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Chapters 18 & 19

Control of GeneExpression

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Control of Gene Expression

Every cell in your body has the same DNA

Yet, your cells are not identical… Why not? 

Differences in Gene Expression

Only 5-10% of genes are expressed

in a cell (eukaryotic) at any given time

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Gene Expression

Gene Expression  – process by which genetic

information flows from genes to active proteins;

involves transcription and translation of a gene and

protein activation

Differences in gene expression

Differences in proteins that are active in a cell

Differences in cell characteristics 

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Chapter 18

Control of Gene Expression: Prokaryotes

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Mechanisms of Regulation:

Prokaryotes 

DNA mRNA polypeptide active protein  

RNA polymerase

DNA

mRNA

Transcriptional control Translational control Post-translational control

Protein

Ribosome

RNA polymerase

Onset and rate of

transcription

Life span (stability)

of mRNA

Translation

rate

Protein activationor inhibition (by

chemical

modification)

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Mechanisms of Regulation:

Prokaryotes 

DNA mRNA polypeptide active protein  

1. Transcriptional Control• Slow response

• Very efficient

2. Translational Control 

• Rapid response• Less efficient

3. Post-Translational Control

• Fastest response

• Least efficient

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Experimental

Foundations

Glucose is E. coli’s

preferred energy source.

If given a choice between

glucose and lactose, it only

utilizes the glucose. Q: How does glucose

prevent the cell from

utilizing lactose?

A: Glucose affects

expression of genes that

code for enzymes in

lactose catabolism. How

does this work?

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Discovering How Genes are RegulatedThe Crucial Role of Mutant Cells in Research

Easiest to figure out what normally happens by

studying situations where the process can’t happen. 

Lactose mutants: will grow in media containing

glucose but not in media containing lactose

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What has to happen for cells to grow

on lactose?

Lactose uptake: galactoside permease protein

Lactose hydrolysis: beta-galactosidase enzyme

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Regulation of Prokaryotic Gene

Activity: Operons

Operons

cluster of genes with related function

coordinately regulated   – transcribed together into one

mRNA (polycistronic mRNA) – improves efficiency

contains promoter  and operator  sites

 found only in prokaryotes 

gene 1 gene 2 gene 3

operon

operator

promoter

Quick review… 

What is a promoter?

What is its function?

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Positive and Negative Control

Negative Control  – utilizes a repressor; blocks transcription

Positive Control  – utilizes an activator; enables transcription 

Repressors and

activators areregulatory proteins  which are coded for by

regulator genes located

outside of the operon 

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Negative Control of Operons

Example: Regulation of Lactose Metabolism (lac  operon)

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Regulation of Lactose Metabolism

E. coli –  a bacterial component of the “normal flora”

of our intestines (we need them); E. coli sense the

types of foods we eat and respond accordingly! 

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Regulation of Lactose Metabolism

Lac operon controls genes that code for enzymes

used for lactose breakdown (catabolism)

When would you want to make enzymes required forlactose breakdown?

When glucose is present? absent?

When lactose levels are low? high?

When would you expect a repressor to bind to the lac

operon?

When glucose is present? absent?

When lactose levels are low? high?

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Negative Control in the lac Operon

Lactose is an inducer of the lac operon that causes

the repressor to release the operator!

Energy Source: Lactose(NEED to breakdown lactose)

TRANSCRIPTION BEGINS-Galactosidase Permease

mRNA

Lactose breakdown genes 

RNA polymerase

bound to promoter

(blue DNA)Lactose-repressor

Complex released

Repressor

synthesized

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How Does Glucose Inhibit Expression

of the Lac Operon?

Glucose is an allosteric inhibitor of galactoside

permease. When external glucose levels are high… 

No lactose uptake no lactose inducer to bind to

repressor repressor blocks transcription of the

lac operon

XHIGH

[glucose]

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Summary of lac Operon Regulation

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Positive and Negative Control

Negative Control  – utilizes a

repressor; blocks

transcription

Positive Control  – utilizes an

activator; enables

transcription 

Repressors and activators are

regulatory proteins which

are coded for by genes

outside of the operon 

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Structures of Operons

Lac operon (genes for lactose1 catabolism)

Ara operon (genes for arabinose2 catabolism)

1 Predominant sugar in milk 2 Abundant sugar in vegetables

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Positive Control of the ara Operon

When arabinose is available… 

It allosterically activates the araC protein.

AraC protein activates the operon by interacting with an

initiator sequence in the DNA and with RNA polymerase.

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Regulons: Global Controllers of Gene Expression

In the uninduced state,

repressors prevent

regulon gene expression.

External signals removerepressors, causing cells

to express regulons.

Example: SOS regulons

coordinate expression of

genes for DNA repair in

response to UV light or

ionizing radiation.

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