Enzyme Induction as an All-or-none Phenomenon.

The phenomenon of enzyme induction (enzymatic adaptation) has been observed in a variety of micro-organisms. One of the most carefully studied examples, largely the work of Jacques Monod and his colleagues at the Institut Pasteur, is the induction of the synthesis of the enzyme j3-galactosidase in the bacterium Escherichia coli.", 2 This enzyme, not otherwise present in appreciable amounts, is formed by the bacteria when grown in the presence of lactose, f3-galactosidase being necessary for the utilization of this sugar. A number of other galactosides also induce the formation of this enzyme in E. coli, including some compounds, such as thiomethyl-f3-Dgalactoside (TMG), that are not split by this enzyme." The fact that TMG is not split by the enzyme or otherwise utilized by the bacteria gives it a great advantage over lactose in kinetic studies. Thus lactose used as an inducer can also serve as an energy source, and so an increase in galactosidase content may bring about an increase in growth rate. In contrast, with inducers such as TMG, together with succinate in place of lactose as the energy source, an increase in enzyme content has no such effect. Induction under these circumstances has been called "gratuitous."' Monod, Pappenheimer, and Cohen-Bazirel studied the kinetics of induction under gratuitous conditions. They discovered that, upon addition of inducer at a sufficiently high concentration to a growing bacterial culture, the bacteria almost immediately begin to make enzyme at the maximum rate. Since the enzyme is being made at a constant rate per bacterium, the enzyme per bacterium in the bacterial culture rises and subsequently levels off at a value determined by this rate. After one bacterial doubling, the enzyme concentration reaches 50 per cent of its ultimate value. (It reaches 63 per cent in one generation, where one generation is defined by the doubling time divided by In 2.) In order to see whether individual bacteria participate equally in the synthesis of galactosidase, Benzer4 studied the distribution of enzyme among the bacteria in a culture under a variety of conditions. He found that under gratuitous conditions high concentrations of inducer produce a uniform distribution of enzyme among individual bacteria as early as 5 minutes after the addition of inducer. Under these conditions the kinetics of induction of a culture represents the-kinetics of the individual organism. Under conditions where the inducer is the sole carbon source, the distribution of enzyme among the bacteria is not uniform at first but becomes uniform when the culture reaches its maximum rate of enzyme synthesis. Subsequently it was discovered in Monod's laboratory2 5 that two independent processes are usually involved in the induction of f3-galactosidase synthesis. On the one hand, the rate of galactosidase synthesis is determined by the concentration of inducer inside the bacterium; and, on the other, the inducer is actively transported into the bacterium to give a much higher inducer concentration inside the bacterium than in the medium. The transport is accomplished by a second enzyme,