To determine the cellular location of extrachromosomal circular DNA in higher plants, wheat (Triticum aestivum) and tobacco (Nicotiana tabacum) nuclei were isolated and purified, and their circular DNAs were examined by electron microscopy. Covalently closed circular (ccc) DNAs were found in nuclear fractions from both species. They showed a heterogeneous size distribution ranging from 0.1μm to more than 5μm in contour length, with a mean of 1.7μm (5.3 kbp) forT. aestivumand 1.5μm (4.7 kbp) forN. tabacum, respectively. This distribution is significantly different from that for small circular DNAs in mitochondria. Small polydisperse circular (spc) DNA/protein complexes were also observed by a rapid microscale method of mica-press-adsorption for electron microscopy. Complexes of spcDNA/protein showed a similar size distribution to cccDNA. The average number of spcDNA/protein complexes per nucleus was estimated at more than hundred. The origin and biological functions of the nuclear circular DNAs are discus
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