All four maize cytoplasm groups (N, cms-S, cms-T and cms-C) have been shown to contain supercoiled DNA molecules of 1,940 base pairs in their mitochondria (Kemble and Bedbrook, Nature 284:565, 1980). We have recently found, however, that not all members of the cms-S group possess these plasmids. Six cms-S cytoplasms were studied in depth; three carried the 1.94 kb plasmid (S, J, I) whereas three did not (VG, RD, ML).
We have used a full length recombinant DNA clone of the 1.94 kb plasmid (Thompson, Kemble and Flavell, unpublished) to probe Southern blots from agarose gels of mtDNAs from over one hundred combinations of cms-S cytoplasms and nuclear backgrounds. In each case, regardless of nuclear background, mtDNAs from S, J and I cytoplasms consistently hybridized strongly to the 1.94 kb probe whereas RD, ML and VG mtDNAs did not show hybridization. Inbred lines homozygous for Rf3, non-restoring backgrounds and the Laughnans' cms-S revertants to fertility all held strictly to the plus, minus pattern. Those cytoplasmic revertants arising from I cytoplasm, for instance, still retained the 1.94 kb plasmid at parental levels, while revertants to fertility from VG cytoplasm lacked the plasmid, as did their sterile parents.
Although the presence or absence of the 1.94 kb plasmid neatly subdivides the cms-S group of cytoplasms, no obvious function is yet apparent for this mtDNA entity. Of interest, however, is our observation that maize nuclear chromosomal DNA contains substantial sequence homology to this 1.94 kb plasmid and the cms-S S1 and S2 linear mtDNAs (Kemble et al., Nature 304:744, 1983). So the 1.94 kb plasmid may yet prove to be more than a "vestigial" phenomenon.
John E. Carlson and Roger J. Kemble
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