Studies on the Regulation and Localization of the Glyoxylate Cycle Enzymes in Saccharomyces cerevisiae

The regulation of the enzymes involved in the operation of the glyoxylate cycle was studied in the yeast Saccharomyces cerevisiae. All enzymes showed an increase in specific activity under growth conditions where the glyoxylate cycle is needed as an anaplerotic pathway. In the presence of 1% glucose, the synthesis of all enzymes, except fumarase, was repressed. However, no indication for a specific regulation mechanism for the entire cycle could be found. Studies on the localization of the glyoxylate cycle enzymes in the yeast cell revealed that the key enzymes, isocitrate lyase and malate synthase, are located in the cytoplasm, whereas succinate dehydrogense was found only in the mitochondrial fraction. Activites of all other enzymes are found in the cytoplasm as well as in the mitochondira. In anaerobically grown cells, no mitochondria could be detected. Succinate dehydrogenase, isocritrate lyase, and malate synthase were absent. However, appreciable activities of citrate synthase, aconitase, fumarase, and malate dehydrogenase were found in the Cytoplasm. The significance of these observations for the operation of the glyoxylate cycle is discussed.