Eukaryotic DNA topoisomerase II is an abundant nuclear enzyme that is essential 
for cell proliferation. This homodimeric enzyme catalyzes the cleavage and 
re-ligation of double-stranded DNA required to separate replicated sister 
chromatids. Both biochemical and genetic studies show that its catalytic 
activity is required for chromosome condensation and segregation, and that its 
decatenation activity can be stimulated by a variety of protein kinases in 
vitro. In budding yeast, topoisomerase II is most highly phosphorylated in 
metaphase, and casein kinase II (CKII) was shown to be the major kinase 
modifying topoisomerase II. We have investigated the effects of phosphorylation 
of yeast topoisomerase II by CKII in vitro, by means of gel-retardation and 
filter binding assays. The phosphorylation of the C terminus of topoisomerase II 
by CKII appears to increase the stability of the complex formed with linear DNA 
fragments, while dephosphorylation has the opposite effect. Rephosphorylation of 
phosphatase-treated topoisomerase II by chicken casein kinase II restores a 
stable protein-DNA complex using a linear DNA fragment. The enhanced stability 
of the topoisomerase II-DNA complex is also observed with relaxed circular DNA, 
but not with supercoiled minicircles, in agreement with published results using 
topoisomerase II from Drosophila. Limited proteolysis and probing with 
domain-specific antibodies shows that, with the exception of a weakly modified 
residue between amino acid residues 660 and 1250, all residues modified by 
casein kinase II are in the last 180 amino acid residues of yeast topoisomerase 
II.