Regulation of gene expression

References: 1. Stryer: “Biochemistry”, 5th Ed.2. Hames & Hooper: “Instant Notes in

Biochemistry”, 2nd Ed.

1. Basic principles of gene control

What is gene expression?It is a process by which a gene's

information is converted into the structures and functions of a cell.

For a protein encoding gene, it is a process of the transcription of a gene into mRNA, and the latter is translated into protein.

How to control gene expression?

In prokaryotic cells, gene expression is regulated primarily at the level of transcription by metabolites;

In eukaryotes it is controlled by much more complex mechanisms, including the action of protein factors, hormones, etc.

Control of gene expression in prokaryotes

R Structural gene

SubstrateProduct

Enzyme

mRNA

DNA

R

Control of gene expression in eukaryotes

DNA

Protein

mRNARNA

synthesis

InactivemRNA

mRNA

InactiveProtein

Transcript.control

1

RNA processingcontrol

2

RNAtransportcontrol

3mRNA

degradationcontrol

5

Translationcontrol

4

Protein activitycontrol6

Nucleus

Cytosol

2. Regulation of Prokaryotic gene expression– the operon theory

Operons are groups of genes that function to produce proteins needed by the cell.

There are two types of genes in operonsA. Structural genes code for proteins

needed for the normal operation of the cell.

B. Regulator genes code for proteins that regulate other genes.

lac I lac Z lac A lac Y PlacI Plac Olac

Plac : promoter; Olac: operator; lac Z: b-galactosidase; lacY: permease; Lac A: transacetylase; lac I: structural gene for lac repressor; PlacI : promoter for lac I.

Lactose Operon

Regulation of the lac operon in E. coli.

p i p o z y aThe lact Operon

RepressormRNA

Repressor

The repressor binds to the operator region and prevents RNA polymerase from transcribing the structural genes

In the presence of lactose

p i p o z y aThe lact Operon

Lactose binds the repressor and allows RNA polymerase access to the operator, resulting in transcription of the structural gene

b-galactosidase permease transacetylase

RepressormRNA

Repressor

Lactose

mRNA

In the presence of glucose & lactose

p i p o z y aThe lact Operon

glucose

AdenylcyclasecAMP ATP

L actose

Low level of transcription

In the absence of glucose and presence of lactose

p i p o z y aThe lact Operon

L actose

High level of transcription

AdenylcyclasecAMP ATP

CAP

active

CAP: catabolite activator protein

Tryptophan OperonThe trp operon encodes the genes for the

synthesis of tryptophan

L A BP,O a CDE

Genes for enzymes

P: promoter, O: operator, a: attenuator, L: leader sequence, A,B,C,D,E: enzymes

of the Trp pathway

Regulation of the trp operon in E. coli.

Binding of Trp to the trp repressor increases the activity of the repressor, and therefore Trp is known as a co-repressor.

Expression of the trp operon is also regulated by attenuation. The attenuator region contains tandem Trp codons, which play a role to attenuate the transcription of the following genes.

Regulation of the trp operon