Rearrangement of mitochondrial DNA molecules during the differentiation of mitochondria in yeast. II. - Labelling studies of the precursor product relationship.

The length distribution in sucrose sedimentation gradient of the newly-synthesized pulse-labelled mitochondrial DNA has been established at an early stage of depression in wild type yeast (Saccharomyces cerevisiae). This stage corresponded to the beginning of mitochondrial differentiation. The radioactive DNA was longer (mean lengths 5, 10 and 22-25 mu) than the preexisting cold DNA (mean length 6.5 mu with two shoulders at 4 mum and 10 mum and one minor peak at 2-2.5 mum). These date confirm that the mean size of the different length populations of linear yeast mitochondrial DNA are under physiological control. Chase experiments were undertaken as follows. The yeast cells were uniformly prelabelled under anaerobiosis. Therefore the mitochondrial DNA molecules were short. Respiratory adaptation was performed in a cold medium and the lengthening process was induced. The specific activities of the long molecules made up during the respiratory adaptation did mot markedly differ from that of prelabelled DNA (decrease of specific activity less than 18 per cent). Molecules as long as 40 mum were also recorded. This lengthening seems to proceed through a non reciprocal exchange of polynucleotide stretches between preexisting molecules. We call it rearrangement. It occurs during the differentiation of mitochondria. Much of the mitochondrial DNA is maintained whereas a small amount of DNA is synthesized. This hypothesis is favoured by recent genetical and physical studies on mitochondrial recombination in yeast.