1. PowerPoint Presentation Materials to accompany
Genetics: Analysis and Principles
Robert J. Brooker
CHAPTER 14
GENE REGULATION IN BACTERIA AND BACTERIOPHAGES
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2. INTRODUCTION
The term gene regulation means that the level of gene expression can vary under different conditions
Genes that are unregulated are termed constitutive
They have essentially constant levels of expression
Frequently, constitutive genes encode proteins that are necessary for the survival of the organism
The benefit of regulating genes is that encoded proteins will be produced only when required
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3. INTRODUCTION
Gene regulation is important for cellular processes such as
1. Metabolism
2. Response to environmental stress
3. Cell division
Regulation can occur at any of the points on the pathway to gene expression
Refer to Figure 14.1
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4. Figure 14.1
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5. 14.1 TRANSCRIPTIONAL REGULATION
The most common way to regulate gene expression in bacteria is at the transcriptional level
The rate of RNA synthesis can be increased or decreased
Transcriptional regulation involves the actions of two main types of regulatory proteins
Repressors  Bind to DNA and inhibit transcription
Activators  Bind to DNA and increase transcription
Negative control refers to transcriptional regulation by repressor proteins
Positive control to regulation by activator proteins
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6. Small effector molecules affect transcription regulation
However, these bind to regulatory proteins and not to DNA directly
In some cases, the presence of a small effector molecule may increase transcription
These molecules are termed inducers
They function in two ways
Bind activators and cause them to bind to DNA
Bind repressors and prevent them from binding to DNA
Genes that are regulated in this manner are termed inducible
In other cases, the presence of a small effector molecule may inhibit transcription
Corepressors bind to repressors and cause them to bind to DNA
Inhibitors bind to activators and prevent them from binding to DNA
Genes that are regulated in this manner are termed repressible
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7. 14-7 Regulatory proteins have two binding sites Figure 14.2
One for a small effector molecule
The other for DNA
Figure 14.2
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8. Figure 14.2
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9. The Phenomenon of Enzyme Adaptation
At the turn of the 20th century, scientists made the following observation
A particular enzyme appears in the cell only after the cell has been exposed to the enzyme’s substrate
This observation became known as enzyme adaptation
François Jacob and Jacques Monod at the Pasteur Institute in Paris were interested in this phenomenon
They focused their attention on lactose metabolism in E. coli to investigate this problem
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10. The lac Operon
An operon is a regulatory unit consisting of a few structural genes under the control of one promoter
It encodes polycistronic mRNA that contains the coding sequence for two or more structural genes
This allows a bacterium to coordinately regulate a group of genes that encode proteins with a common function
An operon contains several different regions
Promoter; terminator; structural genes; operator
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11. There are two distinct transcriptional units 1. The actual lac operon
Figure 14.3a shows the organization and transcriptional regulation of the lac operon genes
There are two distinct transcriptional units
1. The actual lac operon
a. DNA elements
Promoter  Binds RNA polymerase
Operator  Binds the lac repressor protein
CAP site  Binds the Catabolite Activator Protein (CAP)
b. Structural genes
lacZ  Encodes b-galactosidase
Enzymatically cleaves lactose and lactose analogues
Also converts lactose into allolactose (an isomer)
lacY  Encodes lactose permease
Membrane protein required for transport of lactose and analogues
lacA  Encodes transacetylase
Covalently modifies lactose and analogues
Its functional necessity remains unclear
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12. There are two distinct transcriptional units
Figure 14.3a shows the organization and transcriptional regulation of the lac operon genes
There are two distinct transcriptional units
2. The lacI gene
Not considered part of the lac operon
Has its own promoter, the i promoter
Constitutively expressed at fairly low levels
Encodes the lac repressor
The lac repressor protein functions as a tetramer
Only a small amount of protein is needed to repress the lac operon
There is usually ten tetramer proteins per cell
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13. Figure 14.3
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14. The lac Operon Is Regulated By a Repressor Protein
The lac operon can be transcriptionally regulated
1. By a repressor protein
2. By an activator protein
The first method is an inducible, negative control mechanism
It involves the lac repressor protein
The inducer is allolactose
It binds to the lac repressor and inactivates it
Refer to Figure 14.4
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15. RNA pol cannot access the promoter
Constitutive expression
The lac operon is now repressed
Therefore no allolactose
Figure 14.4
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16. 14-16 Figure 14.4 Translation The lac operon is now induced
The conformation of the repressor is now altered
Repressor can no longer bind to operator
Some gets converted to allolactose
Figure 14.4
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17. 14-17 Figure 14.5 The cycle of lac operon induction and repression
Repressor does not completely inhibit transcription
So very small amounts of the enzymes are made
The cycle of lac operon induction and repression
Figure 14.5
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18. The lac Operon Is Also Regulated By an Activator Protein
The lac operon can be transcriptionally regulated in a second way, known as catabolite repression
When exposed to both lactose and glucose
E. coli uses glucose first, and catabolite repression prevents the use of lactose
When glucose is depleted, catabolite repression is alleviated, and the lac operon is expressed
The sequential use of two sugars by a bacterium is termed diauxic growth
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19. The lac Operon Is Also Regulated By an Activator Protein
The small effector molecule in catabolite repression is not glucose
This form of genetic regulation involves a small molecule, cyclic AMP (cAMP)
It is produced from ATP via the enzyme adenylyl cyclase
cAMP binds an activator protein known as the Catabolite Activator Protein (CAP)
Also termed the cyclic AMP receptor protein (CRP)
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20. The lac Operon Is Also Regulated By an Activator Protein
The cAMP-CAP complex is an example of genetic regulation that is inducible and under positive control
The cAMP-CAP complex binds to the CAP site near the lac promoter and increases transcription
In the presence of glucose, the enzyme adenylyl cyclase is inhibited
This decreases the levels of cAMP in the cell
Therefore, cAMP is no longer available to bind CAP
Transcription rate decreases
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21. 14-34 Figure 14.8 (b) Lactose but no cAMP
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22. Figure 14.8
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23. The lac Operon Has Three Operator Sites for the lac Repressor
Detailed genetic and crystallographic studies have shown that the binding of the lac repressor is more complex than originally thought
In all, three operator sites have been discovered
O1  Next to the promoter
O2  Downstream in the lacZ coding region
O3  Slightly upstream of the CAP site
Refer to Figure 14.9
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24. 14-37 Figure 14.9 Repression is 1,300 fold
Therefore, transcription is 1/1,300 the level when lactose is present
No repression
ie: Constitutive expression
The identification of three lac operator sites
Figure 14.9
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25. The results of Figure 14.9 supported the hypothesis that the lac repressor must bind to two of the three operators to cause repression
It can bind to O1 and O2 , or to O1 and O3
But not O2 and O3
If either O2 or O3 is missing maximal repression is not achieved
Binding of the lac repressor to two operator sites requires that the DNA form a loop
A loop in the DNA brings the operator sites closer together
This facilitates the binding of the repressor protein
Refer to Figure 4.14
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26. Each repressor dimer binds to one operator site
Figure 14.10
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