Operon Theory

Operon in bacteria and archaea

structural proteins with related functions are usually encoded together within the genome in a block; are transcribed together under the control of a single promoter, results in polycistronic transcript

Regulatory region

Each operon includes DNA sequences that influence its own transcription; these are located in a region; includes the promoter and the region surrounding the promoter,

Transcription factors

proteins encoded by regulatory genes bind to the promoter

Repressor

transcription factor that suppresses transcription of a gene; physically blocks RNA polymerase from transcribing structural genes.

Operator

in response to an external stimulus by binding to a DNA sequence within the regulatory region

Activator

a transcription factor that increases the transcription of a gene in response to an external stimulus by facilitating RNA polymerase binding to the promoter

Inducer

a third type of regulatory molecule, is a small molecule that either activates or represses transcription by interacting with a repressor or an activator

Prokaryotic operon that gene products are required rather consistently and the expression is unregulated

Constitutively expressed

Constitutively Expressed Operon

they are transcribed and translated continuously to provide the cell with constant intermediate levels of the protein products.

Prokaryotic operons are commonly controlled by the binding of repressors to operator regions, thereby preventing the transcription of the structural genes.

True

Repressible Operons

typically contain genes encoding enzymes required for a biosynthetic pathway.

Inducible Operons

often contain genes encoding enzymes in a pathway involved in the metabolism of a specific substrate like lactose.

Trp Operon: A Repressible Operon

The trp operon in E. coli synthesizes tryptophan when environmental levels are low. When tryptophan is present, it binds to the trp repressor, changing its shape and blocking RNA polymerase from transcribing the operon’s structural genes.

The lac Operon: An Inducible Operon

The lac operon, an inducible operon, encodes genes for lactose processing. Lactose, converted to allolactose, binds to the lac repressor, removing it from the operator region and allowing RNA polymerase to transcribe the structural genes.

The lac Operon: Activation by Catabolite Activator Protein (CAP)

The lac operon is regulated by glucose levels and the presence of lactose. When glucose is scarce, cAMP binds to CAP, activating the lac operon by increasing RNA polymerase binding to the promoter.

Cyclic AMP (cAMP)

a cyclic derivative of AMP and important signaling molecule involved in glucose and energy metabolism in E. coli.

What affects the binding of the trp operon repressor to the operator?

affected primarily by the presence of tryptophan.

How and when is the behavior of the lac repressor protein altered?

by the presence or absence of allolactose, a derivative of lactose

In addition to being repressible, how else is the lac operon regulated?

through a mechanism called catabolite repression, which depends on the availability of glucose. So, the lac operon is controlled by dual regulation