Analysis of four tobacco mitochondrial DNA size classes

Supercoiled mtDNAs were isolated from tobacco suspension culture cells and three of the smallest size classes (10.1, 20.2 and 30.3 kb) were characterized through denaturation, heteroduplex and restriction mapping. The 20.2 molecule was found to be a head-to-tail dimer of the 10.1 or X size class, while the 30.3 kb size class was found to contain two kinds of molecules, a head-to-tail trimer of X (X3) and a second molecule, ABC. X and ABC had a 118 +/- 35 bp region of homology, and both size classes shared a degree of homology with at least one other size class. Restriction maps of both the X and ABC molecules are presented and the possible origin and role of the many plant mtDNA size classes are discussed.