Gene Regulation

… on / off

… fast / slow

Summary:When and where do cells have control over which

genes are expressed and which proteins are active?

Proteins are activated through processing and signals can enlist cellular machinery to destroy proteins

Proteins determine when and how often RNA polymerase can bind

Modifications to pre-mRNA affect the life and use of RNA in the cytoplasm

Newer Ideas to Look For• The study of epigenetics

investigates ways that whole sections of chromosomes are activated and deactivated.

Looking at Individual Genes

• The simplest and most well understood model of the regulation of individual genes is found in prokaryotes.

• We will study two examples of gene control in E. coli ; a bacteria found in the human digestive tract

FEED

ME!

The Operon

Coding Region: Gene(s)

Control Region

Operon

The Lac Operon in E. coli

GenesControl Region

Produce the enzymesnecessary for digesting lactose

Respond to the presence or absence of lactose and

glucose

The Lac Operon in E. coli

lacZ lacY lacA

operator

promoter

activator

ON

OFF

Each lac gene codes for an enzyme only needed when lactose is present

FAST

SLOW

Binding site for RNAP

(CAP binding site)

The Lac Operon in E. coli

The Lac Operon in E. coli• With no proteins bound to either the

operator or activator regions, RNAP is free to bind to the promoter and transcribe the genes

lacZ lacY lacAOPA

pre-mRNA

The Lac Operon in E. coli• If there is no lactose present, this

would be a waste of valuable resources like ATP and amino acids

lacZ lacY lacAOPA

pre-mRNA

The Lac Operon in E. coli• To prevent transcription, the genes

can be “turned off” when a repressor protein is bound to the operator sequence

lacZ lacY lacAOPA

repressor

GET OUT OF THE WAY!

The Lac Operon in E. coli• When lactose is present, it will bind

to the repressor, change its 3D shape and remove it from the operator.

lacZ lacY lacAOPA

The Lac Operon in E. coli• When lactose is present, it will bind

to the repressor, change its 3D shape and remove it from the operator.

lacZ lacY lacAOPA

The Lac Operon in E. coli• RNAP is now free to bind the

promoter and transcribe the sequence

lacZ lacY lacAOPA

The Lac Operon in E. coli• Since the presence of lactose removes

the repressor and “turns on” the genes, lactose is called an inducer.

lacZ lacY lacAOPA

repressor

inducer

What it “really” looks like

The Lac Operon in E. coli

Operon is OFF

lacZ lacY lacAOPA

The Lac Operon in E. coli

Operon is ON

lacZ lacY lacAOPA

The Lac Operon in E. coli• Glucose is the preferred energy

source for cells

glucose

lactoseCell

OM NOM NOM NOM NOM

The Lac Operon in E. coli• If glucose levels are LOW,

lactose metabolism is prioritized

• If glucose levels are HIGH, lactose metabolism is of lower priority

glucose

LOW

cyclic AMP (cAMP)

HIGH

How can glucose levels be detected in the nucleus?

cAMP and CAP

• cAMP binds to the activator protein “CAP” (Catabolite activator protein)

• Together they will bind to DNA ahead of the promoter to help RNAP bind

cAMP and CAP

• When glucose levels are low, cAMP activated CAP proteins will stimulate the transcription of over 100 genes including the lac operon

The Lac Operon in E. coli• Without the cAMP-CAP complex

bound to the activator, RNAP is less likely to bind and produces fewer transcripts in a given time

lacZ lacY lacAOPA

“SLOW”

The Lac Operon in E. coli• With the cAMP-CAP complex bound to

the activator, RNAP binds more readily, speeding up the production of RNA transcripts

lacZ lacY lacAOPA

“FAST”

3D model of cAMP-CAP complex

bound to DNA

http://proteopedia.org/wiki/index.php/Catabolite_gene_activator_protein

Summary

glu

cose

lact

ose

ON

/ O

FF

FAST

/ SLO

W

lacZ lacY lacAOPA

lacZ lacY lacAOPA

lacZ lacY lacAOPA

lacZ lacY lacAOPA

OFF

OFF

ON

ON

FAST

SLOW

The trp Operon in E. coli

• If ingested levels of theamino acid tryptophanare low, e. coli is ableto synthesize it on its own.

• When tryptophan levels are low, the try operon is turned on to make the enzymes necessary to synthesize more.

The trp Operon in E. coli

• Again, the cells should not waste resources making more tryptophan if it is already present

The trp Operon in E. coli

• No tryptophan = operon on• Enzymes are synthesized to make trp

trpEOP trpD trpC trpB trpA

The trp Operon in E. coli

• Tryptophan present = operon off• Trp is a co-repressor

(needed for repressor to function)

trpEOP trpD trpC trpB trpA

repressor

co-repressor

What it “really” looks like

Negative gene regulation

• Repressors are used to turn off genes

lacZ lacY lacAOPA

repressor

inducer

repressor

co-repressor

trpEOP trpD trpC trpB trpA

Negative Gene RegulationThe lac Operon

Positive Gene RegulationThe lac Operon

• On your own, summarize the post transcriptional and post translational control cells have over their genes using your textbook

Co-Repressor