Growth of originally aerobic bakers' yeast under conditions of anaerobiosis caused a decrease in the total specific catalatic activity (patent plus cryptic) of one-half per generation. It is concluded that reversion of catalase was a dilution, rather than a destruction, of the intracellular enzyme. However, the specific patent (whole cell) catalase activity remained constant for one or more generations, and then declined at a considerably slower rate than did the total activity. Thus the cryptic factor diminished progressively during anaerobic growth; after seven or eight generations virtually all the catalase was patent; i.e., the cryptic factor (the ratio of total enzyme to patent enzyme) was approximately unity. At this point, the basal level of enzyme was attained, and thereafter maintained by a basal synthesis, which produced only the patent, heat-stable, variety. Aerobic growth caused a significant, but much smaller, decline of both total catalase activity and of the cryptic factor. The data suggested that during reversion, the cryptic, heat-labile catalase became progressively converted to the patent, heat-resistant form. A model of these events is presented.

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