Mitochondrial DNA rearrangements associated with fertile revertants of S-type male-sterile maize

The mitochondrial genome of the S-type male-sterile cytoplasm of maize appears to be a collection of linear DNA molecules that are maintained by two linear episomal DNA species, S1 (6397 bp) and S2 (5453 bp), recombining with larger circular molecules. The M825 inbred nuclear genotype promotes a high frequency of spontaneous fertile mutants. Where the mutation is maternally inherited, analysis of the mitochondrial DNA reveals that the S1 and S2 episomes and all associated linear molecules have been lost. Integrated copies of the S1 and S2 episomes are retained, though part of one of the two terminal inverted repeats of the integrated S2 sequence has been deleted in all revertants studied. This deletion appears to correlate with the absence of the S2-URF-1 transcript.