Mitochondrial genetics

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Abstract
1. A series of CS revertants has been selected from various strains (both ω+ and ω−) carrying a CR mitochondrial mutation at the RIB1 locus. The properties of mitochondrial recombination exhibited by these CS revertants in various crosses, have been examined systematically. The ω allele of the CS revertants has been defined in crosses with ω+ and ω− tester strains using two criteria: the polarity of recombination and a new criterium called relative output coefficient. We found that mutations of ω appear frequently associated with the mutations at the RIB1 locus selected from ω− strains but not with those selected from ω+ strains. A new allelic form of ω (ωn) which had not been found amongst wild type yeast strains is characterised. Similarly ωn mutation was found frequently associated with CR mutants at the RIB1 locus selected from ω− CS strains but not with those selected from ω+ CS strains. The ωn mutants, and the ω+ and ω− strains, explain the groups of polarity previously observed by Coen et al. (1970). 2. Main features of mitochondrial crosses with ωn strains (ω+×ωn, ω−×ωn and ωn×ωn) are analysed. Recombination is possible between the different mitochondrial genetic markers. No high polarity of recombination is observed and the frequency of recombinants are similar to those found in homosexual crosses (ω+×ω+ and ω−×ω−). A striking property, observed for the first time, exists in crosses between ϱ+ ω+×ωn CS strains and some ϱ− CREO mutants: the ϱ− CREO are unable to integrate by recombination their CR allele into the ϱ+ mit-DNA of ω− CS strains while being capable of integrating it into ω+ CS or ω− CS genomes. 3. It is proposed that the ω locus is the site of initiation of non reciprocal recombination events, the ω+/ω− pairing specifically initiates the non-reciprocal act while ω+/ωn or ω−/ωn pairings do not. 4. The molecular nature of the ωn mutation and its bearing on the structure of the ω locus are discussed. It is suggested that ωn mutations correspond to macrolesions (probably deletions) of a segment of the mit-DNA covering the ω and RIB1 loci. If ωn is a partial deletion of the ω− sequence the ω+ could be an additionnal deletion of the ωn sequence. 5. The occurrence of spontaneous CR and ER mitochondrial mutations has been analysed by the Luria and Delbrück fluctuation test in ω− and ωn isonuclear strains. Results of these tests indicate that an intracellular selection of resistant copies preexisting the action of the antibiotic occurs.