabolic pathways

Repressible Genes

- Usually ON but can be turned OFF by a repressor

- Involved in anabolic pathways

Effectors - Inducers

- Activators bind to the activator-binding sites upstream of promoter

- Promote transcription initiation

Effectors - Repressors

- Bind to the operator and inhibit transcription initiation

Negative Control & Inducible Gene

- Repressor binds to an inducer, taking the repressor off of the DNA so transcription can occur

Negative Control & Repressible Gene

- Repressor binds to DNA, blocking transcription

Positive Control & Inducible Gene

- Activator binds to activar-binding site and promotes transcription

Positive Control & Repressible Gene

- Activator removed from DNA by a repressor

Biosynthetic Enzymes

- If end product is present, operon OFF

- If end product is not present, operon ON

Catabolic Enzymes

- Substrate present and preferred C and energy sources not present, operon ON

- Substrate not present or C/energy sources are present, operon OFF

trp operon of E. coli

- Tryptophan operon: negative and repressible

- Anabolic process

- Tryptophan acts as a corepressor, binds to the repressor and turns off transcription when tryptophan is present

- When T is low, transcription occurs

lac operon of E. coli

- Lactose operon: negative and inducible

- Operon usually OFF, when lactose is absent, the repressor binds to the operator and blocks transcription

- Lactose acts as an inducer, turning the operon ON, binding to the repressor

ara operon of E. coli

- Arabinose operon: protein acts + and -

- Catabolic process, regulated by AraC

- AraC as a repressor: when arabinose isn't present, one binds to aral and another binds to araO2. The two molecules interact and bend DNA so transcription cannot occur

- AraC as inducer: when arabinose is present, AraC breaks bend so transcription can occur