FUNCTIONAL-ORGANIZATION OF PLANT NUCLEAR-DNA .1. EVIDENCE FOR A DNA TOPOISOMERASE-II COMPLEX

A study was made of the changes evoked by various factors in the pattern discrete DNA fragments obtained upon fractionation of nuclear DNA preparations by field inversion gel electrophoresis. The discrete cleavage of nuclear DNA was shown to be SDS-dependent in Mg2+-containing medium and SDS-independent in Ca2+-containing one. Specific topoisomerase II inhibitors teniposide (VM-26) and amsacrine (m-AMSA) were shown to increase the SDS-dependent nuclear DNA cleavage, while NaCl promoted rejoining of cleaved DNA. The data presented allow one to regard the nuclear DNA organization as a constituent of the topoisomerase II-DNA complex, the main property of which is its capability of equilibrium cleavage-rejoining reactions. This offers a possibility of investigating the role of type II topoisomerases in the structural organization and functioning of the eukaryotic genome.