Change in chromosome number associated with a double deletion in the Neurospora crassa mitochondrial chromosome

The mitochondrial genome of Neurospora is usually found in a single covalently closed circular 62-kbp DNA molecule. We report here that the mitochondrial genome of a phenotypic revertant of a stopper mutant (stp-ruv) is contained primarily in two separate, nonoverlapping, autonomously replicating circular chromosomes. The circles, one about 21 kbp and the other somewhat less than 36 kbp are derived from the most frequent classes of recombinant chromosomes (21 and 41 kbp) in the chromosomal population of mitochondria in the original stopper mutant. The new, more stable chromosomal configuration, is associated with the deletion of two sequences (1 kbp and 4 kbp) at the splice junctions of the two circles. The data suggest that both deletions are likely to have originated from a single recombinational event involved in generating the 36-kbp circle. Secondary, spontaneously arising derivatives of stp-ruv have been found to yield, at high copy number, short sections of the 21-kbp circle in covalently closed supercoiled circles varying from unit length to very high multimers. The amplified segments span a common segment likely to contain the replication origin of the 21-kbp chromosome.