STRUCTURE AND GENETIC STABILITY OF MITOCHONDRIAL GENOMES VARY AMONG YEASTS OF THE GENUS SACCHAROMYCES

Several yeast species/isolates belonging to the genus Saccharomyces we re examined for the organization of their mtDNAs and ability to genera te petite mutants. A general characteristic for all of the mtDNAs test ed was that they were very A+T-rich. However, restriction patterns and inducibility of petite mutations revealed a great diversity in the or ganization and genetic behaviour of mtDNAs. One group of yeasts, Sacch aromyces sensu stricto, contains mtDNA ranging in size from 64 to 85 k b. mtDNAs from these yeasts contain a high number of restriction sites that are recognized by the enzymes HaeIII and MspI, which cut specifi cally in G+C clusters. There are three to nine orilrep sequences per g enome. These yeasts spontaneously generate respiration deficient; muta nts. Ethidium bromide (Et-Br), at low concentrations, induces a majori ty of cells to give rise to petites. A second group of yeasts, Sacchar omyces sensu late, contains smaller mtDNAs, ranging in size from 23 to 48 kb, and probably only a few intergenic G+C clusters and no orilrep sequences. These yeasts also generate petite clones spontaneously, bu t St-Sr, even when present at high concentrations, does not substantia lly increase the frequency of petites. In mast petite clones from thes e yeasts only a small fragment of the wild-type molecule is retained a nd apparently multiplied. A third group, represented by Saccharomyces kluyveri, does not give rise to petite mutants either spontaneously or after induction.