Heterogeneity of mitochondrial DNA from Saccharomyces carlsbergensis: renaturation and sedimentation studies

The renaturation kinetics of mitochondrial DNA from the yeast Saccharomyces carlsbergensis have been studied at different temperatures and molecular weights. At renaturation temperatures 25 deg. C below the mean denaturation temperature (T_m) in 1 M-sodium chloride the renaturation rate constant is found to decrease with increasing molecular weight of the reacting strands. This unusual molecular weight dependency gradually disappears with an increase in the renaturation temperature. At a temperature 10 deg. C below the melting point, the rate constant shows the normally expected increase with the square root of the molecular weight. From the renaturation data at this temperature, the molecular weight of the mitochondrial genome is estimated to be about 5·0 × 10⁷. The same size of genome was found from renaturation at low molecular weight and 25 deg. C below the T_m.The sedimentation properties of denatured mitochondrial DNA at pH values 7·0 to 12·5 were used to study the conformation of this DNA in 1 M-sodium chloride. The results obtained support the conclusion from the renaturation studies: that the pieces of denatured mitochondrial DNA with a molecular weight above 2 × 10⁵ to 3 × 10⁵, in 1 M-sodium chloride at 25 deg. C below the mean denaturation temperature are not fully extended random coils. Presumably, interaction between adenine and thymine-rich sequences, which are clustered at certain distances within the molecules, is the molecular basis for these observations.