Cloning of the plasmids in cytoplasmic male sterile rice and changes of organization of mitochondrial and nuclear DNA in cytoplasmic reversion.

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Abstract
Two supercoiled circular DNAs, B-1 and B-2, found in mitochondria of cytoplasmic male sterile rice were cloned into pUC12. Using these plasmids as probes, we examined the organization of mitochondrial and nuclear DNA with respect to sequences homologous to B-1 and B-2. Strains of rice which exhibit cytoplasmic male sterility (cms) contained the two free plasmids not only in the mitochondrial fraction, but also in the nuclear fraction as open circular forms. These DNAs were no longer apparent in preparations of either mitochondrial or nuclear DNA from cytoplasmically reverted strains which were generated by treatment of a cms strain with EMS. Hybridization with restriction fragments revealed the presence of various sequences homologous to B-1 and B-2 in the mitochondrial genomic DNA of high molecular weight and also in the nuclear chromosomal DNA, but in cms strains and in the normal strain. However, the patterns of hybridization of restriction fragments from the cms strains and the normal strain were clearly different. The reversion to fertility resulted in not only the loss of the free plasmids, but also the disappearance or reduction in levels of some of the mitochondrial and nuclear sequences homologous to B-1 and B-2, as well as the appearance of new, nuclear, homologous sequences which were not present in the original cms strain, so that the revertant strain had the same characteristics as the normal strain. Involvement of the free plasmids and rearrangements of mitochondrial or nuclear DNA in the etiology of cytoplasmic male sterility are discussed.