Activators and Repressors

There are two different types of gene regulation: positive and negative. Activators (and sometimes inducers) instigate positive regulation, and repressors instigate negative regulation. When an activator or inducer binds to an operon, the transcription process either increases in rate or is allowed to continue. When a repressor binds to an operon, the transcription process is slowed or halted.

Practice Question

Imagine there’s a regulatory molecule that attaches to the operon when a specific protein is present. When this regulatory molecule attaches itself to the operon, transcription is stopped. What type of regulatory molecule is this?

Activator

One example of an activator is the protein CAP. In the presence of cAMP, CAP binds to the promoter and increases RNA polymerase activity. In the absence of cAMP, CAP does not bind to the promoter. Transcription occurs at a low rate.

Practice Question

What is the role of an activator?

Repressor

When an amino acid is present, it associates with the met repressor, and the repressor is activated. RNA synthesis is blocked by the presence of the repressor on the DNA strand. When the amino acid is not present, the repressor dissociates from the operator and RNA synthesis proceeds.

When tryptophan is not present in the cell, the repressor by itself does not bind to the operator; therefore, the operon is active and tryptophan is synthesized. But when a cell has plenty of tryptophan, it doesn’t need to synthesize more. So the repressor is triggered (by the presence of plenty of tryptophan), thus turning off further synthesis of tryptophan.

PRactice Question

What is the role of a repressor?

to limit or restrict the transcription of a certain protein. Remember: a repressor must be turned on before it can turn synthesis off (or down)

Practice Questions

When you remove a repressor from an operon, what would happen?

If you remove a repressor from an operon, transcription will occur more rapidly than the cell needs: there is nothing to prevent (or repress) the transcription

What would happen if you remove an activator?

If you remove an activator from an operon, transcription will more slowly than the cell needs: there is nothing to activate transcription.