On a model for the structure of circular mitochondrial genomes in higher plants.

The mitochondrial genome of some higher plants consists of a discrete set of circular molecules. The heterogeneity found in the number of circles and their size is puzzling. In some plants evidence exists that indicates that these circular DNA molecules undergo intermolecular and intramolecular recombination events. The result of these events is that two small circles can combine to give one large circle or one large circle can break up into two small circles. Here we pursue the idea that such recombination events are responsible for the variability in genome composition. Treating the recombination events as chemical reactions we derive the equilibrium size distribution of circles. In Brassica campestris there are two basic subunits, circles A and B, containing 135 and 83 kilobases, respectively. By restriction-enzyme mapping one can determine in a multimeric circle, the number of interfaces between A- and B-derived DNA. Using a combinatorial argument we predict the frequency of such interfaces and compare our predictions with published data. A number of suggestions are made for additional experimental tests of the recombinational theory of genomic diversity.