Some operons are repressible

Putting it all together Theoretically, operons might exhibit positive or negative control and be either inducible or repressible. Try sketching out all possible types— negative inducible, negative repressible, positive inducible, and positive repressible. To do so, learn the meanings of positive and negative control and inducible and repressible; then use logic to work out the details of whether the regulatory protein is a repressor or an activator and whether it is produced in an active or inactive form. You can check your answers against Table 16.2, where the important features of these four types of operons are summarized. Another useful exercise is to think about the effects of mutations at various sites in different types of operon systems.

Although it is a useful learning device to think of operons as either positive or negative and either inducible or repressive, in reality both positive and negative controls often exist in the same operon.

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