Final answer:
Lactose-fermenting bacteria, such as E. coli, express the enzyme β-galactosidase, which hydrolyzes lactose into galactose and glucose. The lacZ gene codes for this enzyme within the lactose operon, which is regulated by the presence or absence of lactose.
Step-by-step explanation:
The enzyme expressed by lactose-fermenting bacteria such as Escherichia coli (E. coli) is β-galactosidase. This enzyme catalyzes the hydrolysis of lactose into galactose and glucose, which are simpler sugars that the bacteria can utilize for energy. The gene responsible for coding β-galactosidase is lacZ, which is part of the lactose operon or lac operon. This operon also contains other necessary genes for the metabolism of lactose, such as lacY (lactose permease) and lacA (transacetylase).
In the absence of lactose, a repressor protein binds to the operator sequence of the lac operon, thereby preventing transcription. However, when lactose is present, it is converted to allolactose, a molecule that binds to the repressor and causes it to release from the operator sequence. This removal of the repressor allows RNA polymerase to transcribe the genes, leading to the production of β-galactosidase and the other proteins required for lactose digestion.