Mitochondrial genetics. 3. Recombined molecules of mitochondrial DNA obtained from crosses between cytoplasmic petite mutants of Saccharomyces cerevisiae: physical and genetic characterization

Mitochondrial genetics, circular DNA and the mechanism of the petite mutation in yeast

We propose a general hypothesis involving properties of circular DNA which can explain such phenomena as thepetitemutation, suppressiveness, and the polarity observed in mitochondrial recombination in the yeastSaccharomyces cerevisiae. This hypothesis involves excision and insertion events between circular DNA molecules as well as structural rearrangements in the DNA generated by these events. The special properties of circular DNA have been considered in analysing recombination, and a number of results are obtained which are not intuitively apparent.

This hypothesis can be applied to any situation involving circular DNA such as bacterial plasmids and cytoplasmic circular DNAs, where the opportunity exists for recombination and rearrangement events.

Interspecific recombination of mitochondrial DNA molecules in hybrid somatic cells

The mitochondrial DNA (mtDNA) in rodent-human hybrid somatic cells was studied in strains that contain nucleotide sequences from both parental mtDNAs. A test for linkage of rodent to human mtDNA was devised on the basis of the density and sequence differences between these DNAs. When a mixture of rodent and human mtDNAs was banded in a CsCl gradient and each fraction hybridized with a mixture of complementary [(3)H]RNA transcribed from human mtDNA and complementary [(32)P]RNA transcribed from rodent mtDNA, each DNA was detected as a distinct and separate band at its expected density by specific hybridization with its complementary RNA. In some of the hybrid cell strains, the mtDNA sequences derived from the two species did not separate in the CsCl gradients. This result is interpreted as evidence for linkage between sequences from the two parental mtDNAs. While the exact nature of the linkage and the structure of the molecules containing both types of sequences are not known, the evidence supports the conclusion that the linkage is realized by a covalent bond. An event leading to the covalent bonding of these different sequences may be described as recombination. Among 18 hybrid cell strains examined, 13 contained large proportions of recombinant molecules. These molecules were found in both mouse-human and rat-human strains, and in strains containing more human or more rodent mtDNA sequences.