THE DISTRIBUTION OF TOPOISOMERASE-II ON MAMMALIAN CHROMOSOMES

The distribution of topoisomerase II (Topo II) has been studied using immunofluorescence on cytocentrifuged preparations of mammalian chromo somes. Immunolabelling of Topo II is affected by choice of fixative, b y barriers to accessibility and by the lability of the enzyme. Chromos omes still embedded in cytoplasm remain unlabelled, while in contrast Topo II can easily be lost from some sites in chromosomes free of cyto plasm. The definitive distribution of Topo II consists of a line along the centre of each chromatid, corresponding to the chromosome core or scaffold, and quantities of Topo II elsewhere in the chromosomes whic h vary during the course of mitosis. A strong reaction for Topo II can be seen throughout prophase chromosomes, consistent with a role in co ndensation and/or segregation of the chromosome arms at this stage. At metaphase, Topo II is restricted to the centromeric regions, the only parts of the chromosomes that still have to be separated at this stag e, while in anaphase, after segregation has occurred, this centromeric concentration of Topo II is lost. The distribution and quantity of To po II in mammalian chromosomes is thus wholly consistent with the know n functions of this enzyme in chromosome condensation and segregation.