Analysis of the differentiation and of the heterogeneity within a population of Escherichia coli undergoing induced beta-galactosidase synthesis.

The kinetics of induction of the galactozymase of certain yeasts (Spiegelman, 1951) and the permease-3-galactosidase system of Escherichia coli (Monod, 1956) can, under certain conditions, be described as autocatalytic. Essentially two hypotheses have been proposed to account for such findings: (a) that the enzyme-forming system itself is autocatalytically activated (Campbell and Spiegelman, 1956) or self-reproducing (Spiegelman, 1946), or (b) that in some way the enzyme being induced intervenes in its own induction (Monod and Cohn, 1952). To evaluate these hypotheses, it should be recalled that the kinetics of formation of an enzyme by a population of cells describes events at the cellular level only if the response of each individual cell is simultaneous and equal (Benzer, 1953; Monod and Cohn, 1952). Such a response we term homogeneity. The first experimental analysis of this problem we owe to Benzer (1953) who defined the experimental conditions under which the induction of f-galactosidase would proceed in a homogeneous fashion in an E. coli (lac+) population. These were (a) conditions of gratuity, i. e., neither the presence of the enzyme itself nor its inducer influences general cellular metabolism, and (b) a saturating concentration of inducer. Benzer (1953) also showed that heterogeneity is generated under conditions of nongratuity. The question remained however as to what the response would be at nonsaturating concentrations of inducer.