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Control of Expression In Bacteria –Part 1 Why control expression? Operons in bacteria Negative Control Positive Control Inducible Operons Repressible Operons.

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Presentation on theme: "Control of Expression In Bacteria –Part 1 Why control expression? Operons in bacteria Negative Control Positive Control Inducible Operons Repressible Operons."— Presentation transcript:

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

3 What happens during transcription? 1.RNA polymerase must successfully bind to the promoter. 2.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. mRNA Learning target: I can explain how and why gene expression is controlled in bacteria. promoterGene sequenceDNA RNA polymerase

4 Gene 1Gene 2Gene 3Gene 4Gene 5promoter DNA control sequences are found either upstream or downstream (or both) from the promoter Learning target: I can explain how and why gene expression is controlled in bacteria. Bacterial genes are organized in operons Operon – cluster of genes regulated as a unit controlled by a single promoter. Enzyme 1 Enzyme 2 Enzyme 3Enzyme 4Enzyme 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).

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? Gene 1Gene 2Gene 3Gene 4Gene 5 promoter operator In negative control, the control sequence lies DOWNSTREAM from the promoter and is called an operator. 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. repressor RNA polymerase mRNA transcribedNO mRNA transcribed RNA polymerase Repressor protein Learning target: I can explain how and why gene expression is controlled in bacteria. RNA polymerase inhibits

6 Only rarely is mRNA transcribed Positive Control Gene 1Gene 2Gene 3Gene 4Gene 5 promoter control If the operon is under positive control, then binding of a regulatory protein to a control sequence __________ transcription. How does it work? In positive control, the control sequence lies UPSTREAM from the promoter. 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. RNA polymerase mRNA transcribed Activator protein Learning target: I can explain how and why gene expression is controlled in bacteria. activates

7 What is the effector and how does it play a role? 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. 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. 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 Gene 1Gene 2Gene 3Gene 4 promoter Let’s build an operon! Example: Repressible operon under negative control operator promoter What kind of product would these genes make? What control sequence would this operon have? 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 mRNA Amino acid inactive repressor active repressor This kind of effector is often called a “co-repressor” repressor always expressed Gene for mRNA Amino acid Learning target: I can explain how and why gene expression is controlled in bacteria. But the amino acid effector gets used in protein synthesis, and soon its concentration falls.

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


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