Chapter 16

Control of Gene Expression

Topics to discuss

• DNA binding proteins

• Prokaryotic gene regulation– negative inducible: lac operon– negative repressible: trp operon

• Eukaryotic gene regulation

6 groups of DNA-binding regulatory proteins have been

identified

Prokaryotic Gene Regulation

• Operons

• Operons provide coordinate expression

A model operon

Now let’s look at 2 (of four) groups of operons

Negative inducible operons

Negative repressible operons

INDUCIBLE

Negative inducible operon

Turned on

repressor

Now let’s look at 2 specific operons

• The Lactose operon: negative inducible

• The Tryptophan operon: negative repressible

Francois Jacob and Jacques Monod 1961

• Studied lactose metabolism/operon in prokayotes

J. Monod

Won the Nobel Prize in 1965

Sources of prokaryotic energy

• 1. Disaccharide

• 2. Monosaccharide

• Lactose

• Glucose

Sources of prokaryotic energy

• Monosaccharide

• Disaccharide

• Glucose

• Lactose

Building the lac operon

Z Y AOPI

Lac Operon and Proposed Arrangement with control I gene

Not technically part of the “operon”

Lactose Metabolism Requires Coordination of all these genes!

Negative inducible operon

(assume only lactose is present)

Next: let’s review just the pattern of expression when:

• When only lactose is present.

• When only glucose is present.

• Glucose and lactose are present in the cell

“Why does the presence of glucose influence lactose metabolism?Lac

To answer this question you need to understand this reaction

Adenylyl cyclase Cyclic AMPATP

Glucose may inhibit this enzyme

catabolite activator protein

+

Now put the pieces all together.

How CAP and cAMP affects the promoter

So….Conditions: Only lactose is present

But…Conditions: Only glucose

And Finally….Glucose and Lactose

Summary

Trypthophan Operon

• 5 genes involved in the synthesis of the amino acid trypthophan

Negative repressible operon

2 shapes to the mRNA

When trp is high When trp is low

Ribosome does not stall

Ribosome stalls

Changes in Chromatin structure and eukaryotic gene regulation

• Histone acetylation

• DNA methylation

• RNA silencing

Do eukaryotes show coordinate gene regulation?

Yes, the same response element may be found in related genes.

Also remember gene expression through RNA interference!