What causes the lac operon to shut off?
1 Answer
The lac operon in E. coli , is under both negative and positive transcriptional controls by the repressor protein and catabolite activator protein ( CAP) respectively.
Explanation:
The lac operon codes for proteins required to transport lactose into the cell and to break it down. CAP enables bacteria to use alternative carbon sources such as lactose in the absence of glucose. The lac repressor ensures that the lac operon is shut off in the absence of lactose.
This arrangement enables the control region of lac operon to respond to and integrate two different signals , so that the operon is highly expressed only when two conditions are met ---
1) lactose must be present
2) glucose must be absent
Any of the other three possible signal combinations maintain the cluster of genes in the off state.
Glucose and lactose levels control the initiation of transcription of the lac operon through their effects on the lac repressor protein and CAP.
Lactose addition increases the concentration allolactose, which binds to the repressor protein and removes it from the DNA.
Glucose addition decreases the concentration of cyclic AMP. Cyclic AMP does not bind to CAP , this gene activator protein dissociates from the DNA , turning off the operon.
