In Aspergillus nidulans, activity of the glyoxylate cycle enzyme isocitrate lyase is finely regulated. Isocitrate lyase is induced by growth on C2 compounds and long-chain fatty acids and repressed by glucose. In addition, activity of isocitrate lyase is subject to a second mechanism of catabolite control, glucose-induced inactivation. Here, we demonstrate that the catabolite inactivation of A. nidulans isocitrate lyase, a process that takes place during glucose adaptation of cells grown under gluconeogenic conditions, occurs by proteolysis of the enzyme. Ultrastructural analyses were carried out in order to investigate the cellular processes that govern the catabolite inactivation of this peroxisomal enzyme. Addition of glucose to oleate-induced cells triggered the specific engulfment and sequestration of peroxisomes by the vacuoles. Sequestration of various peroxisomes by a single vacuole was a frequently observed phenomenon. Results obtained by immunoelectron microscopy using antibodies against A. nidulans isocitrate lyase showed that degradation of this peroxisomal enzyme occurred inside the vacuole. In addition, ultrastructural studies demonstrated that microautophagy was the autophagic pathway involved in degradation of redundant peroxisomes during glucose adaptation of oleate-induced cells of A. nidulans.

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Oleate non-utilizing (olu) mutants of Aspergillus nidulans were isolated on selective medium containing the catalase inhibitor, 3-aminotriazole (3-AT); in an attempt to obtain strains that would enable analysis of peroxisome biogenesis and function in filamentous fungi. Out of 816 putative mutants recovered from the selective medium, 40 were olu mutants. All of the olu mutants were unable to utilize acetate for growth, and so ressembled the acu mutants obtained by previous workers. The 40 olu mutants were placed in 5 different complementation groups.