Gene regulation

Two types of genes:1)Structural genes – encode

specific proteins

2)Regulatory genes – control the level of activity of structural genes ie. Control structural gene expression.

Gene regulation in prokaryotes http://www.youtube.com/watch?v=oBwtxdI1zvk

In prokaryotes, operons control the rate of transcription.

An operon is a group of genes that work together and code for the enzymes regulating a particular metabolic pathway.

Regulator gene

Promoter OperatorStructural gene A

Produces the repressor

RNA polymerase binding site

Repressor binding site

OPERON

Structural gene B

Structure of the operonThe operon in prokaryotes comprises a

number of different features:

1) Structural genes: code for particular enzymes in a metabolic pathway

2) Promoter gene: recognition site for the RNA polymerase to bind to.

3) Operator gene: controls the production of mRNA from structural genes.

INDUCTION (initiation)

R P O SG1 SG2R

If a substrate is uncommon the bacteria will not need the enzymes most of the time. So the repressor is usually attached.

This prevents RNA polymerase from forming mRNA. Therefore: no enzymes.

When the substrate molecule is present some of it acts as an inducer; it binds to the repressor, changing its shape so it can’t bind to the DNA.

Transcription takes place.

Sugar lactose enters cell, genes coding for enzymes to break sugar down turned on

R

RInducer

See this movie on the Lac operon in E. coli

for more detailExample: ON

REPRESSION

When a substrate is normally present the enzyme should be normally operating. The only time this should stop is when the end product levels build up too much.

The repressor cannot bind to the operator.

Some of the excess product acts as an effector, which helps the repressor to bind.

Transcription is stopped.

R P O SG1 SG2RR

R

See this movie on the Tryp operon in E. coli

for more detail

http://www.sumanasinc.com/webcontent/anisamples/majorsbiology/lacoperon.html

Lac Operon - induction:

Lac gene off (normal state)

Lactose present- acts as an inducer.

Lactose binds to the repressor protein.

Repressor can’t bind to the operator.

RNA polymerase binds . Lac gene on. Structural proteins made.

Repressor molecule binds to operator and prevents transcription by RNA polymerase

Tryptophan operon - repression

Tryp gene on (normal state)

Tryptophan accumulates in excess. Some of it acts as an effector and activates the repressor molecule.

Effector and repressor molecule bind to the operator gene and prevent transcription by RNA polymerase.

Tryptophan levels in cells decrease, no excess.

Tryptophan doesn’t bind to the repressor which then can’t bind to the operator.

RNA polymerase binds

Lactose all used up.

Gene regulation in prokaryotes -

summary Genes for a metabolic pathway are linked

together in operons with a common switch mechanism (operator).

No introns – no RNA processing

The structural genes undergo transcription and translation simultaneously.

Regulation occurs by switching all genes of a pathway on or off.

Gene regulation in eukaryotes

Genes for metabolic pathways in eukaryotic cells are separated, not grouped as operons.

The genes for a metabolic pathway are switched on separately.

Genes have introns that are removed in RNA processing.

Eukaryotic genes have a relatively large number of control elements.

Regulatory DNA regions

Eukaryotic genes have a promoter region upstream of the coding region, where RNA polymerase binds.

There are 2 two types of regulatory sequences that effect transcription of the structural gene:

1) enhancer 2) silencer

These are located upstream, downstream or within the gene (in introns).

Enhancer sequences

These are non-protein-coding sections of DNA that help regulate transcription by binding proteins called transcription factors (turn on).

Silencer sequencesThese are non-protein-coding sections of

DNA that help regulate transcription by binding proteins called repressors (decrease).

Transcription factors

Two types: 1) Activators – these are small proteins

that bind to enhancer sequences or RNA polymerase. They cause an increase in transcription.

2) Repressors – these are small proteins that bind to silencer regulatory genes. They cause a decrease in transcription.

Coding region of gene

Promoter region of DNA

RNA polymerase Transcription factors that

bind to RNA polymeraseTranscription factors

(activators) that bind to

the enhancer sequence

Enhancer sequence

of DNA

Role of Transcription

Factors

Eukaryotic RNA polymerase cannot, on its own, initiate transcription.

It depends on transcription factors to recognize and bind to the promoter.

Transcription factors also bind to the enhancer sequence of DNA

Transcription factors that

bind to RNA polymerase

Coding region of geneEnhancer

sequence of DNA

Transcription factors

(activators) that bind to

the enhancer

Promoter region of DNA

RNA polymerase

Activating Transcription

Transcription is activated when a hairpin loop in the DNA

brings the transcription factors on the enhancer sequence

(activators) in contact with the transcription factors bound to

the RNA polymerase at the promoter.

Protein-protein interactions are crucial to eukaryotic

tanscription.

The RNA polymerase can only produce a mRNA molecule

once the complete initiation complex is assembled.

Enhancer Promoter

RNA polymerase

ActivatorsTranscription factors

bound to RNA polymerase

Initiation complex

Transcription proceeds

until a terminator

sequence is encountered.

Then transcription stops.

http://highered.mcgraw-hill.com/olc/dl/120080/bio28.swf

Regulation Prokaryotes Eukaryotes

1. Genes for a metabolic pathway are linked together in operons with a common switch mechanism (operator).

2. No introns-no RNA processing

3. The structural genes undergo transcription and translation simultaneously.

4. Regulation occurs by switching all genes of a pathway on or off.

1. Pathways are separated, not grouped by operons.

2. Genes are switched on separately.

3. Genes have introns that are removed in RNA processing.

4. Transcription and translation do not occur simultaneously.

5. Genes have a large number of control elements.