The Lac Operon – An Exercise in Systemic Problem Solving

By A.K. Matai

In Escherichia coli, the lac operon allows for the transport and metabolism of lactose. Wild type (WT) E. coli prefers to use glucose as oppose to lactose as its energy source. Therefore, both negative and positive regulation occurs to insure that the lac operon remains tightly regulated and can be induced under appropriate conditions.

The Lac Operon - An Exercise in Systemic Problem Solving Lac Operon Overview.JPG

Figure 1. The lac operon. B-Gal breaks down lactose. Permease transports lactose into the cell. The function of trans acetylase in the lac operon is currently not clearly understood.

To explain how the lac operon works, I will go through four possible conditions that could occur in WT E. coli: high glucose/high lactose levels, high glucose/low lactose levels, low glucose/high lactose levels and low glucose/low lactose levels.

Case 1: High Glucose/High Lactose

  1. In a cell with high levels of glucose, lower levels of ATP will be catalyzed to cAMP (cyclic adenosine monophosphate). CAP (catabolite activator protein) is a transcriptional activator that requires cAMP to increase it affinity for DNA. Since there are low levels of cAMP present in the cell, CAP will not be able to efficiently bind to the CAP binding site and recruit RNA polymerase (RNAP) to transcribe the lac operon.
  2. High lactose levels in the cell will allow lactose to bind the repressor transcribed and translated from the lac I gene. Since the repressor is sequestered by lactose, it can no longer bind to the operator and repress transcription of the lac operon.

Conclusion: Since, the repressor is no longer bound to the operator, transcription of the lac operon can occur. RNAP will bind to the promoter but not as efficiently. Therefore, the lac operon will be transcribed but the B-galactosidase (B-gal) activity will be at 1%.

Case 2: High Glucose/Low Lactose

  1. High levels of glucose will result in low levels of cAMP. CAP will not be able to efficiently bind to the CAP binding site and recruit RNAP to transcribe the lac operon.
  2. Low levels of lactose will not be able to sequester the repressor. Therefore, the repressor will be able to bind to the operator and prevent the transcription of the lac operon.

Conclusion: The lac operon will not be transcribed and the B-galactosidase (B-gal) activity will be 0%.

Case 3: Low Glucose/High Lactose

  1. Low levels of glucose will result in high levels of cAMP. The cAMP-CAP will be able to efficiently bind to the CAP binding site and recruit RNAP to transcribe the lac operon.
  2. High levels of lactose will be able to sequester the repressor. Therefore, the repressor will not be able to bind to the operator to prevent transcription of the lac operon.

Conclusion: The lac operon will be transcribed and the B-galactosidase (B-gal) activity will be 100%.

Case 4: Low Glucose/Low Lactose

  1. Low levels of glucose will result in high levels of cAMP. The cAMP-CAP will be able to efficiently bind to the CAP binding site and recruit RNAP to transcribe the lac operon.
  2. Low levels of lactose will not be able to sequester the repressor. Therefore, the repressor will be able to bind to the operator and prevent the transcription of the lac operon.

Conclusion: The lac operon will not be transcribed and the B-galactosidase (B-gal) activity will be 0%.

Problems concerning the lac operon will revolved around your basic understanding of how the lac operon works. Therefore, it is essential that your approach to solving problems is systemic.

Mutations

There are a few lac operon mutations that you need to know that are not completely obvious. First is the Is mutation which creates a super repressor meaning that the binding of lactose will not alleviate the repression of the repressor. The second mutation is the Oc mutation in which the the repressor or the super repressor can still bind the operator but not as tightly. This results in a “leaky operator” which transcribes B-gal at levels above basal level (hence it is constitutive).

Percentages

In problems pertaining to the percentage expression of B-gal, you will be provided 4 options: 0%, 100%, 1% and 10-20%. Below is a table that summarizes where all of the percentages are coming from.

the-lac-operon-an-exercise-in-systemic-problem-solving-lac-operon-table-1the-lac-operon-an-exercise-in-systemic-problem-solving-lac-operon-table-2

Weekly Tip to Succeed 

Approach each problem with a systemic approach. Consider what happens with the CAP then the repressor to determine the percentage of B-gal expressed in the cell.

Questions, Comments or Concerns?

Feel free to post a comment down below! I am happy to explain or expand on anything I have written in this post.

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