Structure of Plant Mitochondrial DNAs

Abstract

Relatively little is known about the genomes of plant mitochondria in comparison with the wealth of information available on mtDNAs in animal and fungal systems. This is largely due to the difficulty encountered in isolating the physically heterogeneous plant mtDNAs as intact supercoiled molecules from whole-plant tissue. However, using plant tissue-culture cells, my colleagues and I have been able to isolate 20–40% of the total mtDNA as supercoiled molecules (Sparks and Dale 1980; Dale 1981). The supercoiled mtDNAs can then be separated on agarose gels, and discrete size classes can be isolated. Using the smallest size classes purified, I have begun an analysis of the mitochondrial genomes of bean, corn, and tobacco.

RESULTS
Bean mtDNAs
The agarose-gel profile of bean mtDNA isolated from the supercoil region of a CsCl-ethidium-bromide gradient is shown in Figure 1a (lane 1). One can distinguish approximately 30 bands that bear a striking resemblance to an oligomeric series. The two smallest bean mtDNAs (1.9 kb and 3.8 kb) were isolated from gels, nick-translated, and then used in hybridization and restriction studies. As shown in Figure 1a (lane 2), the 1.9-kb size class hybridizes to all but three of the bands visible in the ethidium-bromide-stained gel pattern, demonstrating that there is homology among most of the bean mtDNAs.

As shown in Figure 1b, the HhaI restriction digests of the 1.9-kb and 3.8-kb bean mtDNAs are identical, indicating that the 3.8-kb molecule is a dimer of the 1.9-kb size class. Digests of the next two largest molecules...