1. Control of Expression In Bacteria –Part 1
Why control expression?
Operons in bacteria
Negative Control Positive Control
Inducible Operons Repressible Operons
Effectors
Building a control system
Learning target: I can explain how and why gene expression is controlled in bacteria.

2. Why control expression?
Some enzymes are needed all the time; feedback inhibition regulates their activity.
Regulation of
transcription
Feedback Inhibition
Other enzymes are needed only sometimes; cells regulate their production in order to use resources efficiently.
Regulation is usually done using by turning transcription on and off.
Learning target: I can explain how and why gene expression is controlled in bacteria.

3. What happens during transcription?
RNA polymerase
promoter
Gene sequence
DNA
mRNA
RNA polymerase must successfully bind to the promoter.
RNA polymerase must be able to read the DNA base code and make mRNA.
Big Question:
How do cells turn transcription on and off?
In eukaryotes, each gene is regulated separately.
In bacteria, genes whose protein products act in the same pathway are regulated together.
Learning target: I can explain how and why gene expression is controlled in bacteria.

4. Bacterial genes are organized in operons
Operon – cluster of genes regulated as a unit controlled by a single promoter.
Gene 1
Gene 2
Gene 3
Gene 4
Gene 5
promoter
DNA control sequences are found either upstream or downstream (or both) from the promoter
Enzyme 1
Enzyme 2
Enzyme 3
Enzyme 4
Enzyme 5
Biochemical pathway
Transcription is controlled by the binding of a regulatory protein to a control sequence.
That binding can either: inhibit transcription (negative control) or
activate transcription (positive control).
Learning target: I can explain how and why gene expression is controlled in bacteria.

5. Negative Control
If an operon is under negative control, then the binding of a regulatory protein to a control sequence __________ transcription. How does it work?
inhibits
In negative control, the control sequence lies DOWNSTREAM from the promoter and is called an operator.
Repressor
protein
RNA polymerase
RNA polymerase
repressor
RNA polymerase
Gene 1
Gene 2
Gene 3
Gene 4
Gene 5
promoter
operator
NO mRNA transcribed
mRNA transcribed
Without the regulatory protein bound, transcription occurs.
If the regulatory protein binds to the operator, it physically prevents the RNA polymerase transcribing the genes.
Because this kind of regulatory protein represses the expression of the genes in negative control systems, the regulatory protein is usually called a repressor protein.
Learning target: I can explain how and why gene expression is controlled in bacteria.

6. Positive Control
If the operon is under positive control, then binding of a regulatory protein to a control sequence __________ transcription. How does it work?
activates
In positive control, the control sequence lies UPSTREAM from the promoter.
Activator
protein
RNA polymerase
RNA polymerase
Gene 1
Gene 2
Gene 3
Gene 4
Gene 5
promoter
control
Only rarely is mRNA transcribed
mRNA transcribed
Without a regulatory protein, RNA polymerase does not find or bind well to the promoter.
The regulatory protein binds to the control sequence and helps RNA polymerase bind more successfully to the promoter.
Because the regulatory protein activates the expression of the genes in positive control systems, the protein is called an activator protein.
Learning target: I can explain how and why gene expression is controlled in bacteria.

7. What is the effector and how does it play a role?
Whether the regulatory protein binds or not depends on the presence of an effector – a molecule from the environment that the cell uses to know whether the operon should be transcribing or not.
What role the effector plays depends on whether this operon is inducible or repressible.
An inducible operon is normally “off”– it can be induced to come on; it is NOT transcribing unless it is turned “on” by the presence of the effector.
A repressible operon is normally “on” – it can be repressed (turned off); it IS transcribing unless it is turned “off” by the presence of the effector.
Examples:
An operon that makes the enzymes for synthesis of an amino acid is repressible – amino acids are required all the time.
An operon that makes the enzymes to break down an unusual sugar is inducible – it needs to come on only when the cell encounters that sugar.
Learning target: I can explain how and why gene expression is controlled in bacteria.

8. What are the effectors?
In a repressible operon that makes enzymes to produce an amino acid, under what conditions would the cell want to stop production of those enzymes?
If plenty of the amino acid was present.
What would be a good effector molecule to tell the cell that there’s lots of amino acid around?
The amino acid itself!
In an inducible operon that makes enzymes to break down an unusual sugar, under what conditions would the cell want to start production of those enzymes?
If the sugar was present.
What would be a good effector molecule to tell the cell that the sugar is around?
The sugar itself!
Learning target: I can explain how and why gene expression is controlled in bacteria.

9. Let’s build an operon!
Example: Repressible operon under negative control
What control sequence would this operon have?
What kind of product would these genes make?
When there’s enough amino acid made for now, the regulator protein will bind to the operator site. (It would be called a _______________.)
repressor protein
But the amino acid effector gets used in protein synthesis, and soon its concentration falls.
mRNA
mRNA
repressor
always expressed
Gene for
Gene 1
Gene 2
Gene 3
Gene 4
promoter
operator
promoter
Amino acid
Amino acid
active repressor
inactive repressor
This kind of effector is often called a “co-repressor”
Learning target: I can explain how and why gene expression is controlled in bacteria.

10. Putting it all together:
Negative control
Positive control
Repressible
Inducible
The effector binds to the regulatory protein and allows the protein to bind to the operator site
Use the worksheet to figure out how each of the other systems work.
Learning target: I can explain how and why gene expression is controlled in bacteria.