A region of extreme instability in the mitochondrial genome of yeast.

About half of the spontaneous petite mutants produced by wild-type Saccharomyces cerevisiae strain B (as well as by several other strains) have the same defective mitochondrial genome. Its repeat unit is a segment, 2200 base pairs (bp) long, which derives from an excision between the origins of replication ori 2 and ori 7 of the wild-type genome, and contains a hybrid ori 2-ori 7 sequence. The spontaneous petites carrying this defective ori.h genome are supersuppressive , i.e., they very rapidly compete out the wild-type genome in crosses. The main reasons for the exceptional frequency of ori.h petites are an extremely high excision frequency, due to the extended homology between the two tandemly oriented ori sequences 265 bp long and the short distance separating them. Such an excision frequency is very strongly increased in petite genomes encompassing the ori 2-ori 7 region, because of their higher concentration in these ori sequences.