In studying operons, how do inducible and repressible operons differ, with examples?

In studying operons, how do inducible and repressible operons differ, with examples?

• A. Inducible operons are normally off and turned on by an inducer; repressible operons are normally on and turned off by a corepressor.
• B. Inducible operons are normally on and turned off by a repressor; repressible operons are normally off and turned on by an activator.
• C. Inducible operons are controlled by RNA polymerase alone; repressible operons are controlled by chromatin state.
• D. Both operon types are unaffected by metabolites.

Answer: (A)

Inducible and repressible operons regulate gene expression in response to metabolite levels. An inducible operon is kept off by a repressor bound to the operator, preventing transcription. It can be turned on when an inducer molecule binds that repressor, inactivating it and freeing the operator for RNA polymerase to transcribe the genes. A classic example is the lac operon, which is activated when lactose (converted to allolactose) appears and binds the repressors, derepressing transcription. A repressible operon, on the other hand, is usually on and makes its product until the end product is plentiful enough to shut it down. A corepressor binds the repressor, boosting its ability to bind the operator and halt transcription. The tryptophan (trp) operon in bacteria illustrates this: high levels of tryptophan act as a corepressor to turn off the operon and stop producing more tryptophan. So the stated distinction—inducible operons are normally off and turned on by an inducer, while repressible operons are normally on and turned off by a corepressor—is the correct description.

Inducible and repressible operons regulate gene expression in response to metabolite levels. An inducible operon is kept off by a repressor bound to the operator, preventing transcription. It can be turned on when an inducer molecule binds that repressor, inactivating it and freeing the operator for RNA polymerase to transcribe the genes. A classic example is the lac operon, which is activated when lactose (converted to allolactose) appears and binds the repressors, derepressing transcription.

A repressible operon, on the other hand, is usually on and makes its product until the end product is plentiful enough to shut it down. A corepressor binds the repressor, boosting its ability to bind the operator and halt transcription. The tryptophan (trp) operon in bacteria illustrates this: high levels of tryptophan act as a corepressor to turn off the operon and stop producing more tryptophan.

So the stated distinction—inducible operons are normally off and turned on by an inducer, while repressible operons are normally on and turned off by a corepressor—is the correct description.