SIZE AND STRUCTURE OF REPLICATING MITOCHONDRIAL-DNA IN CULTURED TOBACCO CELLS

The BY-2 tobacco cell line was used to study the size and structure of replicating mitochondrial DNA (mtDNA). Approximately 70 to 90% of the newly synthesized mtDNA did not migrate during pulsed-field gel elect rophoresis. Moving pictures of the fluorescently labeled molecules sho wed that most of the immobile well-bound DNA was in structures larger than the size of the BY-2 mitochondrial genome of similar to 270 kb. M ost of the structures appeared as complex forms with multiple DNA fibe rs. The sizes of the circular molecules that were also observed ranged continuously from similar to 20 to 560 kb without prominent size clas ses. Pulse-chase and mung bean nuclease experiments showed that the we ll-bound DNA contained single-stranded regions and was converted to li near molecules of between 50 and 150 kb. MtDNA replication in plants m ay be initiated by recombination events that create branched structure s of multigenomic concatemers that are then processed to 50- to 150-kb subgenomic fragments.