Genetically induced subcellular mislocation of Neurospora mitochondrial malate dehydrogenase

Among 60 ultraviolet-induced missense mutations of the structural genes that code for mitochondrial malate dehydrogenase (M-MDH, EC 1.1.1.37) of Neurospora crassa, two enzyme phenotypes are observed. In a previously described class (C-mutants), M-MDH is malfunctional because of an abnormal conformation induced by association with mitochondria. We describe here a second class (K-mutants) in which the enzyme is malfunctional because of an altered subcellular location. Thus, although both classes cause lesions in the assimilation of exogenous malate, the nature of the lesions differs. In C-mutants, the enzyme misfunctions because of low affinity for malate but remains mitochondrial-bound as in wild-type. Conversely, K-mutant M-MDH is dispersed throughout the cytoplasm. Studies of a repressible "glyoxysome" isozyme and a constitutive M-MDH of prototroph and mutants indicate that both isozymes are encoded by the same nuclear structural genes and have polypeptide subunits in common.