¹ From the Department of Bacteriology, Washington University School of Medicine, St. Louis

Experiments on enzymatic adaptations in yeast to galactose and maltose under various conditions are examined. The pertinent facts established may be summarized as follows:—

1. The presence of exogenous nitrogen stimulates the rate of adaptation and raises considerably the attainable level of enzyme activity.

2. This stimulation is absent if the cells are unable to assimilate the added nitrogen.

3. Competitive interactions can be exhibited between two adaptive enzyme systems induced either serially or simultaneously in the same cell.

4. A similar kind of interaction was observed between an adaptive and a so called "constitutive" enzyme.

5. The presence of exogenous nitrogen modifies greatly the nature and extent of the interaction between the enzyme-forming systems.

The significance of these results to our understanding of the mechanism of the modification and maintenance of cellular enzymatic constitution is discussed. The validity of the distinction between "constitutive" and "adaptive" enzymes is reexamined in the light of the data presented.

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