Several yeast species/isolates belonging to the genus were examined for the organization of their mtDNAs and ability to generate petite mutants. A general characteristic for all of the mtDNAs tested was that they were very A+T-rich. However, restriction patterns and inducibility of petite mutations revealed a great diversity in the organization and genetic behaviour of mtDNAs. One group of yeasts, , contains mtDNA ranging in size from 64 to 85 kb. mtDNAs from these yeasts contain a high number of restriction sites that are recognized by the enzymes and , which cut specifically in G+C clusters. There are three to nine per genome. These yeasts spontaneously generate respiration deficient mutants. Ethidium bromide (Et-Br), at low concentrations, induces a majority of cells to give rise to petites. A second group of yeasts, , contains smaller mtDNAs, ranging in size from 23 to 48 kb, and probably only a few intergenic G+C clusters and no sequences. These yeasts also generate petite clones spontaneously, but Et-Br, even when present at high concentrations, does not substantially increase the frequency of petites. In most petite clones from these yeasts only a small fragment of the wild-type molecule is retained and apparently multiplied. A third group, represented by , does not give rise to petite mutants either or after induction.


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