A series of CS revertants has been selected from various strains (both omega+ and omega-) 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 omega allele of the CS revertants has been defined in crosses with omega+ and omega- tester strains using two criteria: the polarity of recombination and a new criterium called relative output coefficient. We found that mutations of omega appear frequently associated with the mutations at the RIB1 locus selected from omega- strains but not with those selected from omega+ strains. A new allelic form of omega (omega n) which had not been found amongst wild type yeast strains is characterised. Similarly omega n mutation was found frequently associated with CR mutants at the RIB1 locus selected from omega- CS strains but not with those selected from omega+ CS strains. The omega n mutants, and the omega+ and omega- strains, explain the groups of polarity previously observed by Coen et al. (1970). 2. Main features of mitochondrial crosses with omega n strains (omega+ x omega n, omega- x omega n and omega n x omega n) 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 (omega+ x omega+ and omega- x omega-). A striking property, observed for the first time, exists in crosses between zota+ omega n CS strains and some zota- CREO mutants: the zota- CREO are unable to integrate by recombination their CR allele into the zota+ mit-DNA of omega n CS strains while being capable of integrating it into omega+ CS or omega- CS genomes. 3. It is proposed that the omega locus is the site of initiation of non reciprocal recombination events, the omega+/omega- pairing specifically initiates the non-reciprocal act while omega+/omega n or omega-/omega n pairings do not. 4. The molecular nature of the omega n mutation and its bearing on the structure of the omega locus are discussed. It is suggested that omega n mutations correspond to macrolesions (probably deletions) of a segment of the mit-DNA covering the omega and RIB1 loci. If omega n is a partial deletions of the omega- sequence the omega+ could be an additionnal deletion of the omega n sequence. 5. The occurrence of spontaneous CR and ER mitochondrial mutations has been analysed by the Luria and Delbrück fluctuation test in omega- and omega n isonuclear strains. Results of these tests indicate that an intracellular selection of resistant copies preexisting the action of the anttibiotic occurs.
