Immuno-electron localization of DNA in chondriolites of Saccharomyces cerevisiae mitochondria.

Under electron microscope, the matrix of sectioned mitochondria exhibits ribosomes and an oval, electron-transparent zone which is devoid of ribosomes and is named chondriolite. Fine fibers or clumps of an electron-dense material appeared in this zone after several fixation and contrasting steps and were identified with mitochondrial DNA by cytologists. To verify this assumption, we labeled DNA by a monoclonal antibody and a secondary antibody coupled to immunogold. The label was observed in the nucleus and in the chondriolite zone of sectioned mitochondria. Because the ultrastructure of chondriolites resembles that of nucleoids of prokaryotes, we suggest the term mitochondrial nucleoid for the zone of mitochondrial matrix devoid of ribosomes and containing DNA.