Inducible versus repressible
operons
a. Inducible operons are turned on in reponse to a metabolite (a small molecule undergoing metabolism) that regulates the operon. E.g. the lac operon is induced in the presence of lactose (through the action of a metabolic by-product allolactose).
b. Repressible operons are switched off in reponse to a small regulatory molecule. E.g., the trp operon is repressed in the presence of tryptophan.
Note that in this usage, the terms
are defined by the reponse to a
small molecule. Although lac is an
inducible operon, we will see conditions under which it is repressed or induced (via derepression).
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