Describe the differences between inducible and repressible operons. How does attenuation differ from inducible or repressible operons? Which of the two inducible operons you've studied would be active if a cell has the following food sources? a

Question

only lactose b. only glucose c. lactose and glucose

Answer 1

Operons like lac and ara that are involved in catabolism of alternative energy sources are typically inducible, since they are called upon only when glucose is depleted and the alternative sugar is available. The presence of the effector molecule (inducer) is needed to activate transcription. In contrast, repressible operons are involved in anabolic pathways (pathways that synthesize compounds needed by the cell) and can be regulated by negative feedback mechanisms that operate through activity of the end product of the pathway to block operon gene transcription. For these, the presence of the effector molecule (corepressor) is needed to inactivate transcription. Some repressible operons can also fine-tune the transcription to help achieve the steady-state amount of a given compound. This lets the cell make more product when needed, but stop when the cell has reached its necessary amount. So, whereas inducible and repressible are simple on-off systems, attenuation allows a wider range of transcriptional activation levels. Furthermore, the molecular mechanisms differ between inducible and repressible operons as compared to attenuation. Inducible and repressible operons are regulated through the interactions between proteins and DNA whereas attenuation involves RNA-RNA interactions in addition to the interaction between mRNA and protein (specifically RNA polymerase)
a. only lactose = lac operon
b. only glucose = none
c. lactose and glucose = none