Topic 21, Trp operon.ppt.edu

Topic 21, Trp operon.ppt.edu - Topic 21: the trp operon...

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Topic 21: the trp operon Please watch and study the videos before coming to class . This is one of the more complicated processes we will cover this semester, but the videos are excellent to help you to understand what is taking place.
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Learning objectives Be able to define the following terms: attenuator, polynucleotide phosphorylase Be able to diagram/explain how & why the trp operon is regulated at the levels of: Transcription initiation Transcription elongation/termination Be able to explain the fine-tuning method of the trp operon and be able to predict low/intermediate/high levels of trp operon gene expression based upon certain scenarios Be able to identify/distinguish between the 2 categories of bacterial enzymes used to degrade mRNAs
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Quick review What is tryptophan? What is it used for? Under what conditions would a bacterial cell need a supply of tryptophan?
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The purpose of the lac operon is to break down lactose for use as an energy source No lactose? Then turn off genes needed to catabolize lactose (lac operon; inducible expression) The purpose of the trp operon is to synthesize tryptophan for use in making new proteins Tryptophan already abundant? Then turn off production of enzymes that synthesize tryptophan (trp operon; repressible expression) Why? It is a waste of energy to make it unnecessarily
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How do you “make” tryptophan? The trp operon does NOT itself produce tryptophan, it encodes the proteins necessary to make tryptophan from scratch
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DO need to know: organization/structure of the operon (red box) DON’T need to know: specifics--names of enzymes or their functions Organization of the trp operon P,O=promoter/operator L=leader Rest are structural enzymes
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Goal of trp operon: only make tryptophan ( actually, trp-producing enzymes ) when insufficient levels of tryptophan are present in environment Mechanism: block trp expression at multiple levels Transcription initiation (on or off, no fine-tuning) Transcription elongation Transcription termination Fine-tuning of expression
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How is trp expression blocked at the level of transcription initiation (Level 1)?
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Transcription occurs, the encoded enzymes produce tryptophan Plenty of tryptophan present: Tryptophan binds to repressor, thus changing it’s shape and allowing it to bind to P/O: transcription is blocked --Compare/contrast to lac repressor: *What binds to repressor to modify DNA binding? *Is repressor alone bound to DNA or not?
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Topic 21, Trp operon.ppt.edu - Topic 21: the trp operon...

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