Lectures

Repressible Operons

by Georgina Cornwall, PhD
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    00:00 Let us look at the opposite case. We looked at a system that was OFF in its normal state and now we are going to look at a system that is ON in its normal state. When the cell is doing its things, the trp operon is going to be the repressible system. If it is ON, then we need to repress it to turn if OFF and shut down production of the genes downstream.

    00:32 In this example, we are talking about tryptophan or the trp operon and tryptophan is amino acid and it has a metabolic pathway to generate tryptophan and if we want to make tryptophan, we need to have those genes ON. If we don't want to make tryptophan, we don't need to have those genes ON. Here the repressor protein that is made from the upstream gene is an inactive repressor protein. That is, it cannot bind to the operator region on the DNA and so it bounces off and genes are transcribed. The polymerase can be passed on the strand and transcribe each of the genes involved in manufacturing tryptophan because we're low in tryptophan. If we have tons of tryptophan, this is not really necessarily. Here all the enzymes that are involved in the manufacture of tryptophan, but if tryptophan is present, we have got plenty of it. We don't need to make any, then tryptophan can bind to the regulatory protein and activate it and that protein will then bind onto the operator region on the DNA and that will prevent polymerase from moving down the strand. Here we can have plenty of tryptophan, we don't need to make any, so that tryptophan itself activates a repressor turns OFF expression of the genes so that we cannot make any of the enzymes involved in the manufacture of tryptophan. Make sense right? Complicated, but I recommend again here you take out a piece of paper and sort of diagram, an inducible and repressible operon. Inducible means that it is OFF. We need to induce it to work. Repressible means that it is ON and we need to repress it if we don't want those products made. We use the trp operon and the lac operon as examples for those situations. That has been looking at all of the mechanisms of possibility for control


    About the Lecture

    The lecture Repressible Operons by Georgina Cornwall, PhD is from the course Gene Regulation.


    Included Quiz Questions

    1. Lactose operon acts as a repressible operon in the presence of glucose but acts as an inducible operon in the presence of cAMP or lactose.
    2. RNA polymerase keeps on transcribing the repressible operon in the normal state.
    3. The presence of tryptophan shuts down the activity of trp operon by binding to the repressor protein.
    4. The tryptophan operon is an excellent example of the repressible operon in the bacterial cells.
    5. The activated repressor protein binds to the operator site of the trp operon and halts the movement of RNA pol to transcribe trp genes.

    Author of lecture Repressible Operons

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD


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