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

Abstract

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.

Surrogate origins of replication in the mitochondrial genomes of ori-zero petite mutants of yeast.

“…The maps of o01 petite genome 2 and 3 were used to reconstruct the wild-type genome region from which these orio genomes were derived. Finally, the map of petite b17 (Marotta et al, 1982) Nobrega and Tzagoloff, 1980;Sor and Fukuhara, 1980;Coruzzi et al, 1981); these sequences ( Figure 6b) share all the main features of the orP sequence; we will call them orh-like. Four other sequences, still sharing the ATAG and GGAG sequences and the contiguous sequences, but apparently missing 5-9 nucleotides, have also been found; these sequences will be indicated as ori5-like, partially deleted.…”

“…Such instability would have already led to the loss of the mitochondrial genome, which is dispensable in yeast, were it not for compensatory selective advantages associated with the physiological function of these sequences. Such an argument, which also applies to the canonical ori sequences themselves (Marotta et al, 1982), has been developed elsewhere (Bernardi, 1982a). 7.…”

“…Culture media were as in Marotta et al (1982). Methods Mitochondrial DNA preparations, restriction enzyme degradations, gel electrophoresis, 32P-labeling of DNA, and hybridizations, were as in Marotta et al (1982). DNA sequencing after secondary cleavage or strand separation was performed according to Maxam and Gilbert (1980).…”

“…For instance, the excision may take place between two ori sequences; this results in the production of orih petite genomes with a hybrid ori sequence (Marotta et al, 1982). In other cases, the excision occurs between two sequences located inside and outside an ori sequence, leading to oripetite genomes with a partially deleted ori sequence (de Zamaroczy et al, 1981).…”

“…We have shown that these different situations are accompanied by different replication rates of the defective genome units relative to that of the resident, parental wild-type units. In turn, these relative replication rates influence the differential segregation of the defective genome and, therefore, its level of transmission to the progeny; the other two factors playing a role here are the rate at which the defective genome was excised and its stability in the cells harboring it (Marotta et al, 1982). When a petite is crossed with a wild-type cell, the replication rate of its genome is the main factor in determining its level of transmission to the progeny.…”