A recombinant protein is to be expressed under the control of the lac promoter and operator in a strain of E. coli having the genotype $lacI^+ \ crp^+$. Even in the absence of inducer IPTG, low levels of expression of the recombinant protein are seen (leaky expression). Which one of the following should be done to minimize such leaky expression?
Minimizing Leaky Lac Promoter Expression
The goal is to reduce the basal or "leaky" expression of a recombinant protein controlled by a lac promoter and operator in E. coli, even without the inducer.
Lac Operon Regulation Factors
- Lac Repressor ($lacI^+$): The wild-type ($lacI^+$) gene product strongly binds to the operator sequence, preventing transcription.
- CRP ($crp^+$) and Glucose: The Catabolite Repressor Protein (CRP), activated by cAMP, binds near the lac promoter to enhance transcription. cAMP levels are inversely proportional to glucose concentration. High glucose leads to low cAMP and weak CRP binding, reducing promoter activation. Low glucose leads to high cAMP and strong CRP binding, enhancing promoter activation. This is known as catabolite repression.
- Inducer (IPTG/Alolactose): Binds the repressor, causing it to release from the operator, allowing transcription.
Analysis of Leaky Expression
Leaky expression occurs because the repressor may not bind the operator perfectly 100% of the time. To minimize this, we need conditions that keep the promoter in its least active state:
- The repressor ($lacI^+$) is present, which is good for repression.
- Promoter activation by CRP-cAMP should be minimized. This occurs when glucose levels are high.
Evaluating Options to Minimize Leakiness
- 1. Addition of lactose: Lactose acts as an inducer. It binds the repressor, causing it to detach, thus increasing expression, not minimizing leakiness.
- 2. Removal of all glucose: This leads to high cAMP levels, strong CRP-cAMP binding, and enhanced promoter activity. This would likely increase leakiness.
- 3. Addition of excess glucose: High glucose results in low cAMP levels. This prevents effective CRP-cAMP binding to the promoter, minimizing its basal activity. Combined with the repressor action, this significantly reduces transcription initiation and thus minimizes leaky expression.
- 4. Addition of allo-lactose: Allo-lactose is the actual inducer molecule. Similar to lactose, it would bind the repressor and increase expression.
Conclusion
Adding excess glucose to the medium minimizes leaky expression by reducing CRP-cAMP activation of the lac promoter, complementing the repressor's function.
Important Questions from Regulation of Gene Expression
- Which of the following conditions induce(s) the expression of $ \beta$-galactosidase gene in the lac operon?
Determine the correctness or otherwise of the following Assertion [a] and the Reason [r].
Assertion [a]: In multicellular organisms, cells of different lineages have different gene expression profiles.
Reason [r]: Alternative splicing is the only mechanism to generate protein diversity.- The event(s) that lead(s) to inactivation of tumor suppressor genes in cancer cells is(are)
- Which of the following statement(s) is(are) CORRECT regarding the $lac$ operon in $E. coli$ when grown in the presence of glucose and lactose?
Determine the correctness or otherwise of the following Assertion [a] and the Reason [r]
Assertion: Ab initio gene finding algorithms that predict protein coding genes in eukaryotic genomes are not completely accurate.
Reason: Eukaryotic splice sites are difficult to predict.
