ELECTRON-MICROSCOPIC STUDY OF CIRCULAR DNA COMPACTION IN MODEL SYSTEMS - II - COMPLEXES OF TRIVALINE WITH HIGH-MOLECULAR-WEIGHT CIRCULAR DNA

An electron-microscopic study of the morphology of complexes of a synt hetic peptide trivaline with pRX10 supercoiled circular DNA was perfor med. Monomers of this plasmid are composed of 10,500 bp. The samples s tudied included also a considerable number of plasmid dimers and even oligomers. The compact structures including monomers of pRX10 most oft en had the characteristic triple ring morphology, although in some cas es triple rings with an adjacent linear stretch of a compact fiber cou ld be observed. Compact structures including plasmid dimers appeared e ither as triple rings of double length or as two triple rings connecte d by a stretch of linear fiber. Such complex structures have been desc ribed previously only in samples of circular DNAs containing bent sequ ences. The formation of intramolecular compact structures containing t wo or more rings in the process of pRX10 compaction demonstrates that the increase in size itself, regardless of secondary structure peculia rities, increases the topological freedom of fragments of a circular m olecule so that they may undergo compaction independently. A detailed study of the morphology of unusual structures of trivaline-pRX10 compl exes and determination of the DNA compaction coefficient shed light on the DNA packing in these structures. The data obtained suggest that D NA compaction under the influence of trivaline is based on the formati on of multistranded compact fibers and is quite a flexible process. Co mpact fibers of similar morphology and compaction may include either t hree (as in triple rings) or two filaments of DNA duplexes.