Formation of yeast mitochondria III: Biochemical properties of mitochondria isolated from a cytoplasmic petite mutant

A number of biochemical properties of mitochondria from a cytoplasmic petite mutant ofSaccharomyces cerevisiae with an extremely high adenine plus thymine content have been studied.When such particles are isolated by means of standard procedures developed for use with wild-type yeasts they are grossly contaminated by non-mitochondrial membrane fragments. Further enrichment of mitochondria is achieved by non-equilibrium centrifugation in sucrose gradients.Throughout this purification procedure the particles can be shown to retain an outer limiting, as well as a non-cristate inner membrane. In many of their morphological and physical features (size, shape, buoyant density) they resemble mitochondria isolated from the wild type.Although enzymes of the respiratory chain are absent from the mutant particles, their content ofl-malate dehydrogenase, NADP-dependent isocitrate dehydrogenase, and ATPase is comparable to that found in the wild type. The mitochondrial ATPase in this mutant strain is cold stable, oligomycin insensitive, Dio-9 sensitive, and susceptible to inhibition by the F₁ inhibitor of beef heart. The enzyme can be rendered cold labile by its detachment from the membrane, followed by fractionation with protamine sulfate and ammonium sulfate.The existence of mutant particles that are incapable of function in oxidation and phosphorylation but resemble their functional homologues in many other ways raises the possibility that mitochondria are required in the cellular economy for purposes not directly linked to oxidative phosphorylation and electron transport. This hypothesis has led us to suggest that, contrary to models currently under discussion, mitochondria did not evolve as a consequence of endosymbiosis. We propose as an alternative that the mitochondrial organelle evolved as a means of improvement of existing subcellular structures in the primordial (perhaps eukaryotic) cell. Partial autonomy may thus constitute a relatively recent modification; the present-day mitochondrial genome had its origin in nuclear DNA and may have been amplified in a manner not unlike the amplification of ribosomal RNA cistrons in developing oocytes ofXenopus.Supported by Research Grant GM 12228 from the National Institute of General Medical Science, National Institutes of Health, U.S. Public Health Service.Recipient of a Public Health Service Career Award No. GM 05060 from the Institute of General Medical Sciences.