A Comparison of the Kinetics of Enzymatic Adaptation in Genetically Homogeneous and Heterogeneous Populations of Yeast

Abstract

Populations of microorganisms possess the ability of undergoing striking changes in their physiological properties under the stimulation of substances in their environment. Such changes may involve either the acquisition of an enzyme system not previously detectable or the loss of one that it had possessed before the environmental change. An example of this phenomenon is the ability of certain yeasts to acquire the enzymatic apparatus necessary to ferment galactose. Since its discovery by Dienert ('00) this particular problem has been investigated by numerous workers. Armstrong ('05) confirmed Dienert's findings and further found that some yeasts were incapable of acquiring this physiological property no matter how long they were cultured in the presence of galactose. Slator ('08) sh-owed that those yeasts capable of fermenting galactose possess this ability only after they had been acclimatized by culture in its presence. No yeast he investigated was able to ferment this hexose immediately upon being introduced to a medium containing it. There was always an induction period of variable length connected with the acquisition of this property. Attempts to elucidate further the nature of this acclimatization or adaptation encountered a basic problem common to all studies of physiological changes in large populations. A comparative biochemical study of large populations always involves over-all populational characteristics. This necessarily introduces difficulties in the interpretations of any observed changes in physiological properties. The mechanisms available to an individual cell for adapting itself to an environmental change are limited by its genome and the physiological flexibility permitted by its particular degree of specialization. When, however, the adaptive ability of a population of cells is being considered, there must be added to the physiological pliability of its members the genetic plasticity of the group in terms of the numbers and kinds of variants it is capable of producing. Because of this composite nature of populational adaptability, it is clear that in any particular case the same end result can be obtained by any one of the following mechanisms: (1) the natural selection of existent variants withi the desired characteristics from a heterogeneous population; (2) induction of a new (as far as measurements of activity are concerned) enzyme by the substrate in all the members of a homogeneous population, resulting in an increase in the