Figure 16.2 An Inducer Stimulates the Expression of a Gene for an Enzyme (A) When lactose is added to the growth medium for the bacterium E. coli, the synthesis of β-galactosidase begins only after an initial lag period. (B) There is a lag period because the mRNA for β-galactosidase has to be made before the protein can be made. The amount of mRNA decreases rapidly after the lactose is removed, indicating that transcription is no longer occurring. These changes in mRNA levels indicate that the mechanism of induction by lactose is transcriptional regulation.