The long term research objectives of this proposal are: a) to precisely define how the activity of DNA topoisomerase II (topo II) is regulated by phosphorylation in cultured cells, and b) to identify the specific phosphorylation sites which influence cell growth and differentiation. Preliminary data strongly suggest that topo II is phosphorylated at multiple sites. Increased topo II phosphorylation is associated with elevated enzymatic activity in cell-free systems; however, in phorbol ester-treated HL-60 cells it is associated with reduced activity. Based on these observations and other phosphorylation/activity relationships, we form the hypothesis that phosphorylation of certain topo II sites renders this enzyme less active, while the phosphorylation of other sites renders it more active. To scrutinize this hypothesis, and to define the biological significance of topo II phosphorylation in cultured cells, we propose to: a) map the multiple phosphorylation sites of topo II; b) identify the specific topo II sites modulated by protein kinase activators and inhibitors, define their impact on the catalytic (strand passing) activity of the enzyme; and c) identify specific topo II determinants which, when phosphorylated, influence the ability of the enzyme to ligate DNA in the presence of topo II-reactive antitumor agents. At the end of this project, it is anticipated that the regulation of cell growth and differentiation will be further clarified and the interactions of topo II and protein kinases will be documented.