Abstract
A particulate fraction not present in wild-type cells has been isolated from the cytoplasm and from lysed mitochondria of the respiratory-deficient extranuclear mutant "abnormal-1" of Neurospora crassa. The particles have a density between 1.13 and 1.2 g/ml. They appear in thin sections after OsO(4) fixation as virus-like polymorphic vesicles containing an electron-dense nucleoid of 120-170 nm in diameter. The central core is surrounded by one or two "unit membrane" envelopes of 100 A thickness. The particles contain a single-stranded 33S RNA that is converted to 7-9S RNA by heat treatment in the presence of sodium dodecyl sulfate and that differs in its base composition from mitochondrial and cytoplasmic ribosomal RNA. They contain 7% phospholipids rich in phosphatidylethanolamine and two major proteins, a lipoprotein of molecular weight 15,000 and a glycoprotein of molecular weight 95,000. It is suggested that these virus-like particles originate within mitochondria.