Intramolecular recombination as a source of mitochondrial chromosome heteromorphism in Neurospora

Approximately 20% of the genome is missing and an equivalent amount is under-represented in the mitochondrial chromosome population of a stopper (stp) mutant of Neurospora crassa during the stopped phase of its cyclical growth pattern. At this stage, a 21 kb (7.2 mu) circular molecule, one-third the length of the normal chromosome, is the predominant form. A complementary 43 kb (14.6 mu) circle appears upon resumption of growth. The circles arise by reciprocal recombination at or near directly repeated tRNAmet sequences and the frequency of the two depends upon their rates of replication from replication origins unique to each of them. The 7.2 and 14.6 mu circles are the most frequent of the heteromorphic derivatives found within wild-type mitochondrial chromosome populations.