When E. coli with a lac+ genotype is cultured in a lactose-free medium, the intracellular concentrations of β-galactosidase, permease, and transacetylase are exceedingly low-roughly one or two molecules of each protein per bacterium. However, when lactose is added to the growth medium, the concentration of these proteins increases simultaneously to about 105 molecules per cell (or about 1% of the total cellular protein). Furthermore, lactose addition triggers the synthesis of lac mRNA as evidenced by studies in which mRNA, labeled with [32P] phosphate at various times after lactose addition, is hybridized to DNA that carries lac genes.
Enzymes such as β-galactosidase, lactose permease, and transacetylase are said to be inducible enzymes because their rate of synthesis increases in response to the addition of a small molecule (lactose) to the medium. Other enzymes, called repressible enzymes, exhibit a decreased rate of synthesis in response to the addition of a small molecule in the medium. For instance, the addition of tryptophan to the growth medium causes E. coli to greatly decrease the rate at which it produces enzymes needed for tryptophan synthesis. Still other enzymes, called constitutive enzymes, are synthesized at fixed rates under all growth conditions. Constitutive enzymes usually perform basic cellular "house-keeping" functions needed for normal cell maintenance.
Lactose is rarely used in experiments to study induction because the β-galactosidase that is synthesized catalyzes the cleavage of lactose. Thus, the lactose concentration continually decreases, which complicates the analysis of many types of experiments (e. g., kinetic experiments). Instead, two sulfur-containing analogs of lactose are used, isopropylthio-galactoside (IPTG) and thiomethylgalactoside (TMG), which are effective inducers without being substrates of β-galactosidase. Inducers having this property are called gratuitous inducers.
