Mitochondrial nucleoids (mt-nucleoids) of the yeast, Saccharomyces cerevisiae, were isolated from spheroplasts of stationary phase cells and their structure and organization were investigated by fluorescence microscopy, electron microscopy, and biochemical techniques. Isolated mt-nucleoids were spherical or ovoid and 0.3-0.6 micron in diameter, and were about the same size and shape as those observed in the cell by the DAPI staining technique. Measurement of DNA content of mt-nucleoids, using a video-intensified microscope system, after DAPI staining revealed that a mt-nucleoid in spheroplasts from stationary phase cells contains, on average, 3.9 mtDNA molecules and an isolated mt-nucleoid contains, on average, 3.1. Negatively stained electron micrographs showed that mt-nucleoids consist of particles 20–50 nm in diameter. SDS-polyacrylamide gel electrophoresis of mt-nucleoids detected 20 species of polypeptides in the molecular weight range from 10 X 10(3) to 70 X 10(3). Acid-urea/SDS two-dimensional electrophoresis of acid extract from mt-nucleoids indicated that a polypeptide of 20 X 10(3) is the only major polypeptide with basic property like histones.

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