Closed‐circular DNA has been isolated from mitochondrial‐enriched fractions from four petite‐negative' yeasts. Electron microscope analysis has shown in each case the presence of a large discrete size class of circular DNA greater than 6 μm in length and smaller heterodisperse circular DNA less than 6 μm. Length and molecular weight measurements of the large circular DNA are: Candida parapsilosis, 11.14 ± 0.45 μ and 23.1 × 106; Hansenula wingei, 8.22 ± 0.43 μm and 17.3 × 106; Kluyveromyces lactis, 11.44 ± 0.20 μm and 24.0 × 106 and Schizosaccharomyces pombe, 6.04 ± 0.16 μm and 12.5 × 106. These circular DNAs are thought to be mitochondrial DNA on the basis of their buoyant densities and their enrichment with mitochondria. The finding of a gross size difference between mitochondrial DNA from petite‐negative yeasts and Saccharomyces cerevisiae focuses attention on the suggestion that certain unique properties of the latter organism's mitochodrial DNA may be of relevance to the mechanism of the petite mutation in yeast.

and maintained on GYP slopes (see below). Media. The basic yeast extract, peptone medium (YP), contains per liter: 3 g of KH2PO4, 5 g of yeast 959

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