SUPERCOIL FORMATION AND COMPACTIZATION OF SUPERCOILED DNA

摘要

Supercoiled DNA pGEMEX 3993 p.n. in length immobilized on different substrates (freshly cleaved mica and standard and modified aminomica) was visualized by means an atomic force microscopy. Plectonomical molecules of supercoiled DNA as well as molecules with unusually high level of compactization with superhelix density significantly higher than that previously studied experimentally and considered theoretically were visualized on modified aminomica, which is characterized by an elevated surface charge density. With an elevated level of compactization of DNA the length of the axis of the supercoiled molecules decreases from ～390 to ～140 nm with subsequent creation of minitoroids ～50 nm diameter and molecules of spherical conformation. Based on an analysis of the images obtained, a model of the possible conformation transitions of supercoiled DNA in vitro in the absence of proteins with increased density of supercoiling is proposed. Our results show that compactization of supercoiled DNA molecules to the level of minitoroids and spheroids is due to growth in surface charge density and the hydrophobicity of aminomica.