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SUMMARY With synchronous single-generation cultures of the non-filamentous water fungus Blastocladiella emersonii the activities of isocitric dehydrogenase, a-keto- glutaric dehydrogenase, and cytochrome oxidase were studied during morpho- genesis. The total isocitric dehydrogenase activity per plant increased rapidly following germination of the spore and reached its peak at the point of no return in ontogeny; this increase was about seven times greater than the increase in total cc-ketoglutaric dehydrogenase activity. During growth from the spore stage to the stage of irreversibility, the specific activity of isocitric dehydrogenase increased 850 yo while that of a-ketoglutaric dehydrogenase decreased 50 yo. By removing the external bicarbonate before the point of no return was reached, and thus re- versing the morphogenetic pathway, the specific activity of isocitric dehydrogenase, which was in the process of increasing, decreased immediately, while the a-keto- glutaric dehydrogenase activity, which had decreased sharply, increased quickly once again, Similarly, the total activity of isocitric dehydrogenase per plant decreased precipitously while, without reversal, it continued to rise. Conversely, reversal of the morphogenetic path caused an immediate rise in the total a-keto- glutaric dehydrogenase activity per plant while, in the absence of reversal, enzyme activity remained on a level plateau, Thus, removal of bicarbonate induced a new morphogenetic path and, simultaneously, a 250% increase in the activity of one key enzyme and a 35 yo decrease in the activity of a second; on the other hand, the total protein and total activity of two other ' control ' enzymes (glucose-6-phosphate dehydrogenase and cytochrome oxidase) remained essentially constant, In contrast to the above, removal of bicarbonate immediately after the point of no return in morphogenesis had no significant effect on these same enzymes. The data provide substantial evidence that a reversible bicarbonate-induced enzyme synthesis is involved in morphogenesis ; i.e. in the mechanism of the bicarbonate-induced formation of resistant-sporangial plants in Blastocladiella.

Reversible Bicarbonate-Induced Enzyme Activity and the Point of no Return during Morphogenesis in Blastocladiella