ATOMIC-FORCE MICROSCOPIC STUDY ON TOPOLOGICAL STRUCTURES OF PBR322 DNA

Plasmid pBR322 DNA (0.5 mg/mL) isolated from Escherichia coli HB101 wa s suspended in Tris-HCl-EDTA (1 mol/L - 0.1 mol/L, pH 8.5); then a dro p of the above solution was deposited on freshly cleaved mica substrat e. After adsorption for about 1 min, the sample was stained with phosp hotungstic acid. The residual solution was removed with a piece of fil ter paper. Afterwards the sample was imaged with a home-made atomic fo rce microscope (AFM) in air. The AFM images of pBR322 DNA with a molec ular resolution have been obtained. These images show that;pBR322 DNA exists in several different topological structures: (i) relaxed circul ar DNA with a different diameter; (ii) supercondensed DNA with differe nt particle sizes; (iii) dimeric catenane connected by one relaxed cir cular molecule and another close-compacted molecule which might be eit her supercoiled or intramolecular knotted form; (iv) oligomeric catena ne with multiple irregular molecules in which DNA is interlocked into a complex oligomer; (v) possibly existing intermediates formed from mo nomer to oligomer.