Prokaryotic Gene Control

Essential knowledge 2.C.1: Organisms use feedback mechanisms

to maintain their internal environments and respond to external

environmental changes.

Essential knowledge 3.B.1: Gene regulation results in differential gene expression, leading to cell specialization.

1. Eukaryotic cells have many more genes (i.e. 23,000 in human cells) in their genomes than prokaryotic cells (i.e. average 3000).

2. Physically there are more obstacles to regulate eukaryotic genes because there is so much more DNA to manage. For example, eukaryotic chromatin is wrapped around histone proteins.

3. In addition there are other non-histone proteins that are used in eukaryotic gene expression that are not used in prokaryotic gene expression.

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The Operon

• Operon– to operate

• Operon- A group of prokaryotic genes with a related function that are often grouped and transcribed together.

• Has only one promoter region for the entire operon. 3

An operon is composed of the following: PROG

Promoter– region of DNA where RNA polymerase can bind to gene.

Repressor— protein that can bind to operator and turn it off.

Operator— on/off switch that controls operon

Genes- genes to be transcribed

What is the benefit of having all of these genes grouped together?4

Regulatory Genes and Repressors

• Regulatory gene- Somewhere else on DNA. This gene produces the protein called the repressor (PROG) that can inhibit the transcription of an operon by attaching to the operator. (PROG p,o, and g in the operon are DNA; r is the only protein in the operon)

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Interaction of Modulators and Repressors

• Repressor proteins have allosteric properties. Modulators can bind to the repressor at an allosteric site changing the conformation of the repressor, thus activating or deactivating the repressor. Usually the modulator is a product of the biochemical pathway.

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The tryptophan (trp) operon

• By itself, the trp operon is turned on. The operon can be switched off by the trp repressor.

• Binds to the operator and blocks the attachment of RNA polymerase– preventing transcription of genes.

Synthesis of Tryptophan and the Repressible trpOperon

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Tryptophan Present and the Repressible trp Operon

Trp operon- a repressible operon. Negative Feedback• With the trp operon,

tryptophan is the corepressor; it binds to the trp repressor and the repressor is then active. Tryptophan is not produced.

• Repressible operon because transcription is usually on but can be repressed.

Lactose and the lac Operon

• The lac operon is an example of an inducible operon. Operon is usually off but can be turned on (induced) when a specific molecule reacts with regulator.

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Negative Gene Regulation

1. Inducible operon- the lac operon. This operon has the ability to convert lactose into glucose and galactose. This involves three structural genes

Absence of Lactose and the lac Operon

• If no lactose is present, the repressor protein is active, binding to the operator site. This prohibits the RNA polymerase from transcribing the operon.

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Lac and trp Operons-Examples of Negative Gene Regulation

•The lac and trp operons are examples of negative gene regulation as the active repressor protein turns the operons off.

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