Site-specific recombination in "petite colony" mutants of Saccharomyces cerevisiae. I. Electron microscopic analysis of the organization of recombinant DNA resulting from end to end joining of two mitochondrial segments.

We have studied the organization of parental and recombined repetitive molecules of mtDNA's from cytoplasmic petite mutants of S. cerevisiae. One parental petite mutant carries a segment conferring resistance to chloramphenicol. The other parental petite mutant carries a segment associated with the erythromycin resistance locus. It was found that both parental mtDNA's are arranged in palindromes. Recombinant mtDNA's display a highly ordered and unique arrangement of repeats. In such a recombinant molecule with a repeat length of 2.24 μm two inverted repeats of one parent (1.69 μm each) follow two inverted repeats of the other parent (0.5 μm each). The overall arrangement of recombinant mtDNA is a palindrome again. As judged by partial denaturation maps very little if any sequence homology exists between the parental sequences in agreement with previous results obtained by DNA-DNA hybridization and high resolution melting. Thus, recombination between different mtDNA sequences consists in end to end joining of two segments. In one recombinant the hybrid molecule is non distinguishable from the corresponding wild type sequence while in the other recombinant the repeat length is slightly smaller (2.12 μm). We interpret the structure of recombined molecules as indicating that the recombination occurs according to the Campbell (1962) model. We discuss the various facets of the “petite” phenomena such as the repetitive nature of their mtDNA and the formation of multimeric and additive series of circle lengths, in the light of this model. We stress the analogy between transposable or translocatable genetic elements in bacteria and the mitochondria DNA in yeast petite mutants.