Final answer:
A mutation in one of the structural genes of the lac operon results in the production of a nonfunctional enzyme, impairing the bacteria's ability to process lactose. Such a mutation differs from regulatory gene mutations which can lead to continuous operon expression, undermining the operon's inducible nature.
Step-by-step explanation:
The question is regarding the lac operon system in E. coli, specifically the outcome of a mutation in one of the structural genes of the lac operon. If a mutation occurs in one of the structural genes (lacZ, lacY, or lacA), the affected gene may not function correctly, leading to deficient or absent production of the enzyme it encodes. This could mean that even in the presence of lactose, E. coli may not be able to metabolize lactose properly due to the lack of functional enzyme(s). This is in contrast to a mutation in the regulatory region, like the lacI gene, which could lead to constitutive expression of the operon regardless of the presence or absence of lactose.
The lac operon is an example of an inducible operon, which remains off by default and is turned on only in the presence of lactose and the absence of glucose. The repressor protein product of the lacI gene typically binds the operator to prevent transcription, but when lactose (and its metabolite allolactose) is present, it binds to the repressor, thereby allowing transcription of the operon. Mutations in the operator or the lacI gene can alter this tight regulation, which provides the basis for detailed genetic studies and an understanding of gene regulation in bacteria.