The orir to ori+ mutation in spontaneous yeast petites is accompanied by a drastic change in mitochondrial genome replication.

The orir petite mutants of Saccharomyces cerevisiae show a very low level of suppressivity (5-12%; suppressivity is the percentage of diploid petites issued from a cross of the parental haploid petite with a wild-type cell), indicating a poor replication efficiency of their mitochondrial genome. The latter is made up of repeat units containing two inverted ori sequences and arranged as tandem pairs in inverted orientation relative to their nearest neighbors. After subcloning orir petites or crossing with wild-type cells a large number of ori+ petites are found in the progeny. In contrast to the orir petites, from which they are derived, these ori+ petites are characterized by high suppressivity levels (approx. 90%) and contain mitochondrial genomes made up of tandem repeat units containing single ori sequences. The structural changes underlying the orir to ori+ mutation are therefore accompanied by a dramatic increase in suppressivity, indicating that the elimination of inverted ori sequences causes a drastic change from very poor to very good replicative efficiency in the mitochondrial genome. Finally, crosses of ori0 petites with wild-type cells were also studied; the results obtained have clarified the reasons for the high frequency of petites having genomes similar to those of orir petites after mutagenesis with ethidium bromide.