Conditions affecting the transcription of Lac operon

Conditions affecting the transcription of Lac operon

Here I am explaining the state of the lac operon under three different plating conditions in the laboratory.

Growth condition 1: E.coli cells grown in the presence of high glucose but no lactose

Positive regulation: Absent

(Reason: The level of cAMP is low, so CAP is not activated).

Negative regulation: Present

(Reason: There is no lactose, so the lac repressor remains bound to the operator)

State of the lac operon: Under these conditions there is no transcription of the lac operon genes.

Condition 2: E.coli cells grown in the presence of both glucose and lactose

Positive regulation: Absent

(Reason: The level of cAMP is low, so CAP is not activated).

Negative regulation: Absent

(Reason: In the presence of lactose, the lac repressor is no longer bound to the operator)

State of the lac operon: Under these conditions there is a low level of transcription of the lac operon genes.

Condition 3: E.coli cells grown in the presence of lactose only

Positive regulation: Present

(Reason: The level of cAMP is high, so CAP is activated)

Negative regulation: Absent

(Reason: In the presence of lactose, the lac repressor is no longer bound to the operator)

State of the lac operon: Under these conditions there is high level of transcription of the lac operon genes.

Table 1: The level of genetic activity of the lac operon as well as the status of the lac repressor and CAP protein under various cellular conditions

Summary

  • For the lac operon to be activated, two conditions must be met. First, the level of glucose must be very low or absent. Second, lactose must be present.
  • Expression of lac operon is never completely shut down. Even with active repressor bound to the operator, there is a low level of transcription and a few molecules of β-galactosidase, permease, and transacetylase are synthesized.
  • Only when glucose is absent and lactose is present, the lac operon is transcribed at a higher level.
  • Collectively, the activator and repressor work together to regulate the gene expression of the lac operon with help from their associated activator or inhibitor molecules (cAMP for the CAP and allolactose for the LacI repressor).

Leave a Reply

Your email address will not be published. Required fields are marked *