The permease of E. coli that transports an alpha-galactoside known as melibiose can also transport lactose, but the melibiose permease is temperature sensitive: Lactose can be transported into the cell by the melibiose permease at 30 ° C but not at 37 ° C.
In a strain that produces the melibiose permease constitutively, what are the phenotypes of lacZ and lacY mutants at 30 ° C and 37 ° C?
To understand the phenotypes of lacZ and lacY mutants at different temperatures, we need to consider the role of these genes in the transport and metabolism of galactosides.
The lacZ gene in Escherichia coli encodes β-galactosidase, an enzyme responsible for the hydrolysis of lactose into glucose and galactose. The lacY gene encodes the lactose permease, a membrane protein that transports lactose into the cell.
At 30 °C:
In a strain that produces the melibiose permease constitutively, the melibiose permease is functional at both 30 °C and 37 °C. Since the melibiose permease can also transport lactose, it allows lactose to enter the cell at 30 °C. Once inside, β-galactosidase (encoded by lacZ) can break down lactose into glucose and galactose. Therefore, at 30 °C, mutants of lacZ or lacY would not have a significant impact on the ability of the cell to transport and metabolize lactose when the melibiose permease is constitutively produced.
At 37 °C:
The melibiose permease is temperature-sensitive and cannot transport lactose at 37 °C. In this case, the ability of the cell to transport and metabolize lactose primarily depends on the lacY gene, which encodes the lactose permease. A lacZ mutant, which lacks β-galactosidase, would not affect the transport of lactose into the cell but would prevent the hydrolysis of lactose into glucose and galactose. Therefore, even in the presence of functional lactose permease, the lacZ mutant would not be able to metabolize lactose at 37 °C.
On the other hand, a lacY mutant, which lacks the lactose permease, would completely eliminate the ability of the cell to transport lactose into the cell, both at 30 °C and 37 °C. Consequently, if the melibiose permease is non-functional at 37 °C, a lacY mutant would be unable to transport lactose at either temperature.
In summary:
- At 30 °C, a lacZ mutant has no effect on lactose transport but impairs its metabolism, while a lacY mutant would completely block lactose import.
- At 37 °C, both lacZ and lacY mutants would be unable to transport lactose, but a lacZ mutant can still metabolize lactose if it is imported by other means.