ELEVATED LEVELS OF PETITE FORMATION IN STRAINS OF SACCHAROMYCES CEREVISIAE RESTORED TO RESPIRATORY COMPETENCE. II. ORGANIZATION OF MITOCHONDRIAL GENOMES IN STRAINS HAVING HIGH AND MODERATE FREQUENCIES OF PETITE MUTANT FORMATION

R. J. Evans ¹ and G. D. Clark-Walker ¹

¹ Department of Genetics, Research School of Biological Sciences, The Australian National University, Canberra, A.C.T. 2601, Australia

Restriction enzyme analysis of aberrant mtDNA molecules in restored strains of Saccharomyces cerevisiae that display an elevated level of petite formation has shown the occurrence of novel junction fragments and nonstoichiometric amounts for some unaltered bands. Five aberrant mitochondrial genomes from high-frequency petite-forming (hfp) strains (>60% petites per generation) contain like-oriented duplications and single copy regions. High-frequency petite formation is postulated to arise from increased intramolecular recombination between duplicated segments. Mitochondrial DNA structures in two other hfp strains cannot be easily interpreted and might arise from intramolecular recombination.—Mitochondria DNA from moderate-frequency petite-forming (mfp) strains (5–16% petites per generation) contains inverted duplications in two cases. The elevated petite formation is postulated to arise from homologous recombination between directly repeated sequences. In mtDNA from one mfp strain, deletion end-points have been shown to overlap. Such deletion endpoint overlap is postulated to be required for the maintenance of the tandem duplication in hfp strains. Two regions of the wild-type mtDNA (between cyb and oli2 and between SrRNA and oxi2) appear to be dispensable for mitochondrial function.

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