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 patte rn discrete DNA fragments obtained upon fractionation of nuclear DNA p reparations by field inversion gel electrophoresis. The discrete cleav age of nuclear DNA was shown to be SDS-dependent in Mg2+-containing me dium and SDS-independent in Ca2+-containing one. Specific topoisomeras e II inhibitors teniposide (VM-26) and amsacrine (m-AMSA) were shown t o 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 c leavage-rejoining reactions. This offers a possibility of investigatin g the role of type II topoisomerases in the structural organization an d functioning of the eukaryotic genome.