Lactose can enter the cell in the absence of lactose permease.
the verdict
CONTESTED
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refutedsupported
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2 sources for · 1 against
Some sources indicate that lactose can enter cells through alternative pathways like mutated transport systems, while other reference texts state that the absence of lactose permease prevents lactose from entering the cell.
The active accumulation of maltose and maltodextrins by Escherichia coli is dependent on the maltose transport system. Several lines of evidence suggest that the substrate specificity of the system is not only determined by the periplasmic maltose-binding protein but that a further level of substrate specificity is contributed by the inner membrane integral membrane components of the system, MalF and MalG. We have isolated and characterized an altered substrate specificity mutant that transports lactose. The mutation responsible for the altered substrate specificity results in an amber stop codon at position 99 of MalF. The mutant requires functional MalK-ATPase activity and hydrolyzes ATP constitutively. It also requires MalG. The data suggest that in this mutant the MalG protein is capable of forming a low affinity transport path for substrate.
The absence of lactose permease leads to the inability of lactose to enter the cell for further metabolic processes. Therefore, lactose permease plays
Lactose permease is a membrane protein which is a member of the major facilitator superfamily. Lactose permease can be classified as a symporter, which uses the proton gradient towards the cell to transport β-galactosides such as lactose in the same direction into the cell.
The protein has twelve transmembrane alpha-helices and its molecular weight is 45,000 daltons. It exhibits an internal two-f
Lactose permease is a membrane protein which is a member of the major facilitator superfamily. Lactose permease can be classified as a symporter, which uses the proton gradient towards the cell to transport β-galactosides such as lactose in the same direction into the…
lactose permease to prevent transport of lactose into the cell. This dual control mechanism causes the sequential utilization of glucose and lactose in
The lactose operon (lac operon) is an operon required for the transport and metabolism of lactose in E. coli and many other enteric bacteria. Although glucose is the preferred carbon source for most enteric bacteria, the lac operon allows for the effective digestion of lactose when glucose is not available through the activity of β-galactosidase. Gene regulation of the lac operon was the first gen
Note that the number of base pairs in diagram given above are not to scale. There are in fact over 5300 base pairs in the lac operon.
It would be wasteful to produce enzymes when no lactose is available or if a preferable energy source such as glucose were available. The lac operon uses a two-part control mechanism to ensure that the cell expends energy producing the enzymes encoded by the lac operon only when necessary.
In the absence of lactose, the lac repressor, encoded by lacI, halts production of the enzymes and transport proteins encoded by the lac operon. It does so by blocking the DNA dependent RNA polymerase. This blocking/ halting is not perfect, and a minimal amount of gene expression does take place all the time. The repressor protein is always expressed, but the lac operon (i.e. enzymes and transport proteins) are almost completely repressed, allowing for a small level of background expression. If this weren't the case, there would be no lacY transporter protein in the cellular membrane; consequently, the lac operon would not be able to detect the presence of lactose.
When lactose is available but not glucose, then some lactose enters the cell using pre-existing transport protein encoded by lacY. This lactose then combines with the repressor and inactivates it, hence allowing the lac operon to be expressed. Then more β-galactoside permease is synthesized allowing even more lactose to enter and the enzymes encoded by lacZ and lacA can digest it.
However, in the presence of glucose, regardless of the presence of lactose, the operon will be repressed. This is because the catabolite activator protein (CAP), required for production of the enzymes, remains inactive, and EIIAGlc shuts down lactose permease to prevent transport of lactose into the cell. This dual control mechanism causes the sequential utilization of glucose and lactose in two distinct growth phases, known as diauxie.
Whe…
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