Final answer:
The element not part of the lac operon among the listed options is lacI, which is a regulatory gene encoding the lactose repressor protein; it is indeed part of the operon but does not fit the structural gene category. A mutation in a structural gene such as lacZ would impair the enzyme's function essential for lactose metabolism.
Step-by-step explanation:
The lac operon is an important genetic regulatory system found in E. coli that controls the metabolism of lactose when glucose is not available. The components of the lac operon include several critical elements: lacZ, lacY, and lacA structural genes, alongside regulatory regions such as the promoter (lacP) and operator (lacO). The question seeks to identify which element listed is not part of the lac operon. Among the available options, lacI is the correct answer as it is indeed part of the lac operon but as a regulatory gene rather than a structural gene. This gene encodes the lactose repressor protein, which binds to the operator sequence to regulate the expression of the operon's structural genes.
A mutation in one of the structural genes of the lac operon, such as lacZ that encodes ß-galactosidase, would most likely result in the loss or reduction of the enzyme's function. As ß-galactosidase is responsible for hydrolyzing lactose into galactose and glucose, any loss of the function would directly affect the bacteria's ability to utilize lactose as an energy source.