Class began with a recap of our guest speaker's talk from the Friday before break, followed by a revisit of the quiz on Chapter 17 that we took the Monday before break.
Today's lesson focused on how E. coli regulate the production of tryptophan, an amino acid necessary for the production of many proteins. E. coli are found in our digestive tracts where tryptophan is present only occasionally, like when we consume meat products. E. coli can create tryptophan from a precursor molecule through a series of steps, where each step is catalyzed by a specific enzyme. Tryptophan, itself, acts as an inhibitor on this system, so if tryptophan accumulates in the cell it shuts down its own synthesis. This is feedback inhibition.
Some key points of the trp operon:
1. The genes that are functionally related, like the 5 genes for the enzymes to make tryptophan, are grouped together into an operon.
2. They can be served by a single promoter. Therefore all of the enzymes necessary can be made at once.
3. Just one mRNA is transcribed which contains all of the genes. (later is makes separate polypeptides from separate start and stop codes)
4. There is a single "on-off" switch for the entire unit. This is called an operator.
5. Operator controls access of RNA polymerase to the genes.
Trp operon (for tryptophan)
If the operator is the control point for transcription what determines if it is turned on or off?
· Alone the operator is on and RNA polymerase can bind to the promoter to transcribe the genes.
· The operator can be switched off by a protein called a repressor. When the repressor binds to the operator it blocks access by RNA polymerase.
· Repressor is made by a gene called the regulatory gene, located some distance away from the operon it controls. It has its own promoter.
· There are always a few trp repressor molecules present in the cell. Why is the operon not switched off permanently?
· Reversible binding to operator
· Allosteric enzyme
· Trp repressor is synthesized in an inactive form which does not bind well to operator.
· Tryptophan will bind to it and change it to the active form that can bind to operator.
· Tryptophan = corepressor
· Therefore the bacterium can respond rapidly to changing environment.
Here is a link to an animation of the process: http://bcs.whfreeman.com/thelifewire/content/chp13/1302002.html
HOMEWORK:
Read all about regulation of gene expression by bacteria in 18.4. You can review the trp operon, but also pay special attention to the lac operon. You will be asked to demonstrate understanding of the operon concept by using an interactive paper model of the lac operon that you will begin constructing in class tomorrow.
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