Sequence homology among different size classes of plant mtDNAs

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Abstract
Supercoiled mt[mitochondrial]DNA were isolated from tissue culture cells of tobacco [Nicotiana tabacum cv. Wisconsin 38], bean [Phaseolus vulgaris] and corn [Zea mays] and the smallest size classes were used to study the relationships among the different size classes of each species through restriction digests and hybridizations. Three of the smallest tobacco mtDNA [10.1, 20.2 and 28.8 kilobases (kb)], the 2 smallest beam mtDNA (1.9 and 3.8 kb) and the 2 smallest corn mtDNA (1.5 and 1.8 kb) were extracted from the gels and nick translated. The 10.1-kb tobacco mtDNA hybridizes to all the other tobacco mtDNA size classes and a large percentage of the tobacco mtRNA. Restriction digests indicate that the 20.2-kb size class is a dimer of the 10.1-kb size class. The 1.9-kb bean mtDNA hybridizes to all but 3 of the bean mtDNA size classes and hybridizes to 2 mtRNA. Restriction digests indicate that the 3.8-kb size class is a dimer of the 1.9-kb size class. The 1.5- and 1.8-kb corn mtDNA, which do not have any Hha I restriction fragments in common, both hybridize to many of the same size classes of the corn mtDNA profile and, in addition, each hybridizes to a few size classes not recognized by the other. The 1.5- and 1.8-kb size classes hybridize to 2 RNA, 1 of which they appear to have in common. With both the 1.9-kb bean mtDNA and the 2 corn mtDNA, the molecular sizes of the 2 RNA exceed those of the respective DNA. The possible role and origin of the many size classes are discussed.