Gradual stabilization of adaptive β -galactosidase in Bact. lactis aerogenes

Abstract

Bact. lactis aerogenes transferred from glucose to lactose as a carbon source undergoes a process of adaptation. There is a short initial lag, followed by growth at a low rate, which increases during serial subculture to an optimum. In the early stages the adaptation is unstable, growth in glucose causing a reversion to the unadapted state. After prolonged culture in lactose the adaptation becomes stabilized and the organism can be grown m glucose without loss of abilitv to grow at the optimum rate on return to lactose. The development of β -galactosidase activity has been studied in relation to the various stages of the adaptation, assays for the enzyme being carried out on intact cells, cells rendered permeable by treatment with benzene and cells disintegrated by ultrasonic vibration. Completely unadapted cells show little activity (about 0.5% of that in fully adapted cells). During the first stages of the growth of previously unadapted strains in lactose the rate of formation of β -galactosidase is very high relative to that of cell material in general, and falls later. The maximum activity reached during the first subculture is roughly half the final level, which is reached only after several subcultures, though some times before the optimum growth rate is attained. As the adaptation proceeds the pattern of variation of the enzyme activity during the growth cycle itself changes, a peak in activity developing towards the end of the logarithmic phase, and the enzyme becoming more stable with respect to ageing. When adapted cells are grown in glucose the β -galactosidase is diluted out If the period of adaptation has been short, the activity falls to the original level and the other character­istics of adaptation are lost. But if the growth in lactose has been continued long enough, then on return to glucose, although the activity falls, it is to a residual value much higher than that of unadapted organisms. This value is maintained during prolonged growth in glucose, and the maintenance of this higher level is accompanied by a continued ability of the cells to grow at the optimum rate on re-transfer to lactose, and to synthesize β -galactosidase rapidly to the full amount. The transition from unstable to stable adaptation follows a complex pattern not easily interpreted in terms of the selection of rare stable mutants. It seems rather that the long continued imposition of a changed enzyme balance eventually reacts on the structure or organization of the enzyme-forming sites themselves, and that this occurs throughout the population.