CHARACTERIZATION OF SOME UNUSUAL DNAS FROM MITOCHONDRIA FROM CERTAIN PETITE STRAINS OF SACCHAROMYCES CEREVISIAE

Studies on the isolation and characterization of DNA from yeast mitochondria (Mt-DNA)4 have been extended to mitochondria from respiratory deficient cells (ρ{variant}- petites). Under appropriate conditions assuring the absence of gluocse repression such cells have been found to contain mitochondrial DNA in amounts comparable to those of the wild type. Mt-DNA from a neutral petitite exhibits a buoyant density in CsCl and a thermal transition midpoint (Tm) very close to that of its isogenic wild type. Mt-DNA's from two other isogenic ρ{variant}- strains, known to be 10 and 50% suppressive respectively, show a very sharp thermal transition with a Tm of 66.5 ° in 0.15 m NaCl-0.015 m sodium citrate. Upon quick cooling almost complete renaturation is observed, as evidenced by second-cycle heating, again with a sharp transition and a Tm of 66.5 °. The average value of the buoyant density of this DNA in CsCl (at 20 °) is 1.6685 which is 0.003 g/ml lower than that reported for synthetic or crab d(A - T). There is virtually no change in the density of this Mt-DNA upon heating and quenching, with or without prior exposure to proteases, again confirming the great ease of its renaturation. In alkaline CsCl its density increases by 0.076 g/ml, indicating strand separation, but upon reneutralization the density returns to that of native Mt-DNA. Ready renaturation, as measured by thermal profiles, is found even for partially degraded Mt-DNA molecules. All these observations suggest that ease of renaturation is due to compositional homogeneity and therefore does not require these DNA's to exist as covalently continuous circles. The composition and homogeneity of the Mt-DNA from one of these suppressive strains (D310-2A-184) was confirmed by direct analysis of base composition: It was found to contain an equimolar amount of G and C as well as of A and T with the former two bases accounting for no more than four mole percent. The implications of these findings with regard to the mutagenic events reponsible for the formation of these aberrant DNA's and their possible genetic capabilities are discussed. © 1968.