Rearrangement of the "R" sequence is the only mitochondrial DNA alteration consistently associated with cms-S reversion to fertility

--Gracia Zabala, Susan Gabay-Laughnan and John R. Laughnan

The mitochondrial genome of S cytoplasmic male-sterile (cms-S) maize plants contains two linear episomes, S1 and S2. The inverted repeats (IRs) at the termini of these episomes recombine with identical IR sequences present in the main mitochondrial genome at the junction of sites designated s - s' and y - y' (Schardl et al., Nature 310:292-296, 1984). These recombination events result in the linearization of a high proportion of the mitochondrial genome. The resulting linear molecules have an episome at one end flanked by a s or y sequence. At the other end is a 2kb sequence (R) common to s' and y', including 195/208bp homology to the IRs of S1 and S2, flanked by the remainder of a s' or y' sequence. The R region shows strong DNA sequence homology with the unique end of the R1 episome of the RU cytoplasm.

Southern blots of mitochondrial DNA (mtDNA) digested with restriction endonucleases and hybridized to the probe for the R sequence can give a complex pattern due to the different arrangements in which R is found, i.e., s-R-s', y-R-s', s-R-y', y-R-y', *R-s' and *R-y' (where * indicates a linear end). Cytoplasmic revertants to fertility in M825, 38-11, H95 and W182BN nuclear backgrounds lose both of the S episomes and all linearized mtDNA molecules. At first, no significance was attached to the absence of *R-s' and *R-y' since there is no linearization in these revertants. Revertants in the WF9 nuclear background lose neither the free S episomes nor the linearized high molecular weight mtDNA molecules. However, the R sequences at the linear chromosome ends are rearranged as are the integrated R sequences. All R sequences are rearranged in WF9 revertants. In some cases the R sequence seems to have been eliminated from the mitochondrial genome completely. Other WF9 revertants retain the R sequence in abundance but in one or more novel arrangements. The rearrangement of the R sequence at the linear chromosome ends is the only consistent change in all cms-S cytoplasmic revertants analyzed, regardless of the nuclear background in which the reversion occurred. It is not the R sequence per se that is associated with cms-S since the mitochondrial genomes of M825, 38-11, H95 and W182Bn revertant contain intact integrated R sequences. The male sterility seems to be correlated with the R sequence located at the ends of the linearized mitochondrial DNA molecules.

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