Mitochondrial and nuclear DNAs from various wild-type and petite yeast strains have been highly purified by hydroxylapatite chromatography. Yeast mitochondrial, nuclear, and Escherichia coli DNA-dependent RNA polymerases were used to transcribe mitochondrial DNA of yeast into RNA. The RNA produced by these enzymes was used to study both the nature of the enzyme products and the properties of the template DNAs. The E. coli enzyme and mitochondrial enzyme transcribed the ribosomal genes of mitochondrial DNA as shown by competition with cold mitochondrial ribosomal RNA. The labeled in vitro synthesized RNAs were hybridized to various DNAs in an attempt to compare the extent of homology among these DNAs. Very little homology was indicated between nuclear and mito- chondrial DNA of yeast. Total cell DNA from two petite mutants lacking the mitochondrial satellite band showed the same low degree of homology to mitochondrial DNA. Wildtype mitochondrial DNAs differed in their homologies to each other and appeared to contain different amounts of repetitive DNA. Mitochondrial DNAs from two petite mutants showed reduced homology to wild-type mitochondrial DNA. However, no additional sequences which differed from those of wild-type mitochondrial DNA could be detected in the mitochondrial DNA of one petite mutant studied, thus indicating a simple deletion mechanism for the origin of this strain. This petite mutant lacks 50-60% of the wildtype mitochondrial genome transcribed in vitro.
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