Prokaryotic Gene Regulation

The lac Operon: An Inducible Operon

The third type of gene regulation in prokaryotic cells occurs through inducible operons, which have proteins that bind to activate or repress transcription depending on the local environment and the needs of the cell. The lac operon is a typical inducible operon. As mentioned previously, E. coli is able to use other sugars as energy sources when glucose concentrations are low. One such sugar source is lactose. The lac operon encodes the genes necessary to acquire and process the lactose from the local environment. The Z gene of the lac operon encodes beta-galactosidase, which breaks lactose down to glucose and galactose.

However, for the lac operon to be activated, two conditions must be met. First, the level of glucose must be very low or non-existent. Second, lactose must be present. Only when glucose is absent and lactose is present will the lac operon be transcribed (Figure). In the absence of glucose, the binding of the CAP protein makes transcription of the lac operon more effective. When lactose is present, it binds to the lac repressor and changes its shape so that it cannot bind to the lac operator to prevent transcription. This combination of conditions makes sense for the cell, because it would be energetically wasteful to synthesize the enzymes to process lactose if glucose was plentiful or lactose was not available.

Art Connection

The lac operon consists of a promoter, an operator, and three genes named lacZ, lacY, and lacA. RNA polymerase binds to the promoter. In the absence of lactose, the lac repressor binds to the operator and prevents RNA polymerase from transcribing the operon. In the presence of lactose, the repressor is released from the operator, and transcription proceeds at a slow rate. Binding of the cAMP–CAP complex to the promoter stimulates RNA polymerase activity and increases RNA synthesis. However, even in the presence of the cAMP–CAP complex, RNA synthesis is blocked if the repressor binds to the promoter.
Regulation of the lac operon. Transcription of the lac operon is carefully regulated so that its expression only occurs when glucose is limited and lactose is present to serve as an alternative fuel source.

Question: In E. coli, the trp operon is on by default, while the lac operon is off. Why do you think this is the case?

If glucose is present, then CAP fails to bind to the promoter sequence to activate transcription. If lactose is absent, then the repressor binds to the operator to prevent transcription. If either of these conditions is met, then transcription remains off. Only when glucose is absent and lactose is present is the lac operon transcribed (Table).

Signals that Induce or Repress Transcription of the lac Operon
GlucoseCAP bindsLactoseRepressor binds Transcription
+--+No
+-+-Some
-+-+No
-++-Yes

Link to Learning

Watch an animated tutorial about the workings of lac operon here.