Because it is normally inhibited and needs a precursor molecule to release the inhibitory molecule so that the bacteria can metabolize lactose. Induced.
1. in lac operon; gene activity is induced when lactose is present in the medium, whereas in case of trp operon, repression of the gene activity takes place in presence of tryptophan in the medium....
There are 3 structural genes in in lac operon. . namely Lac Z - codes for beta galactosidase (breaks lactose into galactose and glucose) Lac Y - coding for beta galactoside permease ( pumps lactose...