The lac operon in Escherichia coli is a classic example of gene regulation, enabling the bacterium to utilize lactose as an energy source. The lacY gene encodes lactose permease, a membrane protein responsible for transporting lactose into the cell. Mutations in lacY (Y–) disrupt this function, leading to distinct phenotypes under different experimental conditions. This article explores the behavior of lacY mutants, focusing on β-galactosidase synthesis and the consequences for growth in the presence of lactose, IPTG, and TONPG.

Role of Lactose Permease in E. coli

Lactose permease, encoded by the lacY gene, is essential for the active transport of lactose and similar β-galactosides across the bacterial membrane. Without functional permease, E. coli cannot efficiently import lactose from the environment, even if the rest of the lac operon is intact124. This has direct implications for the induction of β-galactosidase, the enzyme responsible for hydrolyzing lactose into glucose and galactose.

Experimental Conditions and Expected Outcomes

Let’s analyze each statement regarding lacY (Y–) mutants:

A. No synthesis of β-galactosidase when Y– cells are induced with lactose.

• Explanation: Without lactose permease, lactose cannot enter the cell, so it cannot act as an inducer for the lac operon. As a result, β-galactosidase is not synthesized24.

B. Synthesis of β-galactosidase when cells are induced with lactose.

• Explanation: This is incorrect for lacY mutants, as explained above.

C. No synthesis of β-galactosidase when cells are induced with IPTG.

• Explanation: IPTG is a gratuitous inducer that can enter the cell independently of lactose permease. Thus, even lacY mutants can induce β-galactosidase synthesis with IPTG24. This statement is incorrect.

D. Synthesis of β-galactosidase when cells are induced with IPTG.

• Explanation: Correct. IPTG bypasses the need for lactose permease, allowing induction of the lac operon and synthesis of β-galactosidase24.

E. The cells induced with IPTG cannot grow in the presence of TONPG.

• Explanation: TONPG is a lactose analog that requires lactose permease for uptake. In lacY mutants, TONPG cannot enter the cell, so it cannot be cleaved by β-galactosidase to release its toxic product. Therefore, these cells can grow in the presence of TONPG24.

F. Cells induced with IPTG can grow in the presence of TONPG.

• Explanation: Correct. As explained above, lack of permease prevents TONPG uptake and toxicity, so these cells survive24.

Correct Combination

Based on the above analysis, the correct combination is:

A, D, and F

This matches Option (1): A, D and F.

Summary Table: Behavior of lacY Mutants

Biological Significance

The lacY gene is crucial for lactose utilization, but not for the induction of the lac operon by non-natural inducers like IPTG. This distinction is important in both basic research and biotechnology, where IPTG is commonly used to induce gene expression in lac-based systems. Additionally, the inability of lacY mutants to import toxic analogs like TONPG highlights the protective effect of permease deficiency in certain environments.

Conclusion

lacY mutants of E. coli demonstrate no β-galactosidase synthesis when induced with lactose, but do synthesize the enzyme when induced with IPTG. These mutants can grow in the presence of TONPG, as the analog cannot enter the cell without permease. This phenotype underscores the essential role of lactose permease in substrate uptake and the unique properties of gratuitous inducers like IPTG.

Keywords: lacY mutant, E. coli, lactose permease, β-galactosidase, IPTG induction, TONPG resistance, lac operon, gene regulation, lactose metabolism, membrane transport.